- The spectrum of the center of this nearby Scd
galaxy is basically flat (F
0)
across the entire IUE wave band. At shorter wavelengths the continuum
is rich in absorption features from massive hot start.
M33 is a bright galaxy in the X-ray band, and indeed
its nucleus is
the brightest X-ray source in the Local Group (Markert & Rallis
1983). An interpretation of the nucleus as a low-luminosity active
nucleus has been suggested by Trinchieri, Fabbiano, & Peres
(1988). Ciani, D'Odorico, & Benvenuti (1984) compared stellar
synthesis models against the IUE spectrum of the center and found a
best fit with a multigenerational model, with a young component of age
~ 107 yr and an old component with age ~ 1010 yr
superposed. M33
has been very thoroughly studied in the UV; but most work concerns the
properties of individual constituents of the galaxy, as opposed to the
general stellar population of the central regions. The OB start of M33
were studied by Massey, Hutchings, & Bianchi (1985); the M33 supernova
remnants were studied by Blair, Kirshner, & Raymond (1984a); and the
M33 Hubble-Sandage variables were studied by
Humphreys et
al. (1984). Surprisingly, only three short-wavelength spectra and one
long-wavelength spectrum of the center of M33 are in the IUE archives.
NGC 1023; SB0
- The long-wavelength spectrum of NGC 1023 is
apparently dominated by F and G start. NGC 1023 is a normal SB0 galaxy
in a group of 13 gravitationally bound galaxies (Tully 1980). Since
NGC 1023 lies at Galactic latitude b =
19°, the strong absorption
line of Mg II is very likely to be from our Galaxy. In contrast, Mg I
is so strong that it is unlikely to be from just the Milky Way ISM,
implying that there is an intrinsic Mg I component.
NGC 1068 (M77); Sb, Seyfert 2
- This spectrum has a flat (F -0.4)
continuum with emission lines of
both low and high ionization. NGC 1068 is the nearest and brightest
example of a Seyfert 2 galaxy. This highly studied Sb galaxy was
observed by Antonucci & Miller (1985) in polarized light and found to
harbor a Seyfert 1 nucleus (see Kinney et al. 1991a for a review).
Multiple IUE spectra were combined by Snijders, Netzer, & Boksenberg
(1986), who found broad wings on the permitted emission lines and
broad emission features of Fe II. Ferland & Osterbrock (1986) and
Kinney et al. (1991a) found that the number of recombination photons
measured was larger than would be predicted based on the number of
photons available to ionize the gas. Thus the source of the UV
continuum of NGC 1068 appears to be obscured from direct view.
NGC 1097; SBbc, hot spot + LINER
- This galaxy has been called a
LINER by Keel (1983b) based on optical emission lines. The UV spectrum
is flat with strong absorption features, which places the nucleus of
NGC 1097 in the low activity range among the class
of LINERs (cf. the
emission lines of more active LINERs such as NGC 3031 or
NGC 4579). NGC 1097 is a barred spiral with a ring of star
formation
surrounding the nucleus (a ``hot spot'' galaxy) at a radius of ~ 10", or
~ 1.5 kpc, which is barely outside the IUE aperture. The ring emits in
radio wavelengths, in the 12CO (J = 1-0) line, at 10
µm, and in
H
(Phillips et al. 1984; Hummel,
van der Hulst, & Keel 1987a;
Gerin, Nakai, & Combes 1988). NGC 1097 also contains a weak, compact,
flat-spectrum radio source at the center (Wolstencroft, Tully, &
Perley I 984) and has two pairs of optical jets extending from the
nucleus out to 90 kpc (see Lorre 1978 for spectacular images). Its
large-scale morphology suggests dynamical disturbance, perhaps by
interaction with the companion galaxy NGC 1097A.
NGC 1140 (Mrk 1063); Irr Am, BCG
- NGC 1140 is a blue irregular galaxy
containing a large blob of ionized gas in its center, where the IUE
aperture has been placed. Lamb, Hunter, & Gallagher (1986) have
studied the IUE spectrum of this galaxy, which has a continuum slowly
rising toward shorter wavelengths that is rich in absorption features.
The above authors have also synthesized the spectrum, concluding that
the galaxy has experienced a single burst of star formation starting
about 3 x 106 yr ago and that its population is mainly
constituted of
main-sequence stars with masses <
50 M
, with a contribution from supergiants and, possibly,
Wolf-Rayet stars.
NGC 1313; SBdm, H II
- The small apparent discontinuity between the
short- and the long-wavelength spectra could be due to typical
pointing errors of 1"-2", or to the difficulty of extracting flux with
high background (see Fig. 17). Marcelin & Gondoin (1983) map some of
the many H II regions of NGC 1313 in the optical and show that they
extend well outside the range of the IUE aperture. Although this
object has been included in the Rosa, Joubert, & Benvenuti (1984)
catalog of H II regions, the spectrum does not have the blue slope
typical of H II regions.
NGC 1433; SBab
- This galaxy is classified as Seyfert-like by
Veron-Cetty & Veron (1986) because of the presence of faint emission
lines at optical wavelengths, with [N II] 6583 stronger than
H. The same classification does
not appear obvious from the UV
spectrum, which is that of a normal Sb galaxy. Absorption lines of
C IV, A1 III, Fe I, Fe II, Mg II reveal a complex contribution from a
population with a range of stellar types. NGC 1433 is a nearly
face-on, barred spiral galaxy, including three rings of enhanced blue
color and active star formation (Buta 1983) which all lie outside the
IUE aperture. The nuclear and the inner rings consist of two
components: a blue, narrow one due to young associations and H II
regions, and a broad, red component, composed of old start (Buta
1986). Observations in the radio (Harnett 1987) show low-in-tensity
nonthermal emission with a peak coincident with the optical nucleus.
NGC 1510; Amm pec, BCDG
- NGC 1510 is a low-mass companion galaxy to
the large barred spiral NGC 1512 and has complex central structures of
size ~ 10", which Eichendorf & Nieto (1984) call metal-poor H II regions
with a strong A star component. The UV spectrum rises slowly toward
shorter wavelengths and does have absorption features between 1850 and
2000 Å which are typical of A stars (see Fig. 5 for A star spectra).
NGC 1553; S0 pec
- The UV spectrum has weak flux below the
main-sequence turnoff at ~ 2400 Å, which is typical of an older
intermediate population present in quiescent S0 galaxies (Gregg 1989).
There is an increase in flux in the spectrum at wavelengths below
1400 Å (cf. Burstein et al. 1988 for analogous behavior among
elliptical galaxies, the ``rising branch''). This galaxy is at
sufficiently high redshift (z = 0.004) that the intrinsic Mg II
absorption feature is separated from the Galactic feature by 11 Å.
NGC 1569; Im, starburst
- NGC 1569 lies close to the plane of the
Galaxy (b = 11.°2) and shows very strong Galactic dust extinction
with a depression at 2200 Å and below 1475 Å. The very hot, bright
nucleus of this galaxy resembles a superluminous, young star cluster
(Arp & Sandage 1985). In the UV spectrum, only absorption features
from our Galaxy can be seen (C IV 1550, Fe II
2585, 2600, and Mg II 2800,
although the spectrum is noisy and
other absorption features cannot be unambiguously excluded. NGC 1569
is a Magellanic irregular galaxy, which is said by Hunter et
al. (1989b) to be undergoing active star formation and by Israel
(1988) and Israel & van Driel (1990) to be past its burst of star
formation by 107 yr. Hunter et al. consider NGC 1569 a starburst on
the basis of its population content (extreme Population I), as
obtained from the study of optical and IR properties. Large
uncertainties are expected in this analysis because of the high
reddening of this galaxy [E (B - V) = 0.56 ± 0.10 mag;
Israel 1988].
Israel and Israel & van Driel conclude that NGC 1569 is recovering
from star formation (poststarburst galaxy), based on UV and optical
colors which show an OB star population deficient in very early O
stars and based on analysis of the radio continuum, whose nonthermal
part can be explained by a supernova rate decreasing in time (Israel &
de Bruyn 1988).
NGC 1672; SAB(s)bc, starburst + Seyfert
- This galaxy has been
classified variously as a LINER, a Seyfert 2, or a starburst galaxy
(Diaz 1985, Mouri et al. 1989, Garcia-Vargas et al. 1990,
respectively). Based on the UV spectrum, which is flat with no
indication of a 2200 Å dust absorption feature, and with a rich
collection of broad absorption lines, NGC 1672 is a starburst galaxy
containing a wide range of ages represented in its stellar population.
This late-type barred spiral has an elongated and amorphous structure
centered on its nucleus (Sersic & Pastoriza 1965). NGC 1672 is both a
radio source and an X-ray source (Fabbiano & Trinchieri 1987).
Although its X-ray luminosity is comparable in intensity to that of
most normal spirals, NGC 1672 probably hosts an X-ray pointlike source
in its nucleus, unlike most normal spirals whose X-ray emission is due
mainly to massive binaries. In addition, its X-ray emission appears
extended (Fabbiano 1989).
NGC 1705; Irr Am, BCDG
- Lamb et al. (1985) suggest that this blue
compact dwarf galaxy is either in a mild starburst phase or in a
postburst phase. This irregular galaxy has a steep radial gradient in
surface brightness. Its low metallicity resembles that of its
neighbor, the LMC (Meurer et al. 1992). The strong emission lines
observed in the optical spectrum and the strong absorption features in
the UV spectrum of this galaxy are well accounted for by B0 and B1
stars of approximately 20 M, although this result does not
exclude a contribution from stars of higher mass assuming a normal IMF
(Lamb et al. 1985). In a multi-wave-band study, Meurer et al.(1992)
observe the presence of two stellar populations coexisting in the
galaxy: an inner high surface brightness component, extending up to
500 pc from the center, and a low surface brightness one, up to 1.5
kpc. The dichotomy may originate from two bursts of star formation
that arose at different times. Alternatively, the dichotomy may stem
from a difference in the IMF, with the high brightness population
being richer in high-mass stars. H, imaging reveals the
presence of a bipolar outflow of gas, likely to deplete the galaxy of
a large amount of its gas supply.
NGC 1705 is a gas-rich BCG, and
because of this, York et al. (1990) argue that the ISM contributes
Si IV 1400 and C IV 1550 absorption lines comparable to
the stellar contribution.
UGC 3838 (VII Zw 153, Mrk 7); Im, BCG
- This blue compact galaxy is a
giant, clumpy irregular with a number of starbursting knots of
exceptional intensity (Gordon & Gottesman 1981). UGC 3838 has bright
clumps which are, on average, 100 times brighter in the optical than
30 Doradus and which have the typical absorption features of O and B
stars indicative of the presence of supergiants (Benvenuti, Casini, &
Heidmann 1982b). A feature is present at 2200 Å, which is narrower
than the normal 2200 Å dust feature and is probably due to the high
background and the low sensitivity of this region of the LWP
camera. Synthetic spectra (Benvenuti, Casini, & Heidmann 1982a) show
that the UV continuum emission from the clumps of such galaxies is
from a sum of stars ranging in spectral type from O to early B.
Spectroscopic studies in the optical (Boesgaard, Edwards, & Heidmann
1982) show that the physical conditions such as temperature, electron
density, and chemical abundances are typical of H II regions in normal
spiral galaxies despite the exceptionally high luminosity of the
clumps.
NGC 2403; Sc
- The flux of this galaxy is approximately constant
through the ultraviolet region. This normal Sc galaxy of the M81 group
is the prototypical flocculent spiral (Elmegreen 1981), based on the
disjoint nature of its individual arms. NGC 2403 contains a large
number of H II regions and OB associations that are similar in size and
distribution to those in M33 and in the LMC (Hodge 1985). The more
active star formation regions of NGC 2403 are not in the nucleus
(Beckman et al. 1987), which appears quiescent.
NGC 2415 (Haro 1); Irr pec, BCG
- NGC 2415 is in a compact group,
with three companions situated about 22' (0.5 Mpc) north, but this
galaxy shows no evidence of morphological disturbance (van der Burg
1985). This blue compact galaxy is slightly ellipsoidal, with an
irregular core and irregular nebulosity (Gordon & Gottesmann 1981).
NGC 2415 has a blue magnitude of
MB = -21.0 and a neutral hydrogen
content of ~ 6% of the total mass of the galaxy (Gordon & Gottesmann
1981). Its far-infrared (FIR) luminosity is high
[log(LFIR / L) ~ 11.1; Iyengar, Rengarajan, & Verma 1985], probably
owing to the presence of an intense burst of star formation and to the
presence of dust. If dust is present as these authors suggest, it is
either lacking in the 2200 Å feature seen in Milky Way reddening
curves, or else the dust is possibly very clumped. The
short-wavelength IUE spectrum has been discussed by Fanelli et
al. (1988) and analyzed using spectral population synthesis. The UV
spectrum is rich in absorption features and shows the presence of OB
supergiants together with a composite population of O, B, and A stars.
The spectral synthesis models suggest that star formation took place
in discrete episodes rather than in continuous formation, with the
last episode arising about 10 Myr ago.
Holmberg II (VII Zw 223, Arp 268, DDO 50); Irr,
BCDG
- Holmberg II is
a blue compact dwarf galaxy described by Zwicky (1971) as a large
posteruptive blue irregular with compact core and many knots. It is a
resolved galaxy in the M81-NGC 2403 group. With a distance of about 3
Mpc, it is one of the nearest condensations in the local supercluster
(Sandage & Tammann 1975). Despite its location in a group, Holmberg II
does not appear to be physically interacting (Davis & Seaquist 1983).
The UV emission of Holmberg II is dominated by a single giant H II
region, where intense star formation is taking place. Indeed, the
continuum of the UV spectrum is typical of an H II region (cf. Rosa et
al. 1984). The spectrum shows possible nebular emission of C III]
1909. The galaxy-wide star
formation rate is lower than that
typical of normal spiral galaxies and can be considered approximately
constant over a Hubble time (Hunter & Gallagher 1985). Analysis of
IRAS observations suggests that only a small amount of dust is present
in Holmberg II, so that little hidden star formation
can be present
(Hunter et al. 1989a). Like most dwarf irregulars, the metallicity of
HolmbergII is low compared to that of normal spiral galaxies and is
intermediate between the metallicity of the LMC and that of the SMC (Hunter & Gallagher 1985).
UGC 4483; dIm IV, H II
- This dwarf galaxy is of relatively high
surface brightness and belongs to the M81 group (Karachentseva,
Karachentsev, & Borngen 1985). UGC 4483 is a possible companion of
Holmberg II with an angular separation between them
of 
1.°7
(Thuan & Seitzer 1979), corresponding to about 0.1 Mpc. Like most
dwarf galaxies, the H I content of UGC 4483 is relatively high, with
neutral hydrogen making up half the total mass of the galaxy (Thuan &
Seitzer 1979; Huchtmeier & Richter 1988). The high short-wavelength UV
flux of this galaxy is similar to that in HolmbergII and indicates
star formation activity. However, there is no evidence of the
absorption features typical of massive O and B stars from active star
formation. Depressions in the UV continuum may indicate the presence
of some weak absorption lines.
NGC 2639; Sa, LINER + Seyfert
- This early-type spiral galaxy (Sa)
has a large bulge component with broad H emission and
low-ionization emission lines, which lead Keel (1983b, 1984) to
conclude that it is a composite of a LINER and a Seyfert galaxy.
NGC 2639 is a radio-loud galaxy with an extended
source. Condon et
al. (1982) concluded from the extended nature of the radio emission
that the emission process is thermal and related to star formation
triggered by interaction with the companion A0843+49. When observed
with higher resolution (VLBl), NGC 2639 is found to be one of the rare
examples of a radio-emitting spiral galaxy with a VLBI core source
(Hummel et al. 1982). Higher signal-to-noise ratio UV data would
contribute significantly to understanding this intriguing galaxy.
NGC 2681; Sa, LINER
- Like other LINERs in the atlas, this early-type
spiral galaxy(S0/a) shows little evidence for nuclear activity in the
UV spectrum (Heckman 1980). Although detected in the radio, no compact
source has been seen (van der Kruit l971; Condon 1987,
Table 2). Analysis of the UV spectrum by Burstein et al. (1988) led to
the conclusion that the galaxy has some residual star formation with
major bursts about 1-2 Gyr ago.
NGC 2782; SAB(rs)a, starburst
- This starburst galaxy has a UV
continuum and strong UV absorption lines which resemble those in the
spectrum of the starburst prototype NGC 7714 (Kinney et al. 1984). The
nucleus is a strong, extended (~ 1 kpc) radio source, whose emission is
probably due to supernova remnants from the active star formation in
the galaxy center. The star formation may be triggered by the
anonymous companion (Condon et al. 1982). However, the source of the
radio spectrum is controversial: Heckman et al. (1983) find a
nonthermal component in addition, leading to the composite
classification of LINER plus starburst. This idea is supported by Keel
(1984), who notes the flatness of the radio spectrum. The high
far-infrared luminosity and the strength of the CO emission support
the scenario of an intense burst of star formation (Sanders & Mirabel
1985).
MCG 10-13-71 (Mrk 19); Irr, BCDG
- The optical spectrum of Mrk 19
shows the typical features of a giant H II region (Neugebauer et
al. 1976) and is interpreted as reflecting a constant star formation
rate since the start of the last burst about 6 x 106 yr ago (Dottori
1983). This galaxy is also defined as a ``lazy'' galaxy for its property
of forming stars by intermittent short bursts (Kunth & Joubert
1985). The UV continuum is in agreement with the picture of an H II
region (see Rosa et al. 1984), but the only visible absorption
features are Si IV 1400 and
the blends for > 1800
Å. The paucity of absorption features may be due to the relatively low
metallicity of the galaxy [O/H]
0.15[O/H] ; Kunth &
Joubert 1985).
NGC 2798; SBa, starburst
- This barred spiral galaxy forms an
interacting pair with NGC 2799. The gravitational interaction may be
responsible for the strong burst of star formation present in the
nucleus of the galaxy (Joseph, Wright, & Prestwich 1986), for the
extended nuclear radio emission (Heckman et al. 1983), for the narrow
optical emission lines, and for the powerful far-infrared luminosity
(Armus, Heckman, & Miley 1990). The extended 10 and 20 µm
emission is also in line with the above scenario (Joseph et
al. 1986). The short-wavelength IUE spectrum of NGC 2798 has been
studied by Joseph et al. (1986), and many details can be found in
their paper. The IUE slit was centered on the peak of the 10 microns
emission. The UV continuum is flat, as expected in a starburst
(cf. NGC 7714), and there are C IV and Si IV absorption
features,
typical of massive O and early B stars.
NGC 2841; Sb, LINER
- This Sb galaxy is classified as a LINER by
Heckman (1980) and Keel (1983a,b) on the basis of optical
emission-line ratios and strengths. While Baldwin,Phillips, &
Terlevich (1981) conclude that the ionizing mechanism is shock
heating, Ferland & Hetzer (1983) suggest that the mechanism is
photoionization. A ring of enhanced star formation surrounding the
nucleus is suggested by optical and near-IR data (Prieto et
al. 1985). (The ring lies outside the IUE aperture.) No prominent
emission features can be seen in the UV spectrum. However, NGC 2841
does show a clear rising branch, with flux increasing from 1800 to
1200 Å.
NGC 2903; Sbc, hot spot
- NGC 2903 contains active star formation
within its hot spot knots and also throughout its entire nucleus
(Bonatto, Bica & Alloin 1989). The UV spectrum has deep, complex
absorption features of a starburst galaxy, but with a slope increasing
toward long wavelength. This slope, as well as the complex absorption
features across the entire IUE wave band, indicates the
presence of a
mixture of early and late-type stars, which is consistent with the
conclusions of Oka et al. (1974) based on optical observations. Simons
et at. (1988)note that the ``patchy'' appearance of the central regions
in the V band is due to a variation of internal extinction. The Mg II
absorption feature is too strong to be due to Galactic absorption
alone (see Table 4).
I Zw 018 (UGCA 166, Mrk 116); Irr, BCDG
- This blue compact dwarf
galaxy is one of the most metal-poor galaxies known (([O/H]
0.02[O/H] ; Dufour,
Garnett, & Shields 1988). I Zw 018 has been
described as the prototypical isolated extragalactic H II region that
is most probably undergoing its first burst of star formation (Searle
& Sargent 1970). There are two separate star-forming regions in this
galaxy, surrounded by a 3 kpc envelope of H emission (Dufour &
Hester 1990). The UV spectrum of this super-metal-poor compact galaxy
rises to the blue and contains weak low-ionization absorption features
(Dufour et al. 1988).
NGC 2997; Sc(s), hot spot
- The nucleus of this ``hot spot'' galaxy has
two parts, distinguished by their optical properties: a central red
core, consisting of older stars and in a quiescent phase (no
emission-line activity), and a blue annulus, consisting of a number of
knots, whose emission spectrum identifies them as giant H II regions
(Meaburn & Terret 1982). The radius of the ring is about 4", and is
included in the IUE slit (see Walsh et al. 1986). The UV spectrum is
weak and noisy, and the only convincing features are the Si IV, C IV,
and Mg II absorption lines. From population synthesis, the knots of
NGC 2997 apparently consist of reddened O stars and
less reddened B-
and A-type stars (Walsh et al. 1986). The reddening is due to the
obscuration induced by the clumpiness of the H II regions.
NGC 3031 (M81); Sb, LINER
- M81 is a nearby Sb galaxy, which has the
low-ionization emission lines of a LINER (Heckman 1980) but also has
the broad emission lines of a low-luminosity Seyfert 1 galaxy
(Peimbert & Torres-Peimbert 1981). The galaxy contains a compact,
powerful nuclear radio source (Condon et al. 1982; Kellermann et
al. 1976). Keel et al. (1985) have found evidence that tidal
interactions with companions increase the activity of the nucleus. M81
appears to be interacting with the other components of the M81 group,
which show a common H I envelope (Condon et at. 1982).
The spectra of M81 include only those with the aperture centered on
the galaxy (cf. Peimbert & Torres-Peimbert 1981; Ellis, Gondhalekar, &
Efstathion 1982). The UV spectrum has been analyzed by Peimbert&
Torres-Peimbert (1981) and by Ellis et al. (1982). The emission
probably originates from two different regions: The narrow permitted
lines and the forbidden lines, on the one hand, come from an extended
region where gas is shock-heated by a central source or photoionized
by a power-law spectrum. The broad permitted lines, on the other hand,
originate in a small, compact central region. The line broadening is
due to the motion of the gas around a very compact central object
(M ~ 107 M ;
Peimbert & Torres-Peimbert 1981; see also the
detailed analysis of Filippenko & Sargent 1988). In spite of the
presence of the typically broad Seyfert emission lines, such as Mg II
2800, the UV continuum is that
of a normal early-type spiral galaxy.
NGC 3049 (Mrk 710); SBbc, starburst
- This spiral galaxy belongs to
the Virgo Cluster and is a starburst (Balzano 1983) that
has also been
called a W-R galaxy (a galaxy whose spectrum exhibits the emission
from Wolf-Rayet stars) by Kunth & Schild (1986). Our UV spectrum does
not show clear P Cygni profiles or He II emission, as would be expected
if W-R stars were present. The near-IR emission has been attributed to
a thermal source ( Balzano & Weedman 1981). A discussion of the
visible, UV, and far-infrared properties of Mrk 710, together with a
population synthesis model, can be found in Mas-Hesse, Arnault, &
Kunth (1989), who conclude that the present burst of star formation is
probably only about 5 Myr old.
NGC 3081; SBa, Seyfert 2
- This early-type barred spiral galaxy has
the same relative emission-line intensities and emission-line profiles
as a Seyfert 2 galaxy, but with a much lower luminosity (Phillips,
Charles, & Baldwin 1983). The UV spectrum (also studied by Durret &
Bergeron 1986) shows the emission lines of high excitation and of low
excitation typical of a Seyfert 2 galaxy, as well as the flat
continuum (F
0).
NGC 3125 (Tol 3); Irr Am, BCDG
- This metal-poor, irregular amorphous
galaxy is a dwarf undergoing strong star formation activity (Kunth &
Sargent 1983; Gallagher & Hunter 1987; Kunth, Maurogordato, & Vigroux
1988). Recognized as a W-R galaxy in the optical, its population seems
to be the result of periodic bursts of star formation followed by
periods of quiescence (Kunth & Sargent 1981). The UV spectrum has a P
Cygni profile of C IV 1550,
consistent with the picture of NGC 3125 as a WR galaxy.
NGC 3256; Sb(s) pec, starburst
- This galaxy contains a very bright
star-forming region spanning over 4 kpc and called by Joseph & Wright
(1985) a ``super-starburst''. NGC 3256 is also ultraluminous in the
infrared (Kawara, Nishida, & Gregory 1990). Tidal tails evident in the
optical photographs of Joseph & Wright signal a merger. The
short-wavelength UV spectrum shows a flux decreasing toward short
wavelengths with very strong, deep absorption features. While the
deep broad absorption features reveal the presence of young, hot
stars, the overall slope is probably due to dust [E (B - V) = 0.13 mag].
UGC 5720 (Haro 2, Mrk 33); Im, BCDG
- This blue compact dwarf galaxy
has gone through several bursts of star formation and possesses a
large star formation region of size greater than 350 pc. The low
metallicity resembles that of the LMC (Meurer, Cacciari, & Freeman
1990). The ultraviolet spectrum of UGC 5720 exhibits well-defined,
strong absorption lines and a continuum that rises slowly toward
shortwave-lengths (Fanelli et at. 1988). The stellar synthesis of
Fanelli et al. indicates that the galaxy has gone through at least two
earlier bursts of star formation, the most recent being not more than
20 Myr old.
NGC 3310; SAB(r) bc, starburst
- The central part of this bright
peculiar galaxy has a complex morphology, with a ring of H II regions
surrounding the nucleus (an H II region itself) at a radius of a few
kiloparsecs, and with a ``bow and arrow'' structure in the northwest
region (Walker & Chincarini 1967; Bertola & Sharp 1984). The galaxy
has an extremely bright blue optical continuum (Telesco & Gatley 1984)
and strong extended X-ray, UV, and radio emission of thermal origin
(Fabbiano, Feigelson, & Zamorani 1982; van der Kruit & de Bruyn 1976).
All these aspects, in addition to the intense optical emission lines
(Heckman & Balick 1980) and the very high IR luminosity
(LIR 3 x
1010 L; Telesco & Gatley 1984), place NGC 3310 among
the most luminous star-forming galaxies. The most intense and youngest
burst is taking place in the ``jumbo'' H II region, situated 15"
southwest of the nucleus (Telesco & Gatley 1984). The IUE spectrum is
in perfect agreement with the picture of a starbursting galaxy: the
rising continuum and the deep absorption lines are typical of OB
associations.
NGC 3351 (M95); SBb, hot spot
- This is a typical ringed galaxy: a
quiescent nucleus is surrounded by a ring of H II regions which appear
as bluer knots where intense star formation is taking place. In
particular, the ringlike structure of M95 has an external diameter of
about 20", around a redder nucleus of about 2" (Alloin & Nieto 1982
and references therein). There are three major complexes of H II
regions, whose high content of O stars gives a young age to the
burst. Alloin & Nieto (1982) suggest that the burst is related to the
existence of a radial inflow of the ring toward the nucleus, which
might produce an accumulation of neutral gas on the inner Lindblad
resonance, providing fuel for the star formation. The UV spectrum
shows unusually strong C IV and Mg II absorption lines.
NGC 3353 (Haro 3, Mrk 35); Im,BCDG
- Haro 3 is a typical BCDG with a
UV continuum rising toward shortwave-lengths and a spectrum rich in
both low- and high-ionization absorption features. A complete
discussion of the UV properties of this galaxy, in addition to a study
of spectral synthesis, can be found in Fanelli et al. (1988). The main
conclusions are that the stellar population is composed of
main-sequence stars ranging from O3 to mid-A; the contribution from
cool stars is negligible; there is no significant population of blue
supergiants; the low-ionization absorption lines have stellar origin;
due to the high star formation rate and limited gas supply, the star
formation history is characterized by discrete star formation
episodes.
NGC 3393; SBa, Seyfert 2
- This Seyfert 2 galaxy has a low-level
continuum flux with some evidence of the underlying continuum of the
SBa galaxy and with very strong, narrow emission lines typical of
Seyfert 2 nuclei. A detailed analysis can be found in Diaz, Prieto, &
Wamsteker (1988).
MCG 9-18-32 (Mrk 153); Scp, BCDG
- Mrk 153 is defined as a ``lazy''
galaxy by Kunth & Joubert (1985)-meaning that it is a blue
emission-line galaxy forming stars by intermittent short bursts - and
as a blue compact galaxy by Thuan & Martin (1981). The continuum rises
steeply to short wavelengths with strong absorption features,
suggesting the presence of massive, hot stars.
1050+04 (Mrk 1267); BCG
- The relatively large redshift of this
galaxy separates intrinsic features from Galactic features. The
spectrum of this BCG shows a rich and complex system of absorption
lines, including those usually hidden by the geocoronal emission,
i.e., Ly 1215 and NV
1240. In addition, the
intrinsic Fe II 2600 and Mg II
2800 lines are clearly
distinguished from those due to Galactic absorption. The relatively
flat continuum is a manifestation of the starforming activity of the
galaxy, and the presence of high-ionization absorption lines reveals
that there are many early supergiants.
NGC 3448. Irr Am, starburst
- NGC 3448 is part of a tidally
interacting binary with the dwarf spiral UGC 6016. This galaxy has
been recognized as a starburst because of many properties: the radio
brightness of this amorphous galaxy comes mainly from the nucleus, the
spectrum is dominated by far-infrared emission, and the UV flux is
typical of the presence of massive, hot stars (Noreau & Kronberg
1985a). The nuclear radio source has been resolved into a family of
point sources similar to those seen in M82, which are most probably
supernova remnants and are coincident with a dust lane in the center
of the galaxy (Noreau & Kronberg 1985b, 1987). Bertola et al. (1984)
conclude that the short-wavelength UV flux is due to early B stars.
NGC 3504; SABab, starburst
- Optical and radio data suggest that the
galaxy has a composite nucleus, which shows both nonthermal activity
and recent star formation (Keel 1984). The radio emission is due to
the presence of a strong nuclear radio source in a region of about 1
kpc (Condon et al. 1982). The optical spectrum is dominated by the
regions of active star formation (Keel 1984), which are possibly
triggered by the companion NGC 3512, 50 kpc distant from the main
galaxy (van Moorsel 1983).
The UV spectrum is rich in absorption lines, with the presence of a P
Cygni profile for C IV, indicative of an evolved population of massive
supergiants. The flat continuum and the rich absorption features
resemble the spectrum of the prototype starburst galaxy NGC 7714.
Mrk 36 (Haro 4); Irr, BCDG
- This blue compact dwarf galaxy is
faint and pointlike in the UV. Mrk 36 is probably young; although it
is undergoing intense star formation, its dust content is
exceptionally low, based on IRAS fluxes (Gondhalekar et al. 1986),
suggesting that the stars may not be evolved enough to have built up
the dust seen in normal galaxies. Indeed, the gas-to-dust ratio of
this galaxy is greater than 105 (Gondhalekar et al. 1986), which is
two orders of magnitude above even that inferred for damped Ly
systems (Fall, Pei, & MacMahon 1989), purported to be proto-disk
galaxies (Wolfe 1987).
The UV absorption lines are weak, probably
because of the low metallicity. Only C IV 1550 and the Fe blend
around 1620 Å are
recognizable. The C III] 1909
nebular
emission line is also detected. The rising blue continuum is
consistent with the star formation scenario.
NGC 3660 (Mrk 1291); SBbc, mini-Seyfert 1, NELG
- This galaxy is
classified, on the basis of the optical spectrum, as between a Seyfert
1 and a narrow emission line galaxy (NELG) (Kollatschny et
al. 1983). Nevertheless, the UV spectrum looks like that of a normal
SBbc galaxy with no nonthermal activity. This galaxy, with z = 0.012,
shows strong Ly emission.
NGC 3690 + IC 694; Sc pec, starburst
- The IUE aperture contains both
objects in this pair of ``colliding or strongly interacting'' galaxies
(de Vaucouleurs, de Vaucouleurs, & Corwin 1976), the partner being
IC 694. This system is the most representative
example of intense and
widespread star formation activity, presumably triggered by an
interaction (Gehrz, Sramek, & Weedman 1983). It is a strong radio
source (Condon etal. 1982), has a high infrared luminosity
(LIR 5 x
1011 L; Carico et al. 1988), and has the most luminous
optical emission lines of any non-Seyfert galaxy (Weedman 1972). All
of these properties may be explained within a starburst model. The
system has a complex morphology: three brightest regions can be
identified in the optical, radio, and IR bands - region A (nucleus of
IC 694), region B (nucleus of NGC 3690), and region C. All of them are
starbursting (Gehrz et al. 1983). In a region A there is both a radio
and an IR unresolved source, from which 60% of the far-infrared
luminosity of the system originates (Joy et al. 1989). This source
might be compact and may not be explainable with thermal processes
(Gehrz et al. 1983). The remaining 40% of the far-infrared emission
originates from a compact source in NGC 3690, probably the
extranuclear region C (Joy et al. 1989). The absorption-line-rich UV
spectrum confirms the intense thermal activity of NGC 3690. Because of
the relatively high redshift of this galaxy (z = 0.012), the Ly absorption line is detected.
NGC 3738; Irr IV, H II
- NGC 3738 is a giant irregular galaxy with
high-excitation emission lines, with a large fraction of its mass in
the form of neutral hydrogen, and with moderately low metallicity. It
has been called a normal H II galaxy by Phillips et al. (1983). Hunter
& Gallagher (1986) conclude (following the prescription by Gallagher,
Hunter, & Tutukov 1984) that the star formation rate has been
constant in time. However, Tacconi & Young (1985) suggest that
irregular galaxies form stars more efficiently than do spirals and, as
a consequence, that the present star formation rate of NGC 3738 cannot
be sustained for more than 108 yr. The UV spectrum resembles those
of H II regions (cf. Rosa et al. 1984) with emission dominated by
massive, hot stars.
NGC 3982; Sbc, Seyfert 2
- This Sbc galaxy does not have any
prominent emission lines in the UV, despite being classified as
Seyfert 2-like by Phillips et al. (1983) based on the presence of
high-excitation lines and low intrinsic luminosity in the optical. The
UV continuum is dominated by the normal Sbc galaxy spectrum and shows
evidence of absorption features.
NGC 3991 (Haro 5); Im, BCG
- This Magellanic irregular belongs to a
multiple interacting system, which also includes NGC 3994 and NGC 3995.
The gravitational interaction with the companions may trigger
the activity of the galaxy (Keel et al. 1985). The optical spectrum
shows that NGC 3991 resembles an H II galaxy, with a very blue
nucleus
(Keel et al. 1985). The BCG nature is supported by its low
metallicity (see, e.g., Arnault et al. 1988, where the problem of
nondetection of CO in BCGs is also treated) and by the characteristics
of our UV spectrum, which according to Rosa et al. (1984) resembles an
H II region. A higher level of activity is revealed in the radio, where
the emission is comparable to that of a Seyfert galaxy (Seaquist &
Bell 1968), and in the X-ray (Lx = 2.5 x
1041 ergs s-1; Fabbiano et
al. 1982). The authors attribute the X-ray emission to the intense
star-forming activity. This spectrum with high signal-to-noise ratio
shows absorption lines as well as emission in Ly and He II.
NGC 3994; Sc, LINER
- This galaxy interacts with both NGC 3991 and
NGC 3995. On the basis of the criteria given by
Baldwin et al. (1981),
NGC 3994 is classified as a LINER. It has a complex
structure, with
vigorous star formation going on in the ringlike outer arms (Keel et
al. 1985). NGC 3994 also has a weak broad component of H and is
a radio source (Hummel et al. 1987b). In spite of the signs of
activity in the optical and radio wave bands, the UV spectrum shows no
evidence of emission lines, while the shape of the continuum resembles
that of a normal Sc galaxy, with some additional continuum emission
between 1500 and 1900 Å.
NGC 3995; Sm, H II
- The membership of this spiral galaxy in a
multiple interacting system is considered the main cause of its
enhanced H II region emission spectrum and very blue continuum in the
visible (Kennicutt & Keel 1984). Despite the less than optimal
signal-to-noise ratio, our UV spectrum is in agreement with the strong
H II regions thought to be present in the galaxy. (The UV spectrum
contains emission artifacts at 1280, 1290, and 1663 Å). Data in the
radio (Hummel et al. 1987b) and in the X-ray (Fabbiano et al. 1982)
also support the above picture. Although NGC 3391, NGC 3394, and
NGC 3395 are all members of the same group, and are
interacting, their
response to the interaction must vary, since their spectra are all
very different. Thus we are far from a good understanding of
interaction-induced star formation.
ESO 572-G034; H II
- The P Cygni profile of C IV and the broad
absorption feature of Si IV indicate the presence of evolved,
early-type massive stars, with temperatures hot enough to excite the
forbidden nebular emission line C III] 1909.
NGC 4102; Sb, LINER + H II
- NGC 4102 belongs to the Ursa Major galaxy
group (Odenwald 1986). This Sb galaxy is a powerful far-infrared
galaxy (FIRG) (Young et al. 1989), which appears as a supergiant H II
region (Armus et al. 1990). Keel (1983b) classified this galaxy as a
LINER because of the presence of optical low-ionization lines.
NGC 4102 also has a strong nuclear radio source
(Condon et
al. 1982). Although the radio emission is attributed by Condon et
al. to the supernova remnants associated with a strong burst of star
formation, and the radio core is small enough to be included in the
IUE aperture, the UV spectrum does not resemble that of a starburst
galaxy. NGC 4102 appears to be a normal Sb galaxy with some
evidence of a medium-age main-sequence turnoff.
NGC 4111; S0, LINER
- NGC 4111 is a normal S0 galaxy with an old
stellar population (cf. Gregg 1989), showing MgII and Fe absorption
features longward of 2600 Å which are typical of F-G dwarfs. The
short-wavelength spectrum is lacking a hot star component (Burstein et
al. 1988). Although NGC 4111 has at least eight much fainter
companions, they have no apparent effect on the galaxy itself (van der
Burg 1985). NGC 4111 is classified as a LINER by Heckman (1980),
but there is little evidence of nonthermal activity from the UV
spectrum. NGC 4111 may be on the boundary between active and normal
galaxies.
NGC 4194 (Mrk 201); Sm pec, BCG
- This BCG galaxy shows an unusually
flat continuum (see Table 5). While the absorption features of Si IV
and C IV (with a possible P Cygni profile of C IV) show the presence of
a hot population of massive O and early B stars, the very flat
spectrum suggests the presence of a cooler component of A stars. The
depth of the Mg II 2800 feature
is greater than expected from our Galaxy and is probably due in part to
intrinsic absorption in NGC 4194.
NGC 4194 is a BCG (Thuan & Martin 1981), a
radio-loud galaxy (Heckman
1983), and a FIRG (Armus, Heckman, & Miley 1989). The activity is
apparently powered by a central burst of star formation that is
claimed to be driven by the merging of two galaxies, as evidenced in
part by the tidal tail (Joseph & Wright 1985).
NGC 4214; IABm, starburst
- NGC 4214 is a metal-deficient blue
Magellanic irregular galaxy. In the UV study by Huchra et al. (1983),
who have carefully centered the IUE aperture on the optical center of
the galaxy, the P Cygni profile of C IV 1550 was found to be
consistent with the strong winds typical of O stars, which produce
~ 50% of the UV emission, while the absorption features at
< 1400 Å show the presence
of B stars. The galaxy has a complex
physical structure, which can be optically resolved into H II regions
and associations. The lack of evolved supergiants and Wolf-Rayet
stars in the UV would seem to rule out a decreasing or constant star
formation rate, suggesting instead a relatively recent burst of star
formation, superposed upon an older population. However, note that
Filippenko & Sargent (1991) find Wolf-Rayet features in some locations
in NGC 4214. Hartmann, Geller, & Huchra (1986) conclude
that the burst
of star formation is the result of a merger or strong interaction with
a companion. This consideration is based on the differences between
the velocity fields of the gas and of the older stellar population of
the galaxy.
NGC 4258 (M106); SBc, LINER
- This bright, barred spiral galaxy
contains two anomalous arms of H
emission in its inner regions,
which also emit in the radio. The radio emission is nonthermal in
origin and probably due to the ejection of matter or an explosive
event in the nucleus, according to Krause, Beck, & Klein (1984). Based
on the strengths and ratios of the optical emission lines, this galaxy
has been classified as a LINER, approaching the Seyfert domain
(Heckman 1980). The UV continuum does not manifest strong nuclear
nonstellar emission. The blue rising branch of the UV, the broad, deep
absorption features, and the nebular emission line of C III]
1909 are all characteristic
of H II regions. Ellis et
al. (1982) find that NGC 4258 is dominated by B stars, with little
contribution from O stars. Barbon, Capaccioli, & Longo (1984) suggest
that the UV continuum can be explained by several bursts of star
formation at different stages of their evolution. The short- and the
long-wavelength spectra do not match up, most likely because the
position angles of the 10" x 20" IUE aperture are not the same.
NGC 4321; Sbc, hot spot
- This is the largest spiral in the Virgo Cluster
and is another example of a ``hot spot'' galaxy. The ring, about
13" in radius, contains four distinct H II regions, where strong star
formation is present (Pierce 1986; Arsenault et al. 1988). The UV
spectra presented here are of two of these H II regions which are
contained in the nuclear region of the galaxy, and have been studied
by Panagia et al. (1980). They conclude that the absorption features
are mostly due to the interstellar medium present in the halos and in
the disks of both our Galaxy and NGC 4321.
NGC 4350; S0
- This normal early-type (S0) galaxy has a UV spectrum
consistent with its Nubble type, as described in section 1.
NGC 4350
belongs to the Virgo Cluster.
NGC 4382 (M85); S0 pec
- M85, a gas-poor galaxy of high surface
brightness, belongs to the Virgo Cluster and forms a strongly
interacting pair with NGC 4394 (Cutri & McAlary 1985). This
interaction might explain its bluer visible color in comparison with
typical S0 galaxies (Veron & Veron-Cetty 1985), interpreted as due to
enhanced star formation in the disk as to starformation in the
nucleus. It is a radio quiet galaxy with an X-ray/optical luminosity
of about 1.3 x 10-4 (Dressel & Wilson 1985), easily explained as the
integrated contribution of low-mass binaries (Trinchieri, Fabbiano, &
Canizares 1986). The UV spectrum of the nucleus of this normal galaxy
has been studied and synthesized by Rocca-Volmerange & Guiderdoni
(1987). who conclude that it has a dominant contribution from F and
early G dwarf stars.
NGC 4385 (Mrk 52); SBas, starburst
- A large bibliography and an
extensive discussion of the UV and optical properties of this galaxy
are given in Durret & Tarrab (1988). To summarize NGC 4385 is a
starburst galaxy with plentiful H I and extended H II. It is a typical
H II region galaxy, where the ionized gas is concentrated in a central
region of a few kiloparsecs and many H II regions are distributed along
the spiral arms. The presence of the Wolf-Rayet emission feature at
4650 Å, as well as the possible P Cygni profiles in N V and C IV in
the UV, suggest the present of Wolf-Rayet stars. Population synthesis
implies that the dominant contribution is given by WN7 and WC8.
NGC 4388; Sab, Seyfert 2
- This Seyfert 2 galaxy shows evidence that
it harbors an obscured Seyfert 1 nucleus. Corbin, Baldwin, & Wilson
(1988) and Pogge (1988) have reported high-ionization gas distributed
in two cones, with apices at the nucleus, and extending above and
below the disk. Shields & Filippenko (1988) detected broad H
emission off the nucleus and suggested that this emission originates
from an obscured Seyfert 1 broad-line region (BLR) which is reflected
into our line of sight by dust in the interstellar medium. The IUE
spectra were analyzed by Ferland & Osterbrock (1986) and by Kinney et
al. 1991a). They both found a steep spectral slope and found that the
number of recombination photons is much larger than predicted based on
the strength of the ionizing continuum. Thus the UV source in NGC 4388
appears to be occulted from direct view.
MCG 8-23-35 (Mrk 209); Sm pec, BCDG
- Mrk 209 is a BCDG undergoing a
burst of star formation (Thuan & Martin 1981). The UV spectrum rises
toward shorter wavelengths, indicating the presence of hot stars, but
lacks prominent features except for strong C III] 1909
emission. The weakness of absorption features in an object thought to
be undergoing star formation may be related to its low metallicity
(Z ~ 0.1 Z ; Fanelli et al. 1988), while the strength of the
strong C III] emission may be related to the high nebular temperature
(Viallefond & Thuan 1983).
NGC 4449; IBm, H II
- NGC 4449 is a very blue Magellanic irregular
galaxy, containing a large amount of gas, having a complex
distribution of H II regions, and undergoing active star formation
(Hunter 1982; Bothun 1986). Many of the UV observations of NGC 4449
are concerned with the off-nuclear H II regions (see, e.g., Rosa et
al. 1984; Lamb et al. 1986) and with a peculiar supernova remnant
(Blair et al. 1984b). Because of this, we have been especially careful
to extract only the one short-wavelength and the one long-wavelength
IUE spectra that were taken with the aperture placed on the galaxy
center.
The time evolution of the star formation rate in NGC 4449 is
thought to be constant, based on an analysis of the mass, blue
luminosity, and the strength of H
emission (Gallagher et
al. 1984) and also based on the strength of the infrared emission
(Thronson et al. 1987). From kinematical considerations, Hartmann et
al. (1986) conclude that the strong star formation rate is due to a
merger or interaction with the dwarf companion DDO 125. The poor match
between the short- and the long-wavelength spectra is probably due to
the placement of the aperture.
NGC 4500; SBa, BCG
- IRAS has revealed relatively intense
far-infrared emission from this metal-deficient blue compact galaxy
[log(LFIR / L) =
10.40; Deutsch & Willner 1987]. Mrk 213 has an
intense UV luminosity (Gondhalekar et al. 1984), suggesting a recently
triggered burst of star formation. Analysis of the combination of
near-IR, visible, and UV observations shows that the burst is
superposed on an underlying population of giants from an older
population. Our UV spectrum has a flat continuum with complex
absorption features which are easily identified due to the galaxy's
redshift. The only possible emission line is Ly; other features
are probably due to IUE artifacts.
NGC 4569 (M90); SABab, LINER
- M90 belongs to the Virgo Cluster and,
from properties of the optical emission lines, is classified as a
LINER(Stauffer 1982; Keel l983b). It is a strong emitter at 10 µm
(Scoville et al. 1983), and its emission in the range 1-20 µm is
dominated by dust and starlight, properties typical of normal galaxies
(Willner et al. 1985). This object has Fe II absorption and unusually
strong Mg II absorption.
NGC 4579; Sab, LINER
- NGC 4579 is a double radio source with a flat
spectrum and an unresolved core (Hummel et al. l987b). X-ray
emission has also been detected, indicating the presence of a central,
nonthermal source (Halpern & Steiner 1983). It has been classified as
a LINER and as a dwarf Seyfert 1 (Stauffer 1982; Keel 1983b;
Filippenko & Sargent 1985) based on the broad H emission
line. Its UV spectrum has been analyzed by Goodrich & Keel (1986), who
found that the galaxy appears stellar in the UV. (The spectra of
NGC 4579 presented in this atlas include additional
spectra to those
analyzed by Goodrich & Keel.) The UV spectrum contains both narrow
and broad emission lines, unlike the other LINER in the atlas; this
may be related to the fact that NGC 4579 has a double radio source
with an unresolved, flat-spectrum radio core. Goodrich & Keel (1986)
propose that its UV continuum is similar to the power law seen in
Seyfert 2 galaxies, but our spectra suggest instead that the UV
continuum is simply that of a spiral galaxy.
NGC 4594 (M104); Sa, LINER
- The Sombrero galaxy has in its nucleus
both a very compact and strong nonthermal radio source (Shaffer &
Marscher 1979; Condon et al. 1982) and a strong X-ray source (Halpern
& Steiner 1983). The radio and X-ray emission may be related to the
galaxy's activity as a LINER (Heckman 1980; Keel 1983a, b). However,
in common with the other LINERs in this atlas, there are no emission
lines in the UV. A search for a central massive object by Kormendy
(1988) resulted in a velocity dispersion profile consistent with a
mass of M ~ 108.5-109.5 M. The lack of emission features in our
UV spectrum indicates that the LINER activity is not strong, while the
absorption lines (Mg II 2800
and Fe I 2868,
3025) and the shape of the continuum indicate the predominance of an
old stellar population (Ellis et al. 1982).
IC 3639; SBb, Seyfert 2
- This nearby Seyfert 2 galaxy possesses a
strong radio core embedded in diffuse emission, and has a very high
surface brightness, so that it resembles NGC 1068, the prototype
Seyfert 2 galaxy (Ulvestad & Wilson 1989). The narrow [O III]
5007 emission line has a blue
asymmetry similar to those seen
in NLRGs (Heckman, Miley, & Green 1984), while no such asymmetry is
seen in H
(Fairall
1985). The UV spectrum is flat, with some
weak emission lines (and an emission artifact at 1663 Å).
UGC 7905S; BCG
- UGC 7905S is a BCG in strong interaction with its
companion UGC 7905N, and this is thought to have triggered
the
activity of the galaxy (Bushouse 1986). The short-wavelength UV
spectrum is steeply rising to short wave-lengths with a spurious
emission feature at 1820 Å.
NGC 4670; Am, BCDG
- NGC 4670 is an amorphous, metal-deficient blue
galaxy whose central regions have a spectrum typical of an H II region
(Huchra et al. 1983). Its spectrum is very similar to that of NGC 4214,
and the same considerations hold in this case, suggesting a
relatively recent burst of star formation, with the young stars
superposed on a much older population. As in NGC 4214 , the IUE
aperture has been carefully pointed to the optical center of the
galaxy (Huchra et al. 1983).
NGC 4736 (M94); SAab, LINER
- This galaxy has an expanding ring of
H II regions surrounding the nucleus. Studies of the H emission
from the ring (Buta 1988) and the discovery of many discrete compact
radio sources similar to those found in M82 and NGC 253 (Duric &
Dittmar 1988; Kronberg, Biermann, & Schwab 1985; Antonucci & Ulvestad
1988), explained as bremsstrahlung from H II regions and synchrotron
radiation from supernova remnants, confirm that the ring is the site
of intense star formation activity. The ring is 50" from the nucleus,
so that it falls outside the IUE aperture, and thus the galaxy does
not appear as a starburst in the UV.
Studies of the nucleus show that this object is relatively blue and
has an extremely high surface brightness (Keel & Weedman 1978). De
Bruyn (1977) has reported an extended (5") radio source centered on
the nucleus. The anomalous strength of the CO bands is explained by
Walker, Lebofsky, & Rieke (1988) as the result of a very powerful but
not recent starburst. The presence of a high percentage of stars as
early as A4-A7, as deduced by population synthesis models of the
optical spectrum of NGC 4736, is consistent with the picture of past
starbursting activity, although the weak lines and blue colors in the
optical can be evidence also for a metal-poor population with a hot
blue horizontal branch (Pritchet 1977). The presence of low-excitation
emission lines led Heckman (1980) and Keel (1983a, b) to classify this
galaxy as a LINER. The UV spectrum shows a forbidden emission
line (C III] 1909) and broad
absorption features. The continuum is typical of an Sab galaxy.
NGC 4748; Seyfert 1
- IRAS 1249-131 is classified as a Seyfert 1
galaxy from its optical spectrum by Osterbrock & De Robertis (1985),
de Grijp et al. (1985), and Carter (1984). It has been alternatively
classified as a Seyfert 1.5 by Dahari & De Robertis (1988), on the
basis of the scheme by Veilleux & Osterbrock (1987), and as a
``narrow-line'' Seyfert 1 by Goodrich (1989). Goodrich concluded that
NGC 4748 is a Seyfert with a small line-of-sight
velocity dispersion
in its BLR. The UV spectrum does indeed show emission lines (most
notably C IV 1550) narrower than
those of a typical Seyfert 1 (Kinney et al. 1991b).
NGC 4826 (M64); Sb, LINER
- The ``Black-Eye'' galaxy has a complex
emission structure with numerous H II regions and an irregular nuclear
low-ionization zone (Keel 1983a). This galaxy is considered a LINER
based on optical properties (Keel l983b; Halpern & Steiner 1983). Like
other LINERs. the radio, X-ray, and far-infrared luminosities of
NGC 4826 are typical of normal galaxies (Fabbiano,
Gioia, & Trinchieri
1988). The UV spectrum is typical of an Sb galaxy but with a possible
emission line of C III] 1909).
MCG +06-28-044 (Mrk 54), BCG
- The strong UV continuum and the
complex, broad absorption features suggest that this galaxy is a
starburst (cf. Gondhalekar et al. 1984). The strong absorption line of
Ly can be observed because of the
relatively high redshift. The
nebular emission line [O III] + C II] is evident, while C III] cannot be
detected, since its redshifted wavelength lies in the lowest
sensitivity region of the IUE cameras.
NGC 4853; S0, BCDG
- This galaxy is a member of a group with 19
members which traces the central Coma region (Perea, 1986). The UV
continuum of this BCDG is flat; the C IV line is the only identifiable
absorption feature. The emission at 1940 Å is Si III].
NGC 4861 + IC 3961 (Mrk 59); Im, BCDG
- Mrk 59 (NGC 4861 + IC 3961)
consists of a dwarf irregular (IC 3961) with a bright knot (NGC 4861)
superposed on its southern part, the knot possibly being an H II region
or an OB association (Huchra 1977). Our two spectra are taken with
different pointings specifically targeting the bright knot (NGC 4861)
and the center of the galaxy (IC 3961) separately. Note that Rosa et
al. (1984) do not distinguish between the two pointings and refer to
them both as NGC 4861. The UV spectrum of this BCDG (Thuan &
Martin
1981; French 1980) shows the burst of star formation. The spectrum is
dominated by hot stars, which produce a UV excess in the continuum and
the strong absorption features which are the signature of O and B
stars. The H II region is revealed by the strong C III] nebular emission
line.
NGC 5102; SA0 pec, starburst
- This low-luminosity galaxy is very
blue in the optical and has subsolar metal abundance (Bica & Alloin
1987; Bica 1988; Glass & Moorwood 1984). IR observations (Glass &
Moorwood 1984) and population synthesis of the visible spectrum (Bica
& Alloin 1987) suggest a burst of star formation, which occurred about
4 x 108 yr ago. A detailed analysis of the visible spectrum suggests
an underlying metal-poor red (old) population; residual weak emission
lines and a small H II region continuum contribution agree with the age
of the burst (Bica 1988). The spectrum is flat with strong absorption
lines typical of midto late-B stars.
NGC 5135; SABb, Seyfert 2
- NGC 5135 is a high-excitation
emission-line galaxy belonging to a group of seven galaxies which are
gravitationally bound and lie within 1 Mpc of one another. Tidal
interactions may be causally related to the Seyfert 2 activity
(Kollatschny & Fricke 1989). From X-ray and radio observations and
from the luminosities of the optical emission lines, the nucleus of
NGC 5135 has been called a mini-Seyfert 2 (Phillips
et al. 1983; Thuan
1984). The UV spectrum shows a dual nature; several narrow emission
lines are present, including Ly,
C IV 1550, and He II
1640, while absorption features
of Si IV 1397,
1402 and He II 1640 are also
present. The luminosity of the
Seyfert nucleus is sufficiently low that the underlying absorption
features of the H II regions (contributing 25% of the emission; Thuan
1984) are still detectable.
This galaxy has an anomalous ratio of
Ly / H (~ 5), suggesting the presence of dust or an
extinction law for NGC 5135 different from that of our Galaxy (Thuan
1984).
Mrk 66; BCG
- This blue compact, low-metallicity galaxy is redshifted
enough that its Ly emission is
distinguishable from the
geocoronal line. The strong UV continuum, rising to short wavelengths,
and the C IV absorption line are typical signatures of massive stars,
indicating that Mrk 66 is experiencing active star formation. The
IUE
spectrum of Mrk 66 has been studied by Hartmann et al. (1988),
and further comments can be found in their paper.
NGC 5194 (M51); Sbc, LINER
- The Whirlpool Nebula forms an
interacting system with its companion NGC 5195 (Keel et
al. 1985). Heckman(1980) and Baldwin et al. (198l) have classified
NGC 5194 as a transition galaxy between a LINER and
a Seyfert galaxy
because of the presence of broad wings in its optical emission
lines. Studies of the radio emission of NGC 5194 show a complex
structure of extranuclear bubbles, probably ejected from the nucleus
(Ford et al. 1985). However, the composite nuclear activity in the
radio does not have a counterpart in the X-ray (Palumbo et
al. 1985). The X-ray emission is not concentrated in the nucleus but
is extended over the galaxy disk. The X-ray emission is explained by
Palumbo et al. (1985) as being due to evolved stellar systems such as
X-ray binaries and old starbursts, or to gas outflowing from the
nucleus. The UV spectrum does not show evidence of nonthermal
activity; except for Si III and C III], there are no other detected
emission lines. The presence of a strong absorption feature of A1 III
1857 and the shape of the
continuum suggest that the dominant
population is A-G stars (Ellis et al. 1982).
NGC 5236 (M83); SBe, starburst
- M83 is a nearby, bright southern
galaxy which is almost face-on and which belongs to a class of
galaxies with an ``amorphous'' nucleus (Sersic & Pastoriza 1965).
M83 is
undergoing a burst of star formation in the nucleus and in the spiral
arms, possibly triggered by its companion NGC 5253 (Condon et
al. 1982). Associated with the star formation is strong emission in
the radio (Condon et al. 1982), in the infrared (Telesco & Houper
1980), and in the X-ray (Trinchieri, Fabbiano, & Palumbo 1985). The UV
absorption lines, which have a strong similarity to those of NGC 7714,
suggest that M83 is a starburst with an evolved population of
supergiants, whose signature is the P Cygni profiles of Si IV
1397, 1402 and C IV 1550 (Bohlin et al. l983). A large number of
supernovae have been observed in this
galaxy (five since 1923, among which the one detected in the nucleus
[1968L]) is a Type II supernova. The association of the Type II
supernova with H II regions in the spiral arms and in the nucleus is in
agreement with the general scenario of a starburst (Richter & Rosa
1984). Although the UV spectrum does not show a marked 2200 Å dust
absorption, observation in optical wavelengths indicates a value for
intrinsic E (B - V) 0.3
mag (Pastoriza 1975).
ESO 383-44; SAd, starburst
- ESO 383-44 is an Sd spiral with bright
knots, which is a member of the IC 4296 group of galaxies (Dodd,
Andrews, & MacGillivray 1986). This galaxy has a diffuse, extended H II
region powered by OB associations which surrounds a more compact
nucleus. The number of stars required compared with their lifetimes
indicates intense star formation activity (Meaburn 1983). Although the
UV spectrum is noisy, some features are present, such as absorption of
redshifted C IV 1550, the
signature of a hot star population,
and Si III emission. The unredshifted absorption lines probably come
from our Galaxy.
NGC 5256; Seyfert 2 + LINER
- This peculiar galaxy contains two
compact nuclei separated by about 4 kpc, thought to be the result of a
merger. Both nuclei are included in the IUE aperture, and their
contributions have been added in the final spectrum. The
short-wavelength spectrum of NGC 5256 has been studied by Kollatschny
& Fricke (1984). Emission lines of Ly, C IV, and C III] can be
identified in the short-wavelength spectrum, while MgII can be
recognized in the long-wavelength spectrum. The ratios between the
emission lines in each of the two nuclei have led to the
classification of nucleus a as a LINER and nucleus b as a Seyfert 2
galaxy. The galaxy is characterized by a relatively high far-infrared
emission [log(LFIR / L 11.37; Soifer
et al. 1987], in agreement
with the nonthermal activity of the nuclei and the widespread intense
star formation across the structure.
Most of the radio emission is concentrated in three spots: two
coincide with the two nuclei, while the third one is between the other
two. Mazzarella et al. (1988) has tentatively explained this
phenomenon as synchrotron emission due to the merging.
NGC 5253 (Haro 10); Im Am, starburst
- NGC 5253 is classified as an
amorphous galaxy belonging to the NGC 5128 group (Sandage & Brucato
1979). The galaxy is undergoing violent star formation, which may be
caused by interaction with the companion galaxy NGC 5236 (Moorwood &
Glass 1982). The nucleus is dominated by emission from complex H II
regions, as shown by a very blue continuum (Gonzalez- Kiestra, Rego, &
Zamorano 1987) and by hot star absorption lines in the UV which are
typical of these regions (Perola & Tarenghi 1980). The small
contribution from supernova remnants in the radio and X-ray, and the
low ratio of red giants to supergiants as revealed in the IR, give an
upper limit to the age of the present nuclear burst of 107 yr and
require that previous bursts must be much older than 108 yr
(Moorwood & Glass 1982). The very high signal-to-noise ratio UV
spectrum of NGC 5253 shows complex absorption, even in Ly, and
emission lines of Wolf-Rayet stars (Ne IV] 1486 and He II
1640).
UGC 8850 (Mrk 463); Seyfert 2
- Extensive observations of this double
nucleus galaxy in the optical, near-IR, and radio (Mazzarella et al.
1991) and in polarized light (Miller & Goodrich 1990), and studies of
its featureless continuum in UV + optical and of its hydrogen lines
(Kinney et al. 1991a), suggest that Mrk 463E is probably a
dust-obscured Seyfert 1 nucleus. The presence of a large amount of
dust surrounding Mrk 463E is also confirmed by the strong
far-infrared
luminosity of the galaxy [log(LFIR / L) = 11.77; Sanders et al.
1988]. The nature of Mrk 463W is ambiguous; it could be either a
Seyfert 2 nucleus or a powerful starburst. The extended radio
emission and the far-infrared luminosity may be enhanced by the
merging process (Mazzarella et al. 1991), as manifested by the tidal
tails and by the existence of a small difference in the recession
velocity of the two nuclei (
v ~ 50km s-1; Hutchings & Neff
1987). The two nuclei are separated by 3 kpc and are both included in
the IUE slit.
NGC 5457 (M101); Sc, H II
- This face-on nearby Sc galaxy has a large
angular size (~ 30') and is the most luminous member of a rich
group. NGC 5457 is tidally interacting with NGC 5474,
NGC 5477, and Ho
IV, resulting in a distortion of the H I distribution in its outer
regions (Davies, Davidson, & Johnson 1980). The most striking X-ray
features for this galaxy are in the disk, where several bright and
distinct pointlike sources are found, while the nucleus shows only
extended and not particularly strong emission. The disk sources are
likely to be individual massive binary systems associated with H II
regions and OB associations (Trinchieri, Fabbiano, & Romaine
1990). M101 is dominated in the UV by extreme Population I
in the form
of OB complexes (Hill, Bohlin, & Stecher 1984), which spread from its
nucleus to the spiral arms, and of which 1264 have been identified
(Hodge et al. 1990). The IUE spectrum shows a slowly rising blue
continuum with deep absorption features, indicating star formation
activity and a mixed stellar population from type O (C IV and Si IV
absorption lines) through B (C IV, Si IV, and the lines for
< 1350 Å) to mid-A (features
for > 1750 Å).
NGC 5643; SBc, Seyfert 2
- The nucleus of this almost face-on barred
spiral (Mottis et al. 1985) shows high-excitation optical emission
lines. These characteristics are suggestive of the nonthermal nuclear
activity in Seyfert 2 galaxies. However, its luminosity is lower than
that of a ``classical'' Seyfert 2, and thus it has been classified as a
Seyfert 2-like galaxy by Phillips et al. (1983). It is also a strong
nuclear radio emitter and an ``extreme infrared'' galaxy, that is, a
galaxy for which the ratio between the IR (80 microns) emission and
the B-band flux (per logarithmic frequency interval) is up to 250
(Antonucci & Olzewski 1985). The UV spectrum is noisy with very low
signal-to-noise ratio in the short-wavelength region, but a few
emission lines typical of Seyfert activity are clearly visible, such
as C IV, CIII], [Ne IV] 2424,
and, marginally, Mg II.
Mrk 477; Comp, Seyfert 2
- Mrk 477 (I Zw 92) is a rare example of a
Seyfert 2 galaxy that has displayed variability in the continuum (De
Robertis 1987; Kinney et al. 1991a). A companion object lies 50" from
Mrk 477, with tidal tails extending between the
two. The number of
recombination photons agrees well with the number of photons available
to ionize the gas (Kinney et al. 1991a), indicating that the ionizing
source is not blocked from direct view. Remarkably, this object seems
to have some traits of a Seyfert 1 galaxy, such as continuum
variability and a direct view to the ionizing continuum, while at the
same time showing characteristics of a Seyfert 2 galaxy, such as a
lack of broad emission lines and a high F[O III]
5007 / F ratio. The UV
continuum is flat with prominent
emission lines typical of a Seyfert 2 galaxy.
NGC 5728; SBb, Seyfert 2
- Phillips et al. (1983) classified this
barred spiral as a Seyfert 2 galaxy, but with emission line
intensities lower than those of classical Seyfert 2 galaxies. Like
many Seyfert 2 galaxies, NGC 5728 contains a central radio source
(Wright 1974). As in NGC 5135, there is a suggestion that the
ionization mechanism may be a composite, since there is a
low-ionization gas ring around the nucleus whose line ratios indicate
ionization by the hot stars contained in the ring (Schommer et
al. 1988; Pogge 1989).
In the low signal-to-noise ratio IUE spectrum,
the lack of absorption features does not allow any conclusions about
the star-forming component. The shape of the continuum places NGC 5728
nearer to LINERs than to Seyfert 2 galaxies, with the spectrum of a
typical Sb galaxy.
UGC 9560(II Zw 70, Mrk 829); Irr Pec. BCDG
- The interaction between
UGC 9560 and
UGC 9562 is probably the cause of the distortion in
the spiral structure of the two galaxies (Cutri & McAlary 1985). In
agreement with the hypothesis of the existence of a relation between
gravitational interaction and abnormal activity, this galaxy shows
enhanced emission both in the IR (Cutri & McAlary 1985; Young et
al. 1989)and in the radio (Altschuler & Pantoja 1984). The radio
emission is the combination of a thermal and a nonthermal component,
probably due to a superposition of supernova remnants similar to those
seen in M82 (Skillman & Klein I 988). The high signal-to-noise ratio
UV spectrum in our atlas shows the usual characteristics of H II region
emission, with a flux that is rising toward shorter wavelengths,
absorption lines from high-ionization species, and the nebular
emission line C III] 1909
(cf. Rosa et al. 1984).
NGC 5996 (Mrk 691); SBb, starburst
- The UV spectrum of this
starburst (classified as such by Balzano 1983 on the basis of optical
data) is similar to the one of the prototype NGC 7714. Here also Si IV
and C IV show P Cygni profiles, a signature of Wolf-Rayet stars. A high
far-infrared luminosity confirms the presence of vigorous star
formation (Deutsch & Willner 1987). This activity is possibly
triggered by the strong interaction with the companion
NGC 5994
(Bushouse 1987).
NGC 6052 (Mrk 297); Cl Irr, starburst
- NGC 6052 is the probable
result of the collision of two galaxies, which might have produced the
observed bursts of star formation in clumps in the center of the
galaxy (Alloin & Duflot 1979). The IUE aperture includes ``the central
clump, about half of the next two strongest clumps,'' and a few minor
knots (Benvenuti, Casini, & Heidmann 1979; see also Benvenuti et
al. 1982a). The UV slope (F
-1), indicates that the
clumps, which are the main
contributors to the emitted light, are composed of stars of spectral
type from O to A0. Absorption features are present which can be
attributed to the galaxy because of their redshift.
NGC 6217; SBbc, starburst
- This optically normal galaxy has strong
and extended IR emission, with log(LIR /
LB) 1.4 (Belfort,
Mochkovitch, & Dennefeld 1987). However, a large burst of star
formation coupled with a small extinction cannot be distinguished from
weak star formation with high extinction on the basis of
LIR / LB
alone. Other diagnostic tools suggest a current high level of
star-forming activity in NGC 6217. VLA radio observations have shown
the presence of both an unresolved nuclear source and extended
emission (Vila et al. 1990). The extended radio emission has a
complex structure concentrated in a few ``condensations'' and probably
is thermal (Hummel, van der Hulst, & Dickey 1984). The similarity of
its UV spectrum to that of NGC 7714, with a strong UV emission and the
presence of deep absorption features (showing contributions from O to
mid-A stars), is evidence of strong star formation activity.
Mrk 499; BCG
- This faint, blue compact galaxy shows a nearly
stellar core with little visible nebulosity. The UV spectrum of
Mrk 499
shows complex absorption features, including strong Ly
absorption as well as P Cygni lines of C IV. Hartmann et al. (1988)
surmise that, since the expected stellar Ly absorption
equivalent widths should be much smaller than observed, the galaxy is
likely to be surrounded by a substantial cloud of neutral hydrogen.
Tol 1924-416; BCG
- The central region of this BCDG is irregular and
knotty, and unusually luminous for its type. Tol 1924-416 harbors an
extremely blue ~ 7 x 106 yr old starburst region plus
extended systems
of photoionized gas (Bergvall 1985). The UV spectrum is that of a
typical starburst galaxy with the continuum rising toward short
wavelengths. It is also rich in features: due to the relatively high
redshift, Ly 1216 is
visible; C IV 1550 has a P
Cygni profile; and there is an absorption feature around 1305 Å (Si II
+ O I) and nebular emission of C III] 1909. The presence of
Mg II 2800 emission is unusual
for a starbursting galaxy. The
Mg II absorption at zero redshift is likely due to our Galaxy,
consistent with the low Galactic latitude. The internal reddening is
indeed E (B - V) < 0.04 mag (Iye, Ulrich, & Peimbert 1987). Carswell et
al. (1980) have studied the IUE spectra of the galaxy, and additional
considerations can be found in their paper, such as that the effective
spectral type of the OB association ionizing the gas is O4. This
galaxy has a low helium content. The gas-to-dust ratio of
Tol 1924-416 is estimated by Gondhalekar et
al. (1986) to be of order
6 x 104, which is two to three orders of magnitude higher than in
large galaxies and about one order of magnitude larger than in
high-redshift damped Ly systems
(Fall et al. 1989).
Gondhalekar et al. (1986) suggest that the high gas-to-dust ratio
simply reflects the lack of an evolved population, but this cannot
explain the discrepancy between the value they obtain and that of the
damped Ly systems.
NGC 7130; Sa pec, Seyfert 2 + starburst
- NGC 7130 is an extremely
luminous far-infrared source with a compact radio core (Norris et al.
1990). Both characteristics are related to the Seyfert activity. Thuan
(1984) discusses the dual nature of the nucleus of this galaxy as seen
in the IUE UV spectrum, with both broad absorption lines and blue
continuum showing the presence of massive stars and star formation,
and the Ly emission line. Shields
& Filippenko (1990) have
confirmed the dual nature of the nucleus in optical and IR studies.
NGC 7130 is similar to NGC 5135 in having both Seyfert 2 activity and
a vigorous star-forming H II region.
NGC 7250; S/I, starburst
- This galaxy appears as a interacting
double system (Markaryan et al. 1985). The presence of absorption
lines typical of hot stars suggests that NGC 7250 has active star
formation. However, it is difficult to distinguish absorption features
due to the galaxy itself from those of the Milky Way. In addition, the
IUE camera artifacts are evident here.
NGC 7496; SBc, Seyfert 2 + H II
- This barred spiral belongs to the
Grus Cluster (Aaronson et al. 1981). It has a very bright nucleus with
an intense UV continuum and fairly strong optical emission lines. The
nucleus of the galaxy hosts both an H II region (the blue color is also
due partly to early type stars) and Seyfert 2 activity (Veron et
al. 1981). It is also an extremely luminous far-infrared galaxy
(LFIR 7.8 x
1011 L) with a compact radio core (Norris et
al. 1990). In the UV spectrum the H II region component dominates, with
a rising continuum and deep absorption lines, the signature of hot
stars. No emission lines are present in the UV.
NGC 7552; SBbc, starburst
- NGC 7552 is one of the components of the
Grus quartet. The center shows the absorption
features of early-type
stars, presumably from a burst of recent star formation (Alloin &
Kunth 1979). Spectral population synthesis by Bica (1988) indicates a
population component younger than 5 x 107 yr and with solar
metallicity. The UV spectrum, in addition to having many absorption
lines from young stars, is rising toward longer wavelengths and also
has absorption features between 1850 and 2000 Å of A1 II, A1 III,
Fe II, and Fe III - the latter two are both signs of a dominant A star
population. The UV slope of this galaxy is one of the "reddest" of
the starbursts, with F is +0.3.
The complex optical spectrum of NGC 7552 has been attributed to the
presence of three or four cycles of star formation (Dottori &
Pastoriza 1986).
NGC 7673 (Mrk 325); Cl Irr, H II
- NGC 7673 is a clumpy irregular
galaxy with many regions of star formation, four of which fall in the
IUE aperture (Benvenuti et al. 1982a). This galaxy has a rich
absorption spectrum, the relatively high redshift allowing easy
identification. The C IV 1550
absorption at rest wavelength is
due to our Galaxy, but all other features are intrinsic, showing the
presence of an early (OB) population in the clumps. The feature at
1670-1690 Å may be a blend of A1 II and Fe II, indicating the presence
of a stellar population later than B8. Also, the spectrum increases
slowly toward shorter wavelengths, showing a nonnegligible
contribution from stars later than B, although the OB stars
dominate. Benvenuti et al. find that the UV luminosity of each clump
is ~ 100 times that of 30 Doradus, indicating an enormous burst of star
formation.
NGC 7714 (Mrk 538); Sdm pec, Starburst
- NGC 7714 is the prototype
starburst galaxy (Weedman et al. 1981). The P Cygni profiles of Si IV
and C IV, whose absorption wings are blueshifted by 1000 and 500 km
s-1, respectively, manifest the presence of an evolved population of
massive post-main-sequence stars (Leitherer & Lamers 1991). The burst
of star formation is thought to be caused by interaction with the
companion NGC 7715. The spectrum shows evidence of dust
absorption at
2200 Å. It might be dust intrinsic to NGC 7714 , since the extinction
from our Galaxy (see Table 1) is not high enough to produce the
observed 2200 Å feature.
NGC 7793; SA(s)d H II
- NGC 7793 is a prototype Sd galaxy and the
faintest of the five major members of the Sculptor group. As a typical
late-type spiral, it is dominated by a disk component and has a much
less dominant bulge component. The spectrum of the nucleus of this
galaxy is dominated in the UV by the H II regions (see, for example,
Rosa et al. 1984) and in the optical by relatively early-type and
almost solar metallicity stars (B-A-F mixture; Diaz et al. 1982)
coexisting with the older metal-poor population. The H II regions can
be seen in the absorption features in the UV spectrum and the nebular
C III] 1909.
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is
proportional to