![]() | Annu. Rev. Astron. Astrophys. 1996. 34:
749-792 Copyright © 1996 by Annual Reviews. All rights reserved |
10.5-10.99 | 11.0-11.49 | 11.5-11.99 | 12.0-12.50 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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|
| No. of objects a
|
| 50
| 50
| 30
| 40
| Morphology
| merger
| 12%
| 32%
| 66%
| 95%
|
| close pair
| 21%
| 36%
| 14%
| 0%
|
| single (?)
| 67%
| 32%
| 20%
| 5%
| Separation b
| [kpc]
| 36.
| 27.
| 6.4
| 1.2
| Opt Spectra
| Seyfert 1 or 2
| 7%
| 10%
| 17%
| 34%
|
| LINER
| 28%
| 32%
| 34%
| 38%
|
| H II
| 65%
| 58%
| 49%
| 28%
| Lir / LBc
|
| 1
| 5
| 13
| 25
| Lir / L'CO c
| [L | ![]() 37
| 78
| 122
| 230
| a Objects in the IRAS BGS plus
additional ULIGs from Kim & Sanders (1996).
| b Mean projected separation of nuclei for mergers and close pairs only. c Mean values |
The data presented in Section 4 are sufficient to present a fairly detailed picture of the morphology of LIGs and to investigate how their properties change as a function of infrared luminosity. Table 3 summarizes some of the main results using observations of the complete sample of LIGs in the BGS supplemented by data for a subsample of less luminous BGS objects and data for a larger sample of ULIGs from the survey of Kim (1995).
5.1 Strong Interactions and Mergers
It now seems clear that
strong interactions and mergers of molecular gas-rich spirals
are the trigger for producing the most luminous infrared galaxies.
The images of ULIGs from
the BGS (see Figure 4) show clearly
that maximum infrared luminosity is
produced close to the time when the two nuclei actually merge.
At Lir < 1011 L the vast majority of infrared
galaxies appear
to be single, gas-rich spirals whose infrared luminosity can be accounted for
largely by star formation. Over the range Lir =
1011-1012 L
there
is a dramatic increase in the frequency of strongly interacting systems
that are extremely rich in molecular gas; at the low end of this range
the luminosity appears to be dominated by starbursts with Seyferts becoming
increasingly important at higher luminosities.
Those objects that reach the highest infrared
luminosities, Lir > 1012 L
, contain
exceptionally large central concentrations of molecular gas; because of
heavy dust obscuration it is hard to distinguish the relative roles of
starburst and AGN activity, although the conditions are
clearly optimal for fueling both enormous nuclear starbursts as well as
building and/or fueling an AGN.
The enormous build-up of molecular gas in the centers of the most luminous infrared objects plays a fundamental role in LIGs and is best illustrated by showing data for several relatively nearby well-studied objects. The ultimate fate of these mergers, once their infrared excess subsides, is not completely clear. Examples are also shown below for a small but important subclass of objects which show both strong optical and infrared emission, and which plausibly represent a transition stage in the evolution of LIGs into optically selected AGN (e.g. Sanders et al. 1988b). Finally we mention a few objects initially identified as HyLIGs that illustrate why caution needs to be taken when identifying objects at higher-z.
5.2 Case Studies
![]() Figure 8. Well-studied mergers: (a) NGC 4038 / 39 (Arp 244 = ``The Antennae''); (b) NGC 7252 (Arp 226 = ``Atoms for Peace''); (c) IRAS 19254-7245 (``The Super Antennae''); (d) IC 4553 / 54 (Arp 220). Contours of H I 21-cm line column density (black) are superimposed on deep optical (r-band) images. Inserts show a more detailed view in the K-band (2.2 µm) of the nuclear regions of NGC 4038 / 39, NGC 7252, and IRAS 19254-7245, and in the r-band (0.65 µm) of Arp 220. White contours represent the CO(1->0) line integrated intensity as measured by the OVRO millimeter-wave interferometer. No H I or CO interferometer data are available for the southern hemisphere object IRAS 19254-7245. The scale bar represents 20 kpc. |
5.2.1 MERGERS OF MOLECULAR GAS-RICH SPIRALS
NGC 4038 / 39 (Arp 244 = VV 245 = ``The Antennae'')
This classic, nearby ``early merger'' system is composed of what appear
to be two
overlapping, distorted, late-type spiral disks
(Figure 8a). The
total infrared luminosity
is the minimum required by our definition of LIG. The K-band
image shows two nuclei ~ 15 kpc apart and what appears to be a
large ring of bright H II regions in the northern disk. CO interferometer
observations (Stanford et al. 1990) show that ~ 60% of the 3.9 x
109 M of
molecular gas in the system
(Sanders & Mirabel
1985,
Young et
al. 1995) is concentrated in the two nuclei and in a large
off-nuclear complex where the two disks strongly overlap.
No CO has been detected in the extended tails. In contrast, about 70% of
the total H I mass, M(H I) ~ 109 M
, is associated with the long
(total extent of ~ 100 kpc) tidal tails
(van der Hulst et al., in preparation). About one quarter of the
total H I mass is located at the tip of the southern ``antenna''. In
this early merger
the total ratio M(H2) / M(H I) is ~ 4.
NGC 7252
(Arp 226 = ``Atoms for Peace'')
NGC 7252 has been considered the prototype of a
late-stage merger
(Schweizer
1978). This object has Lir ~ 5 x
1010 L
and therefore is currently not a LIG. The K-band image shows
a single nucleus inside what has been described as a relaxed elliptical
body. The optical image shows relatively large symmetric tails, which
rotate in opposite directions and have a total extent of ~ 130 kpc
(Figure 8b).
NGC 7252 has been described as a decaying merger of
two massive gas-rich late-type spirals caught in the act of forming a
single elliptical galaxy
(Schweizer
1978,
Casoli et
al. 1991,
Fritze-von
Alvensleben & Gerhard 1994). However,
it is not yet clear whether this system has passed through a LIG phase.
Single-dish (Dupraz et al. 1990) and interferometer
(Wang et al. 1992) CO
observations reveal a nuclear molecular gas disk at r < 1.5 kpc,
which contains nearly 75% of the total H2 mass of
4.7 x 109 M
.
Hibbard et
al. (1994) and
Hibbard & van
Gorkom (1996) detect a slightly smaller total mass of
H I (3.6 x 109 M
), but all of the H I is located in the tidal
tails. Until most of the H I and H2 is either consumed,
expelled, or ionized, it would appear that NGC 7252 still has too much
cold gas to resemble most present-day ellipticals.
IRAS 19254-7245 (``The Super Antennae'')
IRAS 19254-7245 is a remarkable ULIG in which
two distinct rotating merging disks can still be identified
(Mirabel et
al. 1991). The colossal tidal tails have a total extent of
~ 350 kpc (Figure 8c).
Among the 20 ULIGs in the IRAS BGS this object has the largest
projected nuclear separation (~ 10 kpc;
Melnick & Mirabel
1990).
If the IRAS luminosity has the same spatial distribution as the
observed 10-µm luminosity, then 80%
of the total luminosity, Lir = 1.1 x
1012 L
, originates in the
southern, heavily obscured Seyfert nucleus. The total molecular gas
content of the system is M(H2) ~ 3.0 x 1010
M
(Mirabel et
al. 1988).
IC 4553 / 54 (Arp 220)
At a distance of 77 Mpc, Arp 220 is the nearest ULIG (by a factor of
~ 2). In contrast to ``The Super Antennae'', it shows two relatively
short and wide tails (Figure 8d). The nuclear
region is completely obscured in the optical, but two
distinct nuclei separated by 0.8" (~ 300 pc) are
detected at K-band
(Graham et
al. 1990). The radial brightness profile at 2.2 µm
is closely approximated by a r-1/4 de Vaucouleurs'
profile (Wright et
al. 1990,
Kim 1995).
The total mass of molecular gas is M(H2) ~ 3.6 x
1010 M, with ~ 2/3 of it inside a projected radius of
~ 300 pc. Compact OH megamaser emission
(
10 pc in diameter) has recently
been discovered at the center of Arp 220
(Londsdale et
al. 1994). H I is detected in absorption
against the nuclear radio continuum source
(Mirabel
1982). In emission,
all of the ~ 2.3 x 109 M
of H I is located outside
the main body (Hibbard & Yun 1996).
Although as luminous as ``The Super Antennae'', the shorter tidal tails,
smaller nuclear separation, and more relaxed central body of Arp 220 suggest that it is
a more advanced merger. Because of the strong H I absorption it is
not possible to
estimate the total mass of H I in the merger disks; however, most of the
cold gas there is likely to be in molecular form, and the overall ratio
M(H2) / M(H I) is probably close to 15.
5.2.2 ENCOUNTERS IN CLUSTERS
Most LIGs detected by IRAS appear to
involve strong interactions/mergers
of molecular gas-rich spirals where the pairs are either isolated or
part of small groups. In these interactions/mergers the relative mean
velocity of the individual members
is typically 200 km
s-1. These conditions are not expected to
be typical of cluster environments where the relative velocities are often
much higher, and many galaxies may be either gas-poor spirals (from
ram-pressure striping) or already transformed into ellipticals (perhaps
due to past mergers).
Although multiple high-speed encounters in clusters may produce some LIGs
(e.g. Moore et
al. 1996), the relatively small number of IRAS LIGs
found in
clusters suggests that lower-speed mergers in pairs or loose groups may be a
more efficient way of enhancing infrared activity.
As examples of the types of strong interactions involving relatively
large (
L*)
galaxies in low-z clusters, we
discuss here observations of an elliptical-spiral encounter
(Arp 105) and a
relatively high-speed lenticular-spiral encounter (NGC 5291A/B).
Arp 105 (Figure 9a)
consists of an infrared luminous
(Lir ~ 1011 L)
spiral galaxy (NGC 3561A) being torn apart by a massive elliptical
(NGC 3561B)
in the X-ray rich cluster Abell 1185.
Duc & Mirabel
(1994) find
that the elliptical has already accreted gas-rich objects and may be the
precursor of a cD galaxy. At the tip of one of the colossal tidal tails
that emanate from the spiral are a compact dwarf galaxy and an irregular
galaxy of Magellanic type. The H I and CO interferometer observations by
Duc (1995)
show that the collision has caused a marked spatial separation of the cold
interstellar gas: Whereas all of the ~ 1010 M
of molecular gas (H2)
is found within the central ~ 6 kpc radius of the spiral, a
similar mass of atomic gas (H I) is found outside that region, most of
it far from the spiral galaxy near the tips of the tidal tails. H I
clouds infalling at high velocities toward the nucleus of the elliptical
are detected in absorption against a central compact radio source.
![]() Figure 9. Encounters in clusters: (a) NGC 3561A/B (Arp 105); (b) NGC 5291A/B (``Sea shell''). Contours of H I 21-cm line column density (black) are superimposed on deep optical (r-band) images. Inserts show a more detailed view in r-band of the spiral galaxy NGC 3561A (Duc & Mirabel 1994), and of the interacting pair NGC 5291A/B. White contours represent the CO(1->0) line integrated intensity as measured by the IRAM millimeter-wave interferometer. CO emission has not been detected in NGC 5291A/B. The scale bar represents 20 kpc. |
Figure 9b shows an optical image of NGC
5291A/B (Duc & Mirabel 1996, in preparation)
and H I column density map (Malphrus et al. 1996, in
preparation). The nuclei of the lenticular galaxy
NGC 5291A and the spiral NGC 5291B (the ``Sea shell'' galaxy)
are at a projected separation of 12 kpc and are moving with a relative
radial speed of ~ 450 km s-1
(Longmore et
al. 1979).
Figure 9b reveals a multitude of
intergalactic H II regions
superimposed on an arc-like distribution of H I, which has a diameter of
~ 200 kpc and a total H I mass of
~ 1011 M (Simpson et al., private communication). However, optical
spectroscopy of the spiral that is being torn apart shows no signs of recent
star formation, which is consistent with its apparently low infrared
luminosity ( Lir < 109 L
); this source was not
detected by IRAS.
5.2.3 MOLECULAR GAS-RICH MERGERS IN QSOs AND PRGs Infrared and millimeter-wave observations of QSOs and PRGs show that a significant fraction of these objects have values of Lir and M(H2) that overlap values found for infrared-selected ULIGs. These data have been used as evidence for an evolutionary connection between QSOs, PRGs, and ULIGs (e.g. Sanders et al. 1988b, Mirabel et al. 1989). Figure 10 shows high-resolution optical images of the optically-selected QSO Mrk 1014, the radio-selected PRG Pks 1345+12, and the radio-selected QSO 3C 48, three objects that exhibit several properties in common with ULIGs.
![]() Figure 10. Optical images of QSOs and PRGs with strong infrared emission: (a) Mrk 1014 (PG 0157+001), (b) 4C 12.50 (Pks 1345+12), (c) 3C 48. The ``+'' sign indicates the position of putative optical nuclei. Tick marks are at 5" intervals, and the scale bar represents 10 kpc. |
Figure 10a shows the UV-excess QSO
Mrk 1014 at a redshift
of 0.163. The deep CCD image shows two tidal tails extending from a
large distorted disk with a total diameter of ~ 90 kpc (e.g.
Mackenty &
Stockton 1984). This warm infrared object has Lir ~ 3
x 1012 L
and M(H2) ~ 4 x 1010 M
(Sanders et al. 1988c).
Figure 10b shows a deep optical image of
the PRG 4C 12.50. This object exhibits a double nucleus
embedded in a distorted disk ~ 85 kpc in diameter (see also
Heckman et
al. 1986). IRAS observations
imply Lir ~ 1.6 x 1012
L.
CO emission
(Mirabel et
al. 1989) and H I absorption
(Mirabel 1989)
with widths of ~ 950 km s-1 have been detected from
this object. The CO data implies an
extremely large mass of molecular gas, M(H2)
~ 6.5 x 1010 M
. Mirabel (1989) proposed
that objects like 4C 12.50 (which is classified as a compact
steep-spectrum radio source)
are relatively young PRGs that have evolved from
progenitors like Arp 220, where a powerful compact source of
nonthermal radio emission is
still confined by a large nuclear concentration of gas and dust.
Figure 10c shows
a recent deep optical image of 3C 48,
the first radio source to be identified as a QSO
(Matthews &
Sandage 1963) and the second QSO to have its redshift
determined
(Greenstein &
Matthews 1963).
3C 48 appears to have a double
nucleus and at least one prominent tidal tail
(Stockton &
Ridgway 1991).
Neugebauer et
al. (1985) used the strength and shape of the
far-infrared continuum, and
Stein (1995)
used the f60 / f25 ratio versus
the radio spectral index at 4.8 GHz to argue that the dominant
luminosity source in this classic QSO is star formation in the host
galaxy rather than the central AGN.
IRAS observations of 3C 48 imply Lir ~ 5
x 1012 L, and the recent detection
of CO emission by Scoville et al. (1993) implies a molecular gas mass
M(H2) ~ 4 x 1010 M
(see also Evans et al. 1996b).
5.2.4 GRAVITATIONAL LENSING IN HIGH-Z OBJECTS
The identification of the IRAS Faint Source 10214+4724 with an
emission-line galaxy at a redshift of 2.286
(Rowan-Robinson et
al. 1991) has attracted considerable
interest. The apparent enormous infrared luminosity, Lir
~ 2 x 1014 L,
the detection of
1011 M
of molecular gas
(Brown & Vanden
Bout 1991;
Solomon et
al. 1992b,
c;
Radford et
al. 1996), and the detection of strong
submillimeter continuum emission from dust
(Clements et
al. 1992,
Downes et al. 1992)
led many observers to propose that this object was a prime candidate for a
``primeval galaxy''. However, IRAS 10214+4724 exhibits at least one
unusual property for an object with such extreme luminosity and mass; the CO
line width of ~ 250 km s-1 is small for such a massive object.
![]() Figure 11. HST Planetary Camera (F814W) image of the gravitational lens object IRAS 10214+4724 (Eisenhart et al. 1996). |
The idea that IRAS 10214+4724 might be gravitationally lensed by a
foreground galaxy or group of galaxies
(Elston et
al. 1994,
Trentham 1995,
Broadhurst &
Lehár 1995) gained support
from sub-arcsecond K-band images obtained with the Keck telescope
(Matthews et
al. 1994,
Graham & Liu
1995), and now appears certain from the
image obtained with the Hubble Space Telescope (HST)
(Figure 11). The geometry of the
lensed arc suggests an amplification factor of ~ 30 for the infrared
emission
(Einsenhart et
al. 1996). CO interferometer maps indicate that the molecular gas
may be somewhat more extended than the infrared emission
(Scoville et
al. 1995,
Downes et
al. 1995), and
Downes et
al. (1995) suggest that the CO
may be amplified by a factor 10. Thus, the intrinsic infrared luminosity
and molecular gas mass of this object may only be Lir
~ 7 x 1012 L
and
M(H2) ~ 2 x 1010 M
, respectively, suggesting
that IRAS 10214+4724 is not a unique infrared selected object but
is better described as
a high-redshift analogue of ULIGs found in the local Universe.
At present, the only other IRAS object at z > 2 that has been detected in CO is the Cloverleaf QSO at z ~ 2.5, which is also a lensed object (Barvainis et al. 1994). After correcting for amplification, the Cloverleaf appears to have a similar infrared and submillimeter spectrum as for IRAS 10214+4724 (Barvainis et al. 1995), again suggesting that this object is yet another high-redshift analog of local ULIGs.
However, several objects at z > 4 not detected by IRAS
have recently been shown to be strong far-infrared (rest-frame) emitters
(Omont et
al. 1996a), and one of these sources - the radio-quiet QSO BR1202-0725
at z = 4.69
(Irwin et
al. 1991) - was recently detected in CO
(Ohta et
al. 1996,
Omont et
al. 1996b) implying M(H2) ~ 6 x
1010 M. This
object has been shown to have a submillimeter spectrum remarkably similar to
IRAS 10214+4724 (McMahon et al. 1994,
Isaak et
al. 1994). An HST
image (Hu et
al. 1996) shows
no obvious evidence that this object is lensed, but it may be premature
to rule out this possibility.
If one extrapolates the local luminosity function of
ULIGs assuming relatively strong evolution (e.g. Kim & Sanders
1996), then the probability that a ULIG detected at z 2 is lensed by at
least a factor of 10 can be as high as ~ 30%
(Trentham 1995,
Broadhurst &
Lehár 1995). Future studies of LIGs at high redshifts will
almost certainly require high-resolution imaging to test whether these
objects are lensed.