Changes in the two-dimensional light distribution of galaxies with wavelength can provide new and unique perspectives on their structures and evolution. The spectral region of interest here lies between the Lyman cutoff of the interstellar medium at 912 Å and the cutoff of the terrestrial atmosphere near 3200 Å. This window contains information on the character of stellar populations, dust grains, interstellar gas, and AGNs which is largely independent of that in the familiar optical bands. In this section we discuss the potential utility of the UV in understanding galaxy evolution and progress to date in exploiting these opportunities.
2.1. Ultraviolet Probes of Galaxy Astrophysics
2.1.1. Ages and Metal Abundances of Stellar Populations
The UV has the highest sensitivity of any spectral region to stellar
temperature and metal abundance, implying that it is especially valuable
as a means of characterizing stellar populations, current star formation
rates (SFRs), and star formation histories. Stars with surface
temperatures above ~ 10,000 K (e.g., main-sequence stars with masses
3
M
) are
brighter in the UV than at longer wavelengths
(Fanelli et al. 1992).
Consequently, UV imaging or spectroscopy of star-forming galaxies
permits direct detection of the massive stars responsible for most
ionization, photodissociation, kinetic energy input, and element
synthesis.
Figure 1 illustrates predicted UV-IR spectral energy
distributions of stellar populations over timescales up to 3000 Myr. The
strong time evolution of the UV compared to longer wavelengths is the
reason for its utility in determining population ages. The sensitivity
of the UV to stellar properties extends even to the cool ~ 1
M stars near
the main-sequence turnoff in the oldest model shown in
Figure 1. These solar-type stars dominate the mid-UV
(2000-3200 Å) light in this model, and chemical composition as well
as age influences the spectrum. The concentration of strong metallic
absorption features is responsible for much of the short-wavelength
structure in this model. The abundance sensitivity of selected UV
spectral regions is discussed by
Fanelli et al. (1992).
|
Figure 1. Synthetic spectral energy
distributions of single generation stellar populations having Salpeter
IMFs and solar abundances for ages 3-3000 Myr from
Bruzual & Charlot (1993),
showing the rapid evolution in UV amplitude and shape. The shape of the
near-IR spectrum ( |
In old, quiescent systems such as elliptical galaxies and spiral bulges, the UV offers a second major probe of stellar populations. Most old systems have been found to contain a very hot, low-mass stellar component with Teff > 15,000 K which dominates the far-UV light. This probably consists of stars with very thin envelopes on the extreme horizontal branch and subsequent advanced evolutionary phases (reviewed in Greggio & Renzini 1990 and O'Connell 1999). Their UV output is predicted to be very sensitive to their envelope masses and compositions. Overall, UV spectra are powerful age and abundance diagnostics for both young and old populations.
2.1.2. Star Formation Histories for t < 1 Gyr
The UV offers a unique probe of the star formation history of galaxies
on intermediate timescales of 10-1000 Myr. By "history" we mean the star
formation rate as a function of time, SFR(t). The most widely
used methods for estimating the recent star formation rate,
SFR(t0), involve optical emission lines such as
H
, radio continuum emission
from hot gas or relativistic electrons, and far-infrared or
submillimeter continuum emission from dust grains (e.g.,
Kennicutt 1998).
Both emission lines and free-free thermal radio continuum depend on
photoionization from massive stars and therefore reflect activity only
over the past ~ 5 Myr, after which photoionization rapidly
decreases. This period represents only 0.05% of the star-forming
lifetime of a typical galaxy. Nonthermal radio emission powered by
supernova-driven relativistic electrons is a useful index of massive
star formation over the past few tens of Myr
(Condon 1992)
but is influenced by the character of the surrounding interstellar
medium. Infrared (
> 10
µm) and submillimeter thermal emission from dust grains is
likewise strongly influenced by the nonstellar environment and has
intrinsically poor time resolution, since grains can be heated by
photons from stars of all ages.
These conventional methods for estimating recent SFRs
are based on the indirect effects of massive stars, involving the
downconversion of UV photons by surrounding media, and have either
restricted or ill-defined age sensitivity. By contrast, the vacuum UV
offers a direct measure of the light from the massive star
populations. Extinction by dust is often cited as a serious obstacle to
using direct FUV observations to infer star formation rates. However,
the photoionizing UV continuum
(
912 Å) which
drives line and free-free emission is possibly yet more sensitive to
extinction, while there are fewer empirical constraints on its
nature. The actual effects of dust on the emergent UV light are smaller
than naively expected (see Section 2.1.3 and
6). All of
these methods are comparably affected by uncertainties in extinction.
The timescales which can be probed by observations of
the 1200-3200 Å continuum range from less than 10 Myr to
1 Gyr. This critical
interval is well sampled neither by the methods described above nor by
the optical region (3200-9000 Å), which is influenced by the star
formation history on longer timescales (more than a few Gyr). It is the
"gap" which
Gallagher, Hunter, & Tutukov (1984),
for instance, were compelled to omit in their landmark study of spiral
galaxy histories derived from
H fluxes, B-band
fluxes, and total masses.
An example of the additional information on galaxy star formation
histories to be gained from UV continuum imaging is shown in
Figure 2. This is a map comparing the
H and far-UV morphologies of
the well-known Sc galaxy M51. There are clearly large variations in the
far-UV to H ratio. The ~
10-50 Myr old populations (FUV-bright) are usually spread farther
downstream from the putative density wave than the ~ 5 Myr old,
H -bright populations, but
the pattern is not entirely symmetrical. Diffuse far-UV light tends to
"fill in" the spiral arms between intense concentrations of
H light. There is a hint of
multiple FUV wavelets, with feathery extensions inclined in pitch angle
to the main spiral pattern. The UIT data for M51 are discussed further
by
Kuchinski et
al. (2000).
A similar comparison map based on a lower resolution UV image from the
FOCA program (see Section 2.3) was published by
Petit et al. (1996).
|
Figure 2.
H |
The sensitivity of different wavebands to a galaxy's star formation history is discussed further in the form of "history weighting functions" in Appendix A.
2.1.3. Cold Interstellar Material
The UV offers high sensitivity to interstellar dust and regions of concentrated cold material (i.e., potential star formation sites). Before the advent of UV observations of galaxies, it was widely assumed that this would actually be a serious disadvantage because the Galactic extinction law (e.g., Cardelli, Clayton, & Mathis 1989) yields A(UV) / A(V) > 2.5, implying that the UV light of typical disk galaxies might be strongly suppressed. However, as is amply demonstrated by the images in this atlas and spectroscopic studies (e.g., Calzetti, Kinney, & Storchi-Bergmann 1994), dust does not dominate the UV morphology of most galaxies.
The UV may ultimately prove to be a valuable tracer of quiescent, cold, molecular material. Because the albedo of dust is high in the UV, cool interstellar clouds far from star-forming regions can be detected by scattered light, as in the case of the faint gaseous outer arms of M101 (Donas et al. 1981; Stecher et al. 1982) or the outer halo of NGC 1068 (Neff et al. 1994). UV imaging can also detect H2 in photodissociation regions directly by virtue of its fluorescence bands in the 1550-1650 Å region (e.g., Witt et al. 1989; Martin, Hurwitz, & Bowyer 1990). Although such regions occupy only a small fraction of the total volume of a typical molecular cloud, nonetheless the direct detection of H2 has, in principle, considerable advantages over methods involving trace constituents like radio-emitting CO (see Allen et al. 1997 and references therein).
2.1.4. Hot Interstellar Material
The UV contains uniquely important emission line probes of interstellar
gas in the T ~ 105-106 K regime, including
C IV (1550),
N V (1241), and
O VI (1035). These
spectral diagnostics have been extensively exploited in absorption-line
spectroscopic studies of our galaxy. Little has been done to date using
imaging, though C IV images of supernova remnants have been published
(e.g., the Cygnus Loop,
Cornett et al. 1992;
N49 in the LMC,
Hill et al. 1995c).
The nuclei of galaxies are often UV-bright. The optical-UV energy distributions of AGNs and associated nonthermal jets are relatively flat, and their contrast against the stellar background is usually better in the UV than in the optical band. Activity has been detected by UV imaging in a number of nearby galaxies (e.g., Maoz et al. 1995; Renzini et al. 1995; Barth et al. 1998). Spiral nuclei often contain starburst cores or ringlike structures, which are again more prominent in the UV (e.g., in M83, Bohlin et al. 1983; NGC 1068, Neff et al. 1994). The leverage of the UV in isolating such hot sources is especially important in spiral bulges and E galaxies, where the cool star background is often overwhelming at optical wavelengths.
2.1.6. Low Surface Brightness Systems
A minimum in the natural night sky background occurs at 1600-2400 Å. This is the deepest window in the UV-optical-IR spectrum and permits detection of extremely low surface brightness objects, perhaps up to 100,000 times fainter than the ground-based sky (O'Connell 1987; Waller et al. 1995). Applications include the study of faint circumgalactic star-forming regions in nearby galaxies and faint blue galaxies at redshifts up to ~ 1 (Martin et al. 1997; Treyer et al. 1998) and detection of low surface brightness disk systems (O'Neil et al. 1996).
2.2. Applications to High-Redshift Galaxies
UV imaging of nearby galaxies is relevant to galaxies at high redshift
for two reasons. First, as just described, the rest-frame UV continuum
is a robust tracer of star formation and is measurable to very high
redshifts (z
10) with optical/IR instruments. For instance, the 2800 Å
rest-frame continuum has been used to estimate the cosmic star formation
density at z ~ 0.5-4 for the Canada-France Redshift Survey,
Hubble Deep Field, and other surveys
(Pei & Fall 1995;
Lilly et al. 1995;
Madau et al. 1996;
Steidel et al. 1999),
leading to the
conclusion that gas processing occurred at a relatively constant rate
for z ~ 1-4 but has precipitously declined since z ~ 1.
Second, observations of high-z galaxies are preferentially made in the rest-frame UV. This is particularly true for ground-based telescopes, where the rapidly increasing night sky brightness for wavelengths above 7000 Å, and thermal emission beyond 2 µm, seriously compromises observations in the near infrared. Because of the strong changes in galaxy appearance with wavelength, as illustrated in this atlas, there is a large "morphological k-correction" which must be quantified in order to distinguish genuine evolutionary effects from simple bandshifting.
High-redshift studies are also strongly influenced by reduced spatial
resolution and by surface brightness selection. The latter is a very
serious problem for z
1 because I ~
I0(1 + z)-n, where
I0 is the surface brightness in the rest-frame and
n = 3 or 5 for monochromatic surface brightnesses per unit
frequency or wavelength, respectively; n = 4 for bolometric
surface brightnesses.
Figure 3 illustrates these effects using a
far-UV image of the luminous, nearby Sc spiral M101.
Bohlin et al. (1991)
and Giavalisco et
al. (1996)
describe methods of creating such simulations from rest-frame UV data.
|
Figure 3. Left panel: A far-UV (1500 Å) image of the luminous Sc galaxy M101 obtained by UIT during the Astro-2 mission. A 5' bar is shown for scale. Right panel: A simulation of a galaxy with the same structure but 10 times higher surface brightness at a redshift z = 1.5 as observed by the Keck 10 m telescope in a 10 hr exposure with 0".5 FWHM seeing against a sky background of 22.5 mag arcsec-2. A 2" bar is shown for scale. The simulation is not easily recognizable as a normal spiral galaxy. Its asymmetries are emphasized; it appears distorted and fragmented. High surface brightness star-forming associations in its disk have taken on the appearance of nearby "companions." |
It is possible to explore bandshifting effects using multicolor (e.g., B and R) optical images to extrapolate the spectral energy distribution to the rest-frame UV on a pixel-by-pixel basis. This has been done using ground-based data (e.g., Abraham 1997; Abraham, Freedman, & Madore 1997; Brinchmann et al. 1998) and moderate-redshift Hubble Space Telescope (HST) data (Bouwens, Broadhurst, & Silk 1998). These studies, as well as those based on HST/NICMOS observations in the rest-frame optical bands (e.g., Bunker 1999, Corbin et al. 2000), conclude that the peculiarities in shape and size distributions found in the deep HST surveys considerably exceed the effects of bandshifting. While this is probably a robust result, these extrapolation methods do not accurately capture the range of rest-frame UV spectra found in real galaxies and are not suitable for making detailed comparisons with the local universe. The reason is that there is considerable scatter in UV colors of nearby galaxies at any given optical color. This is true even in the classic (U-B, B-V) diagram (e.g., Larson & Tinsley 1978), but it is much more pronounced in the rest-frame UV, where, for instance, Donas, Milliard, & Laget (1995) found a 3 mag range in (UV-b) colors at a given (b-r) color in a faint galaxy sample. This UV/optical decoupling is confirmed in spectroscopy of UV-selected samples by Martin et al. (1997) and Treyer et al. (1998). The implication is that the true evolutionary history of galaxies on timescales more recent than a few Gyr can be rather different from that inferred from optical data.
Fiducial photometric and imaging studies of nearby galaxies in the rest-frame UV are needed in order to calibrate these selection and morphological effects and to improve our understanding of the astrophysical drivers of the rest-frame UV luminosity, particularly the influence of dust and the history of star formation.
Extragalactic UV astronomy to date has been largely based on spectroscopy, usually with small entrance apertures (1"-20") centered on galaxy nuclei (e.g., IUE, HST/FOS, HST/GHRS). Several hundred objects have also been photometered in broad bands with large apertures. Early photometric surveys were performed by OAO and ANS; the most recent large-scale study was by FAUST (Deharveng et al. 1994). The total number of UV spectroscopic and photometric observations of galaxies still far exceeds the number of imaging observations (an inversion of the historical development of optical extragalactic astronomy). Brosch (1999) has recently reviewed the available results of UV surveys of galaxies. No all-sky UV survey faint enough to include galaxies has yet been conducted, but this will be remedied by the GALEX mission (Martin et al. 1999).
The first UV image (defined as having many resolution elements over the area of interest) of another galaxy was obtained by the NRL Apollo S201 camera from the lunar surface in 1972 (Page & Carruthers 1981). This was of the Large Magellanic Cloud in the 1250-1600 Å band and dramatically demonstrated a strong wavelength-dependent morphology. Its remarkably fragmented appearance in the UV is entirely different from its familiar barlike shape in the optical continuum.
Subsequent progress in UV imaging up to 1990 was relatively slow (reviewed in O'Connell 1991). Since 1990, we have accumulated a sample of vacuum UV images of about 200 galaxies, principally from three instruments:
Because of technical difficulties in achieving high reflectivity optics
shortward of 1100 Å and in rejecting the very bright geocoronal
Ly line at 1216 Å from
exposures centered at shorter wavelengths, both the HST and UIT
imaging cameras work primarily at wavelengths longer than 1216 Å.