HST Science: Structure and Evolution of Galaxies

This could easily be a shopping list of HST's greatest hits. I will exercise the reviewer's arbitrary prerogative to limit the discussion to a field of particular interest where deconvolved images have had a profound impact, the structure and evolution of galaxies. This has been effective ground for deconvolution, because many targets have less contrast than, say, QSO host galaxies, and we are limited by S/N before PSF uncertainties or algorithmic problems. Many of these results require restoration only to the Nyquist limit of WFC sampling, sidestepping issues of undersampling.

There are many results that have required deconvolution, things we simply wouldn't know without this capability. The discussion moves roughly in order of typical distance.

Galactic Nuclei

These regions have proven fertile ground for imaging and deconvolution, being strongly condensed and bright. Numerous surprises have appeared in such images, especially of active nuclei.

It has long been suspected that active galactic nuclei are driven by central massive objects (massive black holes) surrounded by accretion disks, sometimes with jets arising along the rotation axis of the accretion disk. In the radio galaxies NGC 4261 and 3C 449 (Jaffe &Ford 1993), HST images have revealed dark disklike structures around the nuclei, oriented perpendicular to twin jets seen at centimeter wavelengths. These structures have characteristic dimensions of parsecs, much too large to be the accretion disks responsible for the emission properties of these nuclei and mass loss into the putative black holes, but may represent material which has already settled into orbits aligned with the yet-unseen inner disk.

Most active galactic nuclei fall into two types, classified on the basis of their emission-line spectra. Broad-line objects have emission from gas at a wide range of velocities, covering in the most extreme cases a range , while narrow-line objects show similar ionization and energy-input requirements but show only emission from lower-density gas with characteristic velocity of a few hundred km s (which is also often present in broad-line objects). An important breakthrough was the recognition, driven by polarimetry, that some narrow-line active nuclei possess a dense inner broad-line region that is blocked from our line of sight, but seen by (polarized) scattered light (as reviewed by Antonucci 1993). This can fit nicely with an accretion-disk picture if the scattering region is a thick disk aligned with (perhaps outside) the accretion disk proper. Early PC observations of the nearby Seyfert galaxy NGC 1068 (Lynds et al. 1991) in fact resolved the continuum emission of the nucleus into a region of typical size 10 parsecs, perhaps a direct detection of this scattering region.

Some active nuclei exhibit jets that are detectable in the optical and ultraviolet. Most are dominated by synchrotron radiation, so that observations in this range are sensitive to highly relativistic electrons with relativistic Lorentz factors as well as to the details of the magnetic field structure in which they radiate. The FOC has proven very effective for these objects, since they have high surface brightness, small angular size, and greatest contrast against the background starlight in the blue wavelengths where the FOC is most sensitive. The well-known jet in M87 was a prime HST target, reported by Boksenberg et al. (1992) with the FOC and also observed with the PC in the deep red (e.g. Lauer et al. 1992a). Restoration shows many of the same features observed at 2 cm by the VLA (Biretta et al. 1983, not without its own level of deconvolution). Subtle differences between the radio and UV structures are particularly intriguing, offering the possibility of tracing the fine structures in which electrons are accelerated and respond to the magnetic field geometry. The subtlety of any such differences helps focus attention on the detailed behavior of different deconvolution algorithms, taxing fidelity of both intensity and geometry at low count rates. Observations of additional jets have revealed contrasts between rather smooth structure (PKS 0521-36, Macchetto et al. 1991a) and hints of the kinds of filamentary features seen in parts of the M87 jet (3C 66B, Macchetto et al. 1991b). A further synchrotron jet was discovered serendipitously in NGC 3862 (Crane et al. 1993).

Galaxy Mergers and Star Formation

Ground-based data obtained at a variety of wavelengths have shown that bursts of star formation can be induced by strong tidal interactions and mergers. The high resolution allowed by deconvolved HST images has added a new richness to this picture, by showing very luminous blue star clusters. These have been seen now in the cases of NGC 1275 (Holtzmann et al. 1992), Arp 220 (Dowling &Shaya 1992), NGC 7252 (Whitmore et al. 1993), and a ``quiet'' merger in the compact group NGC 6027 (Seyfert's Sextet; Sulentic et al. 1994). The properties of these clusters open new windows on the history of mergers, with timescales and cluster ages deduced from their colors and in some cases spectra. These may be the precursors of globular clusters, an important issue in understanding the merger history of galaxies.

Morphology of Distant Galaxies

High angular resolution opens the possibility of seeing transformations in the forms of galaxies over cosmic time. For any but elliptical and S0 galaxies, morphological classification requires deconvolution rather than modelling with a manageable number of components. The galaxy content of rich clusters is a particularly rich field for inquiry; the color distributions of cluster members have long been known to show excess blue galaxies at redshifts (the Butcher-Oemler effect; see Lavery, Pierce, &McClure 1992 and references therein). One mechanism for the color change is shown in a spectacular way by WFC imaging of the cluster 0939+4713 at (lookback time years for H km s Mpc and ) presented by Dressler et al. (1994). While nearby rich clusters are uniformly dominated by E and S0 galaxies, all Hubble types are present in 0939+4713; many of the blue galaxies are structurally late-type spirals, along with a few apparent mergers. WFPC2 should allow similar observations to redshifts . The surprisingly rapid evolution of the galaxy population in clusters with cosmic time may be due to a combination of gas stripping from the hot intracluster medium and merging with subsequent starburst-driven galactic winds.

The ability to discriminate arbitrary structures of small angular scale has allowed detection of gravitationally lensed galaxies, as in the cluster AC 114 (Couch et al. 1992). This presents the possibility of measuring structure in extremely distant galaxies, using the ``gravitational telescope'' of deep cluster potentials. Distinguishing between even and odd-parity structure in such a lensed image will in principle allow distinction between features in the background galaxy and foreground lens, and may eventually allow us to probe structures on angular scales beyond the direct resolution of even corrected HST optics.

The observation of merging galaxies locally has led to the suggestion that mergers might be an important part of galaxy evolution, perhaps driving important bursts of star formation and nuclear activity, changing disk galaxies into ellipticals, and certainly changing the comoving space density of galaxies. The ability of deconvolved HST images to resolve fine structure at large redshifts allows a new test for this. Since mergers seen today are dominantly between galaxies that are gravitationally bound to one another, rather than from random encounters of unrelated galaxies, one signature of a strong role for mergers will be increasing numbers of galaxy pairs and tight groups wih increasing redshift and lookback time. Such an effect has indeed been reported, by Burkey et al. (1994) for pairs in a set of parallel WFC images, and by Casertano et al. (1994) for pairs and groups from the Medium-Deep Survey. Statistically well-defined criteria for such associations give membership rates of about 7%for nearby galaxies and 34-40%for faint galaxies ( magnitudes 21-24, redshifts 0.3-0.7), suggesting that the merger rate changes with redshift approximately as . This change is important in interpreting counts of faint galaxies, and is interestingly close to the rate at which QSOs and radio galaxies evolve with cosmic time.

Working to yet higher redshift has been more difficult, but feasible, with spherical aberration. Galaxies have been found with ``normal'' morphology and size out to , as in the case of the radio galaxy 53W002 (Windhorst et al. 1992). This galaxy has an elliptical-like intensity profile and scale length comparable to those of nearby ellipticals with radio sources. Seen at a lookback time only 1-2 years later than the peculiar galaxies at the highest redshift, such objects provide clues to the timescales of galaxy formation. So far, galaxies with strong central condensation have been reported out to . Further imaging studies will be crucial in understanding whether this epoch is when most galaxies approach their present dynamically relaxed structures. We may already be seeing galaxy formation, if we but had a clue what the process looks like.

The highest-redshift galaxies known are the very powerful radio sources found from the 3C and 4C surveys. At redshifts as high as , they show spectacular, if poorly-understood, morphologies. Their optical (that is, emitted UV) images are frequently elongated and clumpy. The elongation is correlated with the direction of the radio structure in the so-called alignment effect, which is in evidence only for . HST images show striking correlations in detail between emitted ultraviolet and radio structures, in the cases of 4C 41.17 (, Miley et al. 1992) and 4C 28.58 (, Miley 1993). Connecting these early views of galaxy evolution with what we see at lower redshifts may be the outstanding problem here. Are these systems rare examples of galaxian pathology, irrelevant to the evolution of galaxies in general, or are they symptomatic of many galaxies during a turbulent initial epoch? Once again, only the ability to image larger samples to fainter levels will clarify these matters. It is intriguing that we see symmetric, centrally condensed galaxies at redshifts less than 2.5, and that the peculiar elongated and clumpy systems occur mostly at yet higher redshifts; one might be tempted to call this an evolutionary pathway, invoking astronomers' well-known ability to generalize from a sample of two or more.