Searches for molecular line emission from high redshift galaxies have become one of the recent highlights in millimeter astronomy, largely because detection of this emission enables one to study the potential for star formation in galaxies at epochs close to galaxy formation. Such information is crucial to models of galaxy evolution. Thus far, most of the searches have been to try to detect any of the rotational lines of CO, although many authors have also inferred the presence of molecular gas through detections of cold dust in the submillimeter region of the spectrum. In addition to providing information about the physical properties of the molecular gas in distant galaxies (when more than one transition or isotope is detected), the CO lines can be used to place stringent constrints on the dynamical masses of these systems. Moreover, since millimeter data has spectral resolutions of typically a few tens of km/s, one can pin down the redshift of the host galaxy with extremely high precision. One of the driving forces in most of the searches for CO emission at high redshift is the fact that molecular gas is known to be an important constituent in the low redshift counterparts to the types of objects that one expects to find at high redshifts, the Ultraluminous Infrared Galaxies (ULIRGs), (e.g. Mirabel and Sanders 1985; Sanders et al. 1986), powerful radio galaxies (e.g. Mazzarella et al. 1993), and nearby quasars (e.g. Barvainis et al. 1989), for example.

The observational evidence for dark halos around galaxies is shortly reviewed. New and old techniques and results constraining the mass, the distribution, the shape and the nature of dark halos are discussed.

The dark matter in the halos of galaxies may well be baryonic, and much of the mass within them could be in the form of clusters of substellar objects within which are embedded cold gas globules. Such halos might play an active role in galaxy formation and evolution.

In a number of dwarf spiral galaxies the HI-emission has been studied with sufficient resolution to derive the rotation curves of the galaxies. These show that the disks of dwarf spirals are imbedded in extended haloes of dark matter, quite similar to the disks of giant spiral galaxies.

Some dwarf galaxies have HI rotation curves that are completely dominated by a surrounding dark matter (DM) halo (e.g. Carignan & Freeman 1988). These objects represent ideal candidates for an investigation of the density structure of low-mass DM halos as the uncertainties resulting from the subtraction of the visible component are small, even in the innermost regions. Flores & Primack (1994) and Moore (1994) compared the observed DM rotation curves with the profiles, predicted from cosmological cold dark matter (CDM) calculations. They found an interesting discrepancy: whereas the calculations lead to a DM density distribution which diverges as ρ ∼ r−1 in the inner parts, the observed rotation curves indicate shallow DM cores which can be described by an isothermal density profile with finite central density.

Many nearby luminous elliptical galaxies exhibit counter-rotating cores, minor axis rotation or peculiar velocity fields. These phenomena require that merging and accretion events played a major role in the formation of ellipticals and, equally important, that stars dominated significantly over gas in the merger progenitors. Low redshift analogues to the last stages in the formation of massive ellipticals can be observed in ultraluminous IRAS mergers. However, this does not imply that ellipticals generally formed in spiral-spiral mergers. Mergers can also occur as a consequence of inhomogenous collapse or hierarchical bottom-up structure formation and it is actually more likely that the progenitors of present-day ellipticals did not resemble present-day spirals.

Although some ellipticals most certainly were and still are formed at low redshift, the general formation epoch of massive ellipticals is likely to be found at redshifts > 1. The evidence for this is threefold: (a) the homogenous stellar populations of ellipticals, most notably the tight correlation between colors/absorption linestrengths and velocity dispersion, (b) the high overabundance of light elements relative to iron and (c) the very weak evolution of colors and linestrengths of ellipticals with redshift.

The bridges and tails of interacting galaxies were elegantly explained when Toomre & Toomre (1972) showed that such features arise from tides acting on disk galaxies. Emboldened by this success, Toomre & Toomre proposed that certain twin-tailed fuzzballs without obvious interaction partners were in fact the merged relics of interacting pairs, and that such relics eventually become elliptical galaxies. Observations of twin-tailed systems found evidence for both their tidal origins and their elliptical destinies (e.g. Schweizer 1986), while self-consistent numerical simulations substantiated theoretical predictions of rapid orbital decay and the elliptical-like outcome of violent relaxation during merging (e.g. Barnes 1988, and references therein). But while our basic picture of merging seems solid, the precise role of this process in galaxy evolution is not so clear. Dynamical studies provide some insight into this issue; at present we can distinguish three levels of reliability in the numerical work: solid results, good bets, and hopeful guesses.

Probably the most striking evidence for galaxy evolution at recent epochs has been the discovery of a rapid change in the nature of galaxy populations in clusters over the redshift range z=0 to z=0.5. The ‘classical’ Butcher-Oemler effect (Butcher & Oemler 1978, 1984) used photomteric studies to reveal an unexpected increase in the fraction of blue galaxies in the cores of distant (z∼ 0.4) rich concentrated clusters when compared with nearby (z< 0.05) clusters of similarly high richness and central concentration. An alternative view, based on spectroscopic studies (Dressier & Gunn 1982; Couch & Sharpies 1987), manifests itself as an increase in the fraction of active galaxies which show signs of recent star formation and/or nuclear activity. Some of these galaxies are indeed blue but some (e.g. in C10016+16, Dressier & Gunn 1992) are red. Although it is the very absence of blue galaxies in nearby clusters which defines the classical Butcher-Oemler effect, comparable spectroscopic studies of nearby cluster populations with the appropriate completeness and high signal-to-noise required for population (as opposed to dynamical) studies have only recently been undertaken. In at least one case these have revealed unexpected similarities to the spectroscopic signatures which appear so prevalent at higher redshifts.

For nearly 20 years, we've known that clusters at z≳0.3 have a substantial population of “blue galaxies” seen only as fuzzy blobs in ground based images (Butcher & Oemler 1978, 1984). Hubble Space Telescope (HST) images reveal that these “fuzzy blue blobs” are low luminosity, often disturbed, spiral galaxies “Sp” (Dressier et al 1994a,b, Couch et al 1994). Today, rich galaxy clusters are dominated by elliptical “E” and lenticular “S0” galaxies (Dressier 1980), mostly low luminosity dwarfs.

During the late 1980's, successively deeper redshift surveys carried out with multi-object spectrographs on 4-m class telescopes produced growing evidence for evolution in the galaxy population. While some evolution had been expected from analysis of the galaxy number counts, the surprising indication from the first deep redshift surveys was that this appeared to involve moderate luminosity galaxies lying at moderate redshifts (Broadhurst et al. 1988, Colless et al. 1990, Cowie et al. 1991). However, while the results were suggestive, these early surveys suffered a number of significant problems that hampered their interpretation:

1. (a) the samples were small, especially at the faintest levels, so the statistical weight was limited and analysis was based on crude parameterizations of the data such as the median redshift of samples;

2. (b) the typical redshifts were small (z << 0.5), so that evolutionary effects could only be seen against “local” populations whose selection was often quite different - indeed the local luminosity function of galaxies is still poorly defined (Loveday et al. 1992, Marzke et al. 1994);

3. (c) the samples were selected in the observed B-band, so that comparison with local samples was based on the poorly constrained ultraviolet properties and relative numbers of galaxies of different types.

Data from the Keck and Hubble Space Telescopes have been combined to explore the nature of very faint I > 22 field galaxies. At a redshift z ∼ 1, such galaxies have luminosities similar to that of typical galaxies today. Though small, our sample of 33 redshifts already suggest that the median redshift for I > 22 galaxies is higher than the z = 0.6 expected for the “maximum merger model” of Carlberg (1995). At redshifts z > 0.8, mergers, interactions, and infall of minor galaxies into larger hosts appear to be common events; a wide diversity of morphological types existed; and some stellar populations were already so red that their major formation epoch occurred at redshifts z > 2.

We summarise recent Hubble Space Telescope results on the morphology of faint field galaxies. Our two principle results are: (1) the galaxies responsible for the faint blue excess have late-type/irregular morphology and (2) the number counts of normal galaxies, ellipticals and early-type spirals, are well fit by standard no-evolution models implying that the giant population was in place and mature by a redshift of ≥ 0.7.

Observers studying the cosmology and evolutionary history of our Universe through the statistical properties of ‘normal’ galaxies have four main tools at their disposal. (1) The number-redshift relation. Although a very powerful diagnostic, spectroscopic surveys are currently limited to B < 24m and significantly incomplete in the range, 23m< B < 24m. (2) Galaxy number-magnitude counts. Although by themselves, they cannot constrain models as tightly as spectroscopy, they can be measured ∼ 4m fainter, where cosmological effects are expected to be significant. (3) Galaxy colours over a wide wavelength range, which provide additional constraints. (4) The dependence of galaxy clustering with magnitude. ω(θ) can be measured to the limit of the counts.

Here we report on the latest Durham count and clustering work.

Keck spectra and HST images have been used to derive characteristic velocity and length scales for an enigmatic component of the faint, blue, field galaxy population: the compact, narrow emission-line galaxies (CNELGs). These galaxies are very luminous, but have been found to be quite low mass systems (with typical masses ∼ 109 M⊙). Their blue colors and strong emission lines indicate that they are undergoing a major burst of star formation. Following the completion of their current burst they will fade, becoming, in the absence of further major bursts, objects very similar to contemporary spheroidal galaxies. With mean sizes Re ∼ 1.4 kpc and Gaussian velocity profiles with mean σ = 45 km s–1, the length scales and velocity widths of CNELGs are also quite consistent with the measured length scales and velocity widths of current spheroidals.

The existence of the Fundamental Plane of early-type galaxies implies that the M/L ratios of early-types are well behaved. It provides therefore an important tool to measure the evolution of the M/L ratio with redshift. These measurements, in combination with measurements of the evolution of the luminosity function, can be used to constrain the mass evolution of galaxies.

We present the Fundamental Plane relation measured for galaxies in the rich cluster CL 0024+16 at z=0.391. The galaxies satisfy a tight Fundamental Plane, with relatively low scatter (15 %). The M/L is 31 ± 12 % lower than the M/L measured in Coma, which is consistent with simple evolutionary models. Hence, galaxies with very similar dynamical properties existed at a z=0.4.

More, and deeper data are needed to measure the evolution of the slope and the scatter of the Fundamental Plane to higher accuracy. Furthermore, data on the richest nearby clusters would be valuable to test the hypothesis that the Fundamental Plane is independent of cluster environment.

This is a progress report on a collaborative effort to probe galaxy evolution by constraining the central mass-to-light ratios of distant field galaxies. Details can be found in a forthcoming paper (Rix, Guhathakurta, Colless & Ing 1996, RGCI). At the moment, the program is focused on the question of how over-luminous the abundant blue field galaxies at z ∼ 0.3 are (see Koo, K. and Kron, R. 1992), compared to galaxies of the same circular velocity and type at z ∼ 0. We have measured the integrated [OII] linewidths for a complete sample of distant blue galaxies to establish a linewidth-luminosity-color relation for this population and to compare this relation to the one for local galaxies. In addition, we explore practical strategies to obtain and analyze data so as to best relate the observable quantities (e.g. linewidths) to the physical quantities of interest (such as vcirc or M/L).

In hierarchical clustering theories of galaxy formation, galaxies form by gas cooling and condensing into dark matter halos which, in turn, form by a hierarchy of mergers (White & Rees 1978). The context in which this process takes place is specified by a cosmological model that determines the spectrum of primordial density fluctuations and the rate at which they grow by gravitational instability. The best known example of such a model is the cold dark matter (CDM) model (see Frenk 1991 for a review), but a number of alternatives (mostly variants of CDM), have recently become popular in response to new data on large-scale structure and the COBE detection of anisotropies in the microwave background radiation.

High resolution N-body simulations show that the density profiles of dark matter halos formed in the standard CDM cosmogony can be fit accurately by scaling a simple “universal” profile. Regardless of their mass, halos are nearly isothermal over a large range in radius, but significantly shallower than r–2 near the center and steeper than r–2 in the outer regions. The characteristic overdensity of a halo correlates strongly with halo mass in a manner consistent with the mass dependence of the epoch of halo formation. Matching the shape of the rotation curves of disk galaxies with this halo structure requires (i) disk mass-to-light ratios to increase systematically with luminosity, (ii) halo circular velocities to be systematically lower than the disk rotation speed, and (iii) that the masses of halos surrounding bright galaxies depend only weakly on galaxy luminosity. This offers an attractive explanation for the puzzling lack of correlation between luminosity and dynamics in observed samples of binary galaxies and of satellite companions of bright spiral galaxies, suggesting that the structure of dark matter halos surrounding bright spirals is similar to that of cold dark matter halos.

It is commonly accepted, that galaxies acquire their angular momentum from tidal torques due to surrounding matter (Hoyle 1949). Most of the angular momentum is gained during the linear phase of the collapse (White 1984). After the turnaround point is reached, any further gain of angular momentum is strongly suppressed because it is increasingly difficult for the tidal field to act on the shrinking radius of the forming dark halo.

There have long been two competing views on the formation history of the ellipticals galaxies we see today. One is that most of the stars in present-day galactic bulges and ellipticals were produced during a relatively short, early phase of intense star formation at high redshift. The second view is that elliptical galaxies are relative latecomers, having been produced as the result of the merging of disk galaxies drawn together by gravity as their surrounding dark matter halos coalesced.