David Schade, S. J. Lilly
Department of Astronomy, University of Toronto, Toronto, Canada M5S 3H8
F. Hammer, O. Le Fèvre & L. Tresse
DAEC, Observatoire de Meudon, 92195 Meudon, France
Dominion Astrophys. Obs., National Research Council of Canada, Victoria, V8W 4M6
F. Barrientos, O. López-Cruz
Department of Astronomy, University of Toronto, Toronto, Canada M5S 1A7
Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.
Keywords: galaxies:evolution---galaxies:fundamental parameters
Until recently, little was known about the properties of the galaxy population at significantly earlier epochs. The steep slope of the faint galaxy counts (Ellis 1990, Lilly, Cowie & Gardner 1991) has been interpreted as evidence for evolution of the galaxy population but the cause and nature of that evolution was a mystery. Over the years, various more-or-less exotic physical explanations, such as bursting dwarf galaxies, vanishing populations, and wholesale merging (see Lilly 1993 for a review) have been proposed. A number of studies with HST imaging of large numbers of galaxies without measured redshifts (e.g., Glazebrook et al. 1995, Driver, Windhorst, & Griffiths 1995) emphasize the peculiar nature of much of the faint galaxy population. It has proven difficult to distinguish between evolutionary scenarios, or even to establish the basic physical properties of the faint galaxy population, without redshift information which is needed to compute luminosity, physical size, restframe color, and intrinsic surface brightness.
The Canada-France Redshift Survey (Lilly et al. 1995a and references therein) is a statistically complete sample of 943 objects with and is large enough to estimate the multi-variate galaxy luminosity function (Lilly et al. 1995b). The purely statistical information provided by the luminosity function has been augmented by morphological information from HST (Schade et al. 1995, hereafter referred to as CFRS IX). By imaging the galaxies that make up the evolving population, it is possible to see directly what types of objects are present at high redshift and active in the evolutionary process. This ability to measure the structure of individual galaxies is critically important in the pursuit of an understanding of the physics at work behind the evolutionary process. The physical structure of a galaxy is expected to change less abruptly than its luminosity (which can vary enormously with changes in the star-formation rate) so that the structure is invaluable as a means of associating a galaxy at high redshift with its local counterpart, i.e., with its evolutionary endpoint in the local population.
We assume km sec Mpc and throughout this paper.
The multi-variate luminosity function (LF) computed from the Canada-France Redshift Survey (Lilly et al. 1995b) shows that LF of those galaxies with restframe colors bluer than a present-day Sbc galaxy evolves strongly between redshift slices at and . This evolution could be due to an increase in overall galaxy density by a factor of at high redshift or, alternatively, by a uniform brightening of the galaxy population by mag relative to the population at lower redshift. More complicated evolutionary scenarios are, of course, also possible. In contrast to the behavior of the blue population, the luminosity function of red galaxies is consistent with no evolution over the entire redshift range 0 < z < 1.
Because the luminosity function is merely a statistical description of the galaxy population, its behavior does not reveal the physical behavior of individual galaxies. Constraints of a physical nature require the analysis of the properties of individual objects.
HST imaging provides detailed morphological information about high-redshift galaxies due to its impressive resolution ( kpc at ). It is possible to detect large-scale peculiarities, to locate strong star-forming regions (e.g., whether they are nuclear or scattered over the face of the galaxy), and most importantly to make quantitative measures of galactic structure (size, surface brightness, bulge/total luminosity, spatially resolved colors). Such images show that the galaxy population at high-redshift has at least 3 distinct morphological components, illustrated in Figure 1 (and in color in Figure 3 of CFRS IX). The first component is the set of red galaxies () which in terms of Hubble types would be mostly E/S0 based on their fractional bulge luminosity (). In the CFRS IX sample 4/32, (10%) of the galaxies fall in this range. The second component of the population is the late-type or disk-dominated galaxies (19/32 objects or 60%). The disks of these objects have a mean surface brightness higher at by more than 1 magnitude relative to the local Freeman (1970) constant surface-brightness law (Figure 2). The third component (9/32 or 30%) consists of the ``blue-nucleated galaxies'' (BNGs) which are dominated by compact blue components and are undergoing star-formation. The CFRS IX sample shows a strong association of the BNG phenomenon with asymmetric structure and interactions/mergers. The BNGs are evident in figure 3 of CFRS IX as objects with strong blue compact (but resolved) components. It is not clear if a counterpart of these galaxies exists among the local population. They may be compact starbursts in disk galaxies, bursting spheroid galaxies, or they may represent a phase in the formation of bulges.
Figure: Images from WFPC2 of 32 galaxies at in the F814W filter with galaxies arranged in order of luminosity (most luminous at the top) and color (increasingly bluer toward the right). A wide range of galaxy morphology is present in this sample. (See Schade et al. 1995 for an accompanying color illustration.)
Figure: The relation between size and luminosity for disk galaxies at with asymmetric objects excluded. The Freeman (1970) relation is indicated. (See Schade et al. (1995)
The high-redshift population shown in CFRS IX is similar in some respects to the local population. All galaxy types from ellipticals through spirals to irregular galaxies are present at high redshift. The colors correlate with morphology in the sense that early-type galaxies are dominated by compact red central bulge components and disks are predominantly blue. Spiral structure and bars are evident in some galaxies. The overall qualitative impression is that this population is not strikingly different from the local galaxy population although we are seeing them during a much younger phase in their development (at 1/3 to 1/2 their present age).
The initial CFRS sample of field galaxies with HST imaging has thus revealed a great deal about the late-type galaxy population at high-redshift. What can HST tell us about the evolution of elliptical galaxies? Although the small number of early-type objects in the CFRS IX HST sample precludes a detailed study at the moment, such galaxies are common in the cores of rich clusters of galaxies. High-redshift clusters have been frequent targets of HST and high-quality data has been analyzed using the same two-dimensional surface photometry procedures as in CFRS IX.
Barrientos, Schade, & López-Cruz (1996) measured the properties of early-type galaxies in the cluster CL0939+4713 at z=0.41 with Hubble Space Telescope and compared the results to an identical analysis of a ground-based image of the Coma cluster. These two images of clusters have nearly identical physical resolution ( kpc), spatial coverage ( Mpc on a side), and signal-to-noise ratio ( at ), so that the results are directly comparable. Two-dimensional surface photometry measurements show that these two clusters have tight sequences of luminous early-type galaxies in the plane of versus (half-light radius) with indistinguishable slopes. However, the two sequences are offset from one another in luminosity such that a galaxy of a given size at is more luminous than a corresponding galaxy in the Coma sequence (see Figure 3). The most direct interpretation of this result is that we are seeing luminosity evolution of individual galaxies and this interpretation is consistent with passively evolving models of elliptical galaxies whose stellar populations were formed in a single massive burst at z > 1 (Bruzual 1993).
Figure: The relation between size and luminosity for elliptical galaxies in the Coma cluster compared with those in CL0939+47 at . (See Barrientos et al. 1996) The best-fit line for Coma is plotted on both panels.
The results of Schade et al. (1995) and Barrientos et al. (1996) show that detailed structural measurements are possible for high-redshift galaxies using Hubble Space Telescope and that such quantitative measurements of well-selected samples can reveal important information about the evolutionary state of distant galaxies. The strongest possible constraints on models of the evolution of galaxies can be constructed by combining detailed quantitative information for individual galaxies with the statistical information that is available because these galaxies originate in well-selected statistical samples. The luminosity function (with units of mag Mpc) represents one of a family of generalized distribution functions, (e.g., the ``size-function'' of disks, Figure 4 (Schade et al. 1996) each of which is a projection of the multi-variate distribution function of galaxy properties into lower dimensionality. Any model of galactic evolution can be expressed as a prediction about the evolution in the distribution function ). The distribution at high-redshift can be measured accurately given a large enough sample combined with HST imaging and can then be compared with the distribution at lower redshift or in the local population. Increasingly rigorous tests will become possible as sample sizes become larger.
Figure: The space density of disks as a function of size (see Schade et al. 1996). This figure shows that the space density of small (1< h < 4 kpc) disks is roughly constant with redshift. There is a surplus of luminous () small disks at high redshift (with a median luminosity of ) relative to the same size range at low redshift. This surplus is accounted for by the number density of lower luminosity disks (; median luminosity ) at lower redshift (). In other words, reaching magnitudes deeper into the luminosity function at low redshift produces the same density of disks in this size range as is seen at high redshift.
The completion of large, complete redshift surveys, the estimation of the evolving galaxy luminosity function, and the availability of detailed quantitative morphological information derived from HST imaging have provided a rapid increase in our knowledge of the galaxy population from 0 < z < 1. All morphological types known among luminous galaxies locally are also seen at . The majority of blue galaxies at are either apparently normal disk galaxies ( of the blue objects) or ``blue-nucleated galaxies'' () which are associated with asymmetric structure and mergers. The mean surface brightness of the disk galaxies is higher than the local value by more than 1 magnitude. These two effects--disk brightening and the BNG phenomenon (also associated with ongoing star formation)--must be responsible for much of the observed evolution in the luminosity function of faint field galaxies. In rich clusters, evolution in the size-luminosity relation of luminous ellipticals suggests that passive evolution of an old stellar population (with little ongoing star-formation) may dominate the changes in these galaxies. A straightforward view of these findings is that luminosity evolution (due to decreasing star-formation in the field population and to an aging stellar population in cluster ellipticals) may constitute a large part of the overall evolution of the galaxy population since redshift one.
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