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Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

Cosmology with the HST Medium Deep Survey

R. E. Griffiths, K. U. Ratnatunga, M. Im, L. W. Neuschaefer, N. Roche, S. Casertano, E. J. Ostrander
Bloomberg Center for Physics and Astronomy, Johns Hopkins University, 3400 Charles St. North, Baltimore, MD 21218, USA

R. S. Ellis, R. Abraham, K. Glazebrook, B. Santiago, G. Gilmore
Institute of Astronomy, Madingley Road, University of Cambridge, England

R. F. Green, V. Sarajedini
NOAO, Tucson, AZ 85719 USA

J. P. Huchra
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA



Although galaxies were suggested and dismissed as potentially useful cosmological tools decades ago, it is time to revisit this potential in the light of high resolution observations using HST. It was recognized early in the development of HST that imaging surveys would be very fruitful in unraveling the evolution of the galaxy populations, and perhaps a well-behaved population such as the ellipticals may indeed be useful for cosmological purposes. This paper summarizes what we have learned thus far, and explores the use of serendipitously discovered gravitational lenses for the measurement of the cosmological constant . The application of HST to the measurement of the Hubble constant was planned, but the application to cosmological parameters has perhaps been more surprising, bearing in mind that we are only two years past the refurbishment Shuttle mission which restored HST to full potential. We are clearly just beginning to tap the full potential of HST for cosmological problems.

First, WFPC2 on HST has provided the means to make rapid progress towards solving a long-standing problem in observational cosmology, viz. the nature of the objects contributing to the faint blue galaxy counts. The solution to this problem has been found in the increasing number of irregular and peculiar systems seen as HST probes to greater depths. Galaxies in the Medium Deep Survey (MDS) have been reliably classified to magnitudes in the F814W band, at a mean redshift 0.5. A high proportion (40%) of these objects are irregular or anomalous and they have great diversity. They include compact galaxies, galaxies or protogalaxies with superluminous starforming regions, interacting pairs, and diffuse low surface brightness galaxies of various forms. These diverse objects contribute most of the excess counts in the I-band at our limiting magnitude, and when MDS images with BVI data are taken into account, they also explain most of the `faint blue galaxy' excess. Of the irregular population (40% of the total at I=22), about 30% show multiple, high surface brightness components indicative of starburst activity.

But roughly half of the faint galaxies (I < 22) appear to be similar to regular Hubble-sequence examples observed at low redshift, with relative numbers of spheroidal and disk systems roughly consistent with nearby samples, indicating that the bulk of the local giant galaxy population was in place at half the Hubble time. Moreover, the giant population has certainly faded since z = 1, by about 1 mag. for the E/S0s and about the same for the early-type spirals. For the first time, we have observed directly the evolution of the luminosity function of E/S0 galaxies. We have found evidence for weak gravitational shear in the vicinity of E/S0 field galaxies, and have used this effect to constrain their masses and the cut-off radii of their dark matter halos.

The ``major'' merger rate has evolved only slowly with look-back time, as out to z = 1 or 1.5, although there is already evidence from the deeper surveys that the rate may have been greater at higher redshifts. ``Major'' merging is probably not a major component of the explanation for the faint blue galaxy excess (to I = 22. ``Minor merging'' is a different story, and more difficult to address observationally. The deeper HST surveys, addressing the redshift range from 1 to 2, show increasing evidence of ``conglomerations'', i.e., groupings of what appear to be sub-galactic `clumps'.

The clear picture which has thus emerged from the HST MDS is one in which the giant ellipticals and spirals have passively faded but have otherwise been relatively stable since z = 1, whereas there has been rapid evolution of the irregular/peculiar galaxy population -- about 20% of them were brighter by about 2 mags. only 6Gyr. ago. This latter population is not at all homogeneous, however, and seems to comprise galaxies in formation as well as fading dwarf irregulars.

Finally, and perhaps of greatest interest to this conference, strong gravitational lenses in MDS and archival data have included the first examples of HST-discovered ``Einstein crosses'' centered on elliptical galaxies. These can be used to measure : combining HST with published data, our results show that if . We can exclude the model at the 95 % confidence level. We also find that an open universe is less likely by a factor of about 5--10 than a flat universe with non-zero .

Keywords: galaxies, morphology, lensing, lambda, cosmology


Although it was hoped that galaxies would provide some clues to the shape of the universe (Sandage 1961) these hopes were dashed when Tinsley (1968) and coworkers pointed out that the galaxies must surely have evolved to their present numbers, shapes and sizes. Before the advent of HST, ground-based imaging could not be used to separate the galaxy populations, but now we have an instrument which can, and we can see how they have evolved individually. If we look at the total galaxy number counts, for example, then the effect of cosmological geometry, e.g., a range in the value of from 0 to 1, has an effect which is smaller than the differential between blue and red galaxy counts at B = 24 (Broadhurst, Ellis & Glazebrook 1992, Metcalfe et al. 1995, Colless et al. 1994, Koo & Kron 1992, Lilly et al. 1993). The identification of this blue population was thus of paramount importance in order to separate and remove it as a `contaminant' to the problem of world geometry. Having done this, we may then be in a position to use a more well-behaved galaxy species, the ellipticals (if we can show that they formed at high redshift), in order to constrain the geometrical parameters. In order to make progress on these problems, we clearly needed large samples. Although the field of view of HST is small, the opportunity presented itself to take pictures of random fields with the Wide Field and Planetary Camera (WFPC2) when another HST instrument was targeted towards a known object of interest.

The Medium Deep Survey (MDS) is the HST Key Project which uses these parallel observations of random fields taken with WFPC2 in broad-band filters. Primarily, and for pointings of only a few orbits, exposures are taken in V and I; but for the longer pointings of six orbits or more, the B-band has been given priority, especially in HST observing cycles 4 and 5, when the scheduling system became more sophisticated. The overall goals are wide-ranging, but they are all based on statistical studies of the properties of a large sample of faint galaxies, as well as Galactic star counts and studies within nearby extragalactic objects. In particular, the Survey was designed to resolve the problem of the abundant population of faint blue galaxies.


Typical MDS observations range from 600 to 2000 seconds per exposure, and may consist of 1--20 exposures of the same field. In Cycles 1--3, 146 fields were observed with the WFC in a total exposure time of 240 hours. To December 1995, Cycle 4/5 MDS observations had been made in nearly 400 fields, with a total of 400 hours in the two predominant filters: F606W, somewhat redder and broader than Johnson V , and F814W, close to Kron-Cousins I . Of these fields, 140 were exposed to both filters, and over 30 had at least three exposures in each. Eight fields had exposures in the BVI set. The filters have been chosen to maximize the sensitivity for typical galaxies at intermediate redshifts (). A typical WFPC2 MDS field contains 50--400 detectable sources within the Wide Field Camera (WFC) to a limiting magnitude of = 24--26 depending on the total exposure. The precision with which morphological properties can be measured is, of course, a function of both angular size and apparent magnitude.

Results from Morphological Classification

The Evolution of Dwarf and Irregular Galaxies

The results of morphological typing into three crude galaxy classes (E/S0, early-type spirals and late-type spirals/irregulars) has been described by Abraham et al. (1996), and summarized by Longair (these Proceedings).

The Evolution of the Giant Galaxies

The mild-evolution models (e.g., Guiderdoni & Rocca-Volmerange 1990) predict luminous galaxies of large angular size at high redshift: such objects are certainly seen in the data, consistent with a brightening at higher redshift. Still, the excess number counts, at least to I = 22, are largely unexplained by `giant' spirals or ellipticals, which are observed to have only passive (luminosity) evolution. Their number counts (Glazebrook et al. 1995, Driver et al. 1995b, Abraham et al. 1996), size vs. redshift relationship (Mutz et al. 1994), and structural parameters (Windhorst et al. 1994, Phillips et al. 1995, Forbes et al. 1995,1996) all indicate a relatively benign population: the bulk of the local giant population was apparently in place at half the Hubble time.

Luminosity Evolution of the E/S0 population

Perhaps the easiest galaxy type to separate out from the general population is the E/S0s. The automated galaxy classification software developed for the MDS and based on the maximum likelihood method (Ratnatunga et al. 1994a) allows us to select galaxies which have the `de Vaucouleurs' profile. We have thus constructed the luminosity functions of elliptical galaxies using data from the MDS together with archived HST surveys (Groth et al. 1994, with spectroscopy from Lilly et al. 1995). Photometric redshifts of these E/S0s have been determined using V-I colors and sizes to an accuracy of up to , and checked with MDS follow-up spectroscopy or published values (Lilly et al. 1995) for about 10% of the sample. The luminosity functions, constructed in 3 different redshift bins (, , ) are shown in Fig.1 (from Im et al. 1996b).

These independent luminosity functions show the brightening in the luminosity of E/S0s by about magnitude at , and no sign of significant number evolution. This is the first direct measurement of the luminosity evolution of E/S0 galaxies, and our results support the hypothesis of a high redshift of formation (z >> 1) for elliptical galaxies, together with weak evolution of the major merger rate at z < 1. The high redshift of formation is consistent with that of cluster ellipticals (Bower et al. 1992, Ellis 1996) as determined from the small scatter in the (U-V) colors, and with HST studies of ellipticals in high redshift clusters (Dickinson et al. 1995). The stars in ellipticals are as old as those in globular clusters.

Figure: Evolution in the Luminosity Function of E/S0 galaxies, from Im et al. (1996c).

The majority of ellipticals must have formed at z >> 1, and their number evolution was not significant at z < 1. Our data are consistent with a merger rate of Gyr for the model, and Gyr for the model with and with no strong major merger rate evolution (§ 3).

Given the negligible number evolution, the observed color distribution of ellipticals can be matched with the predictions of stellar synthesis models. The observed V -I color distribution for I < 22 requires evolution in the spectral energy distribution (SED), otherwise there is an excess of very red ellipticals with respect to the prediction of the no-evolution (NE) model (Im et al. 1996a). Luminosity evolution models based on a single starburst at the redshift of formation (if ) or (if ) can comfortably match the observed color distribution, suggesting that most ellipticals are neither very young nor very old ( 9 < age < 20Gyr). We have used the number-magnitude counts in an attempt to constrain not only the evolution of ellipticals, but also the cosmological parameters or . Our results favor low or non-zero models for a minimal (local) merger rate. The use of the E/S0s to constrain the cosmological parameters is stymied by two problems, however: (i) the uncertainty in the faint-end slope of the luminosity function of these galaxies, and (ii) the uncertainty in the actual epoch or range of epoch of formation. To see how we may better exploit the elliptical population to measure , see § 5.

Statistical Properties at the Survey Limit

Statistical properties of galaxies are measured to I = 24 with WF/PC and 25 with WFPC2. For the pre-refurbishment WF/PC images, the structural parameters of about 13,000 objects are presented by Casertano et al. (1995), using data taken from about 112 fields. Sizes, magnitudes, colors and crude classifications are based on two-dimensional model fitting to undeconvolved images (Ratnatunga et al. 1994a). Number counts in the range 18<I<22 exceed the ground-based numbers by about 50%, a range over which many ground-based objects may have been misclassified as stars. The ellipticals become redder for mag, but the spirals show no trend of apparent color with magnitude, indicating that they are intrinsically slightly bluer at higher redshift when K-corrections are taken into account. For galaxies with in WFPC2 data, the maximum-likelihood fits have been found for combined disk-plus-bulge models of all galaxies.

The angular size distribution certainly favors a steep luminosity function, but the faint data also demonstrate that the numbers of galaxies with both blue () colors and moderately large at ) angular sizes exceed the non-evolving predictions by a factor of two, and are in good agreement with the simple pure luminosity evolution model (Roche et al. 1996a). This model incorporates the steep luminosity function () for later-type galaxies and moderate luminosity evolution (-- at ) for spiral galaxies of all luminosities (similar to the models of Campos & Shanks 1996).

The excess of large blue galaxies in the deeper fields (fig. 2) is positive reinforcement of our finding that large, spiral galaxies do undergo significant luminosity evolution to , and that evolutionary brightening is not confined to dwarf galaxies. This interpretation is supported by the spectroscopic results of Schade et al. (1995), Lilly et al. (1995) and Forbes et al. (1996), all of which suggest a similar brightening of spirals with redshift.

Figure: V-I color distributions for small and large galaxies---from Roche et al. (1996a)

Compact Galactic Nuclei

Another important scientific aim of the MDS is to identify possible AGN candidates from the images in order to measure the faint end of the AGN luminosity function as well as to study the host galaxies of AGNs and nuclear starburst systems. We are able to identify candidate objects based on morphology: candidates are selected by fitting a combined bulge and disk model to the galaxy and examining the residual once this model image is subtracted. Each candidate exhibits an unresolved point source in the nucleus which is well fit with a stellar point spread function. We have discovered that there is a population of unresolved nuclei within those galaxies fitted simultaneously with disk and bulge components ( 22); these galaxies show narrow emission lines in the spectroscopic follow-up program. HST photometry indicates that these stellar nuclei have the colors of moderately redshifted Seyfert I galaxies. About 6% of field galaxies at may, therefore, contain AGN which are 3--4 magnitudes fainter than the host galaxies (Sarajedini et al. 1996).

Slow Evolution of the Major Merger Rate

Figure: Angular correlation functions of galaxies in HST survey data. The lines have slope -0.7. Note the continuation to small scales---from Neuschaefer et al. (1995,1996).

Figure: Nearest-neighbor pair counts versus angular separation, , normalized by random sample statistics. The curves show the expected numbers of pairs for the given evolution in the merger rate, assumed to take the form .

Down to 25 mag the number of galaxy pairs with separations 3 0 is consistent with the inward extrapolation of the angular two-point correlation function observed from the same data (fig. 3); the fraction of such pairs showing morphological evidence for physical association accounts for two-thirds of the amount indicated by ) . Moreover, relative to field galaxies, the (V-I) color and I-magnitude difference distributions of 3 0 pairs are similar.

We find that merging has a moderate dependence on redshift: we estimate the rate of merging , with for galaxies with 25 mag. (fig. 4). There are two models consistent with these data: (a) a low density universe with strong clustering evolution parameterized by a clustering exponent 1.0; or (b) two galaxy populations identified by their clustering properties, in which the more weakly clustered population fades or dissipates but does not take part in widespread merging prior to z.3, whereas the more strongly clustered population, which we associate with local ellipticals and disks within two magnitudes of , takes part in minor mergers with dwarf galaxies at the uniform merging rate observed locally, out to 1.0.

Weak Galaxy-galaxy Gravitational Lensing---Constraints on the Halos of E/S0s

The WFPC2 images are ideal for weak lensing studies, since ellipticities and position angles can be measured routinely to I = 24 in the summed exposures. There is indeed evidence for `weak shear' lensing (Griffiths et al. 1996), as evidenced by the preferential orientation of background field galaxies (I = 22--24), in the vicinity of foreground ellipticals (I = 18--22). The shear cause the background galaxies to be preferentially oriented in the direction perpendicular to the radius vector to the foreground galaxy, and is detected as a deviation of the distribution of the position angle from uniformity (see fig. 5).

Figure: Weak lensing signal for foreground ellipticals, spirals and the control sample of stars, using a Fourier-series decomposition.

For the rest of the foreground galaxies (mostly spirals), the observed signal is considerably weaker. This is an expected trend because the late-type galaxies have less mass and shallower profile near the center than ellipticals. But again we find that the observed signal is well matched by the isothermal sphere model with . No signal is observed in a control sample with stars as ``foreground'' objects.

Strong Lensing by Elliptical Galaxies, and the value of

In the process of visual examination of a sample of elliptical galaxies selected by the automated MDS data processing, we discovered a few of the `Einstein cross' type of gravitational lenses in archived and MDS data. Such objects would not have been discovered in ground-based data because of the required resolution. These are the first lenses centered on relatively bright elliptical galaxies with well understood properties. As shown in Im et al. (1996c), such objects may eventually be powerful cosmological tools. In order to combine the information on cosmological parameters from all available lenses, we constructed a likelihood function which is a product of the probability of each lens having the observed given , , and the cosmological parameters. Gravitational lenses were selected using the following criteria: 1) The strong lensing must be caused by a single galaxy lens. For example, we do not include 0957+561 in our sample since there are two lensing galaxies in this system, 2) The apparent magnitude and the redshift of the lens galaxy must be known or estimated to reasonable accuracies. Accurate values for and are important for estimates of the dynamical properties of the lens galaxy. 3) It must be established that the lens galaxy is an elliptical. For example, we do not include MG0218+0357 in our study since there is good evidence that the lensing galaxy is a spiral or a late-type galaxy 4) For lens candidates that do not have a measured value for , we select only those which show distinctive features such as rings or crosses.

We find that there are seven strong gravitational lenses that meet these selection criteria in the MDS and published literature (Im et al. 1996c). In fig. 6, we present the likelihood function in - space, with contours of various levels of statistical significance. The likelihood function peaks at with low . The diagonal line represents the case when . If , then the model is strongly excluded at a confidence level >95 %. The low model with is excluded at weaker statistical significance (80--90%).

Figure: The maximum likelihood value for using strong gravitational lenses (elliptical galaxies) from HST and published data


The ``excess number counts'' above the predictions of no-evolution models are caused largely by the evolution of low-luminosity systems, which have a higher space density than that predicted from local ground-based surveys. These objects manifest themselves as large numbers of irregular galaxies in the data, a large fraction of which seem to have multiple starburst components. At the faint end of the survey, an increasing frequency of irregular systems and conglomerations are seen, including what appear to be high redshift galaxies or protogalaxies in various stages of formation and evolution.

Early-type galaxies are seen to undergo luminosity evolution and were brighter by about 1 mag. at z = 1. Their numbers and sizes are consistent with a high redshift of formation, z >> 1. Nevertheless, it is difficult to use them to probe the cosmological parameters because of two problems: (i) the faint end slope of the LF for these galaxies is not well determined, and (ii) the actual epoch of formation is unknown. Although the observations support a low value for , this result is tentative.

The rate of galaxy merging has not evolved rapidly since z = 1, in contrast with several claims. However, the deeper HST surveys may already indicate that the merger rate was much higher at z between 1 and 3 than it was at z < 1.

Serendipitously discovered strong gravitational lenses may eventually provide one of the best measures of . These objects are found at the rate of several per square degree, but require the angular resolution of HST, together with follow-up spectroscopy with 10m-class ground-based telescopes.


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