arXiv subjects

Pieter G. van Dokkum

Publications and source records attributed to Pieter G. van Dokkum.

At least 145 records · Page 8Linked to original sources

arXiv2003

On the Assembly History of Early-Type Galaxies in the Hubble Deep Field North

We present deep Keck spectroscopy for a sample of I<22.5 field early-type galaxies selected morphologically in the redshift range 0.56 =0.88. However, we find enhanced H delta absorption of equivalent width 4.0 Angstrom in the mean spectrum of the ten galaxies, indicating the presence of young stars. We infer that the galaxies have composite stellar populations, consisting of a low mass young component in addition to a dominating old component. As the bulk of the stellar mass must have formed at z >~ 2 our results argue against formation scenarios involving major mergers of gas-rich disk systems at 1<z<1.5, and we conclude that z~1 early-type galaxies were either assembled at higher redshift or in mergers involving little gas. The ubiquitous enhanced Balmer lines and the presence of tidal features in two of the galaxies lend some support to the latter hypothesis. The main uncertainty in the analysis is the small sample; larger samples are needed to study the interplay between the evolution of stellar populations and morphology in detail.

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arXiv2003

A Significant Population of Red, Near-IR Selected High Redshift Galaxies

We use very deep near-infrared photometry of the Hubble Deep Field South taken with ISAAC/VLT to identify a population of high redshift galaxies with rest- frame optical colors similar to those of nearby galaxies. The galaxies are chosen by their infrared colors Js-Ks > 2.3, aimed at selecting galaxies with redshifts above 2. When applied to our dataset, we find 14 galaxies with Ks < 22.5, corresponding to a surface density of 3+-0.8 /arcmin**2. The photometric redshifts all lie above 1.9, with a median of 2.6 and a rms of 0.7. The spectral energy distributions of these galaxies show a wide range: one is very blue in the rest-frame UV, and satisfies the normal Lyman-break criteria for high redshift, star-forming galaxies. Others are quite red throughout the observed spectral range, and are extremely faint in the optical, with a median V = 26.6. Hence these galaxies would not be included in photometric samples based on optical ground-based data, and spectroscopic follow-up is difficult. The spectral energy distributions often show a prominent break, identified as the Balmer break or 4000 Ang. break. The median age is 1 Gyr when fit with a constant star formation model with dust, or 0.7 Gyr when fit with a single burst model. Although significantly younger ages cannot be excluded when a larger range of models is allowed, the results indicate that these galaxies are among the oldest at these redshifts. The volume density to Ks=22.5 is half that of Lyman-break galaxies at z = 3. Since the mass-to-light ratios of the red galaxies are likely to be higher, the stellar mass density is inferred to be comparable to that of Lyman-break galaxies. These red galaxies may be the descendants of galaxies which started to form stars at very high redshifts, and they may evolve into the most massive galaxies at low redshift.

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arXiv2003

Spectroscopic Confirmation of a Substantial Population of Luminous Red Galaxies at Redshifts z>2

We confirm spectroscopically the existence of a population of galaxies at z>~2 with rest-frame optical colors similar to normal nearby galaxies. The galaxies were identified by their red near-infrared colors in deep images obtained with ISAAC on the Very Large Telescope. Redshifts of six galaxies with Js-Ks>2.3 were measured from optical spectra obtained with the Keck Telescope. Five out of six are in the range 2.43<=z<=3.52, demonstrating that the Js-Ks color selection is quite efficient. The rest-frame ultraviolet spectra of confirmed z>2 galaxies display a range of properties, with two galaxies showing emission lines characteristic of AGN, two having Ly-alpha in emission, and one showing interstellar absorption lines only. Their full spectral energy distributions are well described by constant star formation models with ages 1.4-2.6 Gyr, except for one galaxy whose colors indicate a dusty starburst. The confirmed z>2 galaxies are very luminous, with Ks=19.2-19.9. Assuming that our bright spectroscopic sample is representative for the general population of Js-Ks selected objects, we find that the surface density of red z>~2 galaxies is ~0.9/arcmin^2 to Ks=21. The surface density is comparable to that of Lyman-break selected galaxies with Ks<21, when corrections are made for the different redshift distributions of the two samples. Although there will be some overlap between the two populations, most 'optical-break' galaxies are too faint in the rest-frame ultraviolet to be selected as Lyman-break galaxies. The most straightforward interpretation is that star formation in typical optical-break galaxies started earlier than in typical Lyman-break galaxies. Optical-break galaxies may be the oldest and most massive galaxies yet identified at z>2, and could evolve into early-type galaxies and bulges.

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arXiv2002

The Fundamental Plane at z=1.27: First Calibration of the Mass Scale of Red Galaxies at Redshifts z>1

We present results on the Fundamental Plane (FP) of early-type galaxies in the cluster RDCS J0848+4453 at z=1.27. Internal velocity dispersions of three K-selected early-type galaxies are determined from deep Keck spectra. Structural parameters are determined from HST NICMOS images. The galaxies show substantial offsets from the FP of the nearby Coma cluster, as expected from passive evolution of their stellar populations. The offsets from the FP can be expressed as offsets in M/L ratio. The M/L ratios of the two most massive galaxies are consistent with an extrapolation of results obtained at z=0.02-0.83. The evolution of early-type galaxies with masses >10^11 M_sun is well described by ln M/L(B) = (-1.06 +- 0.09) z, corresponding to passive evolution of -1.50 +- 0.13 mag at z=1.3. Ignoring selection effects, the best fitting stellar formation redshift is z*=2.6, corresponding to a luminosity weighted age at the epoch of observation of ~2 Gyr. The M/L ratios of these two galaxies are also in excellent agreement with predictions from models that include progenitor bias. The third galaxy is a factor ~10 less massive than the other two, shows strong Balmer absorption lines in its spectrum, and is offset from the Coma Fundamental Plane by 2.9 mag in rest-frame B. Despite their large range in M/L ratios, all three galaxies fall in the ``Extremely Red Object'' (ERO) class with I-H>3 and R-K>5, and our results show that it is hazardous to use simple models for converting luminosity to mass for these objects. Measurements of M/L ratios at high redshift can be considered first steps to empirically disentangle luminosity and mass evolution at the high mass end of the galaxy population, lifting an important degeneracy in the interpretation of evolution of the luminosity function. [SHORTENED]

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arXiv2001

Formation and Evolution of Galaxies in Clusters

Elliptical and S0 galaxies dominate the galaxy population in nearby rich clusters such as Coma. Studies of the evolution of the colors, M/L ratios, and line indices of early-type galaxies indicate that they have been a highly homogeneous, slowly evolving population over the last ~65% of the age of the Universe. On the other hand, recent evidence suggests that many early-type galaxies in clusters have been transformed from spiral galaxies since z~1. Arguably the most spectacular evidence for such transformations is the incidence of red merger systems in several high redshift clusters. Due to this morphological evolution the sample of early-type galaxies at high redshift is only a subsample of the sample of early-type galaxies at low redshift. This ``progenitor bias'' results in an overestimate of the mean formation redshift if simple models without morphological transformations are used. Models which incorporate morphological evolution explicitly can bring the homogeneity, slow evolution, and morphological transformations into agreement. The modeling shows that the corrected mean formation redshift of the stars in early-type galaxies may be as low as z~2 in a Lambda dominated Universe.

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arXiv2001

U, B and r band luminosity functions of galaxies in the Coma cluster

We present a deep multi-colour CCD mosaic of the Coma cluster (Abell 1656), covering 5.2 deg^2 in the B and r bands, and 1.3 deg^2 in the U band. This large, homogeneous data set provides a valuable low redshift comparison sample for studies of galaxies in distant clusters. In this paper we present our survey, and study the dependence of the galaxy luminosity function (LF) on passband and radial distance from the cluster centre. The U, B and r band LFs of the complete sample cannot be represented by single Schechter functions. For the central area, r<245 h_{100}^{-1} kpc, we find best-fitting Schechter parameters of M^{*}_U=-18.60^{+0.13}_{-0.18} and α_U=-1.32^{+0.018}_{-0.028}, M^{*}_B=-19.79^{+0.18}_{-0.17} and α_B=-1.37^{+0.024}_{-0.016} and M^{*}_r=-20.87^{+0.12}_{-0.17} and α_r=-1.16^{+0.012}_{-0.019}. The LF becomes steeper at larger radial distance from the cluster centre. The effect is most pronounced in the U band. This result is consistent with the presence of a star forming dwarf population at large distance from the cluster centre, which may be in the process of being accreted by the cluster. The shapes of the LFs of the NGC 4839 group support a scenario in which the group has already passed through the centre.

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arXiv2001

HST large field weak lensing analysis of MS 2053-04: study of the mass distribution and mass-to-light ratio of X-ray selected clusters at 0.22<z<0.83

We have detected the weak lensing signal induced by the cluster of galaxies MS 2053-04 (z=0.58) from a two-colour mosaic of 6 HST WFPC2 images. The best fit singular isothermal sphere model to the observed tangential distortion yields an Einstein radius r_E=6.2"+-1.8", which corresponds to a velocity dispersion of 886^{+121}_{-139} km/s. This result is in good agreement with the observed velocity dispersion of 817+-80 km/s from cluster members. MS 2053 is the third cluster we studied using mosaics of deep WFPC2 images. For all three clusters we find good agreement between dynamical and weak lensing velocity dispersions, in contrast to weak lensing studies based on single WFPC2 pointings on cluster cores. This result demonstrates the importance of wide field data. We have compared the ensemble averaged cluster profile to the predicted NFW profile, and find that a NFW profile can fit the observed lensing signal well. The best fit concentration parameter is found to be 0.79^{+0.44}_{-0.15} (68% confidence) times the predicted value from an open CDM model. The observed mass-to-light ratios of the clusters in our sample evolve with redshift, and are inconsistent with a constant, non-evolving, mass-to-light ratio at the 99% confidence level. The evolution is consistent with the results derived from the evolution of the fundamental plane of early type galaxies. The resulting average mass-to-light ratio for massive clusters at z=0 is found to be 239+-18+-9 M/L_B. (abridged)

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arXiv2001

Cosmic-Ray Rejection by Laplacian Edge Detection

Conventional algorithms for rejecting cosmic-rays in single CCD exposures rely on the contrast between cosmic-rays and their surroundings, and may produce erroneous results if the Point Spread Function (PSF) is smaller than the largest cosmic-rays. This paper describes a robust algorithm for cosmic-ray rejection, based on a variation of Laplacian edge detection. The algorithm identifies cosmic-rays of arbitrary shapes and sizes by the sharpness of their edges, and reliably discriminates between poorly sampled point sources and cosmic-rays. Examples of its performance are given for spectroscopic and imaging data, including HST WFPC2 images.

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arXiv2001

Luminosity Evolution of Field Early-Type Galaxies to z=0.55

We study the Fundamental Plane of field early-type galaxies at intermediate redshift, using HST WFPC2 observations and deep Keck spectroscopy. Structural parameters and internal velocity dispersions are measured for eighteen galaxies at 0.15 < z < 0.55. Rest frame M/L_B ratios are determined from the Fundamental Plane and compared to those of cluster early-type galaxies at the same redshifts. The systematic offset between M/L ratios of field and cluster early-type galaxies at intermediate redshift is small, and not significant: (field) - (cluster) = -0.18 +- 0.11. The M/L ratios of field early-type galaxies evolve as Δ(ln M/L_B) = (-1.35 +- 0.35) z, very similar to cluster early-type galaxies. After correcting for luminosity evolution, the FP of field early-type galaxies has a scatter 0.09 +- 0.02 in log r_e, similar to that in local clusters. The scatter appears to be driven by low mass S0 galaxies; for the elliptical galaxies alone we find 0.03 +- 0.03. There is a hint that the FP has a different slope than in clusters, but more data are needed to confirm this. The similarity of the M/L ratios of cluster and field early-type galaxies provides a constraint on the relative ages of their stars. At = 0.43, field early-type galaxies are younger than cluster early-type galaxies by only 21 +- 13 %, and we infer that the stars in field early-type galaxies probably formed at z >~ 1.5. Recent semi-analytical models for galaxy formation in a Lambda-CDM Universe predict a systematic difference between field and cluster galaxies of Δ(ln M/L_B) ~ -0.6, much larger than the observed difference. This result is consistent with the hypothesis that field early-type galaxies formed earlier than predicted by these models.

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arXiv2001

The Evolution of Balmer Absorption Line Strengths in E/S0 Galaxies from z=0 to z=0.83

We present new results from a systematic study of absorption line strengths of galaxies in clusters approaching redshifts of unity. In this paper, we specifically compare the strengths of the high-order Balmer absorption features of Hgamma and Hdelta in E/S0s in the four clusters Abell 2256 (z=0.06), CL1358+62 (z=0.33), MS2053--04 (z=0.58), and MS1054--03 (z=0.83). By comparing the correlation of Balmer line strength with velocity dispersion for E/S0s in the three clusters, we find moderate evolution in the zero-point of the (Hdelta_A+Hgamma_A)-sigma relation with redshift. The trend is consistent with passive evolution of old stellar populations. Under the assumption that the samples can be compared directly, we use single-burst stellar population synthesis models to constrain the last major occurrences of star-formation in the observed E/S0s to be z_f > 2.5 (95% confidence). We have compared the evolution of the Balmer absorption with the evolution of the B-band fundamental plane and find that simple stellar population models agree very well with the data. While the best agreement occurs with a low value for Omega_m, the data provide strong confirmation of the time-evolution in recent stellar population models.

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arXiv2001

Morphological Evolution and the Ages of Early-Type Galaxies in Clusters

Morphological and spectroscopic studies of high redshift clusters indicate that a significant fraction of present-day early-type galaxies was transformed from star forming galaxies at z<1. On the other hand, the slow luminosity evolution of early-type galaxies and the low scatter in their color-magnitude relation indicate a high formation redshift of their stars. In this paper we construct models which reconcile these apparently contradictory lines of evidence, and we quantify the effects of morphological evolution on the observed photometric properties of early-type galaxies in distant clusters. We show that in the case of strong morphological evolution the apparent luminosity and color evolution of early-type galaxies are similar to that of a single age stellar population formed at z=infinity, irrespective of the true star formation history of the galaxies. Furthermore, the scatter in age, and hence the scatter in color and luminosity, is approximately constant with redshift. These results are consequences of the ``progenitor bias'': the progenitors of the youngest low redshift early-type galaxies drop out of the sample at high redshift. We construct models which reproduce the observed evolution of the number fraction of early-type galaxies in rich clusters and their color and luminosity evolution simultaneously. Our modelling indicates that approx. 50% of early-type galaxies were transformed from other galaxy types at z<1, and their progenitor galaxies may have had roughly constant star formation rates prior to morphological transformation. After correcting the observed evolution of the mean M/L_B ratio for the maximum progenitor bias we find that the mean luminosity weighted formation redshift of stars in early-type galaxies z_*=2.0^{+0.3}_{-0.2} for Omega_m=0.3 and Omega_Lambda=0.7. [ABRIDGED]

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arXiv2000

HST Photometry and Keck Spectroscopy of the Rich Cluster MS1054-03: Morphologies, Butcher-Oemler Effect and the Color-Magnitude Relation at z=0.83

We present a study of 81 I selected, spectroscopically-confirmed members of the X-ray cluster MS1054-03 at z=0.83. Redshifts and spectral types were determined from Keck spectroscopy. Morphologies and accurate colors were determined from a large mosaic of HST WFPC2 images in F606W and F814W. Early-type galaxies constitute only 44% of this galaxy population. Thirty-nine percent are spiral galaxies, and 17% are mergers. The early-type galaxies follow a tight and well-defined color-magnitude relation, with the exception of a few outliers. The observed scatter is 0.029 +- 0.005 magnitudes in restframe U-B. Most of the mergers lie close to the CM relation defined by the early-type galaxies. They are bluer by only 0.07 +- 0.02 magnitudes, and the scatter in their colors is 0.07 +- 0.04 magnitudes. Spiral galaxies in MS1054-03 exhibit a large range in their colors. The bluest spiral galaxies are 0.7 magnitudes bluer than the early-type galaxies, but the majority is within +- 0.2 magnitudes of the early-type galaxy sequence. The red colors of the mergers and the majority of the spiral galaxies are reflected in the fairly low Butcher-Oemler blue fraction of MS1054-03: f_B=0.22 +- 0.05. The slope and scatter of the CM relation of early-type galaxies are roughly constant with redshift, confirming previous studies that were based on ground-based color measurements and very limited membership information. However, the scatter in the combined sample of early-type galaxies and mergers is twice as high as the scatter of the early-type galaxies alone. This is a direct demonstration of the ``progenitor bias'': high redshift early-type galaxies seem to form a homogeneous, old population because the progenitors of the youngest present-day early-type galaxies are not included in the sample.

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arXiv1999

The Evolution of Early-Type Galaxies in Distant Clusters I.: Surface Photometry and Structural Parameters for 53 Galaxies in the z=0.33 Cluster CL1358+62

Using HST imaging of CL1358+62 (z=0.33) structural parameters are derived for 53 cluster members. We fit integrated r^{1/4}-laws to the integrated S.B. profiles, and fit two-dimensional r^{1/4}-law model galaxies to the images directly. The results from the two methods agree very well, with an rms scatter of 13% in r_e. The half-light radii range from 1 to 20 kpc, with a median of 3 kpc. We compared r_e from the r^{1/4}-law fits to r_{1/2} derived using other profiles. In particular, we fit Sersic r^{1/n}-laws and superpositions of r^{1/4}-law bulges with exponential disks. The r_{1/2} derived from the best-fit Sersic profiles varied with respect to r_e from the r^{1/4}-law fits by 1% in the median, but with a standard deviation of more than 40%. For the bulge-plus-disk fits, the derived r_{1/2} were offset from r_e of the r^{1/4}-law fits by 10% in the mean, also with a standard deviation of more than 40%. By comparing the Sersic r_{1/2} with those derived from the bulge-plus-disk fitting, that scatter is also large, at 30%. We conclude that r_{1/2} is generally measured with a typical accuracy of 30-40%. The large uncertainties in r_{1/2} do not impact the fundamental plane analysis because the combination r ^{0.76}, which enters the fundamental plane, is extremely stable. The rms scatter in r ^{0.76} is less than 3%, regardless of the form of the profile fit to the galaxies. We find that the median bulge fraction of the sample is 84% and that the few E+A galaxies in this sample have disks which make up about 10-35% of their total light. These results are consistent with residuals from fitting 2D r^{1/4}-law models directly to the galaxies, in which disk-like structures are revealed in about half of the sample. Two of the three E+A galaxies show spiral arm structure.

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arXiv1999

The Evolution of Stellar Populations in Intermediate Redshift Clusters

The $M/L$ ratios and absorption line-strengths of distant cluster galaxies can be used to directly study their stellar populations, determine their redshift of formation, their scatter in ages, and any dependence of their ages on such internal properties such as velocity dispersion or mass. Comparing the zero-point the fundamental plane in the $z=0.33$ cluster CL1358+62 to that in Coma, we conclude that the redshift of formation for the stars in massive cluster E/S0s was $z\ga 2$. The fundamental plane in CL1358+62 has the following form: $r_e \propto σ^{1.31\pm 0.13}< I>_e^{-0.86\pm 0.10}$, indicating that the last epoch of star-formation has very little dependence on galaxy mass. The scatter in the CL1358+62 fundamental plane is also very low, equivalent to a scatter in ages of $\sim 15%$.We have also analyzed the $M/L_V$ ratios of galaxies of type S0/a and later. These early-type spirals follow a different plane from the E and S0 galaxies, with a scatter that is twice as large as the scatter for the E/S0s. These residuals also correlate with the residuals from the color-magnitude relation. Preliminary analysis of the absorption line-strengths of the CL1358+62 early-types indicate that they are well-described by uniformly old, single-burst stellar populations with metallicity varying as a function of velocity dispersion.

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arXiv1999

The Evolution of Early-Type Galaxies in Distant Clusters II: Internal Kinematics of 55 Galaxies in the z=0.33 Cluster CL1358+62

We define a large sample of galaxies for use in a study of the fundamental plane in the intermediate redshift cluster CL1358+62 at $z=0.33$. We have analyzed high resolution spectra for 55 members of the cluster. The data were acquired with the Low Resolution Imaging Spectrograph on the Keck I 10m telescope. A new algorithm for measuring velocity dispersions is presented and used to measure the internal kinematics of the galaxies. This algorithm has been tested against the Fourier Fitting method so the data presented here can be compared with those measured previously in nearby galaxies. We have measured central velocity dispersions suitable for use in a fundamental plane analysis. The data have high $S/N$ and the resulting random errors on the dispersions are very low, typically $<5%$. Uncertainties due to mismatch of the stellar templates has been minimized through several tests and the total systematic error is of order $\about 5%$. Good seeing enabled us to measure velocity dispersion profiles and rotation curves for most of the sample and although a large fraction of the galaxies display a high level of rotation, the gradients of the total second moment of the kinematics are all very regular and similar to those in nearby galaxies. We conclude that the data therefore can be reliably corrected for aperture size in a manner consistent with nearby galaxy samples.

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arXiv1999

The Evolution of Early-Type Galaxies in Distant Clusters III.: M/L_V Ratios in the z=0.33 Cluster CL1358+62

Keck spectroscopy and Hubble Space Telescope WFPC2 imaging over a 1.5x1.5 Mpc field of CL1358+62 at z=0.33 are used to study the Fundamental Plane of galaxies based on a new, large sample of 53 galaxies. First, we have constructed the Fundamental Plane for the 30 E and S0 galaxies and find that it has the following shape: r_e = sigma**(1.31+-0.13) * _e**(-0.86+-0.10), similar to that found locally. The 1-sigma intrinsic scatter about this plane is 14% in M/L(V), comparable to that observed in Coma. We conclude that these E and S0 galaxies are structurally mature and homogeneous, like those observed in nearby clusters. The M/L(V) ratios of these early-type galaxies are offset from the Coma Fundamental Plane by delta log M/L(V) = -0.13+- 0.03 (q0=0.1), indicative of mild luminosity evolution. This evolution suggests a formation epoch for the stars of z > 1. We have also analyzed the M/L(V) ratios of galaxies of type S0/a and later. These early-type spirals follow a different plane from the E and S0 galaxies, with a scatter that is twice as large as the scatter for the E/S0s. The difference in the tilt between the plane of the spirals and the plane of the E/S0s is shown to be due to a systematic correlation of velocity dispersion with residual from the plane of the early-type galaxies. These residuals also correlate with the residuals from the Color-Magnitude relation. Thus for spirals in clusters, we see a systematic variation in the luminosity-weighted mean properties of the stellar populations with central velocity dispersion. If this is a relative age trend, then luminosity-weighted age is positively correlated with dispersion. [abridged version]

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arXiv1999

A High Merger Fraction in the Rich Cluster MS1054-03 at z=0.83: Direct Evidence for Hierarchical Formation of Massive Galaxies

We present a morphological study of the galaxy population of the luminous X-ray cluster MS1054-03 at z=0.83. The sample consists of 81 spectroscopically confirmed cluster members in a 3 x 2 Mpc area imaged in F606W and F814W with WFPC2. We find thirteen ongoing mergers in MS1054-03, comprising 17% of the L > L* cluster population. Most of these mergers will likely evolve into luminous (\sim 2 L*) elliptical galaxies, and some may evolve into S0 galaxies. Assuming the galaxy population in MS1054-03 is typical for its redshift it is estimated that \sim 50% of present-day cluster ellipticals experienced a major merger at z < 1. The mergers are preferentially found in the outskirts of the cluster, and probably occur in small infalling clumps. Morphologies, spectra, and colors of the mergers show that their progenitors were typically E/S0s or early-type spirals with mean stellar formation redshifts z* \gtrsim 1.7. The red colors of the merger remnants are consistent with the low scatter in the color-magnitude relation in rich clusters at lower redshift. The discovery of a high fraction of mergers in this young cluster is direct evidence against formation of ellipticals in a single ``monolithic'' collapse at high redshift, and in qualitative agreement with predictions of hierarchical models for structure formation.

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arXiv1999

The evolution of cluster E and S0 galaxies measured from the Fundamental Plane

New photometry and spectroscopy for the two rich clusters Abell 665 and Abell 2218 are presented and used to establish the Fundamental Plane (FP) for the clusters. The FP for these two clusters adds important knowledge about the properties of E and S0 galaxies in the relatively unexplored redshift interval 0.05 to 0.3. We have compared the FP for A665 and A2218 with the FP for the three clusters CL0024+16, CL1358+62 and MS2053-04 with redshifts between 0.33 and 0.58, and with the FP for the Coma cluster. The scatter around the FP is similar for all six clusters. We find that the slope of the FP for the intermediate redshift clusters is slightly different from the one found for the Coma cluster and other nearby clusters. This may either be caused by selection effects for the intermediate redshift clusters or by differences in the evolution of low luminosity galaxies and high luminosity galaxies. The mass-to-light (M/L) ratios, as measured from the FP, change with redshift. Using the data for all five intermediate redshift clusters, we find that the M/L ratios for photometry calibrated to Gunn r change with redshift as Delta log M/L_r = (-0.26+-0.06) Delta z for q0=0.5. This is consistent with the previously published analysis for CL0024+16, CL1358+62 and MS2053-04. For q0=0.5 the results are consistent with passive evolution of a stellar population which formed at a redshift larger than five. For q0=0.15 the formation redshift must be larger than 1.7. Our new data for A665 and A2218 confirm the gradual and slow evolution of the bright E and S0 galaxies. However, possible star formation in E and S0 galaxies during the last 3-4 Gyr of the history of the Universe, as well as the selection effects complicate the interpretation of the data.

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