arXiv subjects

Marijn Franx

Publications and source records attributed to Marijn Franx.

At least 253 records · Page 14Linked to original sources

arXiv2004

Galaxies at z~7-8: z-dropouts in the Hubble Ultra Deep Field

We have detected likely z~7-8 galaxies in the 144"x144" NICMOS observations of the Hubble Ultra Deep Field. Objects are required to be >3 sigma detections in both NICMOS bands, J and H. The selection criteria for this sample are (z-J)>0.8, (z-J) > 0.66 (J-H)+0.8, (J-H)<1.2, and no detection at <8500 Angstroms. The 5 selected sources have total magnitudes H~27. Four of the five sources are quite blue compared to typical lower-redshift dropout galaxies and are clustered within a 1 arcmin**2 region. Because all 5 sources are near the limit of the NICMOS data, we have carefully evaluated their reality. Each of the candidates is visible in different splits of the data and a median stack. We analyzed several noise images and estimate the number of spurious sources to be 1+/-1. A search using an independent reduction of this same data set clearly revealed 3 of the 5 candidates and weakly detected a 4th candidate, suggesting the contamination could be higher. For comparison with predictions from lower redshift samples we take a conservative approach and adopt four z~7-8 galaxies as our sample. With the same detection criteria on simulated datasets, assuming no-evolution from z~3.8, we predict 10 sources at z~7-8, or 14 if we use a more realistic (1+z)^{-1} size scaling. We estimate that the rest-frame continuum UV (~1800 Angstroms) luminosity density at z~7.5 (integrated down to 0.3L*) is just 0.20_{-0.08}^{+0.12}x that found at z~3.8. Effectively this sets an upper limit on the luminosity density down to 0.3 L*. This result is consistent with significant evolution at the bright end of the luminosity function from z~7.5 to z~3.8. Even with the lower UV luminosity density at z~7.5, it appears that galaxies could still play an important role in reionization at these redshifts, though definitive measurements remain to be made. [abridged]

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arXiv2004

Chandra Constraints on the AGN Fraction and Star Formation Rate of Red z>~2 Galaxies in the FIRES MS1054-03 Field

Very deep near-infrared observations have recently uncovered a significant population of red galaxies at redshifts z>2. These distant red galaxies (DRGs) are efficiently selected by the criterion J-K>2.3. We use Chandra data to examine the X-ray emission from DRGs in the 5'x5' FIRES MS1054-03 field. Two of 40 DRGs with K<22 are detected by Chandra, and we infer that ~5 % of DRGs host active nuclei with L>1.2x10^43 ergs/s. This fraction is smaller than that inferred from optical and near-IR spectroscopy, probably largely due to strong spectroscopic selection biases. By stacking all undetected DRGs we find that their average X-ray flux in the 0.5-8 keV band is ~4.6x10^-17 ergs/s/cm^2. The detection is only significant in the soft and full energy bands. The mean detection may result from star formation, the presence of low luminosity AGN, or a combination of both. Assuming the detection is due exclusively to star formation, we find an average instantaneous star formation rate of 214 +- 68 (random) +- 73 (systematic) solar masses per year, in excellent agreement with previous results from spectral energy distribution fitting when constant star formation histories are assumed. These results may imply that DRGs contribute significantly to the cosmic star formation rate at z~2.5. However, the mean X-ray flux strictly provides only an upper limit to the star formation rate due to the uncertain contribution of low luminosity, possibly obscured AGNs. Observations at other wavelengths are needed to provide independent estimates of the star formation rate of DRGs.

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arXiv2004

Stellar Populations and Kinematics of Red Galaxies at z>2: Implications for the Formation of Massive Galaxies

We recently identified a substantial population of galaxies at z>2 with red rest-frame optical colors. These distant red galaxies (DRGs) are efficiently selected by the simple observed color criterion J-K>2.3. In this paper we present NIR spectroscopy with Keck/NIRSPEC of six DRGs at 2.4<z<3.2. We detect continuum emission and emission lines of all observed galaxies. Equivalent widths of H alpha are 20-30 Ang, smaller than measured for LBGs and nearby LIRGs, and comparable to normal nearby galaxies. The modest equivalent widths imply that the galaxies either have a decreasing star formation rate, or that they are very dusty. Fitting both the photometry and the H alpha lines, we find continuum extinction A_V=1-2 mag, ages 1-2.5 Gyr, star formation rates 200-400 solar masses/yr, and stellar masses 1-5x10^11 solar masses for models with constant star formation rates. From [NII]/H alpha ratios we infer that the metallicities are high, 1-1.5 x Solar. For four galaxies we can determine line widths from the optical emission lines. The widths are high, ranging from 130-240 km/s, and by combining data for LBGs and DRGs we find significant correlations between linewidth and restframe U-V color, and between linewidth and stellar mass. The latter correlation has a similar slope and offset as the ``baryonic Tully-Fisher relation'' for nearby galaxies. The median dynamical mass is ~2x10^11 solar masses, supporting the high stellar masses inferred from the photometry. We find that the median M/L_V ~ 0.8, a factor of ~5 higher than measured for LBGs. We infer from our small sample that DRGs are dustier, more metal rich, more massive, and have higher ages than z=3 LBGs of the same rest-frame V-band luminosity. Their high M/L ratios imply that they contribute significantly to the stellar mass density at z~2.5. [ABRIDGED]

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arXiv2004

Field E+A Galaxies at Intermediate Redshifts (0.3<z<1)

We select E+A candidates from a spectroscopic dataset of ~800 field galaxies and measure the E+A fraction at 0.3<z<1 to be 2.7+/-1.1%, a value lower than that in galaxy clusters at comparable redshifts (11+/-3%). HST/WFPC2 imaging for five of our six E+A's shows they have a heterogeneous parent population: these E+A's span a range in half-light radius (0.8-8 kpc) and estimated internal velocity dispersion (50-220 km/s), and they include luminous systems (-21.6<M_Bz-5logh<-19.2). Despite their diversity in some aspects, the E+A's share several common characteristics that indicate the E+A phase is an important link in the evolution of star-forming galaxies into passive systems: the E+A's are uniformly redder than the blue, star-forming galaxies that make up the majority of the field, they are more likely to be bulge-dominated than the average field galaxy, and they tend to be morphologically irregular. We find E+A's make up ~9% of the absorption line systems in this redshift range, and estimate that ~25% of passive galaxies in the local field had an E+A phase at z<1.

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arXiv2003

The Luminosity-Size and Mass-Size Relations of Galaxies out to z~3

The luminosity-size and stellar mass-size distributions of galaxies out to z~3 is presented. We use very deep near-infrared images of the Hubble Deep Field South in the J_s, H, and K_s bands, taken as part FIRES at the VLT, to follow the evolution of the optical rest-frame sizes of galaxies. For a total of 168 galaxies with K_{s,AB}<23.5, we find that the rest-frame V-band sizes r_{e,V} of luminous galaxies ( ~2x10^{10}h^{-2}L_sun) at 2 ~2x10^{10}h^{-2}M_sun, has changed by 20(+-20)% since z~2.5. Both results can be reconciled by the fact that the stellar M/L ratio is lower in the luminous high z galaxies than in nearby ones because they have young stellar populations. The lower incidence of large galaxies at z~3 seems to reflect the rarity of galaxies with high stellar mass.

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arXiv2003

The detailed Fundamental Plane of two high redshift clusters: MS 2053-04 at z=0.58 and MS 1054-03 at z=0.83

We study the fundamental plane relation in high redshift clusters using a sample of 26 galaxies in MS 2053-04 (z=0.583) and 22 galaxies in MS 1054-03 (z=0.83). The zeropoint and scatter are compared to results for lower redshift clusters in order to trace evolutionary effects. Furthermore, our large sample enables us to investigate correlations between residuals from the fundamental plane and other characteristics of the galaxies, such as color, $Hβ$ linestrength, spatial distribution, and mass. The observed scatter of the early-type galaxies with sigma > 100 km/s around the fundamental plane is 0.134 and 0.106 in log r_e for MS 2053-04 and MS 1054-03 respectively. The residuals from the fundamental plane of MS 2053-04 are correlated with residuals from the $Hβ- σ$ relation, suggesting that stellar populations are playing a role in shaping the fundamental plane. The measured evolution in log M/L is influenced by selection effects, as galaxies with lower M/L in the Johnson B-band enter a magnitude-limited sample more easily. When we select high mass early-type galaxies to avoid this bias, we find $\log M/L_B \sim -0.47 z$ and a formation redshift $z_{form} \sim 2.95$, similar to earlier results.

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arXiv2003

The Nature of E+A Galaxies in Intermediate Redshift Clusters

[Abridged] Combining HST/WFPC2 mosaics with extensive ground-based spectroscopy, we study the nature of E+A galaxies in three intermediate redshift clusters (z=0.33, 0.58, & 0.83). From a sample of ~500 confirmed cluster members, we isolate 46 E+A candidates to determine the E+A fraction and study their physical properties. We find E+A's comprise a non-negligible component (~7-13%) of the cluster population at these redshifts, and their diverse nature indicates a heterogeneous parent population. Cluster E+A's span the range in Hubble type and cover a wide range in luminosity, internal velocity dispersion, and half-light radius. From their velocity dispersions and half-light radii, we infer that the descendants of E+A's in our highest redshift cluster are massive early-type galaxies. We find a decrease in the characteristic E+A mass similar to the decrease in luminosity of rapidly star-forming field galaxies since z~1, i.e. galaxy ``down-sizing.'' In addition, we argue our statistics imply that >30% of the E-S0 members have undergone an E+A phase; the true fraction could be 100% if the effects of E+A down-sizing, an increasing E+A fraction with redshift, and the conversion of spirals into early-types are also considered. Thus, the E+A phase may indeed be an important stage in the transformation of star-forming galaxies into early-type cluster members.

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arXiv2003

The Rest-Frame Optical Luminosity Density, Color, and Stellar Mass Density of the Universe from z=0 to z=3

We present the evolution of the rest-frame optical luminosity density, of the integrated rest-frame optical color, and of the stellar mass density for a sample of Ks-band selected galaxies in the HDF-S. We derived the luminosity density in the rest-frame U, B, and V-bands and found that the luminosity density increases by a factor of 1.9+-0.4, 2.9+-0.6, and 4.9+-1.0 in the V, B, and U rest-frame bands respectively between a redshift of 0.1 and 3.2. We derived the luminosity weighted mean cosmic (U-B)_rest and (B-V)_rest colors as a function of redshift. The colors bluen almost monotonically with increasing redshift; at z=0.1, the (U-B)_rest and (B-V)_rest colors are 0.16 and 0.75 respectively, while at z=2.8 they are -0.39 and 0.29 respectively. We derived the luminosity weighted mean M/LV using the correlation between (U-V)_rest and log_{10} M/LV which exists for a range in smooth SFHs and moderate extinctions. We have shown that the mean of individual M/LV estimates can overpredict the true value by ~70% while our method overpredicts the true values by only ~35%. We find that the universe at z~3 had ~10 times lower stellar mass density than it does today in galaxies with LV>1.4 \times 10^{10} h_{70}^-2 Lsol. 50% of the stellar mass of the universe was formed by $z~1-1.5. The rate of increase in the stellar mass density with decreasing redshift is similar to but above that for independent estimates from the HDF-N, but is slightly less than that predicted by the integral of the SFR(z) curve.

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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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arXiv2003

CL 1358+62: Characterizing the Physical Properties of Cluster Galaxies at z=0.33

[Abridged] We examine the physical properties of 173 cluster members in CL1358+62 (z=0.33) from HST WFPC2 imaging taken in the F606W & F814W filters (rest-frame B & V) over a 2.2x2.2 Mpc^2 field. Structural parameters are measured for each member by fitting a PSF-convolved, two component model to their 2D surface brightness distribution. We examine bulge+disk models using three different bulge profiles (de Vaucouleurs, Sersic, and exponential), and rigorously test the robustness of our results by analyzing several thousand artificial galaxies in the same manner as the cluster data. Combining structural parameters with ground-based spectroscopy, we test for correlations between morphological characteristics, current star formation, total galaxy colors, and cluster substructure. We find that: (1) Using a de Vaucouleurs bulge, bulge-to-total ratios and Hubble type (-5<T<8) are strongly correlated (99% CL), but the scatter is large and early-type spirals are not reliably distinguished from E's and S0's based on (B/T). (2) The low luminosity ellipticals in our sample are likely to be face-on S0 galaxies. (3) High galaxy asymmetry and strong [OII] emission are strongly correlated for disk-dominated members. (4) There exists a small population (~5%) of bulge-dominated members whose significant [OII] emission (<-5A) suggest they harbor AGN. (5) At these redshifts, determining the correct Sersic index can be highly unreliable.

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arXiv2002

Physical Sources of Scatter in the Tully-Fisher Relation

We analyze residuals from the Tully-Fisher relation for the emission-line galaxies in the Nearby Field Galaxy Survey, a broadly representative survey designed to fairly sample the variety of galaxy morphologies and environments in the local universe. For spirals brighter than M_R^i=-18, we find strong correlations between Tully-Fisher residuals and both B-R color and EW(Halpha). The extremes of the correlations are populated by Sa galaxies, which show consistently red colors, and spirals with morphological peculiarities, which are often blue. If we apply an EW(Halpha)-dependent or B-R color-dependent correction term to the Tully-Fisher relation, the scatter in the relation no longer increases from R to B to U but instead drops to a nearly constant level close to the scatter we expect from measurement errors. We argue that these results probably reflect correlated offsets in luminosity and color as a function of star formation history. Broadening the sample in morphology and luminosity, we find that most non-spirals brighter than M_R^i=-18 follow the same correlations as do spirals, albeit with greater scatter. However, the color and EW(Halpha) correlations do not apply to galaxies fainter than M_R^i=-18 or to emission-line S0 galaxies with anomalous gas kinematics. For the dwarf galaxy population, the parameters controlling Tully-Fisher residuals are instead related to the degree of recent disturbance: overluminous dwarfs have higher rotation curve asymmetries, brighter U-band effective surface brightnesses, and shorter gas consumption timescales than their underluminous counterparts. As a result, sample selection strongly affects the measured faint-end slope of the Tully-Fisher relation. Passively evolving, rotationally supported galaxies display a break toward steeper slope at low luminosities.

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arXiv2001

Cluster Galaxy Populations from z=0.33 - 0.83: Characterizing Evolution with CL1358+62

We combine HST WFPC mosaics of clusters at multiple redshifts with Keck LRIS spectroscopy to characterize different galaxy populations and how they evolve. By combining quantitative measurements of structural parameters with velocity dispersions, line indices, and morphological types, we attempt to establish links between ellipticals, S0's, E+A's, and later type galaxies. Here we focus on galaxies in CL1358+62, a massive cluster at z=0.33.

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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

A K-band Selected Photometric Redshift Catalog in the HDF-S: Sampling the Rest-Frame V-Band to z=3

We present the first results from the Faint Infra-Red Extragalactic Survey (FIRES) of the Hubble Deep Field South. Using a combination of deep near infrared data obtained with ISAAC at the VLT with the WFPC2 HST data, we construct a K-band selected sample of 136 galaxies with K(AB)<=23.5. We derive zphot's using a newly developed technique which models the observed spectral energy distribution with a linear combination of empirical galaxy templates. Testing our zphot technique against spectroscopic redshifts in the HDF-N yields dz/(1+z)~0.07 for z<6. We show that we can derive realistic error estimates in zphot by combining the systematic uncertainties derived from the HDF-N with errors in zphot which depend on the observed flux errors. The redshift histogram of galaxies in the HDF-S shows distinct structure with a sharp peak at z~0.5 and a broad enhancement at z~1-1.4. We find that 12% of our galaxies with K(vega)<21 lie at z>=2. While this is higher than the fraction predicted in Omega_M=1 hierarchical models of galaxy formation we find that published predictions using pure luminosity evolution models produce too many bright galaxies at redshifts greater than unity. Finally, we use our broad wavelength coverage to measure the rest-frame UBV luminosities Lrest for z<=3. There is a paucity of galaxies brighter than Lrest_V>=1.4e10 h^{-2}Lsun at z~1.5-2, however, at z>2 we find very luminous galaxies with Lrest_V>=5e10 h^{-2}Lsun (for Omega_M=0.3,Lambda=0.7, H_o=100 h km s^{-1}Mpc^{-1}). Local B-band luminosity functions predict 0.1 galaxies in the redshift range 2<=z<=3.5 and with Lrest_B>= 5e10 h^{-2}Lsun,B but we find 9. The discrepancy can be explained if L*_B increases by a factor of 2.4-3.2 with respect to locally determined values (abridged).

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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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