Pieter G. van Dokkum

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 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 Hubble Space Telescope Wide Field Planetary Camera 2 images.

Hubble Space Telescope Photometry and Keck Spectroscopy of the Rich Cluster MS 1054–03: Morphologies, Butcher-Oemler Effect, and the Color-Magnitude Relation at z = 0.83* **

We present a study of 81 I-band selected, spectroscopically confirmed members of the X-ray cluster MS 1054-03 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 RF606W and IF814W, corresponding to U and B in the rest frame. Early-type galaxies constitute only 44% of this galaxy population. This fraction is much lower than in comparable rich clusters at low redshift. 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 mag in rest frame 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 mag, and the scatter in their colors is 0.07 ± 0.04 mag. Spiral galaxies in MS 1054-03 exhibit a large range in their colors. The bluest spiral galaxies are ~0.7 mag bluer than the early-type galaxies, but the majority is within ±0.2 mag 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 MS 1054-03: fB = 0.22 ± 0.05, similar to intermediate redshift clusters and much lower than previously reported values for clusters at z ~ 0.8. 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 (i.e., the sample of future early-type galaxies) 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.

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 that 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 those of a single-age stellar population formed at z = ∞, 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 that reproduce the observed evolution of the number fraction of early-type galaxies in rich clusters and their color and luminosity evolution simultaneously. Our modeling indicates that ~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. The effect of the progenitor bias on the evolution of the mean M/L ratio and color can be estimated. The progenitor bias is a linear function of the scatter in the color-magnitude relation produced by age variations and is maximal if the observed scatter is entirely due to age differences. After correcting the observed evolution of the mean M/LB ratio for the maximum progenitor bias, we find that the mean luminosity-weighted formation redshift of stars in early-type galaxies z* = 3.0 for Ωm = 0.3 and ΩΛ = 0 and z* = 2.0 for Ωm = 0.3 and ΩΛ = 0.7. Our analysis places the star formation epoch of early-type galaxies later than previous studies, which ignored the effects of progenitor bias. The results are consistent with the idea that (some) Lyman break galaxies are star-forming building blocks of massive early-type galaxies in clusters.We present a JavaScript calculator which provides the redshift evolution of the luminosities and colors of passively evolving galaxies for a range of rest-frame filters. In its simplest form it gives the evolution of a single age stellar population formed at a given redshift. However, it can also incorporate morphological evolution in the galaxy population (progenitor bias) and complex star formation histories of individual galaxies. The calculator is based on the models of van Dokkum & Franx (2001), with small updates.

Ultradeep Near-Infrared ISAAC Observations of the Hubble Deep Field South: Observations, Reduction, Multicolor Catalog, and Photometric Redshifts

We present deep near-infrared (NIR) Js-, H-, and Ks-band ISAAC imaging of the Wide Field Planetary Camera 2 (WFPC2) field of the Hubble Deep Field South (HDF-S). The 25 × 25 high Galactic latitude field was observed with the Very Large Telescope under the best seeing conditions, with integration times amounting to 33.6 hr in Js, 32.3 hr in H, and 35.6 hr in Ks. We reach total AB magnitudes for point sources of 26.8, 26.2, and 26.2, respectively (3 σ), which make it the deepest ground-based NIR observation to date and the deepest Ks-band data in any field. The effective seeing of the co-added images is ≈045 in Js, ≈048 in H, and ≈046 in Ks. Using published WFPC2 optical data, we constructed a Ks-limited multicolor catalog containing 833 sources down to K 26, of which 624 have seven-band optical-to-NIR photometry. These data allow us to select normal galaxies from their rest-frame optical properties to high redshift (z 4). The observations, data reduction, and properties of the final images are discussed, and we address the detection and photometry procedures that were used in making the catalog. In addition, we present deep number counts, color distributions, and photometric redshifts of the HDF-S galaxies. We find that our faint Ks-band number counts are flatter than published counts in other deep fields, which might reflect cosmic variations or different analysis techniques. Compared to the HDF-N, we find many galaxies with very red V-H colors at photometric redshifts 1.95 2.3 (in Johnson magnitudes). Because they are extremely faint in the observed optical, they would be missed by ultraviolet–optical selection techniques, such as the U-dropout method.

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 of early-type galaxies in the cluster RDCS J0848+4453 at z ¼ 1:27. Internal velocity dispersions of three K-selected early-type galaxies are determined from deep Keck spectra, using absorption lines in the rest-frame wavelength range 3400-4000 A ˚ . Structural parameters are determined from Hubble Space Telescope NICMOS images. The galaxies show substantial offsets from the fundamental plane of the nearby Coma Cluster, as expected from passive evolution of their stellar popula- tions. The offsets from the fundamental plane can be expressed as offsets in mass-to-light (M=L) ratio. The M=L ratios of the two most massive galaxies are consistent with an extrapolation of results obtained for clus- ters at 0:02 3 and RK > 5, and our results show that it is hazardous to use simple models for converting lumi- nosity to mass for these objects. The work presented here, and previous mass measurements at lower 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.

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

We study the fundamental plane (FP) of field early-type galaxies at intermediate redshift, using Hubble Space Telescope Wide Field Planetary Camera 2 observations and deep Keck spectroscopy. Structural parameters and internal velocity dispersions are measured for 18 galaxies at 0.15 < z < 0.55. Rest-frame M/LB ratios are determined from the FP 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: ln M/LBfield - ln M/LBclus = -0.18 ? 0.11. The M/LB ratio of field early-type galaxies evolves as ? ln M/LB = (-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 re, 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. 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 z = 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 semianalytical models for galaxy formation in a ?CDM universe predict a systematic difference between field and cluster galaxies of ? ln M/LB ~ -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.

A K-Band-selected Photometric Redshift Catalog in the Hubble Deep Field South: 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) =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 =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 = 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).

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

We present a deep multicolour 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(-0.18)(+0.13) and alpha(U) = -1.32(-0.028)(+0.018), M-B* = 19.79(-0.17)(+0.18) and alpha(B) = -1.37(-0.016)(+0.024) and M-r* = -20.87(-0.17)(+0.12) and alpha(r) = -1.16(-0.019)(+0.012). The LF becomes steeper at larger radial distances 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 a large distance from the cluster centre, which may be in the process of being accreted by the cluster. The shapes of the Us of the NGC 4839 group support a scenario in which the group has already passed through the centre.

Galaxy Morphologies in the Cluster Cl 1358+62 at z = 0.33*

We describe the morphological composition of a sample of 518 galaxies in the field of Cl 1358+62 at z = 0.33, drawn from a large Hubble Space Telescope mosaic covering 53 square arcminutes. The sample is complete to I = 22, corresponding to MV = -18.5 in the rest frame. The galaxy morphologies have been independently classified by the authors of this paper and by Alan Dressler. Dresslers classifications place our work in context with the previous MORPHS study and allow us to estimate the scatter between different sets of visual classifications. We restrict most of our analysis to the brighter part of the sample, I < 21 (MV < -19.5), where the scatter between the two sets of classifications is ~1 in morphological type. The scatter doubles at I = 22, presumably owing to the lower signal-to-noise and poorer sampling of faint, small galaxy images. To I = 21 the two sets of classifiers agree on the fraction of early type galaxies (elliptical + S0): 72%. We conclude that Cl 1358+62 does not contain the large population of spiral galaxies found in other studies of clusters at z ~ 0.3 and that there is probably a significant spread in the degree of cluster evolution at intermediate redshift. The two groups of classifiers differ on the relative fraction of S0 and elliptical galaxies. We show that the distributions of ellipticities and bulge/total light cannot resolve this discrepancy. Nonetheless, we can derive significant constraints on physical models for the evolution of the galaxy population in Cl 1358+62. The higher ratio of S0 to elliptical galaxies (1.6) found by DF/MF/PvD requires that the evolution preserve the relative fraction of elliptical, S0 and spiral galaxies. Alternately, the lower ratio (1.1) found by Dressler requires that the evolution preserve the early-type to spiral ratio while increasing the S0 to elliptical ratio. In the latter case, a possible evolutionary mechanism is accretion of galaxies that predominantly evolve to S0 galaxies between z = 0.33 and the present. We use our large body of spectra to make the correspondence between spectral and morphological type. Our data follow the pattern seen in the field at low redshift: emission-line spectra are more prevalent among the later morphological types. The 11 identified k+a galaxies (absorption-line spectra with strong Balmer lines) have S0-Sb morphologies.

HST large-field weak lensing analysis of MS 2053-04: study of the mass distribution and mass-to-light ratio of X-ray luminous 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 six Hubble Space Telescope (HST ) WFPC2 images. The best-fitting singular isothermal sphere model to the observed tangential distortion yields an Einstein radius r (E) =6.2+/-1.8 arcsec, which corresponds to a velocity dispersion of (Omega (m) =0.3, Omega(Lambda) =0.0). This result is in good agreement with the observed velocity dispersion of 817+/-80 km s(-1) from cluster members. The observed average rest-frame mass-to-light ratio within a radius aperture is 184+/-56 h (50) M-./L (B.). After correction for luminosity evolution to z =0, this value changes to 291+/-89+/-19 h (50) M-. /L (B .) (where the first error indicates the statistical uncertainty in the measurement of the mass-to-light ratio, and the second error is due to the uncertainty in luminosity evolution). MS 2053 is the third cluster we have 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 of the clusters in our sample with the predicted Navarro, Frenk & White (NFW) profile, and find that an NFW profile can fit the observed lensing signal well. The best-fitting concentration parameter is found to be (68 per cent confidence) times the predicted value from an open cold dark matter (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 per cent 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 h (50) M-. /L (B .).