The Ellipticities of Cluster Early-type Galaxies from Z ∼1 to Z ∼ 0: No Evolution in the Overall Distribution of Bulge-to-disk Ratios

We have compiled a sample of early-type cluster galaxies from 0 < z < 1.3 and measured the evolution of their ellipticity distributions. Our sample contains 487 galaxies in 17 z > 0.3 clusters with high quality space-based imaging and a comparable sample of 210 galaxies in 10 clusters at z < 0.05. We select early-type galaxies (elliptical and S0 galaxies) that fall within the cluster R200, and which lie on the red-sequence in the magnitude range −19.3 > MB > −21, after correcting for luminosity evolution as measured by the fundamental plane. Our ellipticity measurements are made in a consistent manner over our whole sample. We perform extensive simulations to quantify the systematic and statistical errors, and find that it is crucial to use PSF-corrected model fits; determinations of the ellipticity from HST image data that do not account for the PSF “blurring” are systematically and significantly biased to rounder ellipticities at redshifts z > 0.3. We find that neither the median ellipticity, nor the shape of the ellipticity distribution of cluster early-type galaxies evolves with redshift from z ∼ 0 to z > 1 (i.e., over the last ∼ 8Gyr). The median ellipticity at z > 0.3 is statistically identical with that at z < 0.05, being higher by only 0.01 ± 0.02 or 3 ± 6%, while the distribution of ellipticities at z > 0.3 agrees with the shape of the z < 0.05 distribution at the 1-2% level (i.e., the probability that they are drawn from the same distribution is 98-99%). These results are strongly suggestive of an unchanging overall bulge-to-disk ratio distribution for cluster early-type galaxies over the last ∼ 8Gyr from z ∼ 1 to z ∼ 0. This result contrasts with that from visual classifications which show that the fraction of morphologically-selected disk-dominated early-type galaxies, or S0s, is significantly lower at z > 0.4 than at z ∼ 0. We find that the median disk-dominated early-type, or S0, galaxy has a somewhat higher ellipticity at z > 0.3, suggesting that rounder S0s are being assigned as ellipticals. Taking the ellipticity measurements and assuming, as in all previous studies, that the intrinsic ellipticity distribution of both elliptical and S0 galaxies remains constant, then we conclude from the lack of evolution in the observed early-type ellipticity distribution that the relative fractions of ellipticals and S0s do not evolve from z ∼ 1 to z = 0 for a red-sequence selected samples of galaxies in the cores of clusters of galaxies. Subject headings: galaxies: clusters: general — galaxies: elliptical and lenticular, cD, — galaxies: evolution — galaxies: fundamental parameters — galaxies: photometry — clusters: individual: CL J1226.9+3332 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555. 2 Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. 3 This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile. 4 UCO/Lick Observatories, University of California, Santa Cruz, 95065; [email protected]; [email protected] 5 Leiden Observatory, University of Leiden, P.O.Box 9513, 2300 RA, Leiden, The Netherlands; [email protected] 6 Space Telescope Science Institute, Baltimore, MD 21218; [email protected] 7 Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218; [email protected]; [email protected]; [email protected] 8 Observatories of the Carnegie Institution of Washington, Pasadena, CA, 91101; [email protected] 9 Herzberg Institute of Astrophysics, National Research Council of Canada, Victoria, BC V9E2E7, Canada; [email protected] 10 University of Paris Denis Diderot, 75205 Paris Cedex 13, France 11 GEPI, Observatoire de Paris, Section de Meudon, 5 Place J.