The evolution of the scatter of the cosmic average color-magnitude relation: Demonstrating consistency with the ongoing formation of elliptical ga

📝 Abstract

We present first measurements of the evolution of the scatter of the cosmic average early-type galaxy color-magnitude relation (CMR) from z=1 to the present day, finding that it is consistent with models in which galaxies are constantly being added to the red sequence through truncation of star formation in blue cloud galaxies. We used a sample of over 700 red sequence, structurally-selected early-type galaxies (defined to have Sersic index >2.5) with redshifts 0<z<1 taken from the Extended Chandra Deep Field South (173 galaxies) and the Sloan Digital Sky Survey (550 galaxies), constructing rest-frame U-V colors accurate to <0.04mag. We find that the scatter of the CMR of cosmic average early-type galaxies is ~0.1mag in rest-frame U-V color at 0.05<z<0.75, and somewhat higher at z=1. We compared these observations with a model in which new red sequence galaxies are being constantly added at the rate required to match the observed number density evolution, and found that this model predicts the correct CMR scatter and its evolution. Furthermore, this model predicts approximately the correct number density of `blue spheroids’ - structurally early-type galaxies with blue colors - albeit with considerable model dependence. Thus, we conclude that both the evolution of the number density and colors of the early-type galaxy population paint a consistent picture in which the early-type galaxy population grows significantly between z=1 and the present day through the quenching of star formation in blue cloud galaxies.

💡 Analysis

We present first measurements of the evolution of the scatter of the cosmic average early-type galaxy color-magnitude relation (CMR) from z=1 to the present day, finding that it is consistent with models in which galaxies are constantly being added to the red sequence through truncation of star formation in blue cloud galaxies. We used a sample of over 700 red sequence, structurally-selected early-type galaxies (defined to have Sersic index >2.5) with redshifts 0<z<1 taken from the Extended Chandra Deep Field South (173 galaxies) and the Sloan Digital Sky Survey (550 galaxies), constructing rest-frame U-V colors accurate to <0.04mag. We find that the scatter of the CMR of cosmic average early-type galaxies is ~0.1mag in rest-frame U-V color at 0.05<z<0.75, and somewhat higher at z=1. We compared these observations with a model in which new red sequence galaxies are being constantly added at the rate required to match the observed number density evolution, and found that this model predicts the correct CMR scatter and its evolution. Furthermore, this model predicts approximately the correct number density of `blue spheroids’ - structurally early-type galaxies with blue colors - albeit with considerable model dependence. Thus, we conclude that both the evolution of the number density and colors of the early-type galaxy population paint a consistent picture in which the early-type galaxy population grows significantly between z=1 and the present day through the quenching of star formation in blue cloud galaxies.

📄 Content

arXiv:0901.4340v1 [astro-ph.CO] 28 Jan 2009 ASTROPHYSICAL JOURNAL IN PRESS: JANUARY 27, 2009 Preprint typeset using LATEX style emulateapj v. 26/01/00 THE EVOLUTION OF THE SCATTER OF THE COSMIC AVERAGE COLOR-MAGNITUDE RELATION: DEMONSTRATING CONSISTENCY WITH THE ONGOING FORMATION OF ELLIPTICAL GALAXIES CHRISTINE RUHLAND1, ERIC F. BELL1, BORIS HÄUSSLER1,2, EDWARD N. TAYLOR3, MARCO BARDEN1,4, DANIEL H. MCINTOSH5,6 1 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany; ruhland@mpia.de 2 University of Nottingham, Nottingham NG7 2RD United Kingdom 3 Sterrewacht Leiden, Leiden University, P.O. Box 9513, NL-2300 RA Leiden 4 Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstraße 25/B, A-6020 Innsbruck 5 Astronomy Department, University of Massachusetts, Amherst, MA 01003 USA 6 Department of Physics, University of Missouri - Kansas City, Kansas City, MO 64110 USA ASTROPHYSICAL JOURNAL IN PRESS: January 27, 2009 ABSTRACT We present first measurements of the evolution of the scatter of the cosmic average early-type galaxy color– magnitude relation (CMR) from z = 1 to the present day, finding that it is consistent with models in which galaxies are constantly being added to the red sequence through truncation of star formation in blue cloud galaxies. We used a sample of over 700 red sequence, structurally-selected early-type galaxies (defined to have Sérsic index

2.5) with redshifts 0 < z < 1 taken from the Extended Chandra Deep Field South (173 galaxies) and the Sloan Digital Sky Survey (550 galaxies), constructing rest-frame U −V colors accurate to < 0.04 mag. We find that the scatter of the CMR of cosmic average early-type galaxies is ∼0.1 mag in rest-frameU −V color at 0.05 < z < 0.75, and somewhat higher at z = 1. We compared these observations with a model in which new red sequence galaxies are being constantly added at the rate required to match the observed number density evolution, and found that this model predicts the correct CMR scatter and its evolution. Furthermore, this model predicts approximately the correct number density of ‘blue spheroids’ — structurally early-type galaxies with blue colors — albeit with considerable model dependence. Thus, we conclude that both the evolution of the number density and colors of the early-type galaxy population paint a consistent picture in which the early-type galaxy population grows significantly between z = 1 and the present day through the quenching of star formation in blue cloud galaxies. Subject headings: galaxies: evolution — galaxies: general — galaxies: elliptical and lenticular — galaxies: stellar content — surveys

1. INTRODUCTION One of the best-known and most powerful scaling relations of the early-type (elliptical and lenticular) galaxy population is the systematic reddening of their colors with increasing lumi- nosity: the color–magnitude relation (CMR). The slope of the CMR is driven by a correlation between metallicity and mass (Faber & Jackson 1976; Kodama & Arimoto 1997; Terlevich et al. 1999; Trager et al. 2000; Gallazzi et al. 2006), while the scatter is determined by scatter in both age and metallicity, where it is generally thought that age is the dominant driver (Bower et al. 1992; Trager et al. 2000; Gallazzi et al. 2006, although Trager et al. 2000 argue that an anticorrelation between age and metallicity keeps the scatter of the early-type galaxy scaling relations rel- atively modest while allowing significant scatter in both age and metallicity). The scatter in this correlation is relatively small (Baum 1962; Visvanathan & Sandage 1977; Bower et al. 1992; Terlevich et al. 2001; McIntosh et al. 2005b). Because the color of stellar populations is strongly affected by their ages and metallicities (Worthey 1994), this correlation is a power- ful probe of the formation and evolution of the stellar popula- tions in early-type galaxies. The small intrinsic scatter found in some clusters (as little as σU−V = 0.04 mag in Virgo and Coma; Bower et al. 1992; Terlevich et al. 2001; although other clusters can have scatters approaching 0.1 mag; McIntosh et al. 2005b) gave considerable momentum to the notion that early- type galaxies formed the bulk of their stars at early times and that their stellar populations have aged essentially passively to the present day. In the last few years, this position has been challenged by evidence from deep redshift surveys that the cosmic average red sequence galaxy population (i.e., averaged over all envi- ronments) builds up in stellar mass by roughly a factor of two over the interval z = 1 to z = 0 through the addition of new red sequence galaxies (Chen et al. 2003; Bell et al. 2004b; Cimatti 2006; Brown et al. 2006a; Faber et al. 2007; Scarlata et al. 2007, although some of the papers argue for a build-up in stellar mass only in galaxies with ≲1011M⊙). These ‘new’ red se- quence galaxies are the result of truncation of star forma- tion in some fraction of the blue clo

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