arXiv:1202.6058v1 [astro-ph.CO] 27 Feb 2012 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 29 February 2012 (MN L A T E X style file v2.2) Evolution of the Red Sequence Giant to Dwarf Ratio in Galaxy Clusters out to z ∼ 0.5 C. Bildfell 1 , ⋆ H. Hoekstra 2 , A. Babul 1 , D. Sand 3 , M. Graham 3 , J. Willis 1 , S. Urquhart 1 , A. Mahdavi 4 , C. Pritchet 1 , D. Zaritsky 5 , J. Franse 2 , P. Langelaan 2 1 Department of Physics & Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada 2 Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden, Netherlands 3 Las Cumbres Observatory Global Telescope Network, 6740 Cartona Drive, Suite 102, Santa Barbara, CA 93117, USA 4 Department of Physics & Astronomy, San Francisco State University, San Francisco, CA 94131, USA 5 Steward Observatory, University of Arizona, Tucson AZ 85721, USA 29 February 2012 ABSTRACT We analyze deep g ′ and r ′ band data of 97 galaxy clusters imaged with MegaCam on the Canada-France-Hawaii telescope. We compute the number of luminous (giant) and faint (dwarf) galaxies using criteria based on the definitions of de Lucia et al. (2007). Due to excellent image quality and uniformity of the data and analysis, we probe the giant-to-dwarf ratio (GDR) out to z ∼ 0.55. With X-ray temperature (T x ) information for the majority of our clusters, we constrain, for the first time, the T x -corrected giant and dwarf evolution separately. Our measurements support an evolving GDR over the redshift range 0.05  z  0.55. We show that modifying the (g ′ - r ′ ), m r ′ and K- correction used to define dwarf and giant selection do not alter the conclusion regarding the presence of evolution. We parameterize the GDR evolution using a linear function of redshift (GDR = αz + β) with a best fit slope of α =0.88 ± 0.15 and normalization β =0.44 ± 0.03. Contrary to claims of a large intrinsic scatter (σ int ), we find that the GDR data can be fully accounted for using observational errors alone. Consistently, we find no evidence for a correlation between GDR and cluster mass (via T x or weak lensing). Lastly, the data suggest that the evolution of the GDR at z< 0.2 is driven primarily by dry merging of the massive giant galaxies, which when considered with previous results at higher redshift, suggests a change in the dominant mechanism that mediates the GDR. Key words: galaxies: clusters: general – galaxies: evolution – galaxies: elliptical and lenticular, cD 1 INTRODUCTION Non-starforming galaxies in clusters exhibit a tight sequence in colour-magnitude space known as the red sequence (eg. Visvanathan 1978; Bower et al. 1992), which is in place as early as z ∼ 1 (eg. Bell et al. 2004, Mei et al. 2006). Less understood is how the number of red sequence galaxies and their distribution with stellar mass/absolute magnitude evolves with cosmic time. Understanding the assembly his- tory of the red sequence is critical as these galaxies represent the bulk of the stellar mass in clusters at the present epoch. A useful metric for probing this assembly history is the ratio of the number of luminous red sequence galaxies (giants) to the number of faint red sequence galaxies (dwarfs). This is essentially a non-parametric representation of the luminos- ⋆ E-mail: bildfell@uvic.ca ity function using only 2 bins and is commonly referred to as the Giant-to-Dwarf ratio (GDR). There is an ongoing debate regarding the presence or absence of evolution in the GDR and/or its reciprocal the Dwarf-to-Giant ratio (DGR). De Lucia et al. (2007) for in- stance, finds significant evolution in the GDR, amounting to a change from GDR∼ 0.45 to GDR∼ 0.95 over the red- shift range 0.4 <z< 0.8. The analysis of low redshift (0.08 <z< 0.19) cluster data by Capozzi et al. (2010) sup- ports the extrapolation of this trend to lower redshift. Stott et al. (2007) and Gilbank et al. (2008) also find strong evo- lution in the DGR using somewhat brighter absolute mag- nitude limits for the definition of dwarfs. In contrast, Craw- ford et al. (2009) look at 59 clusters at redshift 0 <z< 0.5 and, though they do not measure the GDR directly, con- clude that there is little evolution in the faint-end slope of the luminosity function. Andreon (2008), in an analysis of c  0000 RAS