The role of environment in the morphological transformation of galaxies in 9 rich intermediate redshift clusters

¹ ESO, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile e-mail: mhuertas@eso.org
² LESIA - Paris Observatory, 5 place Jules Janssen, 92195 Meudon, France
³ Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS and Université de Toulouse, 14 Av. E. Belin, 31400 Toulouse, France

Received: 3 June 2009
Accepted: 16 July 2009

Abstract

Context. Rich clusters offer a unique laboratory for studying the effects of local environment on the morphological transformation of galaxies moving from the blue star-forming cloud to the red passive sequence. Due to the high-density, any environmental process should be more pronounced there compared to the field population.

Aims. Ideally, we would like to reconstruct the evolution of a single, hypothetical representative galaxy as it enters the cluster. For that purpose wide-field imaging is crucial to probe a wide range of densities and environments (from the core to the outskirts) and isolate this way, the different physical processes which are responsible of the migration from the blue-cloud to the red-sequence.

Methods. We analyze a sample of 9 massive clusters at 0.4 < z < 0.6 observed with MegaCam in 4 photometric bands () from the core to a radius of 5 Mpc (~4000 galaxies). Galaxy cluster candidates are selected using photometric redshifts computed with HyperZ. Morphologies are estimated with galsvm in two broad morphological types (early-type and late-type). We examine the morphological composition of the red-sequence and the blue-cloud and study the relations between galaxies and their environment through the morphology-density relations (T-Σ) and the morphology-radius relation () in a mass limited sample (log > 9.5).

Results. We find that the red sequence is already in place at z ~ 0.5 and it is mainly composed of very massive (log > 11.3) early-type galaxies. These massive galaxies seem to be already formed when they enter the cluster, probably in infalling groups, since the fraction remains constant with the cluster radius. Their presence in the cluster center could be explained by a segregation effect reflecting an early assembly history. Any evolution that takes place in the galaxy cluster population occurs therefore at lower masses (10.3 < log < 11.3). For these galaxies, the evolution, is mainly driven by galaxy-galaxy interactions in the outskirts as revealed by the T-Σ relation. Finally, the majority of less massive galaxies (9.5 < log < 10.3) are late-type galaxies at all locations, suggesting that they have not started the morphological transformation yet even if this low mass bin might be affected by incompleteness.