The galaxy populations from the centers to the infall regions in z ≈ 0.25 clusters ^*

¹ Institut für Astrophysik Göttingen, Georg-August Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: mverdugo@astro.physik.uni-goettingen.de
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany
³ Argelander-Institut für Astronomie, Universität Bonn, Germany
⁴ Oxford Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK

Received: 25 September 2007
Accepted: 5 March 2008

Abstract

Context. In the local universe, the relative fractions of galaxy types differs in galaxy clusters in comparison to the field. Observations at higher redshift provide evidence that cluster galaxies evolve with lookback time. This could be due either to the late assembly of clusters, which is predicted by bottom-up scenarios of structure formation, or to cluster-specific interaction processes.

Aims. To disentangle various effects, we explore the evolutionary status of galaxies from the center of clusters out to their infall regions in z≈0.25 clusters.

Methods. We conducted a panoramic spectroscopic campaign with MOSCA at the Calar Alto observatory. We acquired low-resolution spectra of more than 500 objects. Approximately 150 of these spectra were of galaxies that are members of six different clusters, which differ in intrinsic X-ray luminosity. The wavelength range allows us to quantify the star formation activity by using the [Oii] and the Hα emission lines. This activity is examined in terms of the large-scale environment expressed by the clustercentric distance of the galaxies as well as on local scales given by the spatial galaxy densities.

Results. The general decline in star-formation activity observed for galaxies inside nearby clusters is also seen at z≈0.25. A global suppression of star-formation is detected in the outskirts of clusters, at about , where the galaxy densities are low and the intra-cluster medium is very shallow. Galaxies with ongoing star-formation have similar activity, regardless of the environment. Therefore, the decline of the star-formation activity inside the investigated clusters is driven mainly by the significant change in the fraction of active versus passive populations. This suggests that the suppression of the star-formation activity occurs on short timescales. We detect a significant population of red star-forming galaxies whose colors are consistent with the red-sequence of passive galaxies. They appear to be in an intermediate evolutionary stage between active and passive types.

Conclusions. Since a suppression of star-formation activity is measured at large clustercentric distances and low projected densities, purely cluster-specific phenomena cannot fully explain the observed trends. Therefore, as suggested by other studies, group preprocessing may play an important role in transforming galaxies before they enter into the cluster environment. Since models predict that a significant fraction of galaxies observed in the outskirts may have already transversed through the cluster center and intracluster media, the effects of ram-pressure stripping cannot, however, be neglected; this is, in addition, true because ram-pressure stripping may even be effective, under certain conditions, inside group environments.