CFHT Legacy Ultraviolet Extension (CLUE): Witnessing Galaxy Transformations up to 7 Mpc from Rich Cluster Cores

TL;DR

This study examines galaxy star formation activity up to 7 Mpc from cluster centers, revealing that while specific star formation rates are similar to the field, the fraction of actively star-forming and blue galaxies is reduced in cluster outskirts, indicating pre-processing effects.

Contribution

It provides new insights into galaxy transformation processes in cluster outskirts by combining optical and ultraviolet data over large scales, highlighting pre-processing effects.

Findings

Star formation rates are similar to the field at large radii.

Lower fraction of actively star-forming galaxies in cluster outskirts.

Higher fraction of low-SFR blue galaxies around clusters.

Abstract

Using the optical data from the Wide component of the CFHT Legacy Survey, and new ultraviolet data from GALEX, we study the colours and specific star formation rates (SSFR) of ~100 galaxy clusters at 0.16<z<0.36, over areas extending out to radii of r~7Mpc. We use a multicolour, statistical background subtraction method to study the galaxy population at this radius; thus our results pertain to those galaxies which constitute an excess over the average field density. We find that the average SSFR, and its distribution, of the star-forming galaxies (with SFR>0.7 M_sun/yr at z~0.2 and SFR>1.2 M_sun/yr at z~0.3) have no measurable dependence on the cluster-centric radius, and are consistent with the field values. However, the fraction of galaxies with SFR above these thresholds, and the fraction of optically blue galaxies, are lower for the overdense galaxy population in the cluster outskirts compared with the average field value, at all stellar masses M*>10^{9.8} M_sun and at all radii out to at least 7Mpc. Most interestingly, the fraction of blue galaxies that are forming stars at a rate below our UV detection limit is much higher in all radial bins around our cluster sample, compared with the general field value. This is most noticeable for massive galaxies M*>10^{10.7} M_sun; while almost all blue field galaxies of this mass have detectable star formation, this is true for less than 20% of the blue cluster galaxies, even at 7Mpc from the cluster centre. Our results support a scenario where galaxies are pre-processed in locally overdense regions, in a way that reduces their SFR below our UV detection limit, but not to zero.