Intergalactic stellar populations in intermediate redshift clusters

Abstract

A substantial fraction of the total stellar mass in rich clusters of galaxies resides in a diffuse intergalactic component usually referred to as the Intra-Cluster Light (ICL). Theoretical models indicate that these intergalactic stars originate mostly from the tidal interaction of the cluster galaxies during the assembly history of the cluster, and that a significant fraction of these stars could have formed in-situ from the late infall of cold metal-poor gas clouds onto the cluster. The models make predictions about the age distribution of the ICL stars, which may provide additional observational constraints. However, these models also over-predict the fraction of stellar mass in the ICL by a substantial margin. Here we present population synthesis models for the ICL of a dumb-bell dominated intermediate redshift (z=0.29) X-ray cluster for which we have deep MOS data obtained with the FORS2 instrument. In a previous paper we have proposed that the dumbell galaxy act as a grinding machine tearing to pieces the galaxies that pass nearby thus enriching the intergalactic medium. In this paper we analyze the spectra at different locations within the ICL and find that it is dominated by old metal rich stars, at odds with what has been found in nearby clusters where the stars that dominate the ICL are old and metal poor. While we see a weak evidence of a young, metal poor, component, if real, these young stars would amount to less than 1% of the total ICL mass, much less than the up to 30% predicted by the models. We propose that the very metal rich (i.e. 2.5 times solar) stars in the ICL of our cluster, which comprise approximately 40% of the total mass, originate mostly from the central dumb-bell galaxy, while the remaining solar and metal poor stars come from spiral, post-starburst (E+A), and metal poor dwarf galaxies. About 16% of the ICL stars are old and metal poor.Comment: 10 pages, 6 figure

The XXL Survey

“…15, left panel). This is also statistically younger than studies such as Melnick et al (2012). In the cluster they studied, only ∼25% of the ICL stars they detected are younger than 2 Gyr.…”

“…6). This can be explained, however, as the cluster Melnick et al (2012) consider has three bright interacting galaxies with old stellar populations. This probably introduces a large number of old stars in the ICL (see their Fig.…”

“…It is not the first time emission lines have been detected in ICL sources (see e.g. Melnick et al 2012); however, to the best of knowledge, all other detections were several orders of magnitudes weaker in intensity for the oxygen lines.…”

“…Alternatively, hydrodynamical effects such as ram pressure stripping of cold disk gas and dust (Gunn & Gott 1972) can also pollute the ICM with gas that may become the material of future in situ star formation. We note, however, that studies such as Melnick et al (2012) clearly do not favour a very intense in situ star formation. Excitation processes within this gas may lead to detectable emission lines within the ICL.…”

“…There are, however, some noticeable exceptions, for example in Melnick et al (2012). These authors used a z = 0.29 cluster of galaxies (RX J0054.0-2823) and stacked several spectra from four different places in the cluster; however, this study was forced to use the ICL and the BCGs together and to rely on the surface brightness profiles to delineate the boundaries between the two components.…”