Multi-wavelength landscape of the young galaxy cluster RXJ1257.2+4738 at z=0.866: I. The infrared view

We performed a thorough analysis of the star formation activity in the
young massive galaxy cluster RXJ1257+4738 at z=0.866, with emphasis on
the relationship between the local environment of the cluster galaxies
and their star formation activity. We present an optical and IR study
that benefited from the large amount of data available for this cluster,
including new OSIRIS/GTC and Herschel imaging observations. Using a
optical-to-NIR multi-wavelength catalogue, we measured photometric
redshifts through a chi2 SED-fitting procedure. We implemented a
reliable and carefully chosen cluster membership selection criterion
including Monte Carlo simulations and derived a sample of 292 reliable
cluster member galaxies for which we measured the following properties:
optical colours, stellar masses, ages, ultraviolet luminosities and
local densities. Using the MIPS 24um and Herschel data, we measured
total IR luminosities and SFR. Of the sample of 292 cluster galaxies, 38
show FIR emission with an SFR between 0.5 and 45 Msun/yr. The spatial
distribution of the FIR emitters within the cluster density map and the
filament-like overdensities observed suggest that RXJ1257 is not
virialised, but is in the process of assembly. The average star
formation as a function of the cluster environment parametrised by the
local density of galaxies does not show any clear trend. However, the
fraction of SF galaxies unveils that the cluster intermediate-density
regions is preferred for the SF activity to enhance, since we observe a
significant increase of the FIR-emitter fraction in this environment.
Focusing on the optically red SF galaxies, we cannot support the
interpretation of this population as dusty red galaxies, since we do not
observe an appreciable difference in their extinction compared with the
quiescent or the blue populations.