Paper detail

Stellar populations and star formation in AGN hosts at intermediate redshift in the SHARDS survey

SHARDS is an ongoing ESO/GTC large program that is obtaining ultra-deep photometry of the GOODS-North field in 24 medium-band filters (reaching m=26.5 AB in all bands) in the 500-950 nm range with GTC/OSIRIS. It is designed to study the properties of high-z massive galaxies, but it can also provide very valuable information about the population of AGN at intermediate redshifts (z~0.5-2). Here we present preliminary results on a study of the stellar populations and star formation activity in the host galaxies of X-ray selected optically faint AGN at 0.6<z<1.1. We demonstrate that the SHARDS photometry provides a reliable measurement of the 4000 Å break (Dn(4000), an indicator for the age of stellar populations) down to m=26.5 AB. We confirm that most X-ray selected AGN are hosted by massive galaxies (typically M*>10^10.5 Mo) and that the fraction of galaxies hosting an AGN increases with the stellar mass. AGN-hosts have restframe U-V and Dn(4000) comparable to those of non-active galaxies of the same mass, that is, they do not appear to have on average younger stellar populations, unlike in the Local Universe. Nevertheless, z~1 AGN hosts show an excess of IR emission at λ>3μm compared to non-AGN galaxies which might indicate increased star formation rates. In addition, the frequency of AGN in massive galaxies is about twice higher for the ones with young stellar populations compared to the older ones.