No color-morphology bimodality of AGN host galaxies

Abstract

It is still a matter of debate whether the properties of galaxies hosting an Active Galactic Nucleus (AGN) are different from the properties of quiescent galaxies. We constructed a sample of ~50 AGN at a mean redshift of <z>~0.6 that lack a detectable optical nucleus. This characteristic allows to study the properties of the host galaxies with much higher accuracy than in the case of "normal" AGN which show a prominent central point source in optical images. A comparison sample of X-ray faint, quiescent galaxies at intermediate redshifts shows a clear bimodality in terms of both rest-frame colors and morphological concentration indicators. In contrast to this, the AGN host galaxies comprise a large fraction of objects that have early-type morphologies but relatively blue rest-frame colors, possibly due to recent or ongoing star formation. A fraction of the "optically dull" AGN in our sample show evidence for kpc-scale absorption; low Supermassive Black Hole accretion rates are more likely in other cases.