Cold gas and young stars in tidally-disturbed ellipticals at z=0

Abstract

We present an analysis of the neutral hydrogen and stellar populations of elliptical galaxies in the Tal et al. (2009) sample. Our aim is to test their conclusion that the continuing assembly of these galaxies at z~0 is essentially gas-free and not accompanied by significant star formation. In order to do so, we make use of HI data and line-strength indices available in the literature. We look for direct and indirect evidence of the presence of cold gas during the recent assembly of these objects and analyse its relation to galaxy morphological fine structure. We find that >25% of ellipticals contain HI at the level of M(HI)>10^8 M(Sun), and that M(HI) is of the order of a few percent of the total stellar mass. Available data are insufficient to establish whether galaxies with a disturbed stellar morphology are more likely to contain HI. However, HI interferometry reveals very disturbed gas morphology/kinematics in all but one of the detected systems, confirming the continuing assembly of many ellipticals but also showing that this is not necessarily gas-free. We also find that all very disturbed ellipticals have a single-stellar-population-equivalent age <4 Gyr. We interpret this as evidence that ~0.5-5% of their stellar mass is contained in a young population formed during the past ~1 Gyr. Overall, a large fraction of ellipticals seem to have continued their assembly over the past few Gyr in the presence of a mass of cold gas of the order of 10% of the galaxy stellar mass. This material is now observable as neutral hydrogen and young stars.