Are dry mergers dry, moist or wet ?

Abstract

We present a spectral analysis of a sample of red-sequence galaxies identified by van Dokkum as dry merger remnants and ongoing dry mergers. Kinematics, stellar population absorption features and ionization from emission lines are derived. We find that approximately half of the sample showing strong tidal features have younger stellar populations than a control sample at a given velocity dispersion. Conversely, galaxies with weak tidal tails and/or ongoing mergers – with the exception of one galaxy – do not show this young component. This seems to indicate that the young stellar populations observed in a significant fraction of ellipticals is the consequence of star formation triggered by mergers. This young component is consistent with a light ‘frosting’ of young stars (<2 per cent by mass) superimposed upon a dominant, old (∼11 Gyr), stellar population. In terms of stellar populations, these mergers are, in fact, fairly dry. We found, however, that merger remnants with young stellar populations are supported by rotation, contrary to the expectations of a major dry merger. This suggests that the small amount of gas involved has been sufficient to produce a dynamically cold stellar component. Half of the galaxies with strong tidal distortion, however, are slow rotating and have stellar populations compatible with the control sample at a given velocity dispersion. Remarkably, none of the galaxies with velocity dispersions in excess of 250 km s^(−1) (the putative transition between dry and wet mergers) has a young stellar component, independent of the merger stage.