Are Dry Mergers Dry, Moist, or Wet?

TL;DR

This study analyzes the stellar populations and kinematics of galaxies identified as dry merger remnants, revealing that many have young stars likely triggered by minor gas-rich interactions, challenging the traditional view of dry mergers.

Contribution

It provides detailed spectral analysis of dry merger candidates, showing that many have young stellar populations and rotation support, indicating some gas involvement contrary to the dry merger assumption.

Findings

Approximately half of the sample with tidal features have younger stars.

Galaxies with young stars are often rotation-supported, indicating some gas involvement.

No young stellar component found in galaxies with velocity dispersions over 250 km/s.

Abstract

We present a spectral analysis of a sample of red-sequence galaxies identified by van Dokkum (2005) as dry merger remnants and ongoing dry mergers. Kinematics, stellar population absorption features, and ionisation 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% 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 have a young stellar component, independent of the merger stage.