Hubble Space Telescope Images of Red Mergers: How Dry Are They?

Abstract

Mergers between red galaxies are observed to be common in the nearby universe, and are thought to be the dominant mechanism by which massive galaxies grow their mass at late times. These "dry" mergers can be readily identified in very deep ground-based images, thanks to their extended low surface brightness tidal features. However, ground-based images lack the required resolution to determine the morphologies of the merging galaxies, and to measure the amount of dust and associated gas. We present HST ACS and WFPC2 observations of a sample of 31 bulge-dominated red-sequence galaxies at z ∼ 0.1, comprised of ongoing mergers, merger remnants, and undisturbed galaxies. Nearly all galaxies have early-type morphologies and most are well-fit by r1/4 law surface brightness profiles. We find that only 10% of the galaxies show evidence for the presence of dust. The amount of cold gas (or its upper limit) is calculated from the mean color excess, assuming a simple relation between gas mass and dust mass. The gas mass is low for all galaxies, and we find that Mgas/Mstellar ≲ 3 × 104. We infer that red mergers in the nearby universe mostly involve early-type galaxies containing little cold gas and dust. This may imply that the progenitors were mostly devoid of gas and/or that feedback mechanisms are very effective in preventing the gas to cool. The lack of gas in these objects may also imply a relatively large fraction of binary black holes in the centers of massive ellipticals.