How Dry Are Red Mergers?

The focus of current research in galaxy evolution has increasingly turned to understanding the effect that mergers have on the evolution of systems on the red sequence. For those interactions purported to occur dissipationlessly (so-called dry mergers), it would appear that the role of gas is minimal. However, if these mergers are not completely dry, then even low levels of gas may be detectable. The purpose of our study is to test whether early-type galaxies with H I in or around them, or "wet" ellipticals, would have been selected as dry mergers by the criteria set forth in a 2005 study by van Dokkum. To that end, we examine a sample of 20 early types from the H I Rogues Gallery with neutral hydrogen in their immediate environs. Of these, the 15 brightest and reddest galaxies match the optical dry merger criteria, but in each case, the presence of H I (for the majority, at levels >10⁸ M_⊙), as well as significant star formation in some cases, means that they are not truly dry.