How Dry Are Red Mergers?

TL;DR

This study investigates whether supposed dry galaxy mergers are truly devoid of gas by examining a sample of early-type galaxies with neutral hydrogen, revealing that many are not entirely dry despite optical criteria suggesting so.

Contribution

The paper tests the validity of optical dry merger criteria by analyzing HI presence in early-type galaxies, showing many are not genuinely dry mergers.

Findings

Most optically classified dry mergers contain detectable HI.

Presence of HI indicates these mergers are not truly dry.

Optical criteria may misclassify wet mergers as dry.

Abstract

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 HI in or around them, or "wet" ellipticals, would have been selected as dry mergers by the criteria in van Dokkum (2005, AJ, 130, 2647). To that end, we examine a sample of 20 early types from the HI 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 HI means that they are not truly dry.