The Dirt on Dry Mergers

TL;DR

This study uses Spitzer mid-infrared data to show that many dry merger candidates in the Bootes field exhibit signs of ongoing star formation, indicating that these mergers are not entirely dry and may involve minor, gas-rich interactions.

Contribution

It provides new evidence that dry mergers often involve some gas and star formation, challenging the traditional view of completely dry galaxy mergers.

Findings

A quarter of dry merger candidates have star formation rates >1 solar mass per year.

Many candidates show mid-infrared excesses indicating ongoing star formation.

Dry merger candidates include a higher fraction of star-forming galaxies than control samples.

Abstract

Using data from the Spitzer Space Telescope, we analyze the mid-infrared (3-70 micron) spectral energy distributions of dry merger candidates in the Bootes field of the NOAO Deep Wide-Field Survey. These candidates were selected by previous authors to be luminous, red, early-type galaxies with morphological evidence of recent tidal interactions. We find that a significant fraction of these candidates exhibit 8 and 24 micron excesses compared to expectations for old stellar populations. We estimate that a quarter of dry merger candidates have mid-infrared-derived star formation rates greater than ~1 MSun/yr. This represents a "frosting" on top of a large old stellar population, and has been seen in previous studies of elliptical galaxies. Further, the dry merger candidates include a higher fraction of starforming galaxies relative to a control sample without tidal features. We therefore conclude that the star formation in these massive ellipticals is likely triggered by merger activity. Our data suggest that the mergers responsible for the observed tidal features were not completely dry, and may be minor mergers involving a gas-rich dwarf galaxy.