Dry Mergers in GEMS: The Dynamical Evolution of Massive Early-Type Galaxies
Abstract
We have used the 28'x 28' HST image mosaic from the GEMS (Galaxy Evolution from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric redshift survey to constrain the incidence of major mergers between spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A set of N-body merger simulations was used to explore the morphological signatures of such interactions: they are recognizable either as < 5kpc separation close pairs or because of broad, low surface brightness tidal features and asymmetries. Data with the depth and resolution of GEMS are sensitive to dry mergers between galaxies with M_V < -20.5 for z < 0.7; dry mergers at higher redshifts are not easily recovered in single-orbit HST imaging. Six dry mergers (12 galaxies) with luminosity ratios between 1:1 and 4:1 were found from a sample of 379 red early-type galaxies with M_V < -20.5 and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of dry merging are visible for ~250Myr and we use this timescale to convert the observed merger incidence into a rate. On this basis we find that present day spheroidal galaxies with M_V < -20.5 on average have undergone between 0.5 and 2 major dry mergers since z ~ 0.7. We have compared this result with the predictions of a Cold Dark Matter based semi-analytic galaxy formation model. The model reproduces the observed declining major merger fraction of bright galaxies and the space density of luminous early-type galaxies reasonably well. The predicted dry merger fraction is consistent with our observational result. Hence, hierarchical models predict and observations now show that major dry mergers are an important driver of the evolution of massive early-type galaxies in recent epochs.
Keywords
Cite
@article{arxiv.astro-ph/0506425,
title = {Dry Mergers in GEMS: The Dynamical Evolution of Massive Early-Type Galaxies},
author = {Eric F. Bell and Thorsten Naab and Daniel H. McIntosh and Rachel S. Somerville and John A. R. Caldwell and Marco Barden and Christian Wolf and Hans-Walter Rix and Steven V. W. Beckwith and Andrea Borch and Boris Haeussler and Catherine Heymans and Knud Jahnke and Shardha Jogee and Sergey Koposov and Klaus Meisenheimer and Chien Y. Peng and Sebastian F. Sanchez and Lutz Wisotzki},
journal= {arXiv preprint arXiv:astro-ph/0506425},
year = {2009}
}
Comments
ApJ, in press. The paper has been extensively modified, detailing the automated+visual selection and dry merger classification. 11 pages emulateapj with 9 reduced-quality figures. A high quality copy is available at http://www.mpia-hd.mpg.de/homes/bell/papers/dry.ps.gz