H. Messias, J. Afonso, A. M. Hopkins, B. Mobasher, T. Dominici, D. M. Alexander
Abstract
We present a multi-wavelength analysis of the properties of extremely red galaxy(ERG) populations, selected in the GOODS-South/Chandra Deep Field South field. By using all the photometric and spectroscopic information available on large deep samples of extremely red objects(EROs; 645 sources),IRAC EROs(IEROs; 294 sources),and distant red galaxies(DRGs; 350 sources),we derive redshift distributions, identify active galactic nucleus(AGN)-powered and star formation(SF)-powered galaxies, and, using the radio observations of this field, estimate robust(AGN- and dust-unbiased) SF rate densities(dot{rho}_{*}) for these populations. We also investigate the properties of "pure"(galaxies that conform to only one of the three ERG criteria considered) and "combined"(galaxies that verify all three criteria) sub-populations. Overall, a large number of AGNs are identified(up to ~30%, based on X-rays and mid-infrared criteria),the majority of which are type-2(obscured) objects. Among ERGs with no evidence for AGN activity, we identify sub-populations covering a wide range of average SF rates, from below 10 M sun yr-1 to as high as 200 M sun yr-1. Applying a redshift separation(1 <= z < 2 and 2 <= z <= 3),we find significant evolution(an increase of a factor of 2 or higher) of dot{rho}_{*} for EROs and DRGs, while none is observed for IEROs. The former populations can contribute more than 20% to the global dot{rho}_{*} at 2 <= z <= 3. The emission from AGN activity is typically not strong in the ERG population, with AGNs increasing the average radio luminosity of ERG sub-populations by, nominally, less than 20%. AGNs are common, however, and, if no discrimination is attempted, this could significantly increase the dot{rho}_{*} estimate(by over 100% in some cases). Thus, and while the contribution of star-forming processes to the radio luminosity in galaxies with AGN remains uncertain, a comprehensive identification of AGNs in these populations is necessary to obtain meaningful results.
Keywords
galaxies: active - galaxies: high-redshift - galaxies: starburst
The Astrophysical Journal
Volume 719, Number 1, Page 790
2010 August
