A multi-wavelength approach to the properties of Extremely Red Galaxy populations: I - Contribution to the Star Formation Rate density and AGN content

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 EROs (645 sources), IEROs (294 sources), and DRGs (350 sources), we derive redshift distributions, identify AGN powered and Star-formation powered galaxies, and, using the radio observations of this field, estimate robust (AGN- and dust-unbiased) Star Formation Rate Densities (SFRD) 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 AGN 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 star-formation rates, from below 10 Mo/yr to as high as 200 Mo/yr. 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 SFRD for EROs and DRGs, while none is observed for IEROs. The former populations can contribute more than 20\% to the global SFRD at 2<z<3. The emission from AGN activity is typically not strong in the ERG population, with AGN increasing the average radio luminosity of ERG sub-populations by, nominally, less than 20%. AGN are common, however, and, if no discrimination is attempted, this could significantly increase the SFRD 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 AGN in these populations is necessary to obtain meaningful results.

Accepted on June 3rd 2010 for ApJ publication. 21 pages, 11 figures, 3 tables

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