2010MNRAS.406..420G

We explore the nature of Infrared Excess sources (IRX), which are proposed as candidates for luminous [L_X(2-10 keV) > 10⁴³ erg/s] Compton thick (N_H> 2x10²⁴/cm2) QSOs at z ~ 2. Lower redshift, z ~ 1, analogues of the distant IRX population are identified by first redshifting to z = 2 the spectral energy distributions (SEDs) of all sources with secure spectroscopic redshifts in the AEGIS (6488) and the GOODS-North (1784) surveys and then selecting those that qualify as IRX sources at that redshift. A total of 19 galaxies are selected. The mean redshift of the sample is z ~ 1. We do not find strong evidence for Compton thick QSOs in the sample. For nine sources with X-ray counterparts, the X-ray spectra are consistent with Compton thin active galactic nucleus (AGN). Only three of them show tentative evidence for Compton thick obscuration. The SEDs of the X-ray undetected population are consistent with starburst activity. There is no evidence for a hot dust component at the mid-infrared associated with AGN heated dust. If the X-ray undetected sources host AGN, an upper limit of L_X(2-10 keV) = 10⁴³ erg/s is estimated for their intrinsic luminosity. We propose that a large fraction of the z ~ 2 IRX population is not Compton thick quasi-stellar objects (QSOs) but low-luminosity [L_X(2-10 keV) < 10⁴³ erg/s], possibly Compton thin, AGN or dusty starbursts. It is shown that the decomposition of the AGN and starburst contribution to the mid-IR is essential for interpreting the nature of this population, as star formation may dominate this wavelength regime.