Infrared excess sources: Compton thick QSOs, low-luminosity seyferts or starbursts?

Abstract

We explore the nature of Infrared Excess sources (IRX), which are proposed as candidates for luminous [. LX(2-10 keV) > 1043erg s-1] Compton thick (NH > 2 × 1024 cm-2) 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 LX(2-10 keV) = 1043 erg s-1 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 [. LX(2-10 keV) < 1043 erg s-1], 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. © 2010 The Authors. Journal compilation © 2010 RAS.