Infrared Excess sources: Compton thick QSOs, low-luminosity Seyferts or starbursts?

• Published in: Monthly notices of the Royal Astronomical Society 2010-07, Vol.406 (1), p.420-433
• Main Authors: Georgakakis, A., Rowan-Robinson, M., Nandra, K., Digby-North, J., Pérez-González, P. G., Barro, G.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; galaxies: active; galaxies: Seyfert; galaxies: starburst; Infrared radiation; Starbursts; Stars & galaxies; surveys
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2010.16727.x

We explore the nature of Infrared Excess sources (IRX), which are proposed as candidates for luminous [LX(2–10 keV) > 1043 erg 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.