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) > 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.

X-ray detected infrared excess AGN in theChandradeep fields: a moderate fraction ofCompton-thick sources

“…Their SEDs are consistent with starburst activity showing no evidence for a hot dust component. Georgakakis et al (2010) conclude that there is little evidence for the presence of a large fraction of luminous Compton-thick sources in either the X-ray-detected or undetected population of DOG analogues. As far as the Xray detections are concerned, our findings are not that discrepant quantitatively, given that our fraction of Compton-thick sources is 4/12 among the brightest sources.…”

“…The former have an X-ray to mid-IR luminosity ratio of L 2−10 keV /L 6 μm ≈ −1.3 (median), while the latter have L 2−10 keV /L 6 μm ≈ −3 (see Fig. 2 of Georgakakis et al 2010). This could imply that the samples of undetected DOGs indeed contain a large fraction of Comptonthick sources.…”

“…The spectrum of the COSMOS DOGs is harder but consistent within the errors with the derived average spectrum of the DOGs in or sample. Georgakakis et al (2010) follow a different approach for identifying DOGs. They identify lower (spectroscopic) redshift z ≈ 1 sources in the AEGIS and CDFN surveys, which are analogues to the distant DOG population at z ∼ 2.…”

X-ray observations of highly obscuredτ9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?

“…Another widely applied mid-IR method for the selection of Compton-thick AGN ) relies on low X-ray to IR ratios. The principle behind this method is that a low L X /L IR ratio is a sign of heavy obscuration (but see Yaqoob & Murphy 2011;Georgakakis et al 2010).…”

“…However, because of the lack of X-ray spectroscopy the fraction of Compton-thick sources cannot be readily quantified. Based on brighter sources with X-ray spectroscopy in the GOODS (Georgantopoulos et al 2011), AEGIS (Georgakakis et al 2010) and SWIRE fields (Lanzuisi et al 2009), the fractions of Compton-thick sources found among IR-excess AGN range between 0% and 50%. Another widely applied mid-IR method for the selection of Compton-thick AGN ) relies on low X-ray to IR ratios.…”