We explore the nature of Infrared Excess sources (IRX), which are proposed as candidates for luminous L_X(2-10keV)>1e43erg/s Compton Thick (N_H>2e24cm^{-2}$) QSOs at z~2. Lower redshift, z~1, analogues of the distant IRX population are identified by firstly redshifting to z=2 the 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 $z\approx1$. We do not find strong evidence for Compton Thick QSOs in the sample. For 9 sources with X-ray counterparts, the X-ray spectra are consistent with Compton Thin AGN. Only 3 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(2-10keV) =1e43erg/s is estimated for their intrinsic luminosity. We propose that a large fraction of the $z\approx2$ IRX population are not Compton Thick QSOs but low luminosity [L_X(2-10keV)<1e43erg/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.
The composition of the diffuse X-ray background remains a problem for high energy astrophysics. X-ray imaging surveys with the Chandra and XMM-Newton observatories have resolved between 80 to 100% of the XRB into discrete sources below about 6 keV (e.g. Worsley et al. 2005;Hickox & Markevitch 2006;Georgakakis et al. 2008). The vast majority of these sources are Compton thin AGN (NH < ∼ 10 24 cm -2 ) at a mean redshift z ≈ 1 (e.g. Barger et al. 2005;Akylas et al. 2006). At higher energies however, between 20 and 30 keV, where the bulk of the XRB energy is emitted (Marshall et al. 1980), only a small fraction of its total intensity has been resolved into discreet sources (Sazonov et al. 2007). As a result the nature of the populations that make up the XRB at these energies is still not well known. Population synthesis models use our knowledge on the properties of the X-ray sources below ≈ 10 keV email: age@astro.noa.gr to make predictions on the nature of the X-ray populations close to the peak of the XRB (e.g. Gilli et al. 2007). These models indicate that Compton thin AGN alone cannot account for the shape of the XRB spectrum at ≈ 20-30 keV. An additional population of heavily obscured, Compton thick (NH > ∼ 10 24 cm -2 ) AGN is postulated to reconcile the discrepancy (e.g. Gilli et al. 2007). The required number density of such sources is however, under debate (Treister et al. 2009a;Draper & Ballantyne 2009). Unfortunately the identification of the heavily obscured AGN population predicted by the models is far from trivial. The X-ray emission of these sources below about 10 keV is suppressed by photoelectric absorption and as a result most of them are expected to lie well below the sensitivity limits of the deepest current X-ray observations. Although a handful of Compton thick AGN candidates have been identified in deep X-ray surveys (Tozzi et al. 2006;Georgantopoulos et al. 2009), the bulk of this population remains to be discovered.
Selection at the mid-IR (3 -30 µm) is proposed as a powerful tool for finding heavily obscured X-ray faint AGN. The UV/optical photons emitted by the central engine are absorbed by the gas and dust clouds and appear as thermal radiation with a broad bump in the mid-IR (νfν units; e.g. Elvis et al. 1994;Prieto et al. 2009). Diverse selection methods have been developed to identify this AGN spectral signature in the mid-infrared. Lacy et al. (2004), Stern et al. (2005) and Hatziminaoglou et al. (2005) propose simple colour cuts based on the mid-IR colours of luminous high redshift QSOs and/or type-2 Seyferts. Polletta et al. (2006) and Rowan-Robinson et al. (2009) fit templates to the broad-band photometry from UV to the far-IR. Alonso-Herrero et al. (2006) and Donley et al. (2007) select sources with power-law Spectral Energy Distributions (SEDs) in the mid-IR. The methods above have merits and shortcomings. Selection by mid-IR colour for example, is simple but suffers contamination from star-forming galaxies if applied to the deepest mid-IR samples currently available (Georgantopoulos et al. 2008;Donley et al. 2008). Template fits are powerful but require high quality photometry over a wide wavelength baseline for meaningful constraints. Power-law SEDs provide the most clean samples of infrared selected AGN, but are sensitive only to the most luminous and hence, rare sources.
A much promising method that is believed to be efficient in identifying heavily obscured, possibly Compton Thick, AGN is based on the selection of sources that are faint at optical and bright at mid-IR wavelengths. In its simplest version this method applies a cut in the 24 µm over R-band flux density ratio, f24µm/fR > 1000 to identify Dust Obscured Galaxies (DOGs; Dey et al. 2008) at a mean redshift z ≈ 2. In addition to the limit above Fiore et al. (2008Fiore et al. ( , 2009) also use the colour cut R-K > 4.5 to select Infrared-Excess sources (IRXs). This colour selection is motivated by the observational result that redder sources include a higher fraction of obscured AGN (e.g. Brusa et al. 2005). A different approach has been adopted by Daddi et al. (2007). They select BzK sources that show excess mid-IR emission relative to that expected based on the rates of star formation measured from shorter wavelengths. Despite differences in the adopted criteria, all the studies above identify a population of galaxies with similar properties in terms of average redshift (z ≈ 2), mean X-ray properties and mid-IR luminosities. These sources are believed to be massive galaxies (Mstar ≈ 10 10 -10 11 M Treister et al. 2009b;Bussmann et al. 2009b) that experience intense bursts of star-formation and rapid supermassive black hole growth, possibly triggered by mergers (e.g. Bussmann et al. 2009b;Narayanan et al. 2009). The apparently brighter subset of this population (S24 > 300 µJy) are proposed as descendants of Submillimeter Galaxies on their way to becoming unobscured QSOs (e.g. Bussmann et al. 2009b,a;Narayana
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