Infrared Excess Sources: Compton Thick QSOs, low luminosity Seyferts or starbursts?

TL;DR

This study investigates Infrared Excess sources at z~2, finding most are not Compton Thick QSOs but likely low-luminosity AGN or starbursts, emphasizing the importance of decomposing AGN and starburst contributions.

Contribution

The paper introduces a method to identify IRX sources at z=2 and clarifies their nature, showing most are not heavily obscured QSOs but lower luminosity AGN or starbursts.

Findings

Most IRX sources are not Compton Thick QSOs.

X-ray spectra suggest many are Compton Thin or starburst-dominated.

Star formation may dominate mid-infrared emission in these sources.

Abstract

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.