On the nature of red galaxies: the Chandra perspective

We present the X-ray properties of the extremely red objects (ERO) population observed by Chandra with three partially overlapping pointings (up to ≈90 ks) over an area of ≈500 arcmin², down to a 0.5-8 keV flux limit of ≈10^-15 erg cm^-2 s^-1. We selected EROs using a multi-band photometric catalog down to a KS-band magnitude of ≈19.3 (Vega system); 14 EROs were detected in X-rays, corresponding to ≈9% of the overall X-ray source population (149 X-ray sources) and to ≈5% of the ERO population (288). The X-ray emission of all X-ray detected EROs is consistent with that of an active galactic nucleus (AGN) (⪆ 3.5×10⁴² erg s^-1 at photometric redshifts z > 1), in agreement with previous X-ray observations, with an indication of increasing absorption between the three X-ray brightest EROs and the 11 X-ray faintest EROs. We take advantage of the good spatial resolution and limited background provided by Chandra to place constraints on the population of the X-ray undetected EROs by a stacking analysis. Their stacked emission, whose statistical significance is 5.7σ in the observed 0.5-8 keV band, provides an upper limit to the average intrinsic absorption at z = 1 of 2.5×10²² cm^-2 and corresponds to a rest-frame 0.5-8 keV luminosity of 8.9×10⁴¹ erg s^-1. We estimate that any accretion-related X-ray emission to the stacked signal is likely “diluted” by emission due to hot gas in normal galaxies and star-formation activity in dust-enshrouded galaxies at high redshift.

Appendix A is only available in electronic form at: http://www.aanda.org