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.