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 approximate to 90 ks) over an area of approximate to 500 arcmin(2),down to a 0.5-8 keV flux limit of approximate to 10-15 erg cm(-2) s(-1). We selected EROs using a multi-band photometric catalog down to a KS-band magnitude of approximate to 19.3 (Vega system); 14 EROs were detected in X-rays, corresponding to approximate to 9% of the overall X-ray source population (149 X-ray sources) and to approximate 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) (greater than or similar to 3.5 x 10(42) 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 5.7 sigma in the observed 0.5-8 keV band, provides an upper limit to the average intrinsic absorption at z = 1 of 2.5 x 10(22) cm(-2) and corresponds to a rest-frame 0.5-8 keV luminosity of 8.9 x 10(41) 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.