2009A&A...501..485C

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 ≃500arcmin², down to a 0.5-8keV flux limit of ≃10^–15erg/cm2/s. We selected EROs using a multi-band photometric catalog down to a K_S-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) (>3.5x10⁴²erg/s 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 0.5-8keV band, provides an upper limit to the average intrinsic absorption at z=1 of 2.5x10²²cm^–2 and corresponds to a rest-frame 0.5-8keV luminosity of 8.9x10⁴¹erg/s. 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.