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 arcmin^2, 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)
(>=3.5x10^{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
5.7sigma in the observed 0.5-8 keV band, provides an upper limit to the average
intrinsic absorption at z=1 of 2.5x10^{22} cm^{-2} and corresponds to a
rest-frame 0.5-8 keV luminosity of 8.9x10^{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.
