On the nature of red galaxies: the Chandra perspective^*

¹ Max-Planck-Institut für Astrophysik (MPA), Karl-Schwarzschild-Str. 1, Garching b. Muenchen, 85748, Germany e-mail: campisi@mpa-garching.mpg.de
² Dipartimento di Astronomia, Università degli Studi di Bologna, via Ranzani 1, Bologna, Italy
³ INAF - Osservatorio Astronomico di Bologna, via Ranzani, 1, 40127 Bologna, Italy
⁴ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁵ Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, DAPNIA/SAP, Orme des Merisiers, 91191 Gif-sur-Yvette, France
⁶ Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
⁷ INAF - Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁸ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
⁹ The Graduate University for Advanced Studies, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
¹⁰ Center for Astrophysics, University of Science and Technology of China, Hefei 230026, P.R. China

Received: 16 February 2009
Accepted: 17 April 2009

Abstract

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 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) ( 3.510⁴² erg s^-1 at photometric redshifts ), 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 of 2.510²² cm^-2 and corresponds to a rest-frame 0.5–8 keV luminosity of 8.910⁴¹ 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.