Extremely red objects: An X-ray dichotomy

We analyze the X-ray properties of a near-infrared-selected (K-s < 20) sample of spectroscopically identified extremely red objects (EROs; R-K-s>5) in a region of the Chandra Deep Field-South using the public 1 Ms observation. The 21 objects were classified, on the basis of deep Very Large Telescope spectra, into two categories: 13 dusty star-forming galaxies showing [O II] emission and eight early-type galaxies with absorption features in their optical spectra. Only one emission- line object has been individually detected; its very hard X-ray spectrum and the high intrinsic X-ray luminosity unambiguously reveal the presence of an obscured active galactic nucleus. Stacking analysis of the remaining 12 emission-line objects shows a significant detection with an average luminosity L-X ≈ 8 x 10(40) ergs s(-1) in the rest-frame 2-10 keV band. The stacked counts of the eight passive galaxies do not provide a positive detection. We briefly discuss the implications of the present results for the estimate of the star formation rate in emission-line EROs.