The nature, evolution, clustering and X-ray properties of extremely red galaxies in the Chandra Deep Field South/Great Observatories Origins Deep Survey field

We identify a very deep sample of 198 extremely red objects (EROs) in the Chandra Deep Field South, selected on the basis of I-775 - K-s > 3.92, to a limit K-s similar or equal to 22 using the public European Southern Observatory (ESO)/Great Observatories Origins Deep Survey (GOODS) survey. The ERO number counts flatten from a slope of gamma similar or equal to 0.59 to 0.16 at K > 19.5, where they remain below the predictions for pure luminosity evolution, and fall below even a non-evolving model. This suggests there is a significant decrease with redshift in the comoving number density of passive/very red galaxies. We investigate the angular correlation function, omega(theta), of these EROs and detect positive clustering for K-s = 20.5-22.0 sources. The EROs show stronger clustering than other galaxies at the same magnitudes. The omega(theta) amplitudes are best-fitted by models in which the EROs have a comoving correlation radius r(0) similar or equal to 12.5 +/- 1.2 h(-1) Mpc, or r(0) similar or equal to 21.4 +/- 2.0 h(-1) Mpc in a stable clustering model. We find a 40-arcsec diameter overdensity of 10 EROs, centred on the Chandra X-ray source (and ERO) XID: 58. On the basis of colours we estimate that about seven, including XID: 58, belong to a cluster of EROs at z similar or equal to 1.5. The 942-ks Chandra survey detected 73 X-ray sources in the area of our ERO sample, 17 of which coincide with EROs. Of these sources, 13 have X-ray properties indicative of obscured active galactic nuclei (AGN), while the faintest four may be starbursts. In addition, we find evidence that Chandra sources and EROs are positively cross-correlated at non-zero (similar to2-20 arcsec) separations, implying that they tend to trace the same large-scale structures. In conclusion, these findings appear consistent with a scenario where EROs are the z > 1 progenitors of elliptical/S0 galaxies, some forming very early as massive spheroids, which are strongly clustered and may evolve via an AGN phase, others more recently from mergers of disc galaxies.