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

Abstract

We identify a very deep sample of 198 extremely red objects (EROs) in the Chandra Deep Field South, selected on the basis of I₇₇₅-K_s > 3.92, to a limit K_s~= 22 using the public European Southern Observatory (ESO)/Great Observatories Origins Deep Survey (GOODS) survey.

The ERO number counts flatten from a slope of γ~= 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, ω(θ), 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 ω(θ) amplitudes are best-fitted by models in which the EROs have a comoving correlation radius r₀~= 12.5 +/- 1.2 h^-1 Mpc, or r₀~= 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~= 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 (~2-20 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.