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 ≃ 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 r 0 ≃ 12.5 ± 1.2 h −1 Mpc , or r 0 ≃ 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.