Number counts of bright extremely red objects: Evolved massive galaxies at z ∼1

Abstract

We present results on number counts of Extremely Red Objects (EROs) in a 2850 arcmin² near-infrared survey performed in European Large Area ISO Survey (ELAIS) fields at K<17.5. Counts of EROs are extended to brighter levels than available previously, giving 0.002 ± 0.001 arcmin^-2 at K<16.5 and consistent numbers with literature values at fainter magnitudes. Photometric redshifts from HYPERZ as well as GRASIL model SEDs of galaxies imply that our EROs are located in the range z=0.7-1.5, with the bulk of the population at z~1. Taking advantage of the ISO data in the fields, we use mid-IR detections to constrain the number of dusty EROs, and also discuss the superior capabilities of Spitzer Space Telescope to detect dusty EROs. Both the mid-IR data and the use of colour-colour diagrammes indicate that at most 10-20% of the EROs in this bright regime are dusty starbursting systems. The space density of our EROs, interpreted to be counterparts of local >2-3 L^* massive galaxies at around z~1, is estimated to be ≈2×10^-5 Mpc^-3, which is consistent with local values. Furthermore, the cumulative number counts at our bright magnitudes are remarkably well fitted by pure luminosity evolution models.