Counts of Bright Extremely Red Objects : Evolved Massive Galaxies at z ∼ 1

We present results on number counts of Extremely Red Objects (EROs) in a 2850 arcmin 2 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 ± 0.001arcmin −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 − 3L ⋆ 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.