EDP Sciences
Free access
Volume 421, Number 3, July III 2004
Page(s) 821 - 832
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20035842

A&A 421, 821-832 (2004)
DOI: 10.1051/0004-6361:20035842

Number counts of bright extremely red objects: Evolved massive galaxies at z  $\sim1$

P. Väisänen1, 2 and P. H. Johansson3, 4

1  European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
2  Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
3  Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK
4  Observatory, PO Box 14, 00014 University of Helsinki, Finland

(Received 10 December 2003 / Accepted 8 April 2004)

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 $0.002 \pm 0.001 \ \rm arcmin^{-2}$ 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 $z\sim1$. 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 ${>}2{-}3~L^{\star}$ massive galaxies at around $z\sim1$, is estimated to be ${\approx}2\times10^{-5} ~ {\rm Mpc^{-3}}$, which is consistent with local values. Furthermore, the cumulative number counts at our bright magnitudes are remarkably well fitted by pure luminosity evolution models.

Key words: galaxies: evolution -- galaxies: formation -- infrared: galaxies -- cosmology: observations -- galaxies: starburst -- galaxies: elliptical and lenticular, cD

Offprint request: P. Väisänen, pvaisane@eso.org

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© ESO 2004