The Surface Density of Extremely Red Objects

We present initial results from a field survey for extremely red objects (EROs, defined here as (R-K ) ≥ 6) covering 154 square arcminutes of sky, from the first of 7 deep, wide-field K ′ images obtained as part of the Calar Alto Deep Imaging Survey (CADIS). The 5σ point source detection limits are K ′ = 20. 5 and R = 25. 0, while extended-source limits are up to 0. 50–0. 75 brighter. We identify a total of 8 bright EROs with K ′ ≤ 19. 0. Six of these bright EROs are resolved and are likely to be galaxies, while the remaining 2 are unresolved, with colors consistent with their being low-mass galactic stars. We derive a surface density for the 6 bright, extragalactic EROs of 0.039±0.016 arcmin, higher by a factor of 4 than previous values. We estimate that the volume density of bright EROs to be as high as that of nearby Seyfert galaxies. Subject headings: cosmology: observations — early universe — galaxies: formation — infrared: galaxies Current address: MS 320-47 Caltech, Pasadena, CA 91125, USA. Email: [email protected] Current address: STScI, 3700 San Martin Dr., Baltimore, Md. 21218 USA. Email: [email protected] Current address: Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK. Email: [email protected]