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 mag] covering 154 arcmin2 of sky, from the first of seven deep, wide-field K' images obtained as part of the Calar Alto Deep Imaging Survey (CADIS). The 5 σ point source detection limits are K′ = 20.5 mag and R = 25.0 mag, while extended-source limits are up to 0.50-0.75 mag brighter. We identify a total of eight bright EROs with K′ ≤ 19.0 mag. Six of these bright EROs are resolved and are likely to be galaxies, while the remaining two are unresolved, with colors consistent with their being low-mass galactic stars. We derive a surface density for the six bright, extragalactic EROs of 0.039±0.016 arcmin-2, which is 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.