A Hubble Space Telescope lensing survey of X-ray-luminous galaxy clusters -: II.: A search for gravitationally lensed EROs

We present the results of a survey for extremely red objects (EROs) undertaken in the fields of 10 massive galaxy cluster lenses at z similar to 0.2, combining sensitive. high-resolution Hubble Space Telescope imaging with deep, half-arcsecond K-band imaging from UKIRT. We detect 60 EROs with (R-K) greater than or equal to 5.3, of which 26 have (R-K) greater than or equal to 6.0 in a total image-plane survey area of 49 arcmin(2) down to K = 20.6, including one multiply imaged ERO. We use detailed models of the cluster lenses to quantify the lens amplification and thus correct the observed number counts and survey area for the effects of gravitational lensing. After making these corrections, we estimate surface densities at K less than or equal to 21.6 of 2.5+/-0.4 and 1.2+/-0.3 arcmin(-2) for EROs with (R-K) greater than or equal to 5.3 and 6.0 respectively. These ERO number counts agree with previous shallower surveys at K less than or similar to 19, and flatten significantly at magnitudes fainter than K similar to 19-20. This flattening may be due to a transition from an ERO population dominated by evolved galaxies at z similar to1- 2 (K less than or similar to 19.5) to one dominated by dusty starburst galaxies at z > 1 (K greater than or similar to 19.5). We also compare our results with various model predictions. including a model that attempts to explain EROs in terms of a single population of elliptical galaxies formed at high redshift. We find that a formation epoch of z(f) similar to 2.5 for this population matches the observed surface density of (R-K) greater than or equal to 5.3 EROs quite well, and the (R-K) greater than or equal to 6.0 sample less well. More sophisticated models, including semi-analytic prescriptions, underpredict the ERO surface density by approximately an order of magnitude. suggesting that these models produce insufficient stars and/or dust at high redshift. This deficit of EROs appears to be a general problem with models that reproduce the median redshift from K-selected redshift surveys. One possible explanation is that the current K-selected redshift distribution may be incomplete beyond z similar to 1.