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

Abstract

We present the results of a survey for extremely red objects (EROs) undertaken in the fields of 10 massive galaxy cluster lenses at z∼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)≥5.3, of which 26 have (R−K)≥6.0 in a total image‐plane survey area of 49 arcmin2 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≤21.6 of 2.5±0.4 and 1.2±0.3 arcmin−2 for EROs with (R−K)≥5.3 and ≥6.0 respectively. These ERO number counts agree with previous shallower surveys at K≲19, and flatten significantly at magnitudes fainter than K∼19–20. This flattening may be due to a transition from an ERO population dominated by evolved galaxies at z∼1–2(K≲19.5) to one dominated by dusty starburst galaxies at z>1(K≳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 zf∼2.5 for this population matches the observed surface density of (R−K)≥5.3 EROs quite well, and the (R−K)≥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∼1.

The Intrinsic Properties of SMM J14011+0252

“…The measured ellipticity of J1c is ǫ = 0.03 ± 0.01 (at PA=133 deg.) and its total magnitude (within a 6 ′′ -diameter aperture) is R 702 = 21.27 ± 0.03 (our best estimate of the Kband magnitude of J1c is K = 18.44 ± 0.10, giving a color of (R − K) ∼ 2.9, similar to that expected for a dwarf spheroid or early-type disk cluster member, Smith et al 2002). Using GIM-2D (Simard et al 2002) we model the 2-D light distribution in J1c and obtain a best fit model (χ 2 = 1.3) with a half-light radius of r hl = 0.40 ± 0.05 or ∼ 1.6 kpc and a bulge-to-disk ra-tio of 0.30 ± 0.05, suggesting the galaxy is an early-type disk (Sab).…”

AHubble Space Telescopelensing survey of X-ray luminous galaxy clusters - III. A multiply imaged extremely red galaxy atz=1.6

“…ERO J003707 was first detected in HST † imaging of A 68 as part of our lensing survey of X-ray luminous galaxy clusters (Smith et al 2001a;S02a;Smith et al 2002b, in preparation -S02b). A 68 was observed with HST /WFPC2 for 7.5 ks through the F702W filter and 8.8 ks using UFTI on the 3.8m United Kingdom Infrared Telescope ‡ (UKIRT).…”

“…The importance of EROs to our understanding of galaxy formation and the intrinsic faintness of these systems (R > ∼ 23, K > ∼ 18) recently motivated us to search for EROs in the fields of massive gravitational lenses (Smith et al 2002a -S02a). In this letter, we present near-infrared spectroscopy of ERO J003707+0909.5, drawn from S02a's sample.…”

The active and passive populations of extremely red objects

“…Cimatti et al 2002a), Moreover, their relatively large K-band fluxes suggested high stellar masses, comparable with local ellipticals, leading to the interpretation of EROs as already assembled elliptical galaxies and the redshift range z > 2 as their formation epoch. The correct prediction of such an early and efficient forma-tion of massive galaxies at high redshift is a well known challenge for models of galaxy formation based on the CDM cosmology: in fact early implementations predicted lower space densities of EROs than observed (e.g., Firth et al 2002;Smith et al 2002).…”