Constraining the population of star-forming galaxies with deep near-IR images of lensing clusters

¹ Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, 14 avenue E. Belin, 31400 Toulouse, France e-mail: johan@astro.caltech.edu
² Geneva Observatory, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
³ OAMP, Laboratoire d'Astrophysique de Marseille, UMR 6110, Traverse du Siphon, 13012 Marseille, France
⁴ Caltech Astronomy, MC105-24, Pasadena, CA 91125, USA

Received: 29 June 2005
Accepted: 29 May 2006

Abstract

We present the first results of our deep survey of lensing clusters aimed at constraining the abundance of star-forming galaxies at , using lensing magnification to improve the search efficiency and subsequent spectroscopic studies. Deep near-IR photometry of two lensing clusters (A1835 and AC114) was obtained with ISAAC/VLT. These images, combined with existing data in the optical bands including HST images, were used to select very high redshift candidates at among the optical-dropouts. Photometric selection criteria have been defined based on the well-proven dropout technique, specifically tuned to target star-forming galaxies in this redshift domain. 
We have identified 18(8) first and second-category optical dropouts in A1835 (AC114), detected in more than one filter up to H (Vega)  (AB ~ 25.2, uncorrected for lensing). Among them, 8(5) exhibit homogeneous SEDs compatible with star-forming galaxies at , and 5(1) are more likely intermediate-redshift EROs based on luminosity considerations. We have also identified a number of fainter sources in these fields fulfilling our photometric selection and located around the critical lines. We use all these data to make a first attempt at constraining the density of star-forming galaxies present at using lensing clusters. Magnification effects and sample incompleteness are addressed through a careful modeling of the lensing clusters. A correction was also introduced to account for the expected fraction of false-positive detections among this photometric sample. 
It appears that the number of candidates found in these lensing fields, corrected for magnification, incompleteness and false-positive detections, is higher than the one achieved in blank fields with similar photometric depth in the near-IR. The luminosity function derived for  candidates appears compatible with that of LBGs at , without any renormalization. The turnover observed by Bouwens et al. (2005) towards the bright end relative to the  LF is not observed in this sample. Also the upper limit for the UV SFR density at , integrated down to , of    yr^-1 Mpc^-3 is compatible with the usual values derived at , but higher than the estimates obtained in the NICMOS Ultra Deep Field (UDF). The same holds for the upper limit of the SFR density in the interval (  10^-1). This systematic trend towards the bright end of the LF with respect to blank fields could be due to field-to-field variance, a positive magnification bias from intermediate-redshift EROs, and/or residual contamination. Given the low S/N ratio of the high-z candidates, and the large correction factors applied to this sample, increasing the number of blank and lensing fields with ultra-deep near-IR photometry is essential to obtain more accurate constraints on the abundance of  galaxies.