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

A&A 456, 861-880 (2006)
DOI: 10.1051/0004-6361:20053724

Constraining the population of $6 \la {z} \la 10$ star-forming galaxies with deep near-IR images of lensing clusters

J. Richard¹, R. Pelló¹, D. Schaerer^{2, 1}, J.-F. Le Borgne¹ and J.-P. Kneib^{3, 4}

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 $z\sim6{-}10$ , 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 $z\ga 6$ 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) $\sim 23.8$ (AB ~ 25.2, uncorrected for lensing). Among them, 8(5) exhibit homogeneous SEDs compatible with star-forming galaxies at $z\ga 6$ , 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 $6 \la {z} \la 10$ 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 $z\ga 6$ candidates appears compatible with that of LBGs at $z\simeq 3$ , without any renormalization. The turnover observed by Bouwens et al. (2005) towards the bright end relative to the $z\sim 3$ LF is not observed in this sample. Also the upper limit for the UV SFR density at $z\sim6{-}10$ , integrated down to L₁₅₀₀=0.3 L^*_z=3, of $\rho_\star=7.4$ $\times$ $10^{-2}~M_{\odot}$ yr^-1 Mpc^-3 is compatible with the usual values derived at $z \simeq 5{-}6$ , 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 $z \simeq 8{-}10$ interval ( $\rho_\star=1.1$ $\times$ 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 $z\ga 6$ galaxies.

Key words: Galaxy: formation -- galaxies: high-redshift -- galaxies: photometry -- gravitational lensing