The VLT observations of the HDF-S NICMOS field: photometric catalog and high redshift galaxy candidates

Abstract

We present the deep UBVRI observations of the HDF-S NICMOS field obtained as part of the Science Verification of the VLT Unit 1 telescope. The images have been used to construct object catalogs and to obtain photometric redshifts. The effective field of view is $\simeq 70\times70$ arcsec$^2$, and the formal $5σ$ limiting magnitudes (in a 2 FWHM aperture) are 26.3, 27.8, 27.5, 26.9, 25.2 in the $U$, $B$, $V$, $R$ and $I$ bands, respectively. Thanks to the sub-arcsecond image quality, relatively long exposure time, and large collecting area of the VLT, this is the deepest set of multicolor images ever obtained from a ground-based telescope. Galaxy counts have been derived independently in each band, and show no significant departures from previous data from wider areas. A multicolor photometric catalog of all the galaxies selected in the $R$ band has also been obtained and used to derive photometric redshifts for all galaxies with $R \leq 26.5 $, using also the J,H and K magnitudes from the NICMOS deep observations. A significant fraction (\simeq 28%)of the galaxies is placed at $z\geq 2$. Among them, six robust candidates are found at $z \geq 2.5$ .