Photometric Redshifts and Selection of High-Redshift Galaxies in the NTT and Hubble Deep Fields

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Abstract
We present and compare in this paper new photometric redshift catalogs of the galaxies in three public fields: the NTT Deep Field, the HDF-N, and the HDF-S. In the case of the NTT Deep Field, we present here a new photometric catalog, obtained by combining the existing BVrI and JKs with new deep U observations acquired with NTT-SUSI2, and which includes also the contiguous field centered on the zem = 4.7 quasar BR 1202-07. Photometric redshifts have been obtained for the whole sample (NTTDF + HDF-N + HDF-S), by adopting a χ2 minimization technique on a spectral library drawn from the Bruzual and Charlot synthesis models, with the addition of dust and intergalactic absorption. The accuracy, determined from 125 galaxies with known spectroscopic redshifts, is σz ~ 0.08 (0.3) in the redshift intervals z = 0–1.5 (1.5–3.5). The global redshift distribution of I-selected galaxies shows a distinct peak at intermediate redshifts, z 0.6 at IAB ≤ 26 and z 0.8 at IAB ≤ 27.5 followed by a tail extending to z 6. Systematic differences exist among the fields, most notably the HDF-S which contains a much smaller number of galaxies at z 0.9 and at z ≥ 4.5 than the HDF-N. We also present for the first time the redshift distribution of the total IR-selected sample to faint limits (Ks ≤ 21 and J ≤ 22). It is found that the number density of galaxies at 1.25 < z < 1.5 is 0.1 arcmin-2 at J ≤ 21 and 1.0 arcmin-2 at J ≤ 22 and drops to 0.3 arcmin-2 (at J ≤ 22) at 1.5 < z < 2. The HDF data sets are used to compare the different results from color selection criteria and photometric redshifts in detecting galaxies in the redshift range 3.5 ≤ z ≤ 4.5. Photometric redshifts predict a number of high z candidates in both the HDF-N and HDF-S that is nearly 2 times larger than color selection criteria, and it is shown that this is primarily due to the inclusion of dusty models that were discarded in the original color selection criteria. In several cases, the selection of these objects is made possible by the additional constraints from the IR bands. This effect partially reflect the poor spectral sampling of the HDF filter set, and is not present in ground-based observations where a R-I ≤ 0.5 color selection criteria may be applied. Finally, it is shown that galactic M stars may mimic z > 5 candidates in the HDF filter set and that the four brightest candidates at z > 5 in the HDF-S are indeed most likely M stars.
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