On the compared accuracy and reliability of spectroscopic and photometric redshift measurements
Abstract
Recently Cohen et al have presented a careful compilation of all the spectroscopic redshifts measured in the HDF and flanking fields. Incorporating previously published results, corrections to previous wrong values, and data of their own, they include over 140 objects in the HDF. In this paper we present the comparison between their spectroscopic redshifts and the redshifts that our group has measured for the same objects using photometric techniques, in order to characterize the errors associated to the photometric redshift technique. We particularly study each object for which our redshift and the one by Cohen et al seem to disagree. In most of those cases the photometric evidence we put forth is strong enough to call for a careful review of the spectroscopic values, as they seem to be in error. We show that it is possible to characterize the systematic errors associated to our technique, which when combined with the photometric errors, allows us to obtain complete information on the redshift of each galaxy and its associated error, regardless of its apparent magnitude. One of the main conclusions of this study is that, to date, all those redshifts from our published catalogs that have been checked have been shown to be correct within the stated confidence limits. This implies that our galaxy templates are a fair representation of the galaxy population at all redshifts and magnitudes explored to date. On the other hand, spectroscopy of faint sources is subject to unknown and uncharacterized systematic errors, that will in turn be transmitted to any photometric redshift technique which uses spectroscopic samples in its calibration. Our analysis proves that photometric redshift techniques can and must be used to extend the range of applicability of the spectroscopic redshift measurements. (ABRIDGED)Keywords
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