On the Compared Accuracy and Reliability of Spectroscopic and Photometric Redshift Measurements

We present a comparison between the spectroscopic catalog of the HDF recently published by Cohen et al and the redshifts measured by our group for those objects using photometric techniques. We perform this comparison in order to characterize the errors associated to the photometric redshift technique. The spectroscopic sample includes over 140 objects in the HDF proper, representing the deepest, cleanest, most complete spectroscopic catalog ever compiled. We study each object for which our redshift and the one measured by Cohen et al disagree. In most cases the photometric evidence is strong enough to call for a careful review of the spectroscopic values, as they seem to be in error. It is possible to characterize the systematic errors associated to our technique, which when combined with the photometric errors allow us to obtain complete information on the redshift of each galaxy and its associated confidence interval, regardless of apparent magnitude. One of the main conclusions of this study is that, to date, all the redshifts from our published catalog that have been checked have been shown to be correct (within the stated confidence limits). This implies that our galaxy template set is a fair representation of the galaxy population at all redshifts (0<z<6) and magnitudes (R<24) 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 as 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)