New Improved Photometric Redshifts of Galaxies in the Hubble Deep Field

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Abstract
We report new improved photometric redshifts of 1048 galaxies in the Hubble Deep Field. A standard χ2 minimizing method is applied to recent seven-color UBVIJHK photometry by Fernández-Soto et al. We use 187 template spectral energy distributions (SEDs) representing a wide variety of morphology and age of observed galaxies based on a population synthesis model by Kodama & Arimoto. We introduce two new recipes. First, the amount of the internal absorption is changed as a free parameter in the range E(B-V) = 0.0-0.5 with an interval of 0.1. Second, the absorption due to intergalactic H I clouds is also changed by a factor of 0.5, 1.0, and 1.5 around the opacity given by Madau. The total number of template SEDs is thus 187 × 6 × 3 = 3366, except for the redshift grid. The dispersion σz of our photometric redshifts with respect to spectroscopic redshifts is σz = 0.08 and 0.24 for z < 2 and z > 2, respectively, which are smaller than the corresponding values (σz = 0.09 and 0.45) by Fernández-Soto et al. Improvement is significant, especially in z > 2. This is due to smaller systematic errors which are largely reduced mainly by including three opacities due to intergalactic H I clouds. We discuss redshift distribution N(z) and cosmic star formation rate based on our new photometric redshifts.
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