Can a photometric redshift code reliably determine dust extinction

Photometric redshifts can be routinely obtained to accuracies of better than 0.1 in Δ z /(1 + z). The issue of dust extinction, however, is one that has still not been well quantified. In this paper the success of two template-fitting photometric redshift codes (IMPZ and HYPERZ) at reliably returning A V in addition to redshift is explored. New data on the 2nd Canadian Network for Observational Cosmology (CNOC2) spectroscopic sample of 0.2 < z < 0.7 galaxies are presented. These data allow us to estimate A V values from the observed Balmer decrements. We also investigate whether the empirical value of γ = 0.44, the ratio between gas- and star-derived extinction, as determined by Calzetti, is necessarily the best value for this sample. When comparing the two codes with the Balmer-derived A V (Balmer[A V ]), a correlation between the photometrically derived A V (Phot[A V ]) and the Balmer[A V ] is found. The correlation is improved when the empirical value of y = 0.44 is allowed to vary. From least-squares fitting, the minimum in the reduced X 2 distribution is found for y ∼0.25 ± 0.2. For the sample of galaxies here, the factor of 2 difference in covering factor implied by the Calzetti ratio is found to be plausible. The CNOC2 galaxies with detected Balmer lines have some preference for an increased covering-factor difference, which perhaps implies that they are undergoing more rapid, 'bursty' star formation than the galaxies Calzetti used in her derivation.