Extinction and metal column density of HI regions up to redshift z

¹ Osservatorio Astronomico di Trieste, Istituto Nazionale di Astrofisica, Trieste, Italy e-mail: vladilo@ts.astro.it
² Department of Theoretical Astrophysics, Ioffe Physico-Technical Institute, St. Petersburg, Russia
³ European Southern Observatory, Garching-bei-München, Germany
⁴ Department of Physics, Utkal University, Bhubaneswar, India
⁵ Department of Physics and Astronomy, University of South Carolina, Columbia, USA
⁶ Department of Astronomy and Astrophysics, University of Chicago, Chicago, USA
⁷ Enrico Fermi Institute, University of Chicago, Chicago, USA

Received: 21 December 2005
Accepted: 10 April 2006

Abstract

We used the photometric database of the Sloan Digital Sky Survey (SDSS) to estimate the reddening of 13 SDSS quasars selected on the basis of the presence of zinc absorption lines in an intervening Damped Ly α (DLA) system. In 5 of these quasars the reddening is detected at 2σ confidence level in two independent color indices of the SDSS ugriz photometric system. A detailed analysis of the data supports an origin of the reddening in the intervening absorbers. We used these rare measurements of extinction in DLA systems to probe the relation between extinction and metal column density in the interval of absorption redshift . We find that the mean extinction in the V band per atom of iron in the dust is remarkably similar to that found in interstellar clouds of the Milky Way. This result lends support to previous estimates of the dust obscuration effect in DLA systems based on a Milky Way extinction/metal column density relation. We propose a simple mechanism, based on dust grain destruction/accretion properties, which may explain the approximate constancy of the extinction per atom of iron in the dust.