Looking at quasars through galaxies

¹ Department of Physics, Stockholm University, 106 91 Stockholm, Sweden e-mail: [linda;ariel]@physto.se
² Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden e-mail: edvard@astro.su.se

Received: 10 October 2005
Accepted: 17 January 2006

Abstract

Observations of quasars (QSOs) shining through or close to galaxies offer a way to probe the properties of the foreground matter through dust extinction and gravitational lensing. In this paper the feasibility of measuring the dust extinction properties is investigated using the backlitting of QSOs. We test our method to search for QSOs affected by intervening extinction, by matching the coordinates in the SDSS QSO DR3 catalogue with the New York University Value-Added Galaxy Catalog. In total, 164 QSO-galaxy pairs were found with a distance of less than 30 kpc between the galaxy centre and the QSO line-of-sight at the galaxy redshift. Investigating the QSO colours with multiband SDSS photometry, two pairs with galaxy redshifts were found to be particularly interesting in that the QSOs show evidence of heavy Galactic type extinction with at very large optical radii in the foreground spiral galaxies. With the available data, it remains inconclusive whether the two pairs can be explained as statistical colour outliers, by host extinction or if they provide evidence of dust in the outskirts of spiral galaxies. Deeper galaxy catalogues and/or higher resolution follow-up QSO spectra would help resolve this problem. We also analyse five QSOs reported in the literature with spectroscopic absorption features originating from an intervening system. These systems are at higher redshifts than the other two and we find in most cases significantly lower best fit values of R_V. The wide range of preferred values of R_V found, although affected by substantial uncertainties, already indicates that the dust properties in other galaxies may be different from the Milky Way. Furthermore, the available data suggest a possible evolution in the dust properties with redshift, with lower R_V at high z.