EDP Sciences
Free access
Volume 485, Number 2, July II 2008
Page(s) 403 - 415
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20079187
Published online 19 March 2008

A&A 485, 403-415 (2008)
DOI: 10.1051/0004-6361:20079187

Extinction properties of lensing galaxies

L. Östman1, A. Goobar1, and E. Mörtsell2

1  Department of Physics, Stockholm University, 10691 Stockholm, Sweden
    e-mail: [linda;ariel]@physto.se
2  Department of Astronomy, Stockholm University, 10691 Stockholm, Sweden
    e-mail: edvard@astro.su.se

Received 4 December 2007 / Accepted 24 January 2008

Context. Observations of quasars shining through foreground galaxies, offer a way to probe the dust extinction curves of distant galaxies. Interesting objects for this study are found in strong gravitational lensing systems, where the foreground galaxies generate multiple images.
Aims. The reddening law of lensing galaxies is investigated by studying the colours of gravitationally-lensed quasars, and a handful of other quasars where a foreground galaxy is detected.
Methods. We compare the observed colours of quasars reported in the literature, with spectral templates reddened by different extinction laws and dust properties. The data consists of 21 quasar-galaxy systems, with a total of 48 images. The galaxies, which are both early- and late-type, have redshifts in the interval z = 0.04-1.51.
Results. We measure a difference in rest-frame B-V between the quasar images we study, and quasars without resolved foreground galaxies. This difference in colour is indicative of significant dust extinction in the intervening galaxy. Good fits to standard extinction laws were found for 22 of the images, corresponding to 13 different galaxies. Our fits imply a wide range of possible values for the total-to-selective extinction ratio, RV. The distribution was found to be broad with a weighted mode of $\bar R_V$ = 2.4 and a FWHM of $\Delta{R{_V}}$ = 2.7 ($\sigma_{R_V}$ ~ 1.1). Thus the bulk of the galaxies for which good reddening fits could be derived, have dust properties compatible with the Milky Way value (RV=3.1).

Key words: ISM: dust, extinction -- galaxies: ISM -- galaxies: evolution -- galaxies: quasars: general -- techniques: photometric

© ESO 2008