Dust-reddening and gravitational lensing of SDSS QSOs due to foreground damped Lymanαsystems

We use Sloan Digital Sky Survey Data Release 2 QSO spectra to constrain the dust-reddening caused by intervening damped Lyman-alpha systems (DLAs). Comparing the spectral index distribution of a 70 sight-line DLA sample with that of a large control sample reveals no evidence for dust-reddening at z~3. Our limit on the shift in spectral index, |Delta(alpha)| < 0.19 (3-sigma), corresponds to a limit on the colour excess due to SMC-like dust-reddening, E(B-V) < 0.02 mag (3-sigma). This is inconsistent with the early studies of Fall, Pei and collaborators who used the small QSO and DLA samples available. Comparison of the DLA and control magnitude distributions also reveals >2-sigma evidence for an excess of bright and/or a deficit of faint QSOs with foreground DLAs. Higher equivalent width DLAs give a stronger signal. We interpret this as the signature of gravitational magnification due to the intervening DLAs.
6 pages, 6 figures (8 EPS files), 2 tables. Accepted by MNRAS Letters. v2: S/N threshold for fiducial DLA sample somewhat higher and so limits on dust-reddening slightly weaker. Now includes discussion of DLA incompleteness at low N(HI). Qualitative results and conclusions unchanged. Added table of QSOs and DLAs; complete table available at http://www.ast.cam.ac.uk/~mim/pub.html