Dust-reddening and gravitational lensing of SDSS QSOs due to foreground damped Lyman α systems

We use Sloan Digital Sky Survey (SDSS) Data Release 2 quasi-stellar object (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 zsimilar to3. Our limit on the shift in spectral index, Deltaalpha<0.19 (3σ), corresponds to a limit on the colour excess due to Small Magellanic Cloud-like dust-reddening, E(B-V)<0.02 mag (3). This is inconsistent with the early studies of Fall, Pei and collaborators who used the small QSO and DLA samples available at the time. Comparison of the DLA and control magnitude distributions also reveals greater than or similar to 2sigma 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.