Lensing, reddening and extinction effects of MgII absorbers from z=0.4 to 2

Using a sample of almost 7000 strong MgII absorbers with W-0 > 1 angstrom and 0.4 < z < 2.2 detected in the SDSS DR4 data set, we investigate the gravitational lensing and dust extinction effects they induce on background quasars. After carefully quantifying several selection biases, we isolate the reddening effects as a function of redshift and absorber rest equivalent width, W0. We find the amount of dust to increase with cosmic time as tau (z) proportional to (1+ z)(-1.1 +/- 0.4), following the evolution of cosmic star density or integrated star formation rate. We measure the reddening effects over a factor of 30 in E( B - V) and we find that tau proportional to (W-0)(1.9 +/- 0.1), providing us with an important scaling for theoretical modelling of metal absorbers. We also measure the dust-to-metal ratio and find it similar to that of the Milky Way. In contrast to previous studies, we do not detect any gravitational magnification by MgII systems. We measure the upper limit mu < 1.10 and discuss the origin of the discrepancy. Finally, we estimate the fraction of absorbers missed due to extinction effects and show that it rises from 1 to 50 per cent in the range 1 < W-0 < 6 angstrom. We parametrize this effect and provide a correction for recovering the intrinsic partial derivative N/partial derivative W-0 distribution.