Dust in damped lyman‐alpha systems and the CIBR

Dust has been detected in damped Lyα systems by two independent methods: the reddening of background quasars and the gas‐phase depletion of Cr relative to Zn. Both methods give a typical dust‐to‐gas ratio at z≊2.2 roughly 10% of that in the present‐day solar neighborhood. In this talk, I will review some of the consequences of these observations. First, dust causes obscuration and hence incompleteness in optically selected samples of quasars. The ‘‘missing’’ fraction at z=3 is estimated to be 10–70%. Corrections for this effect help to reconcile the ionizing UV background inferred from the proximity effect at high redshifts with that expected from quasars alone. Second, since the samples of damped Lyα systems were obtained primarily from optically selected samples of quasars, they too must be incomplete at some level. This effect biases the statistics of the damped Lyα systems, such as the distribution of HI column densities f(N,z), and especially, the comoving density of gas Ωg(z). The bias has important...