Dusty MgII Absorbers: Implications for the GRB/Quasar Incidence Discrepancy

There is nearly a factor of four difference in the number density of intervening MgII absorbers as determined from gamma-ray burst (GRB) and quasar lines of sight. We use a Monte-Carlo simulation to test if a dust extinction bias can account for this discrepancy. We apply an empirically determined relationship between dust column density and MgII rest equivalent width to simulated quasar sight-lines and model the underlying number of quasars that must be present to explain the published magnitude distribution of SDSS quasars. We find that an input MgII number density dn/dz of 0.273 +- 0.002 over the range 0.4 <= z <= 2.0 and with MgII equivalent width W_0 >= 1.0 angstroms accurately reproduces observed distributions. From this value, we conclude that a dust obstruction bias cannot be the sole cause of the observed discrepancy between GRB and quasar sight-lines: this bias is likely to reduce the discrepancy only by ~10%.