Using weak MgII lines to chart Low Surface Brightness Galaxies

We report the detection, based on HST and Keck data, of two peculiar absorbers in the Ly-alpha forest of the quasar PKS 0454+039. These clouds, at redshifts z=0.6248 and 0.9315 respectively, display both MgII and FeII absorption lines in addition to the Ly-alpha line. Based upon photoionization models, these are inferred to be photoionized by the intergalactic UV background, and to have HI column densities in the range 15.8 < N(HI) < 16.8. Furthermore, if one supposes that the relative abundances of heavy elements is similar to that of depleted clouds of our galaxy, the abundances of these two absorbers are greater than the solar value, which is a unique case for absorbers which are not associated to the quasar. We tentatively suggest that these absorbers may select giant low surface brightness galaxies.