Physical properties of weak Mg II absorbers at z ∼ 2¹

We present the results of photoionization modeling of nine weak Mg 11 (Wr < 0.3 Â) quasar absorption-line systems with redshifts 1.4 < z < 2.4 obtained with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. These systems have been chosen because they provide access to a regime of redshift space that previous weak Mg n studies have not looked at. The densities, metallicities, Doppler parameters, and column densities of these systems are compared to those of other weak Mg n systems at lower redshift. There is no significant statistical variation in the properties ofthe absorbers over the redshift range 0.4 < z < 2.4. The number density per unit redshift is known to decrease for weak Mg n absorbers between z ∼ 1 and 2 by a greater amount than predicted from cosmological effects and changes in the extragalactic ionizing background alone. We suggest that, because the physical properties of the absorber population are not seen to change significantly across this range, the evolution in dN/dz is due to a decrease in the activity that gives rise to weak Mg o absorption, and not due to a change in the processes that form weak Mg n absorbers. The presence of separate, but aligned (in velocity) low- and high-density clouds in all single-cloud weak Mg ii absorbers provides an important diagnostic of their geometry. We discuss possible origins in dwarf galaxies and in extragalactic analogs to high-velocity clouds.