2007ApJ...666...64L

We present the results of photoionization modeling of nine weak Mg II (W_r<0.3 Å) 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 II studies have not looked at. The densities, metallicities, Doppler parameters, and column densities of these systems are compared to those of other weak Mg II systems at lower redshift. There is no significant statistical variation in the properties of the absorbers over the redshift range 0.4<z<2.4. The number density per unit redshift is known to decrease for weak Mg II 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 II absorption, and not due to a change in the processes that form weak Mg II 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.