2010ApJ...711..380B

We present a catalog of Mg II absorption systems obtained from high-resolution Ultraviolet and Visual Echelle Spectrograph/VLT data of 77 quasi-stellar objects in the redshift range 0.6 < z < 2.0, and down to an equivalent width W₀≥ 0.1 Å. The statistical properties of our sample are found to be in agreement with those from the previous work in the literature. However, we point out that the previously observed increase with redshift of ∂N/∂z for weak absorbers pertains exclusively to very weak absorbers with W₀ < 0.1 Å. Instead, ∂N/∂z for absorbers with W₀ in the range 0.1-0.3 Å actually decreases with redshift, similar to the case of strong absorbers. We then use this catalog to extend our earlier analysis of the links between the Faraday rotation measure (RM) of the quasars and the presence of intervening Mg II absorbing systems in their spectra. In contrast to the case with strong Mg II absorption systems (W₀> 0.3 Å), the weaker systems do not contribute significantly to the observed RM of the background quasars. This is possibly due to the higher impact parameters of the weak systems compared to strong ones, suggesting that the high column density magnetized material that is responsible for the Faraday rotation is located within about 50 kpc of the galaxies. Finally, we show that this result also rules out the possibility that some unexpected secondary correlation between the quasar redshift and its intrinsic RM is responsible for the association of high RM and strong intervening Mg II absorption that we have presented elsewhere, since this would have produced an equal effect for the weak absorption line systems, which exhibit a very similar distribution of quasar redshifts.