2008ApJ...679..220M

Through photoionization modeling, constraints on the physical conditions of three z∼1.7 single-cloud weak Mg II systems [W_r(2796)≤0.3 Å] are derived. Constraints are provided by high-resolution R=45,000, high-S/N spectra of the three quasars HE 0141-3932, HE 0429-4091, and HE 2243-6031, which we have obtained from the ESO archive of VLT UVES.
Result. are as follows: (1) The single-cloud weak Mg II absorption in the three z∼1.7 systems is produced by clouds with ionization parameters of -3.8<logU≲-2.0 and sizes of 1-100 pc. (2) In addition to the low-ionization phase Mg II clouds, all systems need an additional one to three high-ionization phase C IV clouds within 100 km/s of the Mg II component. The ionization parameters of the C IV clouds range from -1.9<logU←1.0, with sizes of tens of parsecs to kiloparsecs. (3) Two of the three single-cloud weak Mg II absorbers have near-solar or supersolar metallicities, if we assume a solar abundance pattern. Although such large metallicities have been found for z<1 weak Mg II absorbers, these are the first high metallicities derived for such systems at higher redshifts. (4) Two of the three weak Mg II systems also need additional low-metallicity, broad Lyα absorption lines, offset in velocity from the metal-line absorption, in order to reproduce the full Lyα profile. (5) Metallicities in single-cloud weak Mg II systems are more than an order of magnitude larger than those in DLA systems at z∼1.7. In fact, there appears to be a gradual decrease in metallicity with increasing N_HI_, from these, the most metal-rich Lyα forest clouds, to Lyman limit systems, to sub-DLAs, and finally to the DLAs.