Supersolar metallicity in weak Mg II absorption systems at z ∼ 1.7

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. Results 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 < log U ≲ -2.0 and sizes of 1-100 pc. (2) In addition to the low-ionization phase Mg n clouds, all systems need an additional one to three high-ionization phase C iv clouds within 100 km s^-1 of the Mg II component. The ionization parameters of the C IV clouds range from -1.9 < log U < -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 forz < 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 _{H I}, from these, the most metal-rich Lyα forest clouds, to Lyman limit systems, to sub-DLAs, and finally to the DLAs.