Super-solar Metallicity in Weak Mg Ii Absorption Systems at Z ∼ 1.7 Based on Archive Data of Observations Made with the Eso Telescopes at the La Silla or Paranal Observatories under Programs

Through photoionization modeling, constraints on the physical conditions of three z ∼ 1.7 singlecloud weak Mg II systems (Wr(2796) ≤ 0.3Å) are derived. Constraints are provided by high resolution R = 45,000, high signal-to-noise spectra of the three quasars HE 0141−3932, HE 0429−4091, and HE 2243−6031 which we have obtained from the European Southern Observatory (ESO) archive of Very Large Telescope (VLT), Ultraviolet and Visual Echelle Spectrograph (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 < 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 1 – 3 highionization phase C IV clouds within 100 km s of the Mg II component. The ionization parameters of the C IV phases range from −1.9 < logU < −1.0, with sizes of tens of parsecs to kiloparsecs. In all cases at least one C IV cloud is centered at the same velocity (within 5 km s in our systems) as the Mg II clouds. (3) Two of the three single-cloud weak Mg II absorbers have near-solar or super-solar 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. These strong constraints were possible because of the specific shapes of the Lyα profiles in these cases. All single-cloud weak Mg II absorbers may have high metallicities, but in some cases the kinematic spread of the C IV cloud contributions to Lyα do not allow such a determination. (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) Metallicity in single-cloud weak Mg II systems are more than an order of magnitude larger than those in Damped Lyα systems at z ∼ 1.7. In fact, there appears to be a gradual decrease in metallicity with increasing NH I, from these, the most metal-rich Lyα forest clouds, to Lyman limit systems, to sub-DLAs, and finally to the DLAs. Weak Mg II absorbers could be near local pockets in which star-formation has occurred, but where there is little gas to dilute the metals that are dispersed into the region, resulting in their very high metallicities. We speculate that DLAs may be subject to the opposite effect, where a large dilution of metals produced in the vicinity will occur, leading to a small metallicity. Subject headings: quasars: intergalactic medium – galaxies: abundances — absorption lines – quasars: individual (HE 0141−3932, HE 0429−4901, and HE 2243−6031)