Aims. To study chemical composition of metal-rich absorbers at high redshifts in order to understand their nature and sources of the metal enrichment.
Methods. From seven spectra of high-z QSOs, we select thirteen metal-rich, Z ≥ Z_solar, and optically-thin, N(HI) < 10¹⁷ cm^-2, absorption systems ranging between z=1.5 and z=2.9 and revealing lines of different ions in subsequent ionization stages. Computations are performed using the Monte Carlo Inversion (MCI) procedure complemented with the adjustment of the spectral shape of the ionizing radiation. This procedure along with selection criteria guarantee the accuracy of the ionization corrections and of the derived element abundances (C, N, O, Mg, Al, Si, Fe).
Results. The majority of the systems (11 from 13) show abundance patterns which relate them to outflows from low and intermediate mass stars. One absorber is enriched prevalently by SNe II, however, a low percentage of such systems in our sample is conditioned by the selection criteria. All systems have linear sizes along the line-of-sight of ∼10 pc. In several systems, silicon is deficient, presumably due to the depletion onto dust grains in the envelopes of dust-forming stars and the subsequent gas-dust separation. At any value of [C/H], nitrogen can be either deficient, [N/C] < 0, or enhanced, [N/C] > 0 which supposes that the nitrogen enrichment occurs irregularly. The lines of MgII 2796, 2803 appear to be shifted, probably as a result of an enhanced content of heavy isotopes ²⁵Mg and ²⁶Mg in the absorbing gas relative to the solar isotopic composition. The distribution of low-ionization metal-rich systems over z shows a pronounced increase of their number density at z∼1.8. This result is not affected by selection criteria and indicates a real change in the physical conditions inside the gas-expelling galaxies. Comparing the space number density of metal-rich absorbers with the comoving number density of star-forming galaxies at z∼2, we estimate that the circumgalactic volume of each galaxy is populated by ∼10⁷-10⁸ such absorbers. Possible effects of high metal content on the peak values of star-forming and AGN activities at z∼2 are discussed.

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