ANALYSIS OF LOW-REDSHIFT ABSORBERS IN QUASAR SPECTRA

We present the results of a reanalysis of the low-redshift Mg Ir absor ption-line sample compiled by Steidel & Sargent. We have constructed g rids of photoionization models for various cloud parameters and obtain the conditions on the parameters to produce N(Fe II) greater than or equal to N(Mg II) using single-cloud curve-of-growth analysis. Propert ies of Mg II absorbers with [W(Fe II)/W(Mg II) = R] greater than or eq ual to 0.5 and R < 0.5 are analyzed separately. Contrary to results fo r the whole Mg II sample, the clouds with R < 0.5 show a steep increas e in number density with redshift. These systems also show a clear inc rease in W(Mg II) and doublet ratio of Mg ii with redshift. However, t here is no correlation between W(Mg II) and doublet ratio. In the case of R greater than or equal to 0.5 clouds, W(Mg II) and doublet ratio are not correlated with redshift. However, there is a clear anticorrel ation between doublet ratio of Mg II and W(Mg II). We rind a clear dec rease in the ratio of W(Fe II lambda 2382) to W(Mg II lambda 2796) wit h redshift. The number density of Fe II line-selected absorbers does n ot evolve with redshift, consistent with Mg Ir results. We also do not find any dependence of W(Fe II lambda 2382) and the ratio of W(Fe II lambda 22382) to W(Fe II lambda 2600) on redshift. This implies an abs ence of evolution of the average Fe Ir column density with redshift. B ased on the available data for Lyman-limit systems (LLSs) in the liter ature, we do not find any dependence of optical depth (tau(LLS)) on re dshift in the range z = 0.3-2.0. We collected the LLS information from the literature for 53 QSO sight lines for which details of Mg II abso rption are available. There are four Mg II absorption systems that are not LLSs at redshifts lower than the mean redshift of the sample (z s imilar or equal to 1.1). At the higher redshifts, where one would expe ct to see 2.5 +/- 1.4 such absorbers, we do not find any non-LLS Mg II absorbers. Individual systems with tau(LLS) < 3.0 are analyzed with a n aim to constrain the ionization parameter and metallicity. Our resul ts imply that some of the absorbers at z similar or equal to 0.6 have reached metallicity roughly around solar value, indicating that the ch emical enrichment in some of the absorbers is similar to that in our G alaxy, as z similar to 0.6 is roughly the formation epoch of the Sun. The required ionization parameters for these systems are less than 0.0 01 in most cases. Comparison of our results with results obtained for intermediate- and high-redshift absorbers confirms that the mean ioniz ation state of metal-rich absorbing clouds falls with redshift.