Analysis of low-redshift absorbers in quasar spectra

We present the results of a reanalysis of the low-redshift Mg Ir absorption-line sample compiled by Steidel & Sargent. We have constructed grids 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. Properties of Mg II absorbers with [W(Fe II)/W(Mg II) = R] greater than or equal to 0.5 and R < 0.5 are analyzed separately. Contrary to results for the whole Mg II sample, the clouds with R < 0.5 show a steep increase in number density with redshift. These systems also show a clear increase in W(Mg II) and doublet ratio of Mg ii with redshift. However, there 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 anticorrelation between doublet ratio of Mg II and W(Mg II). We rind a clear decrease in the ratio of W(Fe II lambda 2382) to W(Mg II lambda 2796) with redshift. The number density of Fe II line-selected absorbers does not 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 absence of evolution of the average Fe Ir column density with redshift. Based on the available data for Lyman-limit systems (LLSs) in the literature, we do not find any dependence of optical depth (tau(LLS)) on redshift 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 absorption are available. There are four Mg II absorption systems that are not LLSs at redshifts lower than the mean redshift of the sample (z similar or equal to 1.1). At the higher redshifts, where one would expect 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 an aim to constrain the ionization parameter and metallicity. Our results imply that some of the absorbers at z similar or equal to 0.6 have reached metallicity roughly around solar value, indicating that the chemical enrichment in some of the absorbers is similar to that in our Galaxy, as z similar to 0.6 is roughly the formation epoch of the Sun. The required ionization parameters for these systems are less than 0.001 in most cases. Comparison of our results with results obtained for intermediate- and high-redshift absorbers confirms that the mean ionization state of metal-rich absorbing clouds falls with redshift.