Analysis of Low Z Absorbers in the Qso Spectra

Received ; accepted – 2 – ABSTRACT We present the results of reanalysis of low redshift Mg II absorption line sample compiled by Steidel and Sargent. We constructed grids of photoionization models for various cloud parameters. The conditions on cloud parameters to produce N(Fe II)≥N(Mg II) are obtained using single cloud curve of growth. Properties of Mg II absorbers with W(Fe II)/W(Mg II)(definded as R) ≥ 0.5 and with R<0.5 are analysed separately. Contrary to the whole Mg II sample, the clouds with R<0.5 show a steep increase in number density with redshift. These systems also show 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≥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 find a clear decrease in the ratio of W(Fe II 2383) and W(Mg II 2796) with redshift. The number density of Fe II lines selected absorbers are not evolving with redshift, consistent with Mg II results. We are also not finding any dependence of W(Fe II 2382) and the ratio of W(Fe II 2382) and W(Fe II 2600) with redshift. This implys no evolution for the average Fe II column density with redshift. Based on the available data of Lyman limit sytems (LLS) in the literature, we are not finding any dependence of optical depth (τ LLS) on redshift in the range z = 0.3-2.0. We collected the LLS information for 53 QSO sight lines, from the literature, for which details of Mg II absorption are available. There are 4 Mg II absorption systems which are not LLS at redshifts which are lower than the mean redshift of the sample (z ≃ 1.1). In the higher redshifts, where one expect to see 2.5±1.4 such absorbers, we do not find any nonLLS Mg II absorbers. Individual systems with τ LLS < 3.0 are analysed with an aim to constrain the ionization parameter and metallicity. Our results imply some of the absorbers – 3 – at z≃0.6 have reached metallicity roughly around solar value, indicating the chemical enrichment in some of the absorbers are similar to our Galaxy, as z∼ 0.6 is roughly the formation epoch of sun in our Galaxy. The required ionization parameters for these systems are …