Analysis of Low z Absorbers in the QSO Spectra

Abstract

We present the results of reanalysis of low z Mg II absorption line sample compiled by SS92. The conditions on cloud parameters to produce N(Fe II)>N(Mg II) are obtained using photoionization models and curve of growth. Properties of Mg II absorbers with W(Fe II)/W(MgII) (definded as R) >and< 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 (N(z)) with z. These systems also show clear increase in W(Mg II) and doublet ratio(dr) of Mg II with z. However there is no correlation between W(Mg II) and dr. In the case of R>0.5 clouds W(Mg II) and dr are not correlated with z. However there is a clear anticorrelation between dr and W(Mg II). We find a clear decrease in the ratio of W(Fe II) and W(Mg II) with z. The N(z) as well as average N(Fe II) of Fe II lines selected absorbers are not evolving with z. The available data of LLS, are not showing any dependence of (tau_{LLS}) on z. We collected the LLS information for 53 QSO sight lines, for which details of Mg II absorption are available. There are 4 Mg II absorption systems which are not LLS at z which are lower than the mean z of the sample (z=1.1). In the higher z, where one expect to see 2.5\pm1.4 such absorbers, we do not find any nonLLS Mg II absorbers. Individual systems with tau_{LLS}<3.0 are analysed. Our results imply some of the absorbers at z=0.6 have reached metallicity roughly around solar value. The required ionization parameters for these systems are less than 0.001 in most cases. Our results together with results obtained for high z absorbers confirm that mean ionization state of metal rich absorbing clouds falls with z.