The Physical Conditions of Intermediate-Redshift M[CLC]g[/CLC] [CSC]ii[/CSC] Absorbing Clouds from Voigt Profile Analysis

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Abstract
We present a detailed and statistical analysis of the column densities and Doppler b parameters of Mg II absorbing clouds at redshifts 0.4 ≤ z ≤ 1.2. We draw on the HIRES/Keck data (Δv 6.6 km s-1) and Voigt profile (VP) fitting results presented by Churchill & Vogt (Paper I). The sample comprises 175 clouds from 23 systems along 18 quasar lines of sight. In order to better understand whether the inferred physical conditions in the absorbing clouds could be "false" conditions, which can arise due to the nonuniqueness inherent in parameterizing complex absorption profiles, we performed extensive simulations of the VP analyses presented in this paper. In brief, we find the following: (1) The Fe II and Mg II column densities are correlated at the 9 σ level. There is a 5 σ anticorrelation between the Mg I/Mg II column density ratio and the Mg II column density. (2) Power-law fits to the column density distributions for Mg II, Fe II, and Mg I yielded power-law slopes of approximately -1.6, -1.7, and -2.0, respectively. (3) The observed peaks of the Doppler parameter distributions were ~5 km s-1 for Mg II and Fe II and ~7 km s-1 for Mg I. The clouds are consistent with being thermally broadened, with temperatures in the 30,000–40,000 K range. (4) A two-component Gaussian model to the velocity two-point correlation function yielded velocity dispersions of 54 and 166 km s-1. The narrow component has roughly twice the amplitude of the broader component. The width and amplitude of the broader component decreases as equivalent width increases. (5) From photoionization models we find that the column density ratios are most consistent with being photoionized by the ultraviolet extragalactic ionizing background, as opposed to stellar radiation. Based on the Mg I to Mg II column density ratios, it appears that at least two-phase ionization models are required to explain the data.
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