Effect of a partial coverage of quasar broad-line regions by intervening H$_2$-bearing clouds

TL;DR

This paper investigates how partial coverage of quasar broad-line regions by intervening H$_2$ clouds affects absorption line observations, potentially biasing derived physical parameters, and estimates the likelihood of this effect in quasar spectra.

Contribution

It introduces a method to estimate the probability of partial coverage effects in quasar spectra and assesses its impact on the analysis of H$_2$ absorption systems.

Findings

Probability of partial coverage detection is at least 11%.

About 20% of observed H$_2$ systems may be partially covered.

Accounting for this effect can significantly revise cloud parameter estimates.

Abstract

We consider the effect of a partial coverage of quasar broad-line regions (QSO BLRs) by intervening H$_2$-bearing clouds when a part of quasar (QSO) radiation passes by a cloud not taking part in formation of an absorption-line system in the QSO spectrum. That leads to modification of observable absorption line profiles and consequently to a bias in physical parameters derived from standard absorption line analysis. In application to the H$_2$ {absorption} systems the effect has been revealed in the analysis of H$_2$ absorption system in the spectrum of Q~1232+082 (Ivanchik et al. 2010, Balashev et al. 2011). We estimate a probability of the effect to be detected in QSO spectra. To do this we derive distribution of BLR sizes of high-z QSOs from Sloan Digital Sky Survey (SDSS) Data Release 9 (DR9) catalogue and assume different distributions of cloud sizes. We conclude that the low limit of the probability is about $11\%$. The latest researches shows that about a fifth of observed H$_2$ absorption systems can be partially covered. Accounting of the effect may allow to revise significantly physical parameters of interstellar clouds obtained by the spectral analysis.