Physical conditions in three high-z H2-bearing DLAs: implications for grain size

TL;DR

This study uses numerical simulations to analyze the physical conditions of high-redshift H2-bearing DLAs, focusing on the impact of grain size distribution on observed molecular and atomic species.

Contribution

It introduces a self-consistent modeling approach using CLOUDY to constrain physical parameters and grain size effects in high-z DLAs based on observed column densities.

Findings

Both standard and smaller grain models fit observations well.

H2-bearing clouds have line-of-sight thickness of 1-6 pc.

Dust-to-gas ratio constraints are crucial for understanding grain sizes.

Abstract

We present results of our numerical simulation of three H2-bearing damped Lyman alpha absorbers (DLAs) in the redshift interval ~ 2-3. The systems we have modelled are the DLAs at zabs = 2.3377 towards the quasar LBQS 1232+0815, at zabs = 2.41837 towards SDSS J143912.04+111740.5 and at zabs = 2.6265 towards FBQS J081240.6+320808. We have used the spectral synthesis code CLOUDY to simulate the physical environment of these DLAs, and constrain the density, radiation field, geometry and dust-grain properties of the DLAs self-consistently based on the observed column densities of various atomic and molecular species such as H I, fine structure lines of C I and the rotational level population of H2. In our models, we explore the effect of grain size distribution on the predicted column densities of different species. Within the allowed uncertainties in the inferred dust-to-gas ratio, both models with standard ISM grains and smaller-sized grains reproduce the observations equally well. Improved constraints on dust-to-gas ratio and line-of-sight extinction are important for probing the grain size distribution in high-z DLAs. We find the H2-bearing clouds to have line-of-sight thickness in the range 1-6 pc, consistent with what has been found using partial coverage or 21-cm observations in some high-z DLAs.