The signatures of density fluctuations and mixing gas in circumgalactic absorption systems

TL;DR

This paper explores how density and temperature inhomogeneities in the circumgalactic medium affect absorption measurements, finding that such inhomogeneities are difficult to detect due to competing effects like He II self-shielding and uncertainties in cloud geometry.

Contribution

It introduces simplified models for density and temperature fluctuations in the CGM and assesses their impact on absorption measurements, providing empirical and theoretical estimates.

Findings

Models fit observations equally well with or without thermal inhomogeneities.

Detectable effects of inhomogeneities occur only at parameter boundaries.

He II self-shielding significantly influences ion fractions, complicating inhomogeneity detection.

Abstract

We investigate the prospects for detecting and constraining density and temperature inhomogeneities in the circumgalactic medium (CGM) using absorption measurements of metal ions. Distributions in the gas thermal properties could arise from turbulence, gas cooling from the hot phase, and mixing between the cool and hot phases. Focusing on these physically motivated models, we parameterize each with a single parameter for simplicity and provide empirical and theoretical estimates for reasonable parameter values. We then construct the probability distribution functions for each of these scenarios, calculate the effective ion fractions, and fit our models to the COS-Halos absorption measurements to infer the gas densities and metallicities. We find that the models we consider (i) produce similarly good fits to the observations with or without distributions in the gas thermal properties, and (ii) result in detectable changes in the column densities only at the boundaries of reasonable parameter values. We show that He II self-shielding can have a larger effect on the ion fractions than density and temperature fluctuations. As a result, uncertainties in cloud geometry and their spatial distribution, affecting the details of radiation transfer, may obscure the effect of inhomogeneities.