Dust Biasing of Damped Lyman Alpha Systems: a Bayesian Analysis

TL;DR

This paper uses Bayesian analysis to quantify how dust obscuration biases the detection of damped Lyman alpha systems, showing that the bias is smaller than previously claimed and affects measurements of cosmic metal density.

Contribution

It provides a Bayesian framework combining radio and optical data to constrain dust bias in DLA surveys, clarifying the extent of missing systems and metallicity underestimation.

Findings

Only about 7% of DLAs are missed due to dust obscuration.

Optical metallicity measurements underestimate true values by 0.1 dex.

Cosmic metal density in DLAs is underestimated by roughly a factor of two.

Abstract

If damped Lyman alpha systems (DLAs) contain even modest amounts of dust, the ultraviolet luminosity of the background quasar can be severely diminished. When the spectrum is redshifted, this leads to a bias in optical surveys for DLAs. Previous estimates of the magnitude of this effect are in some tension; in particular, the distribution of DLAs in the column-density -- metallicity plane has led to claims that we may be missing a considerable fraction of metal rich, high column density DLAs, whereas radio surveys do not unveil a substantial population of otherwise hidden systems. Motivated by this tension, we perform a Bayesian parameter estimation analysis of a simple dust obscuration model. We include radio and optical observations of DLAs in our overall likelihood analysis and show that these do not, in fact, constitute conflicting constraints. Our model gives statistical limits on the biasing effects of dust, predicting that only 7% of DLAs are missing from optical samples due to dust obscuration; at 2 sigma confidence, this figure takes a maximum value of 17%. This contrasts with recent claims that DLA incidence rates are underestimated by 30 -- 50%. Optical measures of the mean metallicities of DLAs are found to underestimate the true value by just 0.1 dex (or at most 0.4 dex, 2 sigma confidence limit), in agreement with the radio survey results of Akerman et al. The quantity most affected by dust biasing is the total cosmic density of metals in DLAs, Omega_{Z,DLA}, which is underestimated in optical surveys by a factor of approximately two. (Abridged.)