Diagnosing galactic feedback with the line broadening in the low redshift Lyman-alpha forest

TL;DR

This study compares low redshift Lyman-alpha forest observations with hydrodynamical simulations to diagnose galactic feedback effects, revealing that feedback processes influence line broadening and that the UV background intensity affects absorption features.

Contribution

It demonstrates that the Lyman-alpha forest line broadening distribution can serve as a diagnostic tool for galactic feedback mechanisms in the low redshift universe.

Findings

Simulations with strong feedback produce too hot or too cold gas for observed line widths.

Matching observed line widths requires feedback that broadens lines without over-ionizing hydrogen.

Observed column density distribution favors a higher UV background than previous models.

Abstract

We compare the low redshift (z ~ 0.1) Lyman-alpha forest from hydrodynamical simulations with data from the Cosmic Origin Spectrograph (COS). We find tension between the observed number of lines with b-parameters in the range 25-45 km/s and the predictions from simulations that incorporate either vigorous feedback from active galactic nuclei or that exclude feedback altogether. The gas in these simulations is, respectively, either too hot to contribute to the Lyman-alpha absorption or too cold to produce the required line widths. Matching the observed b-parameter distribution therefore requires feedback processes that thermally or turbulently broaden the absorption features without collisionally (over-)ionising hydrogen. This suggests the Lyman-alpha forest b-parameter distribution is a valulable diagnostic of galactic feedback in the low redshift Universe. We furthermore confirm the low redshift Lyman-alpha forest column density distribution is better reproduced by an ultraviolet background with an HI photo-ionisation rate a factor 1.5-3 higher than predicted by Haardt & Madau (2012).