On the evolution of the size of Lyman alpha halos across cosmic time: no change in the circumgalactic gas distribution when probed by line emission

TL;DR

This study compares low-redshift Ly$ extalpha$ halos with high-redshift counterparts, finding similar flux fractions but slight differences in scale ratios, suggesting no significant evolution in circumgalactic gas distribution over cosmic time.

Contribution

It provides new observational data on Ly$ extalpha$ halos at low redshift and introduces a double exponential model to account for extended stellar UV emission.

Findings

Ly$ extalpha$ halo flux fractions are consistent across redshifts

Halo to UV scale length ratios are slightly lower at low redshift

Extended stellar UV emission does not significantly alter Ly$ extalpha$ halo measurements

Abstract

Lyman $\alpha$ (Ly$\alpha$) is now routinely used as a tool for studying high-redshift galaxies and its resonant nature means it can trace neutral hydrogen around star-forming galaxies. Integral field spectrograph measurements of high-redshift Ly$\alpha$ emitters indicate that significant extended Ly$\alpha$ halo emission is ubiquitous around such objects. We present a sample of redshift 0.23 to 0.31 galaxies observed with the Hubble Space Telescope selected to match the star formation properties of high-$z$ samples while optimizing the observations for detection of low surface brightness Ly$\alpha$ emission. The Ly$\alpha$ escape fractions range between 0.7\% and 37\%, and we detect extended Ly$\alpha$ emission around six out of seven targets. We find Ly$\alpha$ halo to UV scale length ratios around 6:1 which is marginally lower than high-redshift observations, and halo flux fractions between 60\% and 85\% -- consistent with high-redshift observations -- when using comparable methods. However, our targets show additional extended stellar UV emission: we parametrize this with a new double exponential model. We find that this parametrization does not strongly affect the observed Ly$\alpha$ halo fractions. We find that deeper H$\alpha$ data would be required to firmly determine the origin of Ly$\alpha$ halo emission, however, there are indications that H$\alpha$ is more extended than the central FUV profile, potentially indicating conditions favorable for the escape of ionizing radiation. We discuss our results in the context of high-redshift galaxies, cosmological simulations, evolutionary studies of the circumgalactic medium in emission, and the emission of ionizing radiation.