Investigating the Lyman photon escape in local starburst galaxies with the Cosmic Origins Spectrograph

TL;DR

This study examines low-redshift starburst galaxies using HST and VLT data to analyze Lyman-alpha profiles and escape fractions, providing insights into their ionizing photon escape mechanisms.

Contribution

It presents the first combined analysis of Lyman-alpha profiles and escape fractions in a low-redshift galaxy sample using COS and VLT observations.

Findings

Identified three galaxies with double peak Lyman-alpha profiles.

Measured Lyman-alpha escape fractions between 5-13%.

Set upper limits on Lyman continuum escape fractions.

Abstract

We present a study of 7 star-forming galaxies from the Cosmic Evolution Survey (COSMOS) observed with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). The galaxies are located at relatively low redshifts, $z\sim$0.3, with morphologies ranging from extended and disturbed to compact and smooth. To complement the HST observations we also analyze observations taken with the VIMOS spectrograph on the Very Large Telescope (VLT). In our galaxy sample we identify three objects with double peak Lyman-$\alpha$ profiles similar to those seen in Green Pea compact galaxies and measure peak separations of 655, 374, and 275 km s$^{-1}$. We measure Lyman-$\alpha$ escape fractions with values ranging between 5-13\%. Given the low flux levels in the individual COS exposures we apply a weighted stacking approach to obtain a single spectrum. From this COS combined spectrum we infer upper limits for the absolute and relative Lyman continuum escape fractions of $f_{\rm abs}(\rm LyC)$ = 0.4$^{+10.1}_{-0.4}$\% and $f_{\rm res}(\rm LyC)$ = 1.7$^{+15.2}_{-1.7}$\%, respectively. Finally, we find that most of these galaxies have moderate UV and optical SFRs (SFRs $\lesssim$ 10 M$_{\odot}$ yr$^{-1}$).