Charting the Lyman- α escape fraction in the range 2.9 < z < 6.7 and consequences for the LAE reionisation contribution

• Published in: Astronomy and astrophysics (Berlin) Vol. 690; p. A302
• Main Authors: Goovaerts, I., Thai, T. T., Pello, R., Tuan-Anh, P., Laporte, N., Matthee, J., Nanayakkara, T., Pharo, J.
• Format: Journal Article
• Language: English
• Published: EDP Sciences 01.10.2024
• Subjects: Astrophysics; Physics; galaxies: formation; galaxies: starburst; galaxies: luminosity function; mass function; galaxies: evolution
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202451432

Context. The escape of Lyman- α photons at redshifts greater than two is an ongoing subject of study and an important quantity to further understanding of Lyman- α emitters (LAEs), the transmission of Lyman- α photons through the interstellar medium and intergalactic medium, and the impact these LAEs have on cosmic reionisation. Aims. This study aims to assess the Lyman- α escape fraction, f esc, Ly α , over the redshift range 2.9 <  z  < 6.7, focusing on Very Large Telescope/Multi Unit Spectroscopic Explorer (VLT/MUSE) selected, gravitationally lensed, intrinsically faint LAEs. These galaxies are of particular interest as the potential drivers of cosmic reionisation. Methods. We assessed f esc, Ly α in two ways: through an individual study of 96 LAEs behind the A2744 lensing cluster, with James Webb Space Telescope/Near-Infrared Camera (JWST/NIRCam) and HST data, and through a study of the global evolution of f esc, Ly α using the state-of-the-art luminosity functions for LAEs and the UV-selected ‘parent’ population (dust-corrected). We compared these studies to those in the literature based on brighter samples. Results. We find a negligible redshift evolution of f esc, Ly α for our individual galaxies; it is likely that it was washed out by significant intrinsic scatter. We observed a more significant evolution towards higher escape fractions with decreasing UV magnitude and fit this relation. When comparing the two luminosity functions to derive f esc, Ly α in a global sense, we saw agreement with previous literature when integrating the luminosity functions to a bright limit. However, when integrating using a faint limit equivalent to the observational limits of our samples, we observed enhanced values of f esc, Ly α , particularly around z  ∼ 6, where f esc, Ly α becomes consistent with 100% escape. This indicates for the faint regimes we sampled that galaxies towards reionisation tend to allow very large fractions of Lyman- α photons to escape. We interpret this as evidence of a lack of any significant dust in these populations; our sample is likely dominated by young, highly star-forming chemically unevolved galaxies. Finally, we assessed the contribution of the LAE population to reionisation using our latest values for f esc, Ly α and the LAE luminosity density. The dependence on the escape fraction of Lyman continuum photons is strong, but for values similar to those observed recently in z  ∼ 3 LAEs and high-redshift analogues, LAEs could provide all the ionising emissivity necessary for reionisation.