The origin of the escape of Lyman α and ionizing photons in Lyman continuum emitters

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: s.r.n.gazagnes@rug.nl
² Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
³ University of California-Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
⁴ CNRS, IRAP, 14 avenue E. Belin, 31400 Toulouse, France
⁵ Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, 14-b Metrolohichna str., Kyiv 03143, Ukraine

Received: 5 April 2020
Accepted: 13 May 2020

Abstract

Context. Identifying the physical mechanisms driving the escape of Lyman continuum (LyC) photons is crucial for the search of Lyman continuum emitter (LCE) candidates.

Aims. To understand the physical properties involved in the leakage of LyC photons, we investigate the connection between the H I covering fraction, H I velocity width, the Lyman α (Lyα) properties, and the escape of LyC photons in a sample of 22 star-forming galaxies, which includes 13 confirmed LCEs.

Methods. We fit the stellar continuum, dust attenuation, and absorption lines between 920 Å and 1300 Å to extract the H I covering fractions and dust attenuation. Additionally, we measure the H I velocity widths of the optically thick Lyman series and derive the Lyα equivalent widths (EW), escape fractions (f_esc), peak velocities, and fluxes at the minimum of the observed Lyα profiles.

Results. Overall, we highlight strong observational correlations between the presence of low H I covering fractions and the observation of (1) low Lyα peak velocities; (2) more flux at the profile minimum; and (3) larger EW(Lyα), f_esc(Lyα), and f_esc^obs(LyC). Hence, low column density channels are crucial ISM ingredients for the leakage of Lyα and LyC photons. Additionally, galaxies with narrower H I absorption velocity widths have higher Lyα equivalent widths, larger Lyα escape fractions, and lower Lyα peak velocity separations. This may suggest that these galaxies have low H I column density. Finally, we find that dust also regulates the amount of Lyα and LyC radiation that actually escapes the ISM.

Conclusions. The ISM porosity is one of the origins of strong Lyα emission, enabling the escape of ionizing photons in low-z leakers. However, this is not sufficient to explain the largest f_esc^obs(LyC), which indicates that the most extreme LCEs are likely to be density-bounded along all lines of sight to the observer. Overall, the neutral gas porosity provides a constraint for a lower limit to the escape fraction of LyC and Lyα photons, which offers a key estimator for assessing the leakage of ionizing photons.