The Lyα-LyC Connection: Evidence for an Enhanced Contribution of UV-faint Galaxies to Cosmic Reionization
Abstract
The escape of ionizing Lyman continuum (LyC) photons requires the existence of low-N H I sightlines, which also promote escape of Lyα. We use a suite of 2500 Lyα Monte-Carlo radiative transfer simulations through models of dusty, clumpy interstellar (“multiphase”) media from Gronke & Dijkstra, and compare the escape fractions of Lyα ({f}{{esc}}{{Ly}α }) and LyC radiation ({f}{{esc}}{{ion}}). We find that {f}{{esc}}{{ion}} and {f}{{esc}}{{Ly}α } are correlated: galaxies with a low {f}{{esc}}{{Ly}α } consistently have a low {f}{{esc}}{{ion}}, while galaxies with a high {f}{{esc}}{{Ly}α } exhibit a large dispersion in {f}{{esc}}{{ion}}. We argue that there is increasing observational evidence that Lyα escapes more easily from UV-faint galaxies. The correlation between {f}{{esc}}{{ion}} and {f}{{esc}}{{Ly}α } then implies that UV-faint galaxies contribute more to the ionizing background than implied by the faint-end slope of the UV luminosity function. In multiphase gases, the ionizing escape fraction is most strongly affected by the cloud covering factor, f cl, which implies that {f}{{esc}}{{ion}} is closely connected to the observed Lyα spectral line shape. Specifically, LyC-emitting galaxies typically having narrower, more symmetric line profiles. This prediction is qualitatively similar to that for “shell models.”
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 2016
- DOI:
- 10.3847/0004-637X/828/2/71
- arXiv:
- arXiv:1604.08208
- Bibcode:
- 2016ApJ...828...71D
- Keywords:
-
- dark ages;
- reionization;
- first stars;
- galaxies: high-redshift;
- intergalactic medium;
- line: profiles;
- radiative transfer;
- ultraviolet: galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 11 pages, 5 figures, ApJ in press. Added section on delayed escape of Lya, clarified model description and motivation. Added references