2018/03/31 by David Sobral, Jorryt Matthee · 1 citation
Physics and Astronomy · #astro-ph.GA #astro-ph.CO
paper · pdf · doi:10.1051/0004-6361/201833075
published as A&A 623, A157 (2019) · Accepted for publication in A&A. 13 pages, 5 figures. Toy model and example output available at: https://goo.gl/391wBz
arxiv created 2019/02/04 · arxiv updated 2019/03/27
Lyman-alpha (Lya) is intrinsically the brightest line emitted from active galaxies. While it originates from many physical processes, for star-forming galaxies the intrinsic Lya luminosity is a direct tracer of the Lyman-continuum (LyC) radiation produced by the most massive O- and early-type B-stars with lifetimes of a few Myrs. As such, Lya luminosity should be an excellent instantaneous star formation rate (SFR) indicator. However, its resonant nature and susceptibility to dust as a rest-frame UV photon makes Lya very hard to interpret due to the uncertain Lya escape fraction, fesc,Lyα. Here we explore results from the CALYMHA survey at z=2.2, follow-up of Lya emitters (LAEs) at z=2.2-2.6 and a z~0-0.3 compilation of LAEs to directly measure fesc,Lyα with H-alpha (Ha). We derive a simple empirical relation that robustly retrieves fesc,Lyα as a function of Lya rest-frame EW (EW0): fesc,Lyα= 0.0048 EW0[A]±0.05 and we show that it constrains a well-defined anti-correlation between ionisation efficiency and dust extinction in LAEs. Observed Lya luminosities and EW0 are easy measurable quantities at high redshift, thus making our relation a practical tool to estimate intrinsic Lya and LyC luminosities under well controlled and simple assumptions. Our results allow observed Lya luminosities to be used to compute SFRs for LAEs at z~0-2.6 within 0.2 dex of the Ha dust corrected SFRs. We apply our empirical SFR(Lya,EW0) calibration to several sources at z>2.6 to find that star-forming LAEs have SFRs typically ranging from 0.1 to 20 M\odot/yr and that our calibration might be even applicable for the most luminous LAEs within the epoch of re-ionisation. Our results imply high ionisation efficiencies and low dust content in LAEs across cosmic time, and will be easily tested with future observations with JWST.