2019A&A...623A.157S

Lyman-α (Lyα) is intrinsically the brightest line emitted from active galaxies. While it originates from many physical processes, for star-forming galaxies the intrinsic Lyα luminosity is a direct tracer of the Lyman-continuum (LyC) radiation produced by the most massive O- and early-type B-stars (M_*≥10M_☉) with lifetimes of a few Myrs. As such, Lyα 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 Lyα very hard to interpret due to the uncertain Lyα escape fraction, f_esc, Lyα_. Here we explore results from the CAlibrating LYMan-α with Hα (CALYMHA) survey at z=2.2, follow-up of Lyα emitters (LAEs) at z=2.2-2.6 and a z∼0-0.3 compilation of LAEs to directly measure f_esc, Lyα_ with Hα. We derive a simple empirical relation that robustly retrieves f_esc, Lyα_ as a function of Lyα rest-frame EW (EW₀): f_esc,Lyα=0.0048EW₀[Å]±0.05 and we show that it constrains a well-defined anti-correlation between ionisation efficiency (ξ_ion) and dust extinction in LAEs. Observed Lyα luminosities and EW₀ are easy measurable quantities at high redshift, thus making our relation a practical tool to estimate intrinsic Lyα and LyC luminosities under well controlled and simple assumptions. Our results allow observed Lyα luminosities to be used to compute SFRs for LAEs at z∼0-2.6 within ±0.2dex of the Hα dust corrected SFRs. We apply our empirical SFR(Lyα,EW₀) calibration to several sources at z≥2.6 to find that star-forming LAEs have SFRs typically ranging from 0.1 to 20M_☉/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 (log₁₀[ξ_ion/Hz/erg]=25.4-25.6) and low dust content in LAEs across cosmic time, and will be easily tested with future observations with JWST which can obtain Hα and Hβ measurements for high-redshift LAEs.