Predicting Ly α escape fractions with a simple observable

Abstract

Lyman- α (Ly α ) is intrinsically the brightest line emitted from active galaxies. While it originates from many physical processes, for star-forming galaxies the intrinsic Ly α luminosity is a direct tracer of the Lyman-continuum (LyC) radiation produced by the most massive O- and early-type B-stars ( M ⋆ ≳ 10 M ⊙ ) with lifetimes of a few Myrs. As such, Ly α 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 Ly α very hard to interpret due to the uncertain Ly α escape fraction, f esc, Ly α . Here we explore results from the CAlibrating LYMan- α with H α (CALYMHA) survey at z = 2.2, follow-up of Ly α emitters (LAEs) at z = 2.2 − 2.6 and a z ∼ 0−0.3 compilation of LAEs to directly measure f esc, Ly α with H α . We derive a simple empirical relation that robustly retrieves f esc, Ly α as a function of Ly α rest-frame EW (EW 0 ): f esc,Ly α = 0.0048 EW 0 [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 Ly α luminosities and EW 0 are easy measurable quantities at high redshift, thus making our relation a practical tool to estimate intrinsic Ly α and LyC luminosities under well controlled and simple assumptions. Our results allow observed Ly α luminosities to be used to compute SFRs for LAEs at z ∼ 0−2.6 within ±0.2 dex of the H α dust corrected SFRs. We apply our empirical SFR(Ly α , EW 0 ) calibration to several sources at z ≥ 2.6 to find that star-forming LAEs have SFRs typically ranging from 0.1 to 20 M ⊙ yr −1 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 (log 10 [ ξ ion /Hz erg −1 ] = 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 H α and H β measurements for high-redshift LAEs.