2018MNRAS.478L..60V

The Lyman alpha (Ly α) line of Hydrogen is a prominent feature in the spectra of star-forming galaxies, usually redshifted by a few hundreds of km s^–1 compared to the systemic redshift. This large offset hampers follow-up surveys, galaxy pair statistics, and correlations with quasar absorption lines when only Ly α is available. We propose diagnostics that can be used to recover the systemic redshift directly from the properties of the Ly α line profile. We use spectroscopic observations of Ly α emitters for which a precise measurement of the systemic redshift is available. Our sample contains 13 sources detected between z ≃ 3 and z ≃ 6 as part of various multi-unit spectroscopic explorer guaranteed time observations. We also include a compilation of spectroscopic Ly α data from the literature spanning a wide redshift range (z ≃ 0-8). First, restricting our analysis to double-peaked Ly α spectra, we find a tight correlation between the velocity offset of the red peak with respect to the systemic redshift, V_ peak_^ red^, and the separation of the peaks. Secondly, we find a correlation between V_ peak_^ red^ and the full width at half-maximum of the Ly α line. Fitting formulas to estimate systemic redshifts of galaxies with an accuracy of <=100 km s^–1, when only the Ly α emission line is available, are given for the two methods.