Star formation indicators and line equivalent width in Lyα galaxies

The equivalent width (EW) of the Ly alpha line is directly related to the ratio of star formation rates (SFR) determined from Ly alpha flux and UV flux density [SFR(Ly alpha)/SFR(UV)]. We use published data - in the literature EW and SFR(Ly alpha)/SFR(UV) are treated as independent quantities - to show that the predicted relation holds for the vast majority of observed Ly alpha-emitting galaxies (LAEs). We show that the relation between EW and SFR(Ly alpha)/SFR(UV) applies irrespective of a galaxy's 'true' underlying SFR, and that its only source of scatter is the variation in the spectral slope of the UV continuum between individual galaxies. The derived relation, when combined with the observed EW distribution, implies that the ratio SFR(UV)/SFR(Ly alpha) is described well by a lognormal distribution with a standard deviation of similar to 0.3-0.35. This result is useful when modelling the statistical properties of LAEs. We further discuss why the relation between EW and SFR(Ly alpha)/SFR(UV) may help in identifying galaxies with unusual stellar populations.