Star Formation Indicators and Line Equivalent Width in Lyman Alpha Galaxies

TL;DR

This study confirms a fundamental relation between Lyman Alpha equivalent width and star formation rate ratios in galaxies, revealing that UV spectral slope variations cause most scatter, and provides a statistical model for LAE properties.

Contribution

It demonstrates that the EW and SFR ratio relation holds broadly for LAEs and models the SFR ratio distribution as log-normal, aiding statistical analyses of these galaxies.

Findings

The EW and SFR ratio relation applies to most observed LAEs.

Spectral slope variations are the main source of scatter.

SFR(UV)/SFR(Lya) follows a log-normal distribution with std dev ~0.3-0.35.

Abstract

The equivalent width (EW) of the Lyman Alpha (Lya) line is directly related to the ratio of star formation rates determined from Lya flux and UV flux density [SFR(Lya)/SFR(UV)]. We use published data --in the literature EW and SFR(Lya)/SFR(UV) are treated as independent quantities-- to show that the predicted relation holds for the vast majority of observed Lya emitting galaxies (LAEs). We show that the relation between EW and SFR(Lya)/SFR(UV) applies irrespective of a galaxy's `true' underlying star formation rate, 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(Lya) is described well by a log-normal distribution with a standard deviation of ~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(Lya)/SFR(UV) may help identifying galaxies with unusual stellar populations.