Star Formation Indicators and Line Equivalent Width in Lyman Alpha Galaxies

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.

6 pages, 4 figures, accepted to MNRAS

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• Modelling the observed luminosity function and clustering evolution of Lyman-$\alpha$ emitters: growing evidence for late reionization
• Linking gas and galaxies at high redshift: MUSE surveys the environments of six damped Lyman alpha galaxies at z~3
• Small-scale Intensity Mapping: Extended Ly$\alpha$, H$\alpha$ and Continuum emission as a Probe of Halo Star Formation in High-redshift Galaxies
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• Empirical Constraints on the Star Formation & Redshift Dependence of the Lyman Alpha `Effective' Escape Fraction
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• Spitzer's View of the Candidate Cluster and Protocluster Catalog (CCPC)
• Deep Hubble Space Telescope Imaging on the Extended Ly$\alpha$ Emission of a QSO at $z=2.19$ with Damped Lyman Alpha System as a Natural Coronagraph