Effects of Star Formation Stochasticity on the Ly-alpha & Lyman Continuum Emission from Dwarf Galaxies during Reionization

Abstract

Observations of distant galaxies play a key role in improving our understanding of the Epoch of Reionization (EoR). The observed Ly-alpha emission line strength - quantified by its restframe equivalent width (EW) - provides a valuable diagnostic of stellar populations and dust in galaxies during and after the EoR. In this paper we quantify the effects of star formation stochasticity on the predicted Ly-alpha EW in dwarf galaxies, using the publicly available code SLUG ('Stochastically Light Up Galaxies'). We compute the number of hydrogen ionizing photons, as well as flux in the Far UV for a set of models with star formation rates (SFR) in the range 10-3-1 Msol/yr. From these fluxes we compute the luminosity, L-alpha, and the EW of the Ly-alpha line. We find that stochasticity alone induces a broad distribution in L-alpha and EW at a fixed SFR, and that the widths of these distributions decrease with increasing SFR. We parameterize the EW probability density function (PDF) as an SFR-dependent double power law. We find that it is possible to have EW as low as ~EW0/4 and as high as ~3 times the EW0, where EW0 denotes the expected EW in the absence of stochasticity. We argue that stochasticity may therefore be important when linking drop-out and narrow-band selected galaxies, when identifying population III galaxies, and that it may help to explain the large EW (EW > 100 - 200 A) observed for a fraction of Ly-alpha- selected galaxies. Finally, we show that stochasticity can also affect the inferred escape fraction of ionizing photons from galaxies. In particular, we argue that stochasticity may simultaneously explain the observed anomalous ratios of the Lyman continuum flux density to the (non-ionizing) UV continuum density in so-called Lyman-Bump galaxies at z = 3.1, as well as the absence of such objects among a sample of z = 1.3 drop-out galaxies.

9 pages, 4 figures, accepted for publication in MNRAS

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• SLUG - Stochastically Lighting Up Galaxies. II: Quantifying the Effects of Stochasticity on Star Formation Rate Indicators
• On the (non-)enhancement of the Lyman α equivalent width by a multiphase interstellar medium
• Consequences of bursty star formation on galaxy observables at high redshifts
• The MUSE Hubble Ultra Deep Field Survey X. Ly$\alpha$ Equivalent Widths at $2.9<z<6.6$
• Detection of isolated population III stars with the James Webb Space Telescope
• The spectral evolution of the first Galaxies. III. Simulated James Webb Space Telescope spectra of reionization-epoch galaxies with Lyman continuum leakage
• The UV, Lyman-alpha, and dark matter halo properties of high redshift galaxies
• Duty Cycle and the Increasing Star Formation History of z>=6 Galaxies
• Small-scale Intensity Mapping: Extended Ly$\alpha$, H$\alpha$ and Continuum emission as a Probe of Halo Star Formation in High-redshift Galaxies
• Searching for candidates of Lyman continuum sources - revisiting the SSA22 field
• Boosting Lya and HeII 1640A Line Fluxes from Pop III Galaxies: Stochastic IMF Sampling and Departures from Case-B
• A Multiply-Imaged z $\sim$ 6.3 Lyman Alpha Emitter candidate behind Abell 2261
• Evolution of dwarf galaxy observable parameters
• Star Formation for Predictive Primordial Galaxy Formation
• Challenges in modelling the rest-frame ultraviolet/optical spectra of galaxies at the high-redshift frontier