A Closer Look at Bursty Star Formation with L_(Hα) and L_(UV) Distributions

Abstract

We investigate the bursty star formation histories (SFHs) of dwarf galaxies using the distribution of log(L_(Hα)/L_(UV)) of 185 local galaxies. We expand on the work of Weisz et al. to consider a wider range of SFHs and stellar metallicities, and show that there are large degeneracies in a periodic, top-hat burst model. We argue that all galaxies of a given mass have similar SFHs and we can therefore include the LHα distributions (subtracting the median trend with stellar mass, referred to as Δlog(L_(Hα)) in our analyses. Δlog(L_(Hα)) traces the amplitude of the bursts, and log(L_(Hα)/L_(UV)) is a function of the timescale, amplitude, and shape of the bursts. We examine the two-dimensional distribution of these two indicators to constrain the SFHs. We use exponentially rising/falling bursts to determine timescales (e-folding time, τ). We find that galaxies below 10^(7.)5 M_⊙ undergo large (maximum amplitudes of ∼100) and rapid (τ < 30 Myr) bursts, while galaxies above 10^(8.5) M_⊙ experience smaller (maximum amplitudes ∼10), slower (τ ≳ 300 Myr) bursts. We compare with the FIRE-2 hydrodynamical simulations and find that the burst amplitudes agree with observations, but they are too rapid in intermediate-mass galaxies (M_* > 10⁸ M_⊙). Finally, we confirm that stochastic sampling of the stellar mass function cannot reproduce the observed distributions unless the standard assumptions of cluster and stellar mass functions are changed. With the next generation of telescopes, measurements of L_(UV) and L_(Hα) will become available for dwarf galaxies at high redshift, enabling similar analyses of galaxies in the early universe.

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