Star Formation Stochasticity Measured from the Distribution of Burst Indicators

One of the key questions in understanding galaxy formation and evolution is how starbursts affect the assembly of stellar populations in galaxies over time. We define a burst indicator (η) that compares a galaxy’s star formation rates (SFRs) on short (∼10 Myr) and long (∼100 Myr) timescales. To estimate η, we apply the detailed time-luminosity relationship for Hα and near-ultraviolet emission to simulated star formation histories (SFHs) from semi-analytic models and the MUFASA hydrodynamical cosmological simulations. The average of η is not a good indicator of star formation stochasticity (burstiness); indeed, we show that this average should be close to zero unless the galaxy population has an average SFH that is rising or falling rapidly. Instead, the width of the η distribution characterizes the burstiness of a galaxy population’s recent star formation. We find this width to be robust to variations in stellar initial mass function and metallicity. We apply realistic noise and selection effects to the models to generate mock Hubble Space Telescope (HST) and James Webb Space Telescope (JW ST) galaxy catalogs and compare these catalogs with 3D-HST observations of 956 galaxies at 0.65 < z < 1.5 detected in Hα. Measurements of η are unaffected by dust measurement errors under the assumption that E(B - V)_stars = 0.44 E(B - V)_gas (i.e., Q _sg = 0.44). However, setting {Q}_sg}={0.8}_-0.2^+0.1 removes an unexpected dependence of the average value of η upon dust attenuation and stellar mass in the 3D-HST sample while also resolving disagreements in the distribution of SFRs. However, even varying the dust law cannot resolve all discrepancies between the simulated and the observed galaxies.