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Updated Measurements of Star formation Stochasticity Measured from the Distribution of Burst Indicators

Presentation #232.05 in the session “ Star Formation in Galaxies”.

Published onJan 11, 2021
Updated Measurements of 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) as the logarithm of the ratio of 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 Santa-Cruz semi-analytic models and the MUFASA hydrodynamical cosmological simulations. 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 galaxy catalogs for the 3D-HST survey and the Fiber Multi-Object Spectrograph (FMOS)-COSMOS survey, as well as predicted mock catalogs for the James Webb Space Telescope (JWST) and Roman Space Telescope. We compare these mock catalogs with Hα detections of 956 3D-HST galaxies at 0.65 < z < 1.5 and 1646 FMOS-COSMOS galaxies at 1.46 < z < 1.72. 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 Qsg=0.8-0.2+0.1 removes an unexpected dependence of the average value of η upon dust attenuation and stellar mass in both the 3D-HST and FMOS-COSMOS samples while also resolving disagreements in the distribution of SFRs between observed samples and the mocks. Using Qsg=0.8 additionally gives Avg(η) ≈ 0 for FMOS-COSMOS in agreement with the mocks but yields Avg(η) = -0.31 for the 3D-HST sample, implying a strongly quenching population. These results indicate a potentially weaker dust attenuation of nebular light relative to stellar light at z > 1 than is typically assumed from dust laws derived from low-z starburst galaxies and motivate a further investigation into the dust attenuation laws for nebular and stellar light at these redshifts.

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