THE BURSTY STAR FORMATION HISTORIES of LOW-MASS GALAXIES at 0.4 < z < 1 REVEALED by STAR FORMATION RATES MEASURED from Hβ and FUV

We investigate the burstiness of star formation histories (SFHs) of galaxies at 0.4 < z < 1 by using the ratio of star formation rates (SFRs) measured from Hβ and FUV (1500 A°) (Hβ-to-FUV ratio). Our sample contains 164 galaxies down to stellar mass (M _∗) of 10^8.5 M _o in the CANDELS GOODS-N region, where Team Keck Redshift Survey Keck/DEIMOS spectroscopy and Hubble Space Telescope/WFC3 F275W images from CANDELS and Hubble Deep UV Legacy Survey are available. When the ratio of Hβ- and FUV-derived SFRs is measured, dust extinction correction is negligible (except for very dusty galaxies) with the Calzetti attenuation curve. The Hβ-to-FUV ratio of our sample increases with M _∗ and SFR. The median ratio is ∼0.7 at M _∗∼ 10^8.5 M _o (or SFR ∼ 0.5 M _o yr^-1) and increases to ∼1 at M _∗ ∼ 10¹⁰ M _o (or SFR ∼ 10 M _o yr^-1). At M _∗ < 10^9.5 M _o, our median Hβ-to-FUV ratio is lower than that of local galaxies at the same M _∗, implying a redshift evolution. Bursty SFH on a timescale of a few tens of megayears on galactic scales provides a plausible explanation for our results, and the importance of the burstiness increases as M _∗ decreases. Due to sample selection effects, our Hβ-to-FUV ratio may be an upper limit of the true value of a complete sample, which strengthens our conclusions. Other models, e.g., non-universal initial mass function or stochastic star formation on star cluster scales, are unable to plausibly explain our results.