The Bursty Star Formation Histories of Low-mass Galaxies at $0.4<z<1$ Revealed by Star Formation Rates Measured from H${\hat{I}}^2$ and FUV

Abstract

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 H${\hat{I}}^2$ and FUV (1500 Ã ) (H${\hat{I}}^2$--to--FUV ratio). Our sample contains 164 galaxies down to stellar mass (M*) of $10^{8.5}{M}_{\odot }$ in the CANDELS GOODS-N region, where Team Keck Redshift Survey DEIMOS spectroscopy and HST/WFC3 F275W images from CANDELS and Hubble Deep UV Legacy Survey are available. When the {\it ratio} of H${\hat{I}}^2$- and FUV-derived SFRs is measured, dust extinction correction is negligible (except for very dusty galaxies) with the Calzetti attenuation curve. The H${\hat{I}}^2$--to--FUV ratio of our sample increases with M* and SFR. The median ratio is $\sim$0.7 at M*$\sim 10^{8.5}{M}_{\odot }$ (or SFR$\sim 0.5M_{\odot }/yr$) and increases to $\sim$1 at M*$\sim 10^{10}{M}_{\odot }$ (or SFR $\sim 10M_{\odot }/yr$). At M*$<10^{9.5}{M}_{\odot }$, our median H${\hat{I}}^2$--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 of our results, and the importance of the burstiness increases as M* decreases. Due to sample selection effects, our H${\hat{I}}^2$--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.

13 pages, 8 figures. ApJ accepted. The main conclusions are not changed. Major modifications include: (1) to be consistent with the literature, now reporting Hβ--to--FUV ratio (rather than FUV--to--Hβ in the first version); (2) detailed discussions on dust extinction correction; (3) new SF bustiness calculation; and (4) enriched discussions in Introduction