The Star Formation Burstiness and Ionizing Efficiency of Low-mass Galaxies

Abstract

We investigate the burstiness of star formation and the ionizing efficiency of a large sample of galaxies at $0.7 < z < 1.5$ using HST grism spectroscopy and deep ultraviolet (UV) imaging in the GOODS-N and GOODS-S fields. The star formation history (SFH) in these strong emission line low-mass galaxies indicates an elevated star formation rate (SFR) based on the H$\alpha$ emission line at a given stellar mass when compared to the standard main sequence. Moreover, when comparing the H$\alpha$ and UV SFR indicators, we find that an excess in SFR(H$\alpha$) compared to SFR(UV) is preferentially observed in lower-mass galaxies below $10^{9}$ M$\odot$, which are also the highest-EW galaxies. These findings suggest that the burstiness parameters of these strong emission line galaxies may differ from those inferred from hydrodynamical simulations and previous observations. For instance, a larger burstiness duty cycle would explain the observed SFR(H$\alpha$) excess. We also estimate the ionizing photon production efficiency $\xi_{ion}$, finding a median value of Log($\xi_{ion}$/erg$^{-1}$ Hz)$=24.80 \pm 0.26$ when adopting a Galactic dust correction for H$\alpha$ and an SMC one for the stellar component. We observe an increase of $\xi_{ion}$ with redshift, further confirming similar results at higher redshifts. We also find that $\xi_{ion}$ is strongly correlated with EW(H$\alpha$), which provides an approach for deriving $\xi_{ion}$ in early galaxies. Lower-mass, lower-luminosity galaxies have a higher $\xi_{ion}$. Overall, these results provide further support for faint galaxies playing a major role in the reionization of the Universe.