The star formation burstiness and ionizing efficiency of low-mass galaxies

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 Ha emission line at a given stellar mass when compared to the standard main sequence. Moreover, when comparing the Ha and UV SFR indicators, we find that an excess in SFRHa compared to SFRUV is preferentially observed in lower mass galaxies below 109 M, 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 SFRHa excess. We also estimate the ionizing photon production efficiency ?ion, finding a median value of log(?ion/erg-1 Hz) = 24.80 ± 0.26 when adopting a Galactic dust correction for Ha and an SMC one for the stellar component. We observe an increase of ?ion with redshift, further confirming similar results at higher redshifts. We also find that ?ion is strongly correlated with EWHa, which provides an approach for deriving ?ion in early galaxies. We observe that lower mass, lower luminosity galaxies have a higher ?ion. Overall, these results provide further support for faint galaxies playing a major role in the reionization of the Universe.