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 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 SFRH alpha compared to SFRUV is preferentially observed in lower mass galaxies below 10(9) M-circle dot, 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 SFRH 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 +/- 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 EWH alpha, which provides an approach for deriving xi(ion) in early galaxies. We observe that 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.