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

TL;DR

This study examines the bursty star formation and ionizing efficiency of low-mass galaxies at redshifts 0.7 to 1.5, revealing their significant role in cosmic reionization through detailed spectroscopic and UV observations.

Contribution

It provides new insights into the burstiness and ionizing photon production of low-mass galaxies, highlighting their importance in the reionization era and comparing observations with simulations.

Findings

Lower-mass galaxies show higher SFR(Hα) excess compared to UV.

Ionizing efficiency ξ_{ion} increases with redshift and EW(Hα).

Faint, low-mass galaxies have higher ξ_{ion}, supporting their role in reionization.

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.