Towards unveiling the Cosmic Reionization: the ionizing photon production efficiency (${\xi }_{ion}$) of Low-mass H$\alpha$ emitters at $z\sim 2.3$

Abstract

We investigate the galaxy properties of $\sim$400 low-mass ($<10^9{M}_{\odot }$) H$\alpha$ emitters (HAEs) at z $\sim$ 2.3 in the ZFOURGE survey. The selection of these HAEs is based on the excess in the observed $K_s$ broad-band flux compared to the stellar continuum estimated from the best-fit SED. These low-mass HAEs have elevated SFR(H$\alpha$) above the star formation main sequence (SFMS), making them potential analogs of the galaxies that reionized the universe during the epoch of reionization. The ionizing photon production efficiencies (${\xi }_{ion}$) of the low-mass HAEs have a median value of $\log ({\xi }_{ion}/er{g}^{-1}Hz)=25.24_{-0.13}^{+0.10}(25.35_{-0.15}^{+0.12})$, assuming the Calzetti (SMC) curve for the stellar continuum dust correction. This value is higher than that of main sequence galaxies by $\sim$0.2 dex at similar redshift, indicating that the low-mass HAEs are more efficient in producing ionizing photons. Our results also consolidate the trend of increasing ${\xi }_{ion}$ with redshift, but reveal a "downsizing" relationship between ${\xi }_{ion}$ and stellar mass ($M_{\odot }$) with increasing redshift. We further explore the dependence of ${\xi }_{ion}$ on other galaxy properties, such as the UV spectral slope (${\beta }_{\mathrm{UV}}$), the UV magnitude ($M_{\mathrm{UV}}$), the equivalent widths ($EWs$) of H$\alpha$ and [O{\sc iii}] emission lines. Galaxies with the bluer UV slopes, fainter UV luminosities and higher equivalent widths exhibit elevated ${\xi }_{ion}$ by a factor of $\sim$2 compared to the median ${\xi }_{ion}$ of our sample. JWST data will provide an opportunity to extend our method and further investigate the properties of low-mass galaxies at high redshifts.

15 pages, 10 figures, Accepted for publication in ApJ

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