Toward Unveiling Cosmic Reionization: The Ionizing Photon Production Efficiency (ξ ion) of Low-mass Hα Emitters at z ∼ 2.3

Abstract

We investigate the galaxy properties of ∼400 low-mass (<10 ^9 M _⊙ ) H α emitters (HAEs) at z ∼ 2.3 in the ZFOURGE survey. The selection of these HAEs is based on the excess in the observed K _s broadband flux compared to the stellar continuum estimated from the best-fit spectral energy distribution. These low-mass HAEs have elevated SFR(H α ) above the star formation main sequence, making them potential analogs of the galaxies that reionized the Universe during the Epoch of Reionization. The ionizing photon production efficiencies ( ξ _ion ) of the low-mass HAEs have a median value of $\mathrm{log}({\xi }_{\mathrm{ion}}/{\mathrm{erg}}^{-1}\,\mathrm{Hz})={25.24}_{-0.13}^{+0.10}\ ({25.35}_{-0.15}^{+0.12})$ , assuming the Calzetti (SMC) curve for the stellar continuum dust correction. This value is higher than that of main-sequence galaxies by ∼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 ξ _ion with redshift, but reveal a “downsizing” relationship between ξ _ion and stellar mass ( M _⊙ ) with increasing redshift. We further explore the dependence of ξ _ion on other galaxy properties, such as the UV spectral slope ( β _UV ), the UV magnitude ( M _UV ), and the equivalent widths of H α and [O iii ] emission lines. Galaxies with bluer UV slopes, fainter UV luminosities, and higher equivalent widths exhibit elevated ξ _ion by a factor of ∼2 compared to the median ξ _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.

Keywords

• High-redshift galaxies
• Emission line galaxies
• Reionization
• Galaxy evolution