OutThere Survey: Addressing ${\xi }_{\mathrm{ion}}$ and ${\mathrm{f}}_{\mathrm{esc}}$ with a population of average galaxies at z$\sim$2

Abstract

Constraining the major contributors to the ionisation of the early universe is an ongoing endeavour of high-redshift galaxy research. We measure the ionising photon production efficiency and Lyman Continuum escape fraction for a sample of 230 intermediate redshift ($1.3<z<2.6$) sources observed as a part of the \textit{OutThere survey}; a pure-parallel, wide-area JWST/NIRISS survey with accompanying JWST/NIRCam, NIRISS and HST photometry. The low threshold emission selection criteria for this sample makes for a large and robust control, against which other works may be contrasted, particularly for low-mass galaxies above z$>$5. This control sample allows us to verify the correlations between ionising and spectral/physical properties suggested by previous studies. We find no significant correlations between the ionising photon production efficiency (${\xi }_{\mathrm{ion}}$) with the UV slope, ${\mathrm{M}}_{\mathrm{UV}}$, M${}_{\ast }$ or sSFR. We do find that ${\xi }_{\mathrm{ion}}$ correlates with [OIII]5007Å\, equivalent width (EW) (Spearman coefficient $\rho$ =0.24; p$<4\times 10^{-4}$) and H$\alpha$ EW ($\rho$ =0.63; p$<<1\times 10^{-6}$) hold even at low EW albeit with more scatter. We also find that our novel approach to determining the ionising photon escape fraction ${\mathrm{f}}_{\mathrm{esc}}$ results in values within theoretical ranges (0-10\%) though vary substantially in comparison to the empirical results (median ${\mathrm{f}}_{\mathrm{esc}}=0.9{\%}_{-0.5}^{+1.1}$ including non-detections, median ${\mathrm{f}}_{\mathrm{esc}}=1.9{\%}_{-1.8}^{+8.9}$ above a $0.01\%$ threshold). We find that this escape fraction method has consistently significant correlations with the redshift, SFR and M${}_{UV}$ and sample-dependent correlations with [OIII]5007Å\,EW,H$\alpha$ EW and stellar mass.

20 pages, 13 figures, 5 tables