A negligible contribution of two luminous z ∼ 7.5 galaxies to the ionizing photon budget of reionization

Abstract

We present indirect constraints on the absolute escape fraction of ionizing photons (f_esc^LyC) of the system GN 42912 which comprises two luminous (M_UV magnitudes of -20.89 and -20.37) galaxies at z ~ 7.5, GN 42912-NE and GN 42912-SW, to determine their contribution to the ionizing photon budget of the epoch of reionization (EoR). The high-resolution JWST NIRSpec and NIRCam observations reveal they are separated by only ~0.1'' (0.5 kpc) on the sky and have a 358 km s^-1 velocity separation. GN 42912-NE and GN 42912-SW are relatively massive for this redshift (log(M_* / M_⊙) ~ 8.4 and 8.9, respectively), with gas-phase metallicities of 18 and 23 per cent solar, O_32 ratios of 5.3 and > 5.8, and β slopes of -1.92 and -1.51, respectively. We use the Mg II λλ2796,2803 doublet to constrain f_esc^LyC. Mg II has an ionization potential close to that of neutral hydrogen and, in the optically thin regime, can be used as an indirect tracer of the LyC leakage. We establish realistic conservative upper limits on f_esc^LyC of 8.5 per cent for GN 42912-NE and 14 per cent for GN 42912-SW. These estimates align with f_esc^LyC trends observed with β, O_32, and the Hβ equivalent width at z < 4. The small inferred ionized region sizes (< 0.3 pMpc) around both galaxies indicate they have not ionized a significant fraction of the surrounding neutral gas. While these z > 7 f_esc^LyC constraints do not decisively determine a specific reionization model, they support a minor contribution from these two relatively luminous galaxies to the EoR.