MIDIS: Strong H β +[O iii ] Line Emitters at z ≳ 9

We search for strong Hβ+[O iii] line emitters at z = 9.4 − 11.3 in the Hubble Ultra Deep Field using ultra-deep MIRI/F560W imaging from the MIRI Deep Imaging Survey. Three galaxies show F560W flux excesses relative to their continuum, consistent with Hβ+[O iii] emission. We derive rest-frame Hβ+[O iii] equivalent widths of EWHβ +[OIII]ᵣₑₛₜ 700-1500\Å, placing them among the most extreme nebular line emitters at this epoch. Combining our sources with 15 spectroscopically confirmed galaxies at z > 9 with published Hβ+[O iii] measurements, we derive a median EWHβ +[Oiii]ᵣₑₛₜ = 1433\Å with median absolute deviation of 670 Å. We find no evidence for an increase or decline in EWHβ +[OIII]ᵣₑₛₜ at z > 9. Binning the combined z > 9 sample by UV luminosity, we find higher equivalent widths in more UV-luminous systems. While consistent with trends at z = 6 − 9, the difference is not significant, and subject to heterogeneous sample selection. No significant anti-correlation between Hβ+[O iii] equivalent width and stellar mass is found, although a fit suggests a trend consistent with lower-redshift observations. We place a first constraint on the Hβ+[O iii] line luminosity function at z ≃ 9 − 11 (Φ ∼ 10−3.3 Mpc−3 dex−1 at log (LHβ +[Oᵢii]/ erg\, s⁻¹) = 42.45) consistent with a decline relative to z ≃ 7 − 8. We derive ionising photon production efficiencies of log (ξion/Hz erg−1) = 25.3 − 25.5, and find tentative trends between ξion and EWHβ +[OIII]ᵣₑₛₜ, and between ξion and β, consistent with those observed at z = 6 − 9. These results suggest that the physical conditions governing nebular emission and its link to the UV continuum are in place at z > 9.