MIDIS: Strong H$\beta$+[OIII] Line Emitters at $z \geq 9$

We present a search for strong H$\beta$+[O III] emitters at $z=9.4-11.3$ in the HUDF using ultra-deep JWST/MIRI F560W imaging from the MIDIS survey. Three galaxies are identified via pronounced F560W flux excesses, consistent with strong rest-frame optical line emission. SED modelling yields rest-frame H$\beta$+[O III] equivalent widths of $\sim 600-1300$AA (median $\simeq 1260$AA), placing these sources among the most extreme known at these epochs. Combining these with a literature sample of 16 spectroscopically confirmed galaxies at $z\geq 9$, we find a median ${\rm EW}^{\rm H\beta+[O III]}_{\rm rest}\simeq 1300$AA, similar to values at $z\sim6-9$. We find no evidence for either a strong increase or decline in EW beyond $z\sim9$. A tentative trend of higher EW with increasing UV luminosity is observed, while no statistically significant anti-correlation with stellar mass is found. We place a first constraint on the H$\beta$+[O III] luminosity function at $z\simeq9-11$ ($\Phi\sim10^{-3.4}\,{\rm Mpc^{-3}\,dex^{-1}}$ at $\log( L_{\rm H\beta+[OIII]}/{\rm erg\,s^{-1}})=42.5$), consistent with a decline relative to $z\sim7-8$. The MIDIS sources have $\log(\xi_{\rm ion}/{\rm Hz\,erg^{-1}})=25.1-25.4$. We find significant correlations between $\xi_{\rm ion}$ and EW and $\beta$, but not with UV luminosity, consistent with trends at lower redshift. These results suggest that the physical conditions governing nebular emission and ionising efficiency are already in place at $z\sim9-11$, extending trends established at $z\sim6-9$.