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

Abstract

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 $z\ge 9$, we find a median ${\mathrm{EW}}_{\mathrm{rest}}^{\mathrm{H}\beta +[\mathrm{OIII}]}\simeq 1300$AA, similar to values at $z\sim 6-9$. We find no evidence for either a strong increase or decline in EW beyond $z\sim 9$. 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 $z\simeq 9-11$ ($\Phi \sim 10^{-3.4}\mathrm{Mp}{\mathrm{c}}^{-3}\mathrm{de}{\mathrm{x}}^{-1}$ at $\log (L_{\mathrm{H}\beta +[\mathrm{OIII}]}/\mathrm{erg}{\mathrm{s}}^{-1})=42.5$), consistent with a decline relative to $z\sim 7-8$. The MIDIS sources have $\log ({\xi }_{\mathrm{ion}}/\mathrm{Hz}\mathrm{er}{\mathrm{g}}^{-1})=25.1-25.4$. We find significant correlations between ${\xi }_{\mathrm{ion}}$ 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 $z\sim 9-11$, extending trends established at $z\sim 6-9$.

19 pages, 12 figures. Submitted to MNRAS