2024A&A...686A..85A

We present MIRI/JWST medium-resolution spectroscopy (MRS) and imaging (MIRIM) of the lensed galaxy MACS1149-JD1 at a redshift of z = 9.1092 ± 0.0002, when the Universe was about 530 Myr old. We detect, for the first time, spatially resolved Hα emission in a galaxy at a redshift above nine. The structure of the Hα emitting gas consists of two clumps, S and N, carrying about 60% and 40% of the total flux, respectively. The total Hα luminosity implies an instantaneous star-formation rate in the range of 3.2 ± 0.3 and 5.3 ± 0.4 M_⊙ yr^–1 for sub-solar and solar metallicities. The ionizing photon production efficiency, log(ζ_ion), shows a spatially resolved structure with values of 25.55 ± 0.03; 25.47 ± 0.03; and 25.91 ± 0.09 Hz erg^–1 for the integrated galaxy and clumps S and N, respectively. The Hα rest-frame equivalent width, EW₀ (Hα), is 726$ ^{+660}_{-182} $ Å for the integrated galaxy, but it presents extreme values of 531$ ^{+300}_{-96} $ Å and ≥1951 Å for clumps S and N, respectively. The spatially resolved ionizing photon production efficiency is within the range of values measured in galaxies at a redshift above six and well above the canonical value (25.2 ± 0.1 Hz erg^–1). The EW₀ (Hα) is a factor of two lower than the predicted value at z = 9.11 based on the extrapolation of the evolution of the EW₀ (Hα) with redshifts, ∝(1 + z)^2.1, including galaxies detected with JWST. The extreme difference of the EW₀ (Hα) for clumps S and N indicates the presence of a recent (< 5 Myr) stellar burst in clump N and a star formation over a larger period of time (e.g., ∼50 Myr) in clump S. The different ages of the stellar population place MACS1149-JD1 and clumps N and S at different locations in the log(ζ_ion) to EW₀ (Hα) plane and above the main relation defined from intermediate- and high-redshift (z = 3 - 7) galaxies detected with JWST. Finally, clump S and N show very different Hα kinematics, with velocity dispersions of 56 ± 4 km s^–1 and 113 ± 33 km s^–1, likely indicating the presence of outflows or increased turbulence in clump N. The dynamical mass M_dyn = (2.4 ± 0.5) × 10⁹M_⊙, obtained from the size of the integrated Hα ionized nebulae and its velocity dispersion, is within the range previously measured with the spatially resolved [OIII]88 μm line.