2025A&A...701A..31P

We present MIRI/JWST medium-resolution spectroscopy (MRS) and imaging (MIRIM) of B14-65666, a source identified as a Lyman-break and interacting galaxy at a redshift of z = 7.15. We detect the Hα line emission in this system, revealing a spatially resolved structure of the Hα-emitting gas, which consists of two distinct galaxies, E and W, at a projected distance of 0.4 arcsec apart (i.e., 2.2 kpc). One of the galaxies (E) is very compact (upper limit for the effective radius of 63 pc) in the rest-frame ultraviolet light, while the other galaxy (W) is more extended (effective radius of 348 pc), showing a clumpy structure reminiscent of a tidal tail. The total Hα luminosity implies that the system is forming stars at a rate of 76 ± 8 M_⊙ yr^–1 and 30 ± 4 M_⊙ yr^–1 for E and W galaxies, respectively. The ionizing photon production efficiency, log(ζ_ion), for galaxies E and W, has values of 25.1 ± 0.1 Hz erg^–1 and 25.5 ± 0.1 Hz erg^–1, which is within the range measured in galaxies at similar redshifts. The high values derived for the Hα equivalent widths (832 ± 100 and 536 ± 78 Å) and the distinct locations of the E and W galaxies in the log(ζ_ion) - equivalent width (Hα) plane indicate that the system is dominated by a young (under 10 Myr) stellar population. The overall spectral-energy distribution suggests that in addition to a young stellar population, the two galaxies may have mature (over 100 Myr) stellar populations and very different dust attenuations, with galaxy E showing a larger attenuation (A_V = 1.5 mag) compared to the almost dust-free (A_V = 0.1 mag) galaxy W. The derived star formation rate (SFR) and stellar masses identify the two galaxies as going through a starburst phase characterized by a specific SFR (sSFR) of 40-50 Gyr^–1. Galaxy E has an extreme stellar mass surface density (6 × 10⁴ M_⊙ pc^–2), close to that of the nuclei of low-z galaxies, while galaxy W (10³ M_⊙ pc^–2) is consistent with the surface densities measured in galaxies at these redshifts. The kinematics of the ionized gas traced by the Hα line show a velocity difference of 175 ± 28 km s^–1 between the two components of B14-65666 and a broader profile for galaxy W (312 ± 44 km s^–1) relative to galaxy E (243 ± 41 km s^–1). The detailed study of B14-65666 shows that the complex stellar and interstellar medium structure in merging galaxy systems was already in place by the Epoch of Reionization. The general properties of B14-65666 agree with those predicted for massive merging systems at redshifts of 7 and above in the FIRSTLIGHT cosmological simulations. The in-depth study of systems such as B14-65666 reveal how galaxy mergers in the early Universe drive intense star formation, shape the interstellar medium, and influence the buildup of stellar mass, just 700-800 Myr after the Big Bang.