Spatially resolved H<i>α</i> emission in B14-65666: Compact starbursts, ionizing efficiency, and gas kinematics in an advanced merger at the Epoch of Reionization

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 H α line emission in this system, revealing a spatially resolved structure of the H α -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 H α 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 H α 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 (H α ) 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 × 10 4 M ⊙ pc −2 ), close to that of the nuclei of low- z galaxies, while galaxy W (10 3 M ⊙ pc −2 ) is consistent with the surface densities measured in galaxies at these redshifts. The kinematics of the ionized gas traced by the H α 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.