Spatially resolved Hα emission in B14-65666: Compact starbursts, ionizing efficiency, and gas kinematics in an advanced merger at the Epoch of Reionization

¹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, E-28850 Madrid, Spain
² Departamento de Física de la Tierra y Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain
³ Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Department of Astronomy, Stockholm University, Oscar Klein Centre, AlbaNova University Centre, 106 91 Stockholm, Sweden
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, E-28692 Villanueva de la Cañada, Madrid, Spain
⁶ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁷ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800 9700 AV Groningen, The Netherlands
⁸ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁹ Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹⁰ European Space Agency, Space Telescope Science Institute, Baltimore, MD, USA
¹¹ DTU-Space, Elektrovej, Building 328, 2800, Kgs., Lyngby, Denmark
¹² DARK, Niels Bohr Institute, University of Copenhagen, 2200 Copenhagen, Denmark
¹³ Telespazio UK for the European Space Agency (ESA), ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁴ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁵ School of Physics & Astronomy, Space Park Leicester, University of Leicester, 92 Corporation Road, Leicester LE4 5SP, UK
¹⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁷ Max Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹⁸ Division of Physics, Faculty of Pure and Applied Sciences, University of Tsukuba,Tsukuba, Ibaraki 305-8571, Japan
¹⁹ Tomonaga Center for the History of the Universe (TCHoU), Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
²⁰ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²¹ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
²² ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland

^⋆ Corresponding author: cprieto@cab.inta-csic.es

Received: 7 April 2025
Accepted: 8 July 2025

Abstract

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.