GA-NIFS: interstellar medium properties and tidal interactions in the evolved massive merging system B14-65666 at <i>z</i> = 7.152

ABSTRACT We present JWST/NIRSpec IFU observations of the $z=7.152$ galaxy system B14-65666, as part of the GA-NIFS survey. Line and continuum emission in this massive system ($\log _{10}(M_*/{\rm M}_{\odot })=9.8\pm 0.2$) is resolved into two strong cores surrounded by diffuse emission, as seen in recent JWST/NIRCam imaging. Our data set contains detections of [O ii]$\lambda \lambda 3726{,}3729$, [Ne iii]$\lambda \lambda 3869{,}3968$, Balmer lines, [O iii]$\lambda \lambda 4959{,}5007$, He i$\lambda 5875$, and weak [O iii]$\lambda 4363$. Each spectrum is fit with a model that consistently incorporates interstellar medium conditions (i.e. electron temperature, $T_{\rm e}$, electron density, $n_{\rm e}$, and colour excess, $E(B-V)$). The resulting line fluxes are used to constrain the gas-phase metallicity ($Z_{\rm g}\sim 0.2\mathrm{ -}0.3$ solar) and H$\beta$-based star formation rate for each region. Common line ratio diagrams (O32–R23, R3–R2, Ne3O2–R23) reveal that each line-emitting region lies at the intersection of low- and high-redshift galaxies, suggesting low ionization and higher metallicity compared to the predominantly lower-mass galaxies studied with the JWST/NIRSpec IFU so far at $z\gt 5.5$. Spaxel-by-spaxel fits reveal evidence for both narrow (FWHM $\lt 400$ km s$^{-1}$) and broad (FWHM $\gt 500$ km s$^{-1}$) line emission, the latter of which likely represents tidal interaction or outflows. Comparison to ALMA [C ii]158$\mu$m and [O iii]88$\mu$ m data shows a similar velocity structure, and we explore optical-far infrared diagnostics. The two core galaxies both lie on the mass-metallicity relation at $z\gt 4$, but show contrasting properties (e.g. $M_*$, $Z_{\rm g}$), suggesting distinct evolutionary pathways. Combining the NIRSpec IFU and ALMA data sets, our analysis opens new windows into the merging system B14-65666.