2025ApJ...979...87M

We report JWST/NIRSpec observations of a star-forming galaxy at z = 2.76, MACS J1149-WR1. We securely detect two temperature-sensitive auroral lines, [S III] 6312 (7.4σ) and [O II] 7320+7331 doublets (10σ), and tentatively [N II] 5755 (2.3σ) for the first time in an individual galaxy at z > 1. We perform a detailed analysis of its interstellar media (ISM), and derive electron temperatures, various heavy element abundances (O/H, N/O, S/O, and Ar/O) in the hot ionized region, and the neutral fraction in the warm ionized region. MACS J1149-WR1 shows a broad feature at the wavelength of He II 4686, which consists of a broad (∼1000 km s^–1), blueshifted (∼-110 km s^–1) line component. Taken together with its mildly elevated N/O abundance, we conclude that MACS J1149-WR1 is experiencing a young starburst (≲10 Myr), likely hosting a large number of Wolf-Rayet (W-R) stars. None of its spectral features support the presence of active galactic nuclei, including (i) the absence of broad components and velocity shifts in hydrogen recombination lines, (ii) low [Fe II]1.257 μm/Paβ ratio, and (iii) the absence of high-ionization lines. Our analysis using He I lines reveals a higher electron temperature and a higher attenuation value, indicating that He I may probe a smaller spatial scale than H I, presumably the region dominated by the aforementioned W-R stars. The star formation rates derived from various He I lines broadly agree with those from hydrogen recombination lines. We thus advocate that He I can be an excellent, independent probe of multiphase ISM in the era of JWST.