GA-NIFS: Co-evolution within a highly star-forming galaxy group at z ∼ 3.7 witnessed by JWST/NIRSpec IFS

Abstract

We present NIRSpec IFS observations of a galaxy group around the massive GS_4891 galaxy at z ∼ 3.7 in GOODS-South that includes two other two systems, GS_4891_n to the north and GS_28356 to the east. These observations, obtained as part of the GTO Galaxy Assembly – NIRSpec IFS (GA-NIFS) program, allow us to study for the first time the spatially resolved properties of the interstellar medium (ISM) and the ionised gas kinematics of a galaxy at this redshift. Leveraging the wide wavelength range spanned with the high-dispersion grating (with resolving power R = 2700) observations, covering from [O II] λλ3726, 29 to [S II] λλ6716, 31, we explore the spatial distribution of the star formation rate, nebular attenuation, and gas metallicity, together with the mechanisms responsible for the excitation of the ionised gas. GS_4891 presents a clear gradient of gas metallicity (as traced by 12 + log(O/H)) by more than 0.2 dex from the southeast (where a star-forming clump is identified) to the northwest. The gas metallicity in the less massive northern system, GS_4891_n, is also higher by 0.2 dex than at the centre of GS_4891, suggesting that inflows of lower-metallicity gas might be favoured in higher-mass systems. The kinematic analysis shows that GS_4891 presents velocity gradients in the ionised gas consistent with rotation. The region between GS_4891 and GS_4891_n does not present high gas turbulence, which, together with the difference in gas metallicities, suggests that these two systems might be in a pre-merger stage. Finally, GS_4891 hosts an ionised outflow that extends out to rout = 1.5 kpc from the nucleus and reaches maximum velocities, vout, of approximately 400 km s−1. Despite entraining an outflowing mass rate of Ṁout ∼ 4 M⊙ yr−1, the low associated mass-loading factor, η ∼ 0.04, implies that the outflow does not have a significant impact on the star formation activity of the galaxy.