Extended high-ionization [Mg iv] emission tracing widespread shocks in starbursts seen by JWST/NIRSpec

Abstract

We report the detection of extended ($>$0.5--1\,kpc) high-ionization (80\,eV) emission in four local luminous infrared galaxies observed with JWST NIRSpec. Excluding the nucleus and outflow of the Type 1 active galactic nucleus (AGN) in the sample, we find that the luminosity is well correlated with that of H recombination lines, which mainly trace star-forming clumps in these objects, and that the (75\,eV), usually seen in AGN, is undetected. On 100--400\,pc scales, the line profiles are broader ($ $) and shifted ($ v$ up to pm $) compared to those of the H recombination lines and lower ionization transitions (e.g., $ $). The kinematics follow the large-scale rotating velocity field of these galaxies, and the broad profiles are compatible with the broad wings detected in the H recombination lines. Based on these observational results, extended highly ionized gas more turbulent than the ambient interstellar medium, possibly a result of ionizing shocks associated with star formation, is the most likely origin of the emission. We also computed new grids of photoionization and shock models to investigate where the line originates. Shocks with velocities of $ reproduce the observed line ratios and the luminosity agrees with that expected from the mechanical energy released by supernove (SNe) in these regions. Therefore, these models support shocks induced by SNe as the origin of the line. Future studies on the stellar feedback from SNe will benefit from the line that is little affected by obscuration and, in the absence of an AGN, can only be produced by shocks due to its high ionization-potential.