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 [Mg IV ] 4.487 μm (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 [Mg IV ] luminosity is well correlated with that of H recombination lines, which mainly trace star-forming clumps in these objects, and that the [Ar VI ] 4.530 μm (75 eV), usually seen in AGN, is undetected. On 100–400 pc scales, the [Mg IV ] line profiles are broader ( σ ([Mg IV ]) = 90 ± 25 km s −1 ) and shifted (Δ v up to ±50 km s −1 ) compared to those of the H recombination lines and lower ionization transitions (e.g., σ (Hu-12) = 57 ± 15 km s −1 ). The [Mg IV ] kinematics follow the large-scale rotating velocity field of these galaxies, and the broad [Mg IV ] 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 [Mg IV ] emission. We also computed new grids of photoionization and shock models to investigate where the [Mg IV ] line originates. Shocks with velocities of 100–130 km s −1 reproduce the observed line ratios and the [Mg IV ] 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 [Mg IV ] line. Future studies on the stellar feedback from SNe will benefit from the [Mg IV ] 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.

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