Letter to the Editor

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

¹ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
e-mail: miguel.pereira@iff.csic.es
² Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
³ Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
⁴ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁷ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus

Received: 14 March 2024
Accepted: 25 April 2024

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 (Δv 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.