JWST/MIRI detection of [Ne V] and [Ne VI] in M83: Evidence for the long sought-after AGN?

TL;DR

This study reports the first detection of high-ionization neon emission lines in M83's nucleus, suggesting the possible presence of an active galactic nucleus (AGN) and challenging its previous classification as a starburst galaxy.

Contribution

It provides the first high-confidence detections of [Ne V] and [Ne VI] lines in M83 and explores their origins through photoionization models, indicating potential AGN activity.

Findings

Detection of [Ne V] and [Ne VI] lines at high confidence.

Emission source possibly ionized by an AGN radiation cone.

Starburst classification of M83 may need revision.

Abstract

We report the first detections of [Ne V] 14.3 {\mu}m and [Ne VI] 7.7 {\mu}m at high confidence (S/N>=6) in the nuclear region of the nearby spiral galaxy M83. Emission line maps of these high ionization lines show several compact structures. Specifically, the [Ne VI] emission is located at 140 pc from the optical nucleus and appears as a point source of size ~<18 pc (FWHM =<0.8"). We investigate the possible source of this extreme emission through comparison with photoionization models and ancillary data. We find that photoionization models of fast radiative shocks are able to reproduce the observed high excitation emission line fluxes only for the lowest preshock density available in the library, n =0.01 cm^-3. Additionally, tailored active galactic nuclei (AGN) photoionization models assuming a two-zone structure are compatible with the observed high ionization fluxes. Our simple AGN model shows that the emission at the location of the [Ne VI] source can be the result of a cloud being ionized by the radiation cone of an AGN. We stress, however, that to definitively confirm an AGN as the main source of the observed emission, more complex modeling accounting for different geometries is required. Previously known as a purely starburst system, these new findings of the nuclear region of M83 will require a reassessment of its nature and of objects similar to it, particularly now that we have access to the unparalleled infrared sensitivity and spatial resolution of the James Webb Space Telescope.