Cosmic-ray impact on optical and mid-infrared emission-line diagnostics in NGC 5728

¹ Section of Astrophysics, Astronomy & Mechanics, Department of Physics, National and Kapodistrian University of Athens, University Campus Zografos, GR 15784 Athens, Greece
² Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, E–44001 Teruel, Spain
³ Istituto di Astrofisica e Planetologia Spaziali (INAF–IAPS), Via Fosso del Cavaliere 100, I–00133 Roma, Italy

^⋆ Corresponding author: evkoutso@phys.uoa.gr

Received: 5 June 2025
Accepted: 5 September 2025

Abstract

Cosmic rays (CRs), produced by the jets of active galactic nuclei (AGN) and supernovae (SNe), serve as a significant feedback mechanism influencing emission lines in narrow line region (NLR) clouds. These highly energetic particles, propelled by shocks, not only heat the interstellar medium (ISM) but also modify its chemical composition. This study investigates the role of CRs, particularly in their ability to excite gas and align with observed line flux ratios across UV and optical diagnostic diagrams. We employed CLOUDY simulations to explore the CR ionization rate, the ionization parameter, and the initial hydrogen density effects on optical and mid-infrared (MIR) emission. Our analysis includes high-quality optical data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) for NGC 5728, supplemented by infrared observations from the James Webb Space Telescope (JWST). This multiwavelength approach provides a deeper understanding of CR impact on the ISM. Our previous results indicate that CRs are instrumental in heating the inner regions of gas clouds, enhancing the emission of low-ionization optical lines such as [N II] and [S II]. Additionally, the integration of mid-infrared (MIR) data reveals that emission lines susch as [Ar II] and [Ne II] within the JWST Mid-Infrared Instrument (MIRI) field of view are sensitive to CRs. In contrast, high-ionization lines (e.g., [Ne V], [O IV]) serve as robust tracers of photoionization being insensitive to CR effects. Moreover, mixed optical–MIR diagnostic diagrams offer insight into the relative roles of CRs and shocks, which often produce similar signatures in emission lines. We find that while both mechanisms can elevate certain line ratios, their influence on MIR diagnostics diverges: shocks and CRs affect low-ionization lines differently, allowing for a better understanding when multiwavelength data are available. Our approach not only helps to resolve the degeneracy between metallicity and CR ionization but also enables the potential differentiation of shocks and CR-driven processes in AGN host galaxies.