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

TL;DR

This study combines VLT/MUSE optical and JWST/MIRI MIR spectroscopy of NGC 5728 to test the role of cosmic-ray (CR) ionization as a driver of emission lines in AGN environments. Using updated CLOUDY grids with an intermediate AGN SED and a broad CR ionization-rate range, the authors show that low-ionization lines in both optical and MIR are sensitive to CRs (with $\zeta_{CR} \sim 10^{-14}$–$10^{-13}$ s$^{-1}$), while high-ionization lines remain dominated by photoionization; this enables CRs to be distinguished from shocks when multi-wavelength data are available. The analysis finds that combining optical and MIR diagnostics mitigates degeneracies with metallicity and helps separate CR- and shock-driven excitation, supporting a CR-influenced, jet-associated ISM in NGC 5728. The results align with similar analyses of Cen A and NGC 1068, illustrating CRs as a plausible, pervasive supplemental excitation mechanism in AGN environments and highlighting the diagnostic power of multi-wavelength, spatially resolved spectroscopy for disentangling ionization sources.

Abstract

Cosmic rays (CRs), from active galactic nuclei (AGN) jets 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, heat the interstellar medium (ISM) and modify its chemical composition. This study investigates the role of CRs, particularly in their ability to excite gas and align with observed line ratios across UV and optical diagnostics. We employ CLOUDY to explore CR ionization rate, ionization parameter, and 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). Our previous results indicate that CRs are instrumental in heating the inner regions of gas clouds, enhancing emission of low-ionization optical lines. Mid-infrared data reveal that emission lines like [Ar II] and [Ne II] within the JWST Mid-Infrared Instrument (MIRI) field of view are sensitive to CRs. In contrast, high-ionization lines (for example, [Ne V]) serve as robust tracers of photoionization insensitive to CRs. Moreover, mixed optical and MIR diagnostics 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 multi-wavelength 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.

Figures (10)

• Figure 1: Apertures chosen to extract spectra from the spectroscopic images of NGC 5728, depicted in different shades of purple from deep purple being the nuclear aperture "N" to pale lilac in increasing distance, also noted with numbers. The apertures are drawn over (a) the continuum in Channel 1 of MIRI, extracted in the rest-frame range 5.90–6.05$\rm\mu$m, and over $[\ion{Ar}{II}]{\lambda\rm7\mu m}$, [SIV]$\lambda 10.5\rm \mu m$, [NeII]$\lambda12.8\rm \mu m$, [NeV]$\lambda14.9\rm \mu m$, [SIII]$\lambda18.7\rm \mu m$, H$\alpha$, and [Oiii]$\lambda$5007Å (b-h), line emission maps with the stellar continuum subtracted. Panel (i) presents a schematic illustration of the adopted jet axis together with the three color-shaded regions used to group the apertures (see Sec. \ref{['apertures']}). These regions are defined by the projected vertical distance from the jet axis, relative to the aperture radius: JR (innermost, cyan) includes apertures within one radius (nuclear/jet regions); IJR (intermediate, lime green) covers one to two radii (intermediately jet-affected regions); and NJR (outermost, magenta) includes apertures beyond two radii (non-jet or star-forming regions). The multi-wavelength data are aligned based on the astrometry of the nucleus.
• Figure 2: BPT diagrams depicting [Nii]/H$\alpha$, [Sii]/H$\alpha$, and [Oi]/H$\alpha$ ratios. The observations from NGC 5728, and NGC 1320 are marked with stars, and a red diamond, respectively. The cyan squares, lime green diamonds, and magenta stars represent jet-affected (JR), intermediate jet-affected (IJR), and non-jet-affected (NJR) regions, respectively, and correspond to the shaded areas in Fig. \ref{['fig:chosen_apertures']}. The Kewley and Schawinski lines are indicated with solid and dashed black, respectively, while the Koutsoumpou (SFzeta) line, is depicted by the red-solid line. In the background we show the line ratios measured for nearby galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 7 Abazajian_2009.
• Figure 3: Diagrams with the AGN photoionization models compared with the observations from the selected apertures in NGC 5728 (Fig.\ref{['fig:chosen_apertures']}). The different shades of purple going from deep purple to pale lilac/white represent the increasing distance from the nucleus, as also noted with numbers, with "N" being the nuclear aperture. The different shapes, square, thin diamond, and star, represent the nucleus/jet impacted, intermediate and distant areas, respectively. The different shades from white to deep red represent the range of ionization parameter values, $-3.5\leq \log U\leq -1.5$. All the models have solar abundances. The panels from top to bottom correspond to $\zeta_\mathrm{CR}=10^{-16}\,\rm s^{-1},\,10^{-15}\,\rm s^{-1},\,10^{-14}\,\rm s^{-1},\,10^{-13}\,\rm s^{-1}$, and $10^{-12}\,\rm s^{-1}$, respectively.
• Figure 4: Temperature and line emissivity versus depth in the simulated cloud for AGN models, for an initial density $n_{\rm H}=100\,\rm{cm^{-3}}$, and for $\zeta_\mathrm{CR}=10^{-16}\,\rm s^{-1},10^{-15}\,\rm s^{-1},\,10^{-14}\,\rm s^{-1},\,10^{-13}\,\rm s^{-1}$, and $10^{-12}\,\rm s^{-1}$, and $\log U=-3.0$. The different panels a-f correspond to kinetic temperature and the emissivity of $[\ion{S}{III}]_{\rm18.7\mu m},\, [\ion{S}{IV}]_{\rm10.5\mu m},\,[\ion{Ar}{II}]_{\rm7\mu m},\,[\ion{Ar}{III}]_{\rm8.9\mu m},\,[\ion{Ar}{V}]_{\rm7.9\mu m},\,[\ion{Ne}{II}]_{\rm12.8\mu m},\,[\ion{Ne}{III}]_{\rm15.5\mu m}$, and $[\ion{Ne}{V}]_{\rm14.3\mu m}$, respectively. The shaded area indicates the approximate region where CR heating becomes dominant. The CR-dominated area is depicted in lilac shade in the temperature plot, while in the emissivity plots, in teal shade.
• Figure 5: Ionic fraction versus depth in the simulated cloud for AGN models, for an initial density $n_{\rm H}=100\,\rm{cm^{-3}}$, and for $\zeta_\mathrm{CR}=10^{-16}\,\rm s^{-1},10^{-15}\,\rm s^{-1},\,10^{-14}\,\rm s^{-1},\,10^{-13}\,\rm s^{-1}$, and $10^{-12}\,\rm s^{-1}$, from left to right. Top row showcases the Ar and bottom row the Ne ionic fractions. The blue-shaded area indicates the approximate region where CR heating becomes dominant.