Revealing Hidden Excitation in the Core of NGC 4696: New Infrared Diagnostic Diagrams with JWST

Abstract

The warm molecular and ionized gas in filamentary nebulae of cool-core BCGs provide a sensitive probe of the poorly understood exchange of energy between the ICM, cold molecular gas, and AGN feedback. Optical line ratios commonly yield similar LINER-like classifications across these systems, but do not resolve how the excitation of warm molecular gas varies between compact nuclear structures and surrounding filaments. We present JWST/NIRSpec integral-field spectroscopy of the central $3^{\prime \prime }\times 3^{\prime \prime }$ ($620\times 620\mathrm{p}{\mathrm{c}}^2$) of the Centaurus cluster BCG, NGC~4696. The NIR data resolve H$I$ and ro-vibrational H${}_2$ emission across compact nuclear apertures, a circumnuclear swirl, and filamentary structures. H${}_2$ excitation diagrams yield characteristic low-excitation temperatures of $T_{\mathrm{ex}}\simeq 1.8-2.2\times 10^3$~K, while curvature at higher energies requires hotter or non-thermal components. The H${}_2$/Pa$\alpha$ balance varies strongly: filamentary regions have H${}_2$ 1-0 S(1)/Pa$\alpha$ ratios 4-8 times higher than the compact nuclear apertures. In contrast, matched-aperture optical diagnostic diagrams show substantially less regional separation, with all regions occupying a low-ionization locus. We thus introduce new spatially resolved NIR diagnostic diagrams that reveal distinct molecular excitation conditions across the nucleus, swirl, and filaments. Ratios are broadly consistent with contributions from AGN-related photoionization, ICM-related cooling radiation, and non-radiative heating, although the present data do not uniquely determine their relative importance. These results demonstrate how spatially resolved NIR spectroscopy reveals excitation variations hidden in conventional optical classifications, providing a new route to constrain the multiphase cooling-feedback cycle in cluster cores.

arXiv commentSubmitted to ApJ and revised in response to referee comments (32 pages, 10 figures)