H₂ 2–1 S(1)/1–0 S(1) vs. 1–0 S(2)/1–0 S(0) emission-line ratios for selected regions of M58. Colored markers indicate the nucleus (Nuc), north and south lane (NL-SL), and the northeast and southwest H₂ lobes (NE-SW). The solid black curve marks the predicted trend for pure thermal excitation. Shaded regions represent the domains of alternative excitation mechanisms: UV thermal excitation (Sternberg 1989), X-ray heating (Draine & Woods 1990), shocks (Kristensen et al. 2023), and nonthermal UV fluorescence (Black & van Dishoeck 1987). The gray curve shows mixed thermal and fluorescent excitation models, decreasing from 90% thermal (left) in 10% steps. The top and right axes indicate the vibrational and rotational temperatures (T_vib, T_rot) respectively.