Emission-line diagnostic diagrams of UG101. Left panels: From top to bottom, BPT–[N II], BPT–[S II], and BPT–[O I]. The solid black and red curves correspond to the classification lines introduced by Kewley et al. (2001) and Kewley et al. (2006), respectively. In the BPT–[N II] diagram, the dotted green and dashed blue curves indicate the divisions proposed by Sharp & Bland-Hawthorn (2010) and Kauffmann et al. (2003), respectively. Each point corresponds to an individual spaxel, and only those with S/N ≥ 4 in all required lines are shown. Mean uncertainties in the line ratios are indicated in the bottom-right corner of each panel. The green square shows the position of the stacked spectrum considering all spaxels classified as LINERs in the BPT–[O I] diagram. Right panels: Spatial distribution of spaxels color-coded according to the left-hand diagrams, overlaid on the MUSE white-light image of UG101. Dashed contours indicate the extent of the galaxy main body, for reference.