Top panel: Spectrum of the uniformly stacked CO(2–1) emission (in black). Bottom panel: Spectrum of the CO(2–1) emission (in black) extracted from the uniformly stacked CO(2–1) cubes. In the left panel, we show a single-Gaussian fit (in red) and its residuals and corresponding reduced χ² in the bottom panel. In the right panel, we show a two-Gaussian fit (the sum of the two Gaussians in red) composed of a narrow component (green dashed line) and a broad component (blue dashed line).