Radial distribution of the HCN/CO(2–1) intensity ratio across NGC 253. Sight lines are color-coded based on the environment. We show points with a signal-to-noise ratio above three in intensity and the peak brightness temperature. A small panel in this figure shows the distribution of the HCN/CO(2–1) line intensity ratio per environment in NGC 253: the center, bar, ring, spiral arms, and disk. The mean line ratio measured in each environment is shown as a horizontal line.