Distribution of “cold" molecular gas derived from different CO line transitions. Upper panels: Velocity-integrated intensity maps derived from the CO(1–0), CO(2–1), and CO(3–2) lines in the central 400 pc×400 pc region of NGC 7172. Contour levels have a logarithmic spacing from 2.5σ to 90% of the peak CO intensity inside the displayed field of view. The (blue) filled ellipses at the bottom left corners represent the beam sizes of the observations. Lower panels: Normalized radial distributions of molecular gas derived out to r = 200 pc from the 3 CO maps of NGC 7172 shown in the upper panels. The estimated molecular gas concentration indices are displayed in panels d)-to-f).