1999A&A...351...10I

CO emission from the Sb(rs)I-II galaxy NGC 7331 has been mapped in the J=2-1 transition with a 21" beam over an area 3.5' by 1.3'. A relatively low contrast enhancement of molecular line emission occurs in a ringlike zone at a distance of approximately 3.5 kpc from the center; there is no evidence for a pronounced central hole. The ring is located at the edge of the region of rigid rotation and roughly coincides with an inhomogeneous ring of nonthermal radio continuum emission. It is well inside the radius of maximum rotational velocity. The intensities of the 492 GHz [CI] line and various ¹²CO and ¹³CO transitions observed towards the center and two outlying positions are modelled by multiple molecular gas components: low-density gas at a kinetic temperature T_kin≃10K, and high-density gas at both T_kin≃10K and T_kin≃20K. The molecular gas must be distributed in clumpy or filamentary form. The CO-to-H₂ conversion factor X applicable to the bulge is only half that applicable to the ring and beyond. The latter is still significantly lower than X_Milky Way_. Molecular hydrogen is the dominant mass contributor to the interstellar medium in the bulge and in the ring. Far-infrared emission from dust peaks inside the ring at 100µm (warm dust), and in the ring at 850µm (colder dust). Beyond the ring, neutral atomic hydrogen is dominant. Inferred total hydrogen mass densities in the ring are about twice those in the bulge. Interstellar gas to dynamical mass ratios are of order 1% in the bulge, about 1.5% in the ring followed by a rise to 3%. The bulge gas may have originated in mass loss from bulge stars; in that case, the molecular ring is probably caused by a decrease in evacuation efficiency at the bulge outer edge.