CO emission from the Sb ( rs ) I-II galaxy NGC 7331 has been mapped in the J =2–1 transition with a 21 \arcsec beam over an area 3.5 \arcmin by 1.3 \arcmin .
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 ^ { 12 } CO and ^ { 13 } CO 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 } \approx 10 K , and high-density gas at both T _ { kin } \approx 10 K and T _ { kin } \approx 20 K. The molecular gas must be distributed in clumpy or filamentary form .
The CO-to-H _ { 2 } 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 _ { MilkyWay } .
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 100 \mu m ( warm dust ) , and in the ring at 850 \mu m ( 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 .