1999/09/22 by F. P. Israel, Israel, F. P., F. Baas +1
Decision Sciences · Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics (astro-ph) #Astrophysics and Star Formation Studies #FOS: Physical sciences #Scientific Measurement and Uncertainty Evaluation #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9909373
12 pages, 4 figures, A&A in press
arxiv created 1999/09/22 · openalex publication_date 1999/09/22 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/04
Maps of the J=2-1 12CO emission from the SbII galaxy NGC 7331 show a low-contrast ring at a radius of about 3.5 kpc. There is no evidence for a pronounced central hole in the CO distribution as claimed by others. The molecular ring is just outside the radius of peak emission from warm dust, but coincides with the peak of colder dust emission. Various 12CO and 13CO transitions have been observed from three positions including the center, which was also observed in the 492 GHz transition. The line measurements have been modelled by emission from a clumpy mixture of low-density molecular gas at about T(kin) = 10 K and high-density molecular gas at temperatures of 10 K and 20 K. The CO to H2 conversion factor in NGC 7331 is lower than that in the Milky Way, and lowest in the center of NGC 7331. The total interstellar gas mass is dominated by molecular hydrogen in the bulge and in the ring, and by atomic hydrogen outside the ring. Total hydrogen mass densities in the ring are about twice those in the bulge. Total gas to dynamic mass ratios increase from 1% in the bulge to 3% outside the ring. The bulge molecular gas may have originated in mass loss from bulge stars, in which case the molecular ring is probably the consequence of evacuation efficiency decreases at the outer bulge edge.