Molecular spiral structure in M51

Abstract

Observations of CO emission at 7 arcsec resolution from the central 2 arcmin x 1 arcmin of M51 show directly that molecular gas, probably complexes of relatively warm giant molecular clouds, is strongly confined to spiral arms outlined by prominent dust lanes and radio continuum ridges. Molecular clouds must also populate fairly uniformly the interarm regions. The arm-to-interarm N(H2) ratio, averaged over 300 pc, is at least three. These observations suggest that in the inner region of M51 molecular clouds coalesce to form bigger complexes in the presence of a spiral gravitational potential, and that such complexes are redispersed into molecular clouds upon leaving the arms. The spiral arm molecular cloud complexes last at most 50 million yr, the arm crossing time, and molecular clouds probably last as long as the interarm crossing time which is at most 100 million yr at 45 arcsec (2.2 kpc) from the center.