A molecular spiral arm segment of M51 at 3.5 arcsecond resolution

Abstract

A CO (1-0) map of two 1' fields in M51 at 3.5'' resolution made with the Millimeter Interferometer of the Owens Valley Radio Observatory is presented. The fields include three arm and two interarm Giant Molecular Associations (GMAs) identified earlier in the 8'' mosaic map of Rand & Kulkarni. The GMAs are resolved at this higher resolution. One of the arm GMAs is a less distinct feature in the map than in the mosaic. For the two well-defined arm GMAs and a newly discovered concentration of gas in the arm, the virial masses based on observed CO velocities are 2-4 times below the masses based on their CO fluxes. If we demand that the virial masses of GMAs be no lower than the CO-flux-based masses, then this mass discrepancy hints that the standard conversion factor between CO brightness and molecular column density appropriate for the Galaxy may be too high for M51, by perhaps a factor of 2 or 3. For the two interarm GMAs, the virial masses are 1 and 3 times their CO-flux-based masses, but are rather more uncertain. If the conversion factor is indeed lower in M51, then both of these GMAs are unbound The brightness temperature of the GMAs and the (CO)-C-12/(CO)C-13 ratio found by Young & Sanders also hint at a lower conversion factor for M51. However, it remains unlikely that the conversion factor is low enough to make the virial masses of the arm GMAs significantly larger than the CO-flux-based masses. Several other results from this map aid in our understanding of how GMAs form. Where the spiral arm crosses the major axis, the observed shift in tangential velocity relative to a flat rotation curve agrees with the shift measured from the 8'' mosaic, indicating a tangential streaming motion as expected in the presence of a density wave and confirming that this segment of the spiral arm is a region of reduced shear. The streaming is much stronger, however, than seen in simulations of the gaseous response to a density wave. The GMAs appear to be stable or marginally stable against galactic tidal forces if the adopted conversion factor is correct to within a factor of 2-3. The detailed spatial coincidence between molecular emission and Halpha emission is poor. Three possible causes for this breakdown on small scales are (1) real SFE variations, (2) star formation at different stages along the arm, and (3) patchy extinction. The one-dimensional velocity dispersion in the arms at this resolution is approximately 10 km s-1.