A new high-resolution CO map of the inner 2.' 5 of M51. I. Streaming motions and spiral structure

The Owens Valley millimeter array has been used to map the CO 1-0 emission in the inner 2.'5 of the grand-design spiral galaxy M51 at 2 "-3 " resoluti on These new images reveal the molecular spiral arms with unprecedented cla rity; the emission in the two major arms (northeast and southwest) originat es from supermassive cloud complexes, giant molecular associations (GMAs), which are for the first time resolved both along and perpendicular to the a rms. The overall morphology of the CO emission is symmetric in reflection a bout the nucleus, with major complexes occurring opposite each other in the two major arms. On the other hand, the CO line flux in the area of the sou thwest arm closest to the nucleus is approximately twice as bright as that from the analogous location in the northeast arm. Streaming motions can be studied in detail and appear with great clarity along the major and minor a xes of M51. The streaming velocities are very large, 60-150 km s(-1). Our m aps offer, for the first time, sufficient resolution to resolve the structu re in the molecular streaming motions. Both the radial and tangential veloc ity components show steep gradients, in qualitative accordance with predict ions of the density-wave models of Roberts & Stewart. Our data thus support the presence of galactic shocks in the arms of M51. In general, velocity g radients across arms are higher by a factor of 2-10 than previously found. They vary in steepness along the spiral arms, becoming particularly steep i n between GMAs. The steep gradients cause conditions of strong reverse shea r in several regions in the arms, and thus the notion that shear is general ly reduced by streaming motions in spiral arms will have to be modified. Of the three GMAs studied on the southwest arm, only one shows reduced shear. We find an unusual structure, an expansion in the northeast molecular arm at 25 " radius (1.2 kpc) southeast of the center. This broadening occurs ri ght after the end of the northeast arm at the inner Lindblad resonance (ILR ). Multiple-peak spectra, velocity twists, and structures with apparently h igh velocity dispersion are associated with this feature. Bifurcations in t he molecular spiral arm structure, at a radius of approximate to 3.3 kpc (7 3 "), may be evidence of a secondary compression of the gas caused by, and occurring near, the 4/1 ultraharmonic resonance. Several molecular spurs pr otrude from the main spiral arms, in particular on the western side of the map. Inside the radius of the ILR, we detect narrow (approximate to 5 ") mo lecular spiral arms, possibly related to the K-band arms found in the same region. We find evidence of noncircular motions in the inner 20 " of M51. T he magnitude of these deviations is 20-30 km s(-1), and they are consistent with gas on elliptical orbits in a bar.