Star Formation and Asymmetry in the Spiral Arms of M51: Variable Star Formation Caused by More than One Spiral Density Wave

In the inner 3kpc of M51 we find that logarithmic spirals provide good fitsto the peak intensities in molecular gas observed by BIMA in the CO (J=1-0)emission line along the spiral arms. However, we measure significantasymmetries between the location and density of the arms observed on one sideof the galaxy compared to those on the opposite side. Between a radius of 1 and2.2 kpc the gas distribution traced in CO is lopsided with densities twice aslarge in one arm compared to the opposite one. In the same region, the spiralarms are offset by 20 degrees from the position of the arm on the opposite sideof the galaxy after a rotation of 180 degrees. We use the ratio of CO emissionto that in the Pa alpha hydrogen recombination line to estimate the efficiencyof star formation along each arm. Except for a narrow region at about 2kpcwhere star formation is surprisingly inefficient, the gas depletion time isapproximately 200 million years despite large variations in the molecular gasdensity in the arms. We account for the deviations from two armed bi-symmetric structure with asimple model that includes an additional three-armed spiral density wave. Thismodel accounts for the angular offset between the arm on one side compared toits opposite, the lopsided distribution in molecular gas, and inter-arm starformation to the north east of the galaxy nucleus. Since the star formationefficiency is unaffected by the variations in gas density, and the variationsin gas density can be accounted for by the presence of an additionalthree-armed density wave, we suggest that the star formation rate is variable,and is highest where and when the maxima of the two spiral density wavescoincide or constructively add.Comment: 18 pages, 7 figures, 1 tabl