Large-scale asymmetry of rotation curves in lopsided spiral galaxies

Many spiral galaxies show a large-scale asymmetry with a cos phi dependence in their rotation curves as well as in their morphology, such as M 101 and NGC 628. We show that both these features can be explained by the response of a galactic disk to an imposed lopsided halo potential. A perturbation potential of 5% is deduced for the morphologically lopsided galaxies in the Rix & Zaritsky (1995) sample. This is shown to result in a difference of 10% or greater than or equal to 20-30 km s(-1) in the rotation velocity on the two sides of the major axis. Interestingly, the observed isophotal asymmetry in a typical spiral galaxy is not much smaller and it results in a velocity asymmetry of 7% or similar to14-21 km s(-1). Hence, we predict that most spiral galaxies show a fairly significant rotational asymmetry. The rotation velocity is shown to be maximum along the elongated isophote in agreement with the observations along the SW in M 101, while it is minimum along the opposite direction. This result leads to the distinctive asymmetric shape of the rotation curve which rises more steeply in one half of the galaxy than the other, as observed by Swaters et al. (1999). This shape is shown to be a robust feature and would result for any centrally concentrated disk. The net disk lopsidedness and hence the asymmetry in the rotation curve is predicted to increase with radius and hence can be best studied using HI gas as the tracer.