DYNAMICAL CENTERS AND NONCIRCULAR MOTIONS IN THINGS GALAXIES: IMPLICATIONS FOR DARK MATTER HALOS

We present harmonic decompositions of the velocity fields of 19 galaxies from The Hi Nearby Galaxy Survey (THINGS) which quantify the magnitude of the noncircular motions in these galaxies and yield observational estimates of the elongations of the dark matter halo potentials. Additionally, we present accurate dynamical center positions for these galaxies. We show that the positions of the kinematic and photometric centers of the large majority of the galaxies in our sample are in good agreement. The median absolute amplitude of the noncircular motions, averaged over our sample, is 6.7 kms(-1), with similar to 90% of the galaxies having median noncircular motions of less than similar to 9 kms(-1). As a fraction of the total rotation velocity, this translates into 4.5% on average. The mean elongation of the gravitational potential, after a statistical correction for an unknown viewing angle, is 0.017 +/- 0.020, which is consistent with a round potential. Our derived noncircular motions and elongations are smaller than what is needed to bring cold dark matter (CDM) simulations in agreement with the observations. In particular, the amplitudes of the noncircular motions are not high enough to hide the steep central mass-density profiles predicted by CDM simulations. We show that the amplitudes of the noncircular motions decrease toward lower luminosities and later Hubble types.