A Kinematic Measurement of Ram Pressure in the Outer Disk of Regular   Galaxies
  While most ram pressure studies have focused on ram pressure stripping in galaxy clusters, we devise a novel approach based on a kinematic measurement of ram pressure perturbations in HI velocity fields for intergalactic material (IGM) densities and relative velocities that are one to two orders of magnitude lower than in galaxies showing ram pressure stripping. Our model evaluates ram pressure induced kinematic terms in gas disks with constant inclination as well as those with a warped geometry. Ram pressure perturbations are characterized by kinematic modes of even order, m=0 and m=2, corresponding to a ram wind perpendicular and parallel to the gas disk, respectively. Long-term consequences of ram pressure, such as warped disks as well as uncertainties in the disk geometry typically generate uneven modes (m=1 and m=3), that are clearly distinguishable from the kinematic ram pressure terms. We have applied our models to three nearby isolated galaxies, utilizing Markov Chain Monte Carlo fitting routines to determine ram pressure perturbations in the velocity fields of NGC 6946 and NGC 3621 of ~30km s$^{-1}$ (effective line-of-sight velocity change) at HI column densities below (4-10)$\times$10$^{20}$cm$^{-2}$ (at radial scales greater than ~15kpc). In contrast, NGC 628 is dominated by a strongly warped disk. Our model fits reveal the three-dimensional vector of the galaxies' movement with respect to the IGM rest-frame and provide constraints on the product of speed with IGM density, opening a new window for extragalactic velocity measurements and studies of the intergalactic medium. 
Sebastian Haan and Robert Braun
astro-ph.GA