A kinematic measurement of ram pressure in the outer disc 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 H i 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 discs 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 2, corresponding to a ram wind perpendicular and parallel to the gas disc, respectively. Long-term consequences of ram pressure, such as warped discs as well as uncertainties in the disc geometry typically generate uneven modes (m = 1 and 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 similar to 30 km s(-1) (effective line-of-sight velocity change) at H i column densities below (4-10) x 10(20) cm(-2) (at radial scales greater than similar to 15 kpc). In contrast, NGC 628 is dominated by a strongly warped disc. 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.