A Kinematic Measurement of Ram Pressure in the Outer Disk of Regular Galaxies

TL;DR

This paper introduces a novel kinematic method to measure ram pressure effects in galaxy disks caused by intergalactic medium, enabling new insights into galaxy motion and IGM properties at low densities.

Contribution

The study develops a new model to detect ram pressure perturbations in galaxy HI velocity fields, applicable to warped and inclined disks, and applies it to real galaxies to measure their motion relative to the IGM.

Findings

Detected ram pressure perturbations of ~30 km/s in two galaxies.

Constrained the galaxies' 3D motion relative to the IGM.

Provided new measurements of galaxy-IGM interactions at low densities.

Abstract

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.