The Similar Signatures of Coplanar Gas Inflow and Disk Warps in Galactic Gas Kinematic Maps

TL;DR

This paper investigates how radial gas inflows and disk warps produce similar signatures in galactic gas kinematic maps, complicating the distinction between these phenomena and affecting inflow estimates.

Contribution

It demonstrates that inflows and warps create similar kinematic signatures, and provides observational evidence that warps likely dominate over inflows in observed galaxies.

Findings

Inflow effects mimic disk warps in velocity fields.

Warp signatures are more prevalent than inflow signatures in the sample.

Treating warp-induced distortions as inflows can underestimate actual inflow rates.

Abstract

Hydrodynamic simulations suggest that galactic gas disks form when coplanar gas spirals into the inner regions of the disk. We recently presented a simple "modified accretion disk" model of viscous galactic disks in which star-formation is fed by a radial flow of gas. However, little observational evidence has been presented for such inflows, which are expected to be only a few km s$^{-1}$ in the central regions of the disk, i.e. within three disk scale-lengths, but could reach of order 50-100 km s$^{-1}$ in the very outer disk. The effects of systematic inflow on the 2-d velocity field are examined and it is shown that these are quite similar to those produced by geometric warps of the disks, with twist distortions of both the kinematic major and minor axes. This makes it potentially difficult to distinguish between these in practice. By comparing the handedness of the observed twisting of the kinematic axes and of the spiral arms for a sample of nearby galaxies, we find (assuming that the spiral arms are generally trailing) that the effects of warps are in fact likely to dominate over the effects of radial inflows. However, the common practice of treating these twist distortions of the kinematic major and minor axes as being due only to warps can lead, for galaxies of low-to-intermediate inclinations, to substantial underestimates of any systematic inflow.