Rotation Curve Anomaly and Galactic Warp in M51

TL;DR

This paper explains the rotation curve anomaly and counter rotation in galaxy M51 as effects of a warped galactic disk, revealing a complex warp structure through tilted-ring analysis.

Contribution

It introduces a detailed warp model of M51's disk, linking the observed rotation anomalies to disk warping rather than dark matter effects.

Findings

Disk is nearly flat in the inner region

A significant warp occurs at 7.5 kpc with a 27° bend

Outer disk becomes face-on at 18 kpc, causing apparent counter rotation

Abstract

We revisit the anomaly of rotation curve in the nearly face-on galaxy M51 that shows an apparently faster decrease of rotation velocity than the Keplerian law in the outer disk, further showing apparent counter rotation in the outermost HI disk. We interpret this anomaly as due to warping of the galactic disk, and determined the warping structure of M51's disk using the tilted-ring method, assuming that the intrinsic rotation curve is normal. It is shown that the disk is nearly flat in the inner disk at a constant inclination angle, but the disk suddenly bends at radius 7.5 kpc by about 27$\deg$. The inclination angle, then, decreases monotonically outward reaching a perfect face-on ring at 18 kpc, beyond which the disk is warped in the opposite sense to the inner disk, resulting in apparent counter rotation.