Smoothing Rotation Curves and Mass Profiles

TL;DR

Spiral activity in disk galaxies can smooth out features in rotation curves and mass profiles, explaining the observed uniformity and contributing to exponential disk profiles.

Contribution

This study demonstrates that spiral dynamics naturally erase features in rotation curves and mass profiles, providing a physical mechanism for the observed smoothness in galaxy rotation curves.

Findings

Spiral activity redistributes disk material, erasing density features.

Unstable spiral modes develop at density ridges, spreading particles.

The mechanism explains the disk-halo conspiracy in galaxy rotation curves.

Abstract

We show that spiral activity can erase pronounced features in disk galaxy rotation curves. We present simulations of growing disks, in which the added material has a physically motivated distribution, as well as other examples of physically less realistic accretion. In all cases, attempts to create unrealistic rotation curves were unsuccessful because spiral activity rapidly smoothed away features in the disk mass profile. The added material was redistributed radially by the spiral activity, which was itself provoked by the density feature. In the case of a ridge-like feature in the surface density profile, we show that two unstable spiral modes develop, and the associated angular momentum changes in horseshoe orbits remove particles from the ridge and spread them both inwards and outwards. This process rapidly erases the density feature from the disk. We also find that the lack of a feature when transitioning from disk to halo dominance in the rotation curves of disk galaxies, the so called "disk-halo conspiracy", could also be accounted for by this mechanism. We do not create perfectly exponential mass profiles in the disk, but suggest that this mechanism contributes to their creation.