Peculiarities in the stellar velocity distribution of galaxies with a two-armed spiral structure

TL;DR

This study investigates how the stellar velocity distribution in two-armed spiral galaxies is affected by spiral density waves, revealing abnormal velocity ellipsoid ratios and limitations of the epicycle approximation near spiral arms.

Contribution

It extends previous numerical analysis by examining the impact of different spiral density wave amplitudes on stellar velocity distributions and the validity of the epicycle approximation.

Findings

The : ratio of the stellar velocity ellipsoid becomes abnormally small near spiral arms.

Stronger spiral density waves significantly decrease the : ratio.

The epicycle approximation is invalid near spiral arms with high density wave amplitudes.

Abstract

We expand our pervious numerical study of the properties of the stellar velocity distribution within the disk of a two-armed spiral galaxy by considering spiral stellar density waves with different global Fourier amplitudes, C_2. We confirm our previous conclusion that the ratio \sigma_1:\sigma_2 of smallest versus largest principal axes of the stellar velocity ellipsoid becomes abnormally small near the outer edges of the stellar spiral arms. The extent to which the stellar velocity ellipsoid is elongated (as compared to the unperturbed value typical for the axisymmetric disk) increases with the strength of the spiral density wave. In particular, the C_2=0.06 spiral can decrease the unperturbed value of \sigma_1:\sigma_2 by 20%, while the C_2=0.13 spiral can decrease the unperturbed \sigma_1:\sigma_2 by a factor of 3. The abnormally small values of the \sigma_1:\sigma_2 ratio can potentially be used to track the position of {\it stellar} spiral density waves. The \sigma_{\phi\phi}:\sigma_{rr} ratio is characterized by a more complex behaviour and exhibits less definite minima near the outer edges of the spiral arms. We find that the epicycle approximation is violated near the spiral arms and cannot be used in spiral galaxies with C_2 >= 0.05-0.06 or in galaxies with the amplitude of the spiral stellar density wave (relative to the unperturbed background) of order 0.1 or greater.