Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure

TL;DR

This study uses hydrodynamical simulations to analyze the complex, tidally induced spiral structure of galaxy M51, challenging the traditional density wave theory by showing the absence of a steady pattern speed and the presence of dynamic, transient features.

Contribution

The paper demonstrates that M51's spiral arms are better explained by tidally induced, transient density patterns rather than steady, quasi-stationary density waves, based on detailed simulations and observational comparisons.

Findings

No consistent offset between stars and gas in M51.

Spiral arms lack a single global pattern speed.

Spiral arms are not well fitted by logarithmic spirals.

Abstract

We present hydrodynamical models of the grand design spiral M51 (NGC 5194), and its interaction with its companion NGC 5195. Despite the simplicity of our models, our simulations capture the present day spiral structure of M51 remarkably well, and even reproduce details such as a kink along one spiral arm, and spiral arm bifurcations. We investigate the offset between the stellar and gaseous spiral arms, and find at most times (including the present day) there is no offset between the stars and gas to within our error bars. We also compare our simulations with recent observational analysis of M51. We compute the pattern speed versus radius, and like the observations, find no single global pattern speed. We also show that the spiral arms cannot be fitted well by logarithmic spirals. We interpret these findings as evidence that M51 does not exhibit a quasi-steady density wave, as would be predicted by density wave theory. The internal structure of M51 derives from the complicated and dynamical interaction with its companion, resulting in spiral arms showing considerable structure in the form of short-lived kinks and bifurcations. Rather than trying to model such galaxies in terms of global spiral modes with fixed pattern speeds, it is more realistic to start from a picture in which the spiral arms, while not being simple material arms, are the result of tidally induced kinematic density `waves' or density patterns, which wind up slowly over time.