Dynamics of stars around spiral arms in an N-body/SPH simulated barred-spiral galaxy

TL;DR

This study uses detailed N-body/SPH simulations to analyze the transient nature and dynamics of spiral arms in a barred-spiral galaxy, revealing star migration patterns and challenging classical density wave theory.

Contribution

It provides new insights into the transient behavior of spiral arms and star migration, showing their pattern speeds and interactions with the bar differ from classical theories.

Findings

Spiral arms are transient features with pattern speeds similar to star rotation.

Star particles migrate towards and away from arms, indicating systematic radial migration.

No significant offset of star forming tracers across spiral arms, contradicting classical density wave theory.

Abstract

We run N-body smoothed particle hydrodynamics (SPH) simulations of a Milky Way sized galaxy. The code takes into account hydrodynamics, self-gravity, star formation, supernova and stellar wind feedback, radiative cooling and metal enrichment. The simulated galaxy is a barred-spiral galaxy consisting of a stellar and gas disc, enveloped in a static dark matter halo. Similar to what is found in our pure N-body simulation of a non-barred galaxy in Grand et. al. (2012), we find that the spiral arms are transient features whose pattern speeds decrease with radius, in such a way that the pattern speed is similar to the rotation of star particles. Compared to the non-barred case, we find that the spiral arm pattern speed is slightly faster than the rotation speed of star particles: the bar appears to boost the pattern speed ahead of the rotational velocity. We trace particle motion around the spiral arms at different radii, and demonstrate that there are star particles that are drawn towards and join the arm from behind (in front of) the arm and migrate toward the outer (inner) regions of the disc until the arm disappears as a result of their transient nature. We see this migration over the entire radial range analysed, which is a consequence of the spiral arm rotating at similar speeds to star particles at all radii, which is inconsistent with the prediction of classical density wave theory. The bar does not prevent this systematic radial migration, which is shown to largely preserve circular orbits. We also demonstrate that there is no significant offset of different star forming tracers across the spiral arm, which is also inconsistent with the prediction of classical density wave theory.