The Galactic Branches as a Possible Evidence for Transient Spiral Arms

TL;DR

This study uses simulations to compare gaseous and stellar responses to different spiral arm models in the Milky Way, suggesting that features like branches may indicate transient spiral arms.

Contribution

It introduces a detailed comparison between cosine and density-based spiral potential models, proposing a new explanation for galactic branches as transient features.

Findings

Density-based models produce four-armed gaseous responses, unlike cosine models.

Four-armed structures are linked to ultra harmonic resonances or shocks.

Branches may be indicators of transient spiral arms.

Abstract

With the use of a background Milky-Way-like potential model, we performed stellar orbital and magnetohydrodynamic (MHD) simulations. As a first experiment, we studied the gaseous response to a bisymmetric spiral arm potential: the widely employed cosine potential model and a self-gravitating tridimensional density distribution based model called PERLAS. Important differences are noticeable in these simulations, while the simplified cosine potential produces two spiral arms for all cases, the more realistic density based model produces a response of four spiral arms on the gaseous disk, except for weak arms -i.e. close to the linear regime- where a two-armed structure is formed. In order to compare the stellar and gas response to the spiral arms, we have also included a detailed periodic orbit study and explored different structural parameters within observational uncertainties. The four armed response has been explained as the result of ultra harmonic resonances, or as shocks with the massive bisymmetric spiral structure, among other. From the results of this work, and comparing the stellar and gaseous responses, we tracked down an alternative explanation to the formation of branches, based only on the orbital response to a self-gravitating spiral arms model. The presence of features such as branches, might be an indication of transiency of the arms.