Disk-stability constraints on the number of arms in spiral galaxies

TL;DR

This paper introduces an analytical model based on disk-stability criteria to predict the number of spiral arms in disk galaxies, linking galaxy structure to spiral morphology and enabling constraints on dark matter halo contributions.

Contribution

It provides a new analytic expression to estimate spiral arm numbers from galaxy structural properties, facilitating large survey analyses and halo mass constraints.

Findings

Model predicts spiral arm number from galaxy properties.

Application to Milky Way and survey data constrains dark halo mass.

Analytic approach simplifies large-scale galaxy morphology studies.

Abstract

A model based on disk-stability criteria to determine the number of spiral arms of a general disk galaxy with an exponential disk, a bulge and a dark halo described by a Hernquist model is presented. The multifold rotational symmetry of the spiral structure can be evaluated analytically once the structural properties of a galaxy, such as the circular speed curve, and the disk surface brightness, are known. By changing the disk mass, these models are aimed at varying the critical length scale parameter of the disk and lead to a different spiral morphology in agreement with prior models. Previous studies based on the swing amplification and disk stability have been applied to constrain the mass-to-light ratio in disk galaxies. This formalism provides an analytic expression to estimate the number of arms expected by swing amplification making its application straight-forward to large surveys. It can be applied to predict the number of arms in the Milky Way as a function of radius and to constrain the mass-to-light ratio in disk galaxies for which photometric and kinematic measurements are available, like in the DiskMass survey. Hence, the halo contribution to the total mass in the inner parts of disk galaxies can be inferred in light of the ongoing and forthcoming surveys.