Dark matter fraction derived from the M31 rotation curve

TL;DR

This study models M31 as a merger remnant to accurately derive its dark matter fraction from the rotation curve, revealing that 68% of its mass within 137 kpc is dark matter.

Contribution

It presents a dynamical model of M31 incorporating its merger history, improving mass estimates and dark matter fraction determination.

Findings

M31's total mass within 137 kpc is 4.5 x 10^11 solar masses.

68% of M31's mass within this radius is dark matter.

The model reproduces key features of M31's rotation curve.

Abstract

Mass estimates of a spiral galaxy derived from its rotation curve must account for the galaxy's past accretion history. There are several lines of evidence indicating that M31 experienced a major merger 2 to 3 Gyr ago. Here, we have generated a dynamical model of M31 as a merger remnant that reproduces most of its properties, from the central bar to the outskirts. The model accounts for the past major merger, and reproduces the details of M31's rotation curve, including its 14 kpc bump and the observed increase of velocity beyond 25 kpc. Furthermore, we find non-equilibrium and oscillatory motions in the gas of the merger-remnant outskirts caused by material in a tidal tail returning to the merger remnant. A total dynamical M31 mass of 4.5 $\times 10^{11} M_{\odot}$ within 137 kpc has been obtained after scaling it to the observed HI rotation curve. Within this radial distance, 68% of the total dynamical mass is dark.