Dark matter fraction derived from the M31 rotation curve

¹ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, Cergy Paris Université, 92190 Meudon, France
² Aix-Marseille Univ., CNRS, CNES, LAM, 38 rue Frédéric Joliot Curie, 13338 Marseille, France
³ Instituto de Astrofisica, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
⁴ Institut d’Astrophysique de Paris (UMR7095: CNRS & Sorbonne Université), 98 bis Bd Arago, 75014 Paris, France
⁵ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Universita di Padova, Vicolo Osservatorio 3, I-35122 Padova, Italy

^⋆ Corresponding author; francois.hammer@obspm.fr

Received: 25 October 2024
Accepted: 30 November 2024

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. In this work, we generated a dynamical model of M31 as a merger remnant that reproduces most of its properties, including from the central bar to the outskirts. The model accounts for M31’s past major merger and reproduces the details of its rotation curve, including its 14 kpc bump and the observed increase of velocity beyond 25 kpc. 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 × 10¹¹ M_⊙ within 137 kpc was obtained after scaling it to the observed HI rotation curve. Within this radial distance, we find that 68% of the total dynamical mass is dark.