A widefield H I mosaic of Messier 31. II. The disk warp, rotation, and the dark matter halo
Abstract
Aims: We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M 31, determined using a recent deep, fulldisk 21cm imaging survey smoothed to 466 pc resolution.
Methods: We fit a tilted ring model to the HI data from 8 to 37 kpc and establish conclusively the presence of a dark halo and its density distribution via dynamical analysis of the rotation curve.
Results: The disk of M 31 warps from 25 kpc outwards and becomes more inclined with respect to our line of sight. Newtonian dynamics without a dark matter halo provide a very poor fit to the rotation curve. In the framework of modified Newtonian dynamic (MOND) however the 21cm rotation curve is well fitted by the gravitational potential traced by the baryonic matter density alone. The inclusion of a dark matter halo with a density profile as predicted by hierarchical clustering and structure formation in a ΛCDM cosmology makes the mass model in newtonian dynamic compatible with the rotation curve data. The dark halo concentration parameter for the best fit is C = 12 and its total mass is 1.2 × 10^{12} M_⊙. If a dark halo model with a constantdensity core is considered, the core radius has to be larger than 20 kpc in order for the model to provide a good fit to the data. We extrapolate the bestfit ΛCDM and constantdensity core mass models to very large galactocentric radii, comparable to the size of the dark matter halo. A comparison of the predicted mass with the M 31 mass determined at such large radii using other dynamical tracers, confirms the validity of our results. In particular the ΛCDM dark halo model which best fits the 21cm data well reproduces the mass of M 31 traced out to 560 kpc. Our best estimate for the total mass of M 31 is 1.3 × 10^{12} M_⊙, with 12% baryonic fraction and only 6% of the baryons in the neutral gas phase.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 February 2010
 DOI:
 10.1051/00046361/200913297
 arXiv:
 arXiv:0912.4133
 Bibcode:
 2010A&A...511A..89C
 Keywords:

 galaxies: ISM;
 galaxies: individual M 31;
 galaxies: kinematics and dynamics;
 dark matter;
 radio lines: galaxies;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 19 pages, 16 figures, accepted for publication in A&