SIMBAD references

We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M31, determined using a recent deep, full-disk 21-cm imaging survey smoothed to 466pc resolution. We fit a tilted ring model to the HI data from 8 to 37kpc and establish conclusively the presence of a dark halo and its density distribution via dynamical analysis of the rotation curve. The disk of M31 warps from 25kpc 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 21-cm 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 1.2x10¹²M_☉. If a dark halo model with a constant-density core is considered, the core radius has to be larger than 20kpc in order for the model to provide a good fit to the data. We extrapolate the best-fit ΛCDM and constant-density 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 M31 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 21-cm data well reproduces the mass of M31 traced out to 560 kpc. Our best estimate for the total mass of M31 is 1.3x10¹²M_☉, with 12% baryonic fraction and only 6% of the baryons in the neutral gas phase.