A wide-field H I mosaic of Messier 31

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, full-disk 21-cm 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 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 1.2 × 10 12 M . If a dark halo model with a constant-density 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 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 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 21-cm 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.

Molecular gas in the inner 0.7 kpc-radius ring of M 31

“…Whatever inclination it has, the gradient in velocity across a small region of 40 pc should be negligible. Ad hoc hypotheses with HI infall/outflow with important radial velocity are then required, which are incompatible with the HI observations (e.g., Corbelli et al 2010). We therefore think that, unless the gas is completely out of equilibrium (which is not really supported by the observations of Corbelli et al 2010;Casoli & Combes 1988), the peculiar component cannot come from the outer warp.…”

“…Ad hoc hypotheses with HI infall/outflow with important radial velocity are then required, which are incompatible with the HI observations (e.g., Corbelli et al 2010). We therefore think that, unless the gas is completely out of equilibrium (which is not really supported by the observations of Corbelli et al 2010;Casoli & Combes 1988), the peculiar component cannot come from the outer warp. However, we have some tentative detections close to the systemic velocity (M 31K, M 31I, M 31G, M 31J), which could be accounted for by the external warp.…”

“…The warp is visually conspicuous, and remarkable in HI (Corbelli et al 2010). Farther in the north-east disc (beyond 10 kpc), Casoli & Combes (1988) already noted that CO emission could come from the main disc and the warped component, both on the same line of sight.…”

“…There is, however, some HI emission in the central kiloparsec, but most of it comes near the systemic velocity, and is likely to be projected emission from the external tilted gas orbits, which are warped to an almost edge-on inclination (e.g., Corbelli et al 2010). To subtract this projected large-radii emission, and better see the residual coming from the actual centre, with a large velocity gradient, we have summed all channels avoiding ±100 km s −1 around the systemic velocity of −310 km s −1 .…”

The extended disc and halo of the Andromeda galaxy observed withSpitzer-IRAC

“…The old, low-mass stars which make up the bulk of the stellar mass (Rix & Rieke 1993) have their spectral energy distribution (SED) peaks in the near-infrared band, where the effects of extinction are greatly reduced compared to visible light. This is especially important in a highly-inclined galaxy such as M31 (i = 77 • , Corbelli et al 2010). …”