The Milky Way and Messier 31 (Figure 1a) are the two brightest galaxies of the Local Group and constitute the majority of its mass. One of the recurring questions is : which of these two galaxies is the most massive ? Constraints on the mass of a spiral galaxy can be obtained by measuring the kinematics (the rotation curve) of the stars or the interstellar gas of the disc. The rotation curve represents the variation of the circular rotation velocity of a kinematical tracer as a function of the distance from the center of the galaxy. The kinematical tracer the most easily observable is the gas observed thanks to the emission lines of the ionized hydrogen in the visible light at 656.3 nm and of the neutral hydrogen (referred to as H I) in radio waves at 21 cm. At the beginning of the 1990’s, observations of the H I gas of M 31 showed a decreasing rotation curve as a function of galactocentric radius. This result made M 31 a unique galaxy in the Universe because spiral galaxies generally present a rotation curve for which the velocity is constant at large galactocentric distances (Fig. 1b). An international team (Montreal, Paris, Bonn, NRAO), leaded by astronomers from the Université de Montréal and the Observatoire de Paris (Claude Carignan and Laurent Chemin) carried out new measurements of the H I rotation curve of M 31 in order to test this previous result and to provide a better estimate of its mass. The new observations were done with the 100 m radio telescopes at Effelsberg (Germany) and Green Bank (USA). These observations are more sensitive than those obtained previously and make it possible to probe more distant regions in the disc of M 31, up to a radius of 35 kpc (Fig. 1b, full square symbols). Contrary to the previous results, the new H I rotation curve of M 31 does not decrease at large radius but the velocity remains constant with respect to the galactocentric distance, reaching a value of 225 km/s. The kinematical properties of M 31 are thus comparable with those of the majority of the spiral galaxies in the Universe. A model that decomposes the rotation curve of M 31 into luminous and dark components implies the presence of a dark matter halo (Fig. 1b, purple curve) which mass does not dominate that of the luminous matter (Fig. 1b, yellow curves) inside a radius of 35 kpc. The mass of the dark matter halo is indeed approximately half of the visible mass (star and gas discs and bulge).
The total mass of M 31 (luminous + dark) integrated in a radius of 35 kpc is 3.5x1011 solar masses, which would correspond to an extrapolated mass of 5x1011 solar masses inside a radius of 50 kpc. As a comparison, the mass of the Milky Way is 5x1011 solar masses integrated inside a radius of 50 kpc. The two galaxies thus have comparable masses.
This work will continue with a complete mapping of the H I and ionized gas of M 31 by means radio and optical interferometry. These observations will make it possible to model more accurately the mass distribution of the galaxy. Other models describing the mass density distribution for the dark matter halo will be explored.
Reference The Extended H I Rotation Curve and Mass Distribution of M 31 C. Carignan, L. Chemin, W.K. Huchtmeier & F.J. Lockman 2006, The Astrophysical Journal, 641, L109
Contact Laurent Chemin (Université de Montréal et Observatoire de Paris, GEPI)