Dark matter and visible baryons in M33

Abstract

In this paper we present new measurements of the gas kinematics in M33 using the CO J=1-0 line. The resulting rotational velocities complement previous 21-cm line data for a very accurate and extended rotation curve. The implied dark matter mass, within the total gaseous extent, is a factor 5 higher than the visible baryonic mass. Dark matter density profiles with an inner cusp as steep as R^{-1}, are compatible with M33 data. The dark matter concentrations required for fitting the M33 rotation curve are very low but marginally consistent with halos forming in a standard Cold Dark Matter cosmology. The M33 virialized dark halo is at least 50 times more massive than the visible baryons and its size is comparable with the M33-M31 separation. Inner cusps as steep as R^{-1.5} are ruled out, while halo models with a large size core of constant density are consistent with the M33 data. A central spheroid of stars is needed and we evaluate its dynamical mass range. Using accurate rotational velocity gradients and the azimuthally averaged baryonic surface densities, we show that a disk instability can regulate the star formation activity in M33. Considering the gaseous surface density alone, the predicted outer star formation threshold radius is consistent with the observed drop of the H-alpha surface brightness if a shear rate criterion is used. The classical Toomre criterion predicts correctly the size of the unstable region only when the stellar or dark halo gravity, derived in this paper, is added to that of the gaseous disk.