Dark matter and visible baryons in M33

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 of this nearby galaxy. The implied dark matter mass, within the total gaseous extent, is a factor of 5 higher than the visible baryonic mass. Dark matter density profiles with an inner cusp as steep as R-1, suggested by some numerical simulation of structures formation, are compatible with the actual data. The dark matter concentrations required for fitting the M33 rotation curve are very low but still marginally consistent with haloes 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 excess 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 disc 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 Halpha surface brightness if a shear rate criterion is used with the lowest possible value of velocity dispersion. The classical Toomre criterion predicts the size of the unstable region correctly only when the stellar or dark halo gravity, derived in this paper, is added to that of the gaseous disc.