2014A&A...572A..23C

We determine the mass distribution of stars, gas, and dark matter in M33 to test cosmological models of galaxy formation and evolution. We map the neutral atomic gas content of M33 using high resolution Very Large Array and Green Bank Telescope observations of the 21cm HI line emission. A tilted ring model is fitted to the HI datacube to determine the varying spatial orientation of the extended gaseous disk and the rotation curve. We derive the stellar mass surface density map of M33's optical disk via pixel-SED fitting methods based on population synthesis models that allow for positional changes in star formation history. Stellar and gas maps are used in the dynamical analysis of the rotation curve to constrain the dark halo properties. The disk of M33 warps from 8kpc outward without substantial change of its inclination with respect to the line of sight; the line of nodes rotates clockwise toward the direction of M31. Rotational velocities rise steeply with radius in the inner disk, reaching 100km/s in 4kpc, then the rotation curve becomes more perturbed and flatter with velocities as high as 120-130km/s out to 2.7R₂₅. The stellar surface density map highlights a star-forming disk with a varying mass-to-light ratio. At larger radii, a dynamically relevant fraction of the baryons are in gaseous form. A dark matter halo with a Navarro-Frenk-White density profile, as predicted by hierarchical clustering and structure formation in a ΛCDM cosmology, provides the best fits to the rotation curve. Dark matter is relevant at all radii in shaping the rotation curve and the most likely dark halo has a concentration C≃9.5 and a total mass of 4.3 (±1.0)x10¹¹M_☉. This imples a baryonic fraction of order 0.02 and the evolutionary history of this galaxy should therefore account for loss of a large fraction of its original baryonic content.