Context :

Aims : We determine the mass distribution of stars , gas and dark matter in the nearby galaxy M33 to test cosmological models of galaxy formation and evolution .

Methods : We map the neutral atomic gas content of M33 using high resolution VLA and GBT observations of the 21-cm 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 its 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 which allow for positional changes in star formation history , and estimate the stellar mass surface density beyond the optical edge from deep images of outer disk fields .
Stellar and gas maps are then used in the dynamical analysis of the rotation curve to constrain the dark halo properties in a more stringent way than previously possible .

Results : The disk of M33 warps from 8 kpc outwards without substantial change of its inclination with respect to the line of sight , but rather in a manner such that the line of nodes rotates clockwise towards the direction of M31 .
Rotational velocities rise steeply with radius in the inner disk , reaching 100 km s ^ { -1 } in 4 kpc , then the rotation curve becomes more perturbed and flatter with velocities as high as 120-130 km s ^ { -1 } out to 2.7 R _ { 25 } .
The stellar surface density map highlights a star-forming disk whose mass is dominiated by stars 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 \Lambda CDM 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 \simeq 10 and a total mass of 4.3 ( \pm 1.0 ) 10 ^ { 11 } M _ { \odot } .
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 .

Conclusions :