Dynamical signatures of a ΛCDM-halo and the distribution of the baryons in M 33

Aims: We determine the mass distribution of stars, gas, and dark matter in M 33 to test cosmological models of galaxy formation and evolution.
Methods: We map the neutral atomic gas content of M 33 using high resolution Very Large Array and Green Bank Telescope 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 the rotation curve. We derive the stellar mass surface density map of M 33'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.
Results: The disk of M 33 warps from 8 kpc outward without substantial change of its inclination with respect to the line of sight; the line of nodes rotates clockwise toward the direction of M 31. 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 R₂₅. 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) × 10¹¹ M_⊙. 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.

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