Andromeda: about the weak molecular gas density and star formation in its circum-nuclear region

The galaxy M31 shows an atypical morphology for a spiral galaxy, with its two rings structure (the eccentric 1 kpc inner ring and the outer 10 kpc ring) which contains most of its gas mass. Its circum-nuclear region is empty and star formation seems absent, leading us to expect Andromeda's nucleus gas reservoir to be depleted. Explanations for this phenomenon are still up for debate. The main hypothesis include stellar feedback triggered by a recent starburst, gas infall in the black hole or a head-on collision with M32 which could have redistributed the gas. In this thesis, we use interferometric data to explore the gas density within the central 165 pc of M31. We produce a 12 molecular clouds catalog and rely on GALEX, SDSS, 2MASS and SPITZER data to study the spectral energy distribution. It showed to be consistent with a combination models with a strong contribution of a 8 Gyr old stellar population and a modest contribution of 200 Myr old stellar population. The study of the polycyclic aromatic hydrocarbon in the black hole sphere of influence is compatible with the existence of a recent starburst. We create a new star formation rate map for M31 nucleus. Our work confirms the low density of molecular gas and star formation in the circum-nuclear region. We estimate the total gas mass 8.4 x 10⁴ M* and set an upper limit for the surface density star formation rate 1.2 x 10^-3 M*yr^-1kpc^-2. (author)