Understanding the state of the gas surrounding Andromeda's black hole.

A millimeter CO survey has been performed at IRAM of the gas surrounding Andromeda's black hole. Several velocity components are detected supporting the presence of a relic 0.7-kpc ring, signing a possible frontal collision with M32. A multiwavelength analysis of this central field of M31 has been carried out in order to get clues on the location and state of this gas. Viaene et al. (2014) estimate in this central zone a star formation rate of order 3 × 10^{-4} M_⊙ yr^{-1}, based on infrared data. Beside a 200 Myr A-star cluster detected close to the black hole, no trace of recent star formation is observed in UV. In parallel, Halpha gas is present but due to shocks as the [NII]/Halpha ratio is larger than 3. The issue is thus to understand why the cold molecular gas is quenched and not forming stars.