Exhaustion of the gas next to the supermassive black hole of M31

¹ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
e-mail: A.L.Melchior@obspm.fr
² Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France
e-mail: Francoise.Combes@obspm.fr

Received: 30 March 2017
Accepted: 24 September 2017

Abstract

New observations performed at the IRAM Plateau de Bure reveal the absence of molecular gas next to the black hole of the Andromeda galaxy. We derived a 3σ upper limit on the molecular gas mass of 4300 M_⊙ for a line width of 1000 km s^-1. This is compatible with infra-red observations, which reveal a hole in the dust emission next to the black hole. Some gas from stellar feedback is expected from the old eccentric stellar disc population, but it is not accreted close to the black hole. This absence of gas explains the absence of stellar formation observed in this region, contrary to what is observed next to Sgr A* in the Milky Way. Either the gas has been swallowed by the black hole, or a feedback mechanism has pushed the gas outside the central 1 pc. Nevertheless, we detect a small clump of gas with a very low velocity dispersion at 2.4″ from the black hole. It is probable that this clumpy gas is seen in projection, as it does not follow the rotation of the disc surrounding the black hole, its velocity dispersion is ten times lower than the expected velocity gradient, and the tidal shear from the black hole requires a gas density for this clump that is not compatible with our observations.