A cold gas reservoir to fuel M31 nuclear black hole and stellar cluster

Abstract

With IRAM-30m/HERA, we have detected CO(2-1) gas complexes within 30 arcsec (~100 pc) from the center of M31 amounting to a minimum total mass of 4.2 x 10^4 Msol (one third of the positions are detected). Averaging the whole HERA field, we have shown that there is no additional undetected diffuse component. We show that the above gas detection is associated with gas lying on the far side as no extinction is observed in the optical, but some emission is present on infra-red Spitzer maps. The kinematics is complex. (1) The velocity pattern is mainly redshifted: the dynamical center of the gas differs from the black hole position and the maximum of optical emission, and only the red-shifted side is seen in our data. (2) Several velocity components are detected in some lines of sight. Our interpretation is supported by the reanalysis of the effect of dust on a complete planetary nebula sample. Two dust components are detected with respective position angles of 37 deg and -66 deg. This is compatible with the superposition of the (PA=37 deg) disk dominated by the 10-kpc ring and the inner 0.7-kpc ring detected in infrared data, which position angle (-66 deg) can be measured for the first time. The large scale disk, which dominates the HI data, is very inclined (i=77 deg), warped and superposed on the line of sight on the less inclined inner ring. The detected CO emission might come from both components.