M31 nucleus: molecular and ionised gas content upper limits

Abstract

We report NOEMA and ALMA observations of the nucleus of Andromeda (M31), putting strong constraints on the presence of gas in the form of cold or warm phase, as proposed by Chang et al. M31 hosts the largest supermassive black hole (SMBH) closer than 1 Mpc from us. Its nucleus is silent with some murmurs at the level of 4 10$^{-9}$ L$_{Edd}$, and is surrounded by a 5-pc-radius disk of old stars. The mass-loss from these stars is expected to fill a molecular gas disk within the tidal truncation of 1 pc (0.26"), of 10$^4$ Mo, corresponding to a CO(1-0) signal of 2mJy with a linewidth of 1000km/s. We observed the nucleus with NOEMA in CO(2-1) and with ALMA in CO(3-2) with angular resolutions of 0.5" (1.9 pc) and 0.12" (0.46 pc) respectively. We exclude the presence of gas with a 3$\sigma$ upper limit of 195 Mo. The CO(3-2) upper limit also constrains warm gas, escaping detection in CO(1-0). The scenario proposed by Chang et al. is not verified, and instead the hot gas, expelled by the stellar winds, might never cool nor fall onto the disc. Alternatively, the stellar wind mass-loss rate can have been overestimated by a factor 50, and/or the ionised gas escaped from the nucleus. The SMBH in M31 is obviously in a low state of activity, similar to what is observed for Sgr A* in the Milky Way (MW). Recently, a cool (10$^4$ K) ionised accretion disc has been detected around Sgr A* in the H30$\alpha$ recombination line with ALMA. Re-scaling sizes, masses and fluxes according to the mass of M31's black hole (35 times higher than in the MW) and the distances, a similar disc could be easily detectable around M31 nucleus with an expected signal 8 times weaker that the signal detected in SgrA*. We searched for an ionised gas disc around M31 nucleus with NOEMA, and put a 3$\sigma$ upper limit on the H30$\alpha$ recombination line at a level twice lower than expected with a simple scaling of the SgrA*.