Millimetric Observations of the Center of M81 : A Starved Nucleus with Intraday Variability

The central kiloparsec of M81 has been observed in the CO(J=1–0) line and the 3 mm continuum at 100 pc resolution in an attempt to probe molecular gas, and to search for the nuclear inner Lindblad resonance (NILR), around the low-luminosity AGN M81. We found the following. (1) Molecular gas in the central kpc is mainly on a “pseudoring” or a spiral arm at a radius of about 500 pc. (2) The region within ∼300 pc from the nucleus is mostly devoid of molecular gas except for diffuse one; in particular, there is neither a giant molecular cloud that is now accreting on the nucleus nor a conspicuous gas feature that can be identified as an NILR. (3) The 3 mm continuum emission shows significant intraday variation, suggesting an emitting region of ∼ 100 AU. (4) The 3σ upper limit for CO absorption toward the continuum source is ∫ τCO(0→1) dV < 0.1 for a linewidth of 10 km s−1. The dearth of accreting molecular gas in the vicinity of the nucleus may explain the low luminosity of M81. Subject headings: galaxies: active — galaxies: individual (M81, NGC3031) — galaxies: ISM — galaxies: kinematics and dynamics — galaxies: spiral