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 a 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 kiloparsec 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 a 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 3 σ upper limit for CO absorption toward the continuum source is τCO(0→1)dV < 0.1 for a line width of 10 km s-1. The dearth of accreting molecular gas in the vicinity of the nucleus may explain the low luminosity of M81*.