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 similar to 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 similar to 100 AU. (4) The 3 sigma upper limit for CO absorption toward the continuum source is integral (tau CO(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*.