2012A&A...537A..93R

The nearby spiral galaxy M81 harbours in its core a low-luminosity active galactic nucleus (LLAGN), and appears to be closely related to the more distant and powerful active galactic nuclei (AGNs) seen in quasars and radio galaxies. The intrinsic size of this object is unknown because of too much scattering, and it has shown a core-jet morphology with weak extended emission rotating with wavelength. The proximity of M81 (D=3.63Mpc) allows for a detailed investigation of its nucleus. The nucleus is four orders of magnitude more luminous than the Galactic centre, and is therefore considered to be a link between SgrA* and the more powerful nuclei of radio galaxies and quasars. Our main goal was to determine the size of M81* at a shorter wavelength to directly test whether the frequency-size dependent law Θ∝ν^–0.8 is still valid for wavelengths shorter than 1cm. In addition, we also aimed to confirm the rotation of the source as a function of frequency. We observed the continuum 7mm radio emission of M81* with the Very Long Baseline Array on Sep. 13, 2002, using nearby calibrators to apply their interferometric observables to the target source to increase the chances of detection. The source was detected on all baselines and hybrid mapping was possible. We present the first 7mm VLBI image of the core of M81*, which is the highest resolution image ever of this source. Modelling the interferometric visibilities with two Gaussian functions sets constraints on the angular size of its core down to 38 microarcseconds, corresponding to a maximum (projected) linear size of 138AU, and shows extended emission towards the NE with a position angle of ∼50°. A fit of a one Gaussian elliptical function yields a position angle of 28±8 degrees for its elongated, compact structure. Combining the 7mm size with earlier measurements at other frequencies, we determine a frequency-size dependence of Θ∝ν^{(–0.88±0.04)}. Our VLBI imaging of M81* has clearly detected its core-jet structure, and has allowed us to estimate a size for its core, with a minimum size of 138AU (≃100 Schwarzschild radii). Our work opens the way for future observations of M81* at high-angular resolution, including the monitoring of its structure, given that much higher bandwidths are currently available on the interferometric networks. In particular, this would allow one to test for possible proper motions of the core or of its components in the inner jet of M81*, and for the speed of the detected jet components.