E. Ros, M. A. Perez-Torres
The nearby spiral galaxy M81 harbors in its core a Low-Luminosity AGN
(LLAGN), and appears closely related to the more distant and powerful AGNs seen
in quasars and radio galaxies. The intrinsic size of this object is unknown due
to scattering, and it has shown a core-jet morphology with weak extended
emission rotating with wavelength. The proximity of M\,81 (D=3.63 Mpc) allows a
detailed investigation of its nucleus to be made. The nucleus is four orders of
magnitude more luminous than the Galactic centre, and is therefore considered 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 thus
directly testing whether the frequency-size dependent law Theta propto
nu^(-0.8) was 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* using 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 represents the
highest resolution image ever of this source. Modeling 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 ~50deg. A fit of 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 Theta propto nu^(-0.88+/-0.04).
View original:
http://arxiv.org/abs/1111.4856
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