Spectroastrometry of rotating gas disks for the detection of
  supermassive black holes in galactic nuclei. III. CRIRES observations of the
  Circinus galaxy
  We present new CRIRES spectroscopic observations of BrGamma in the nuclear
region of the Circinus galaxy, obtained with the aim of measuring the black
hole (BH) mass with the spectroastrometric technique. The Circinus galaxy is an
ideal benchmark for the spectroastrometric technique given its proximity and
secure BH measurement obtained with the observation of its nuclear H2O maser
disk. The kinematical data have been analyzed both with the classical method
based on the analysis of the rotation curves and with the new method developed
by us and based on spectroastrometry. The classical method indicates that the
gas disk rotates in the gravitational potential of an extended stellar mass
distribution and a spatially unresolved mass of (1.7 +- 0.2) 10^7 Msun,
concentrated within r < 7 pc. The new method is capable of probing gas rotation
at scales which are a factor ~3.5 smaller than those probed by the rotation
curve analysis. The dynamical mass spatially unresolved with the
spectroastrometric method is a factor ~2 smaller, 7.9 (+1.4 -1.1) 10^6 Msun
indicating that spectroastrometry has been able to spatially resolve the
nuclear mass distribution down to 2 pc scales. This unresolved mass is still a
factor ~4.5 larger than the BH mass measurement obtained with the H2O maser
emission indicating that it has not been possible to resolve the sphere of
influence of the BH. Based on literature data, this spatially unresolved
dynamical mass distribution is likely dominated by molecular gas and it has
been tentatively identified with the circum-nuclear torus which prevents a
direct view of the central BH in Circinus. This mass distribution, with a size
of ~2pc, is similar in shape to that of the star cluster of the Milky Way
suggesting that a molecular torus, forming stars at a high rate, might be the
earlier evolutionary stage of the nuclear star clusters which are common in
late type spirals.
