A Cool Accretion Disk around the Galactic Centre Black Hole
  A supermassive black hole SgrA* with the mass ~4x10^6 Msun resides at the
centre of our galaxy. Building up such a massive black hole within the ~10^10
year lifetime of our galaxy would require a mean accretion rate of ~4x10^-4
Msun/yr. At present, X-ray observations constrain the rate of hot gas accretion
at the Bondi radius (10^5 R_Sch = 0.04 pc at 8kpc) to \dot{M}_Bondi ~ 3x10^-6
Msun/yr, and polarization measurements constrain it near the event horizon to
\dot{M}_horizon ~ 10^{-8} Msun/yr. A range of models was developed to describe
the accretion gas onto an underfed black hole. However, the exact physics still
remains to be understood. One challenge with the radiation inefficient
accretion flows is that even if one understands the dynamics there is no
accepted prescription for associating emissivity (and absorption) with the
flow. The other issue is the lack of model-independent probes of accretion flow
at intermediate radii (between few and ~ 10^5 R_Sch), i.e. the constraints that
do not assume a model of accretion flow as an input parameter. We report
detection and imaging of the 10^4 K ionized gas disk within 2x10^4 R_Sch in a
mm hydrogen recombination line H30alpha: n = 31 -> 30 at 231.9 GHz using the
ALMA. The emission was detected with a double-peaked line profile spanning full
width of 2,200 km/s with the approaching and the receding components straddling
Sgr A*, each offset from it by 0.11arcsec= 0.004pc. The red-shifted side is
displaced to the north-east, while the blue-shifted side is displaced to the
south-west. The limit on the total mass of ionized gas estimated from the
emission is 10^-4 - 10^-5 Sun at a mean hydrogen density 10^5-10^6 cm^-3,
depending upon whether or not we assume the presence of a uniform density disk
or an ensemble of orbiting clouds, and the amplification factor of the mm
radiation due to the strong background source which is Sgr A* continuum.
