Title: A Cool Accretion Disk around the Galactic Centre Black Hole Sagittarius A*

Abstract: A supermassive black hole Sagittarius A* (SgrA*) with the mass ~4×10^6
Msun resides at the center of our galaxy. A large reservoir of hot
(10^7 K) and cooler (10^2 – 10^4 K) gas surrounds it within few pc.
Building up such a massive black hole within the ~10^{10} year
lifetime of our galaxy would require a mean accretion rate of
~4×10^{-4} Msun/yr. At present, X-ray observations constrain the rate
of hot gas accretion at the Bondi radius ~3×10^{-6} Msun/yr, and
polarization measurements constrain it near the event horizon to
~10^{-8} Msun/yr. A range of models were 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 (RIAFs) 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 will report a detection and imaging
of the 10^4 K ionized gas disk within 2×10^4 R_Sch in a millimetre
hydrogen recombination line H30alpha: n = 31 -> 30 at 231.9 GHz using
the Atacama Large Millimeter/submillimeter Array (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 limit on the
total mass of ionized gas estimated from the emission is 10^{-4} –
10^{-5} Msun at a mean hydrogen density 10^5-10^6 cm^-3.