Detection of Cool Accretion Disk around the Galactic Centre Black Hole Sagittarius A*
Elena Murchikova (IAS)

Abstract:
A supermassive black hole Sagittarius A* (SgrA*) with the mass M_SgrA*= 4x10^6 M_sun, deduced from the orbits of nearby stars, 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} M_sun/yr.$ However, X-ray observations constrain the rate of hot gas accretion at the Bondi radius (10^5 R_Sch = 0.04 pc) to \dot{M}_Bondi~3x10^{-6} M_sun/yr, and polarization measurements constrain it near the event horizon to \dot{M}_{horizon}~10^{-8} M_sun/yr. What happens to the gas in between, what its physical state, its source, and the exact accretion mechanism present a long standing puzzle, primarily due to the absence of model-independent probes at intermediate radii. Here we report a detection and imaging of the 10^4 K ionized gas disk within 2x10^4 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 2,200 km/s with the approaching and the receding components straddling SgrA*, each offset from it by $0.11 arcsec = 0.004 pc which indicating a rotating disk of a mass 10^{-4} - 10^{-5} M_sun$ 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. This detection provides spatial and kinematic information on the accretion disk on scales a factor of ~10 smaller than what was possible before.