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High-velocity hot CO emission close to Sgr A*. Herschel/HIFI submillimeter spectral survey toward Sgr A*

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Authors : Goicoechea, J. R. ; Santa-Maria, M. G. ; Teyssier, D. ; Cernicharo, J. ; Gerin, M. ; Pety, J
Abstract : The properties of molecular gas, the fuel that forms stars, inside the cavity of the circumnuclear disk (CND) are not well constrained. We present results of a velocity-resolved submillimeter scan ( 480-1250 GHz) and [C II] 158 μm line observations carried out with Herschel/HIFI toward Sgr A* ; these results are complemented by a 2’ × 2’ 12CO (J = 3-2) map taken with the IRAM 30 m telescope at 7″ resolution. We report the presence of high positive-velocity emission (up to about +300 km s-1) detected in the wings of 12CO J = 5-4 to 10-9 lines. This wing component is also seen in H2O (11,0-10,1), a tracer of hot molecular gas ; in [C II]158 μm, an unambiguous tracer of UV radiation ; but not in [C I] 492, 806 GHz. This first measurement of the high-velocity 12CO rotational ladder toward Sgr A* adds more evidence that hot molecular gas exists inside the cavity of the CND, relatively close to the supermassive black hole (<1 pc). Observed by ALMA, this velocity range appears as a collection of 12CO (J = 3-2) cloudlets lying in a very harsh environment that is pervaded by intense UV radiation fields, shocks, and affected by strong gravitational shears. We constrain the physical conditions of the high positive-velocity CO gas component by comparing with non-LTE excitation and radiative transfer models. We infer Tk ≃ 400 K-2000 K for nH ≃ (0.2-1.0) × 105 cm-3. These results point toward the important role of stellar UV radiation, but we show that radiative heating alone cannot explain the excitation of this 10-60 M⊙ component of hot molecular gas inside the central cavity. Instead, strongly irradiated shocks are promising candidates.
Journal : Astronomy&Astrophysics A&A