2018A&A...616L...1G

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 (∼480-1250GHz) and [CII] 158µm line observations carried out with Herschel/HIFI toward Sgr A^*; these results are complemented by a ∼2'x2' ¹²CO (J=3-2) map taken with theIRAM 30 m telescope at ∼7'' resolution. We report the presence of high positive-velocity emission (up to about +300km/s) detected in the wings of ¹²CO J=5-4 to 10-9 lines. This wing component is also seen in H₂O (1_1,0-1_0,1), a tracer of hot molecular gas; in [CII] 158µm, an unambiguous tracer of UV radiation; but not in [CI] 492, 806GHz. 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 (<1pc). 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 T_k~=400K-2000K for n_H~=(0.2-1.0)x10⁵cm^–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 ∼10-60M_☉ component of hot molecular gas inside the central cavity.Instead, strongly irradiated shocks are promising candidates.