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

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 to 1250 GHz) and [C ii] 158 ?m 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 ?m, 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 T k?400 K to 2000 K for n H?(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.

SUBMITTER: Goicoechea JR 

PROVIDER: S-EPMC6914365 | biostudies-literature | 2018 Aug

REPOSITORIES: biostudies-literature