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

TL;DR

This study detects high-velocity hot molecular gas near Sgr A* using Herschel/HIFI and IRAM observations, revealing complex excitation mechanisms involving shocks and UV radiation within the central cavity of the circumnuclear disk.

Contribution

It provides the first velocity-resolved submillimeter survey of high-velocity CO emission near Sgr A*, identifying hot molecular gas components and constraining their physical conditions.

Findings

High positive-velocity CO emission up to +300 km/s detected.

Hot molecular gas with T_k~400-2000 K inside the CND cavity.

Shocks, rather than UV radiation alone, likely excite the hot gas.

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 [CII]158um line observations carried out with Herschel/HIFI toward Sgr A*; these results are complemented by a ~2'x2' CO (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) detected in the wings of CO 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]158um, an unambiguous tracer of UV radiation; but not in [CI]492,806 GHz. This first measurement of the high-velocity CO 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 CO (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)x10^5 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_Sun component of hot molecular gas inside the central cavity. Instead, strongly irradiated shocks are promising candidates.