GREAT confirms transient nature of the circumnuclear disk

TL;DR

This study uses multi-transition CO spectroscopy and radiative transfer modeling to demonstrate that the Galactic center's circumnuclear disk is composed of transient, non-self-gravitating gas phases with high turbulence and temperature.

Contribution

It provides the first detailed multi-transition CO analysis showing the transient, non-stable nature of the CND's gas, challenging previous assumptions of gravitational stability.

Findings

The CO excitation indicates multiple warm gas phases.

Most of the CND gas is not gravitationally bound or stable.

The gas is likely transient and disrupted by tidal forces.

Abstract

We report SOFIA/GREAT, Herschel/HIFI, and ground-based velocity-resolved spectroscopy of carbon monoxide (CO) rotational transitions from J=2-1 to J=16-15 toward two positions in the circum-nuclear disk (CND) in our Galactic center. Radiative transfer models were used to derive information on the physical state of the gas traced by CO. The excitation of the CO gas cannot be explained by a single physical component, but is clearly the superposition of various warm gas phases. In a two-component approach, our large velocity gradient (LVG) analysis suggests high temperatures of ~200 K with moderate gas densities of only ~10^4.5 cm^-3 for the bulk of the material. A higher excited phase, carrying ~20-30% of the column densities, is warmer (~300-500 K) but only slightly denser ~10^5.3 cm^-3. These densities are too low to self-stabilize the clumps against their high internal turbulence and fall below the Roche density (>10^7 cm^-3) at 1.5 pc galactocentric distance. We conclude that the bulk of the material in the CND is not organized by self-gravity nor stable against tidal disruption, and must be transient.