Warm molecular gas traced with CO J=7→6 in the Galaxy's central 2 parsecs:: Dynamical heating of the circumnuclear disk

We present an 11 '' resolutionmap of the central 2 pc of the Galaxy in the CO J = 7 --> 6 rotational transition. The CO emission shows rotation about Sgr A* but also evidence for noncircular turbulent motion and a clumpy morphology. We combine our data set with available CO measurements to model the physical conditions in the disk. We find that the molecular gas in the region is both warm and dense, with T similar to 200 300 K and n(H2) similar to (5-7) x 10(4) cm(-3). The mass of warm molecular gas we measure in the central 2 pc is at least 2000 M-., about 20 times the UV-excited atomic gas mass, ruling out a UV heating scenario for the molecular material. We compare the available spectral tracers with theoretical models and conclude that molecular gas is heated with magnetohydrodynamic shocks with v similar to 10-20 km s(-1) and B similar to 0.3-0.5 mG. Using the conditions derived with the CO analysis, we include the other important coolants, neutral oxygen and molecular hydrogen, to estimate the total cooling budget of the molecular material. We derive a mass-to-luminosity ratio of similar to 2 - 3 M-. L-.(-1) , which is consistent with the total power dissipated via turbulent decay in 0.1 pc cells with v(rms) similar to 15 km s(-1). These size and velocity scales are comparable to the observed clumping scale and the velocity dispersion. At this rate, the material near Sgr A* is dissipating its orbital energy on an orbital timescale and cannot last for more than a few orbits. Our conclusions support a scenario in which the features near Sgr A* such as the circumnuclear disk and northern arm are generated by infalling clouds with low specific angular momentum.