Far-Infrared Spectroscopy of the Galactic Center. Hot Molecular Gas:
  Shocks versus Radiation near SgrA*
  We present a 52-671um spectral scan toward SgrA* taken with the PACS and
SPIRE spectrometers onboard Herschel. The achieved angular resolution allows us
to separate, for the first time at far-IR wavelengths, the emission toward the
central cavity (gas in the inner central parsec of the galaxy) from that of the
surrounding circum-nuclear disk. The spectrum toward SgrA* is dominated by
strong [OIII], [OI], [CII], [NIII], [NII], and [CI] fine structure lines (in
decreasing order of luminosity) arising in gas irradiated by UV-photons from
the central stellar cluster. In addition, rotationally excited lines of 12CO
(from J=4-3 to 24-23), 13CO, H2O, OH, H3O+, HCO+ and HCN, as well as
ground-state absorption lines of OH+, H2O+, H3O+, CH+, H2O, OH, HF, CH and NH
are detected. The excitation of the 12CO ladder is consistent with a hot
isothermal component at Tk ~ 10^{3.1} K and n(H2)< 10^4 cm^{-3}. It is also
consistent with a distribution of temperature components at higher density with
most CO at Tk<300 K. The detected molecular features suggest that, at present,
neither very enhanced X-ray, nor cosmic-ray fluxes play a dominant role in the
heating of the hot molecular gas. The hot CO component (either the bulk of the
CO column or just a small fraction depending on the above scenario) results
from a combination of UV- and shock-driven heating. If irradiated dense
clumps/clouds do not exist, shocks likely dominate the heating of the hot
molecular gas. This is consistent with the high-velocity gas detected toward
SgrA*.
