Herschel Far-infrared Spectroscopy of the Galactic Center. Hot Molecular Gas: Shocks versus Radiation near Sgr A*

Abstract

We present a ~52-671 μm spectral scan toward Sgr A* taken with the PACS and SPIRE spectrometers on board 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 circumnuclear disk. The spectrum toward Sgr A* is dominated by strong [O III], [O I], [C II], [N III], [N II], and [C I] 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 T _k ~= 10^3.1 K and n(H₂) <~ 10⁴ cm^-3. It is also consistent with a distribution of temperature components at higher density with most CO at T _k <~ 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 Sgr A*.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.