ESA Science & Technology - Herschel Far-Infrared Spectroscopy of the Galactic Center. Hot Molecular Gas: Shocks versus Radiation near Sgr A*

Publication date: 08 May 2013

Authors: J.R. Goicoechea et al.

Journal: Astrophysical Journal Letters
Year: 2013

Copyright: IOP Publishing

We present a ~52-671 um spectral scan towards Sgr A* 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 towards the central cavity (gas in the inner central parsec of the galaxy) from that of the surrounding circum-nuclear disk. The spectrum towards 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), ¹²CO, 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 towards SgrA*.