Far-Infrared Spectroscopy of the Galactic Center. Hot Molecular Gas: Shocks versus Radiation near SgrA*

TL;DR

This study presents a detailed far-infrared spectral analysis of the Galactic Center near SgrA*, revealing the heating mechanisms of hot molecular gas through shock and radiation interactions, with implications for understanding energetic processes in galactic nuclei.

Contribution

First high-resolution far-IR spectral scan of SgrA* separating central cavity and surrounding disk, identifying heating processes of hot molecular gas via shocks and UV radiation.

Findings

Hot molecular gas is primarily heated by shocks and UV radiation.

The CO excitation indicates a hot isothermal component at ~1250 K.

Neither X-ray nor cosmic-ray fluxes dominate the gas heating.

Abstract

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*.