FIR Spectroscopy of the Galactic Center: Hot and Warm Molecular Gas

TL;DR

This study uses Herschel FIR spectroscopy to analyze the hot and warm molecular gas near the Galactic Center, revealing the presence of hot CO components likely heated by UV radiation and shocks, with ongoing high-resolution ALMA observations.

Contribution

First detailed FIR spectral analysis of the Galactic Center's molecular gas, identifying hot CO components and their likely heating mechanisms.

Findings

Detection of hot CO at T~1000 K near the CND

Evidence of UV and shock-driven heating processes

Potential for higher resolution studies with ALMA

Abstract

The angular resolution (~10") achieved by the Herschel Space Observatory ~3.5m telescope at FIR wavelengths allowed us to roughly separate the emission toward the inner parsec of the galaxy (the central cavity) from that of the surrounding circumnuclear disk (the CND). The FIR spectrum toward SgrA* is dominated by intense [Oiii], [Oi], [Cii], [Niii], [Nii], and [Ci] fine-structure lines (in decreasing order of luminosity) arising in gas irradiated by the strong UV field from the central stellar cluster. The high-J CO rotational line intensities observed at the interface between the inner CND and the central cavity are consistent with a hot isothermal component at T~10^{3.1} K and n(H_2)~10^4 cm^{-3}. They are also consistent with a distribution of lower temperatures at higher gas density, with most CO at T~300 K. The hot CO component (either the bulk of the CO column density or just a small fraction depending on the above scenario) likely results from a combination of UV and shock-driven heating. Although this component is beam diluted in our FIR observations, it may be resolved at much higher angular resolution. An ALMA project using different molecular tracers to characterize UV-irradiated shocks in the innermost layers of the CND is ongoing.