H3+ Spectroscopy and the Ionization Rate of Molecular Hydrogen in the Central Few Parsecs of the Galaxy

TL;DR

This study analyzes infrared spectra of H3+ and CO near the Galactic center, revealing a high cosmic ray ionization rate that is insufficient to explain the observed gamma-ray emissions, thus providing insights into the energetic environment near Sgr A*.

Contribution

It provides the first detailed measurements of H3+ ionization rates in the Galactic center, highlighting discrepancies with proton energy spectra inferred from gamma-ray observations.

Findings

Cosmic ray ionization rate near Sgr A* is an order of magnitude higher than outside the Galactic center.

The elevated ionization rate is four orders of magnitude too low to account for the gamma-ray source.

Infrared spectroscopy of H3+ offers a new probe of energetic processes in galactic nuclei.

Abstract

We report observations and analysis of infrared spectra of H3+ and CO lines in the Galactic center, within a few parsecs of the central black hole, Sgr A*. We find a cosmic ray ionization rate typically an order of magnitude higher than outside the Galactic center. Notwithstanding, the elevated cosmic ray ionization rate is 4 orders of magnitude too short to match the proton energy spectrum as inferred from the recent discovery of the TeV gamma-ray source in the vicinity of Sgr A*.