Cosmic-Ray Heating of Molecular Gas in the Nuclear Disk: Low Star Formation Efficiency

Abstract

Understanding the processes occurring in the nuclear disk of our Galaxy is interesting in its own right, as part of the Milky Way, but also because it is the closest galactic nucleus. It has been more than two decades since the general phenomenon of higher gas temperature in the inner few hundred parsecs by comparison with local clouds in the disk of the Galaxy was recognized. This is one of the least understood characteristics of giant molecular clouds having a much higher gas temperature than dust temperature in the inner few degrees of the Galactic center. We propose that an enhanced flux of cosmic-ray electrons, as evidenced recently by a number of studies, is responsible for directly heating the gas clouds in the nuclear disk, elevating the temperature of molecular gas (~75 K) above the dust temperature (~20 K). In addition, we report the detection of nonthermal radio emission from Sgr B2 F based on low-frequency GMRT and VLA observations. The higher ionization fraction and thermal energy due to the impact of nonthermal electrons in star-forming sites have important implications in slowing down star formation in the nuclear disk of our Galaxy and nuclei of galaxies.