74 MHz Nonthermal Emission from Molecular Clouds: Evidence for a Cosmic Ray Dominated Region at the Galactic Center
Abstract
We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free–free absorption along the line of sight. We focus on one cloud, G0.13-0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (α_(327MHz)^(74MHz) = 1.3 ± 0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l 0.2° and is a strong source of 74 MHz continuum, SiO (2-1), and Fe I Kα 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13-0.13 molecular cloud and that low-energy cosmic ray electrons are responsible for the Fe I Kα line emission. The high cosmic ray ionization rate ~10^(–13) s^(–1) H^(–1) is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high-velocity dispersion. Large velocity gradient (LVG) modeling of multitransition SiO observations of this cloud implies H_2 densities ~10^(4–5) cm^(–3) and high temperatures. The lower limit to the temperature of G0.13-0.13 is ~100 K, whereas the upper limit is as high as 1000 K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N_2H^+/HCO^+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.
Additional Information
© 2013 American Chemical Society. Received: November 13, 2012. Revised: April 30, 2013. Publication Date (Web): May 1, 2013. This research is supported in part by grants from the Fermi Guest Investigator Program as well as the grant AST-0807400 from the NSF the National Science Foundation. The Caltech Submillimeter Observatory is operated by the California Institute of Technology under cooperative agreement with the National Science Foundation (AST-0838261). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors declare no competing financial interest.Additional details
- Eprint ID
- 42872
- DOI
- 10.1021/jp311240h
- Resolver ID
- CaltechAUTHORS:20131206-071118994
- NASA Fermi Guest Investigator Program
- AST-0807400
- NSF
- AST-0838261
- NSF
- Created
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2013-12-06Created from EPrint's datestamp field
- Updated
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2021-11-10Created from EPrint's last_modified field