A Viscous Heating Mechanism for the Hot Plasma in the Galactic Center Region

Abstract

In addition to lines originating in a soft phase at ~0.8 keV and to cold molecular clouds, the X-ray spectra from the Galactic center region also exhibit properties similar to those of a diffuse, thin, very hot plasma at 8 keV on a scale of hundreds of parsecs. This phase is surprising for more than one reason. First, such a hot plasma should not be bound to the Galactic plane and the power needed to sustain the escaping matter would be higher then any known source. Second, there is no known mechanism able to heat the plasma to more than a few keV. Recently we have suggested that, hydrogen having escaped, the hot plasma could be a helium plasma, heavy enough to be gravitationally confined. In this case, the required power is much more reasonable. We present here a possible heating mechanism which taps the gravitational energy of the molecular clouds. We note that the 8 keV plasma is highly viscous and we show how viscous friction of molecular clouds flowing within the hot phase can dissipate energy in the gas and heat it. We detail the MHD wake of a spherical cloud by considering the different MHD waves the cloud can excite. We find that most of the energy is dissipated by the damping of Alfvenic perturbations in two possible manners, namely by non-linear effects and by a large scale curvature of the field lines. Depending on the field strength, this heating can balance the radiative cooling. We note that the plasma parameters may be optimal to make the dissipation most efficient, suggesting a self-regulation mechanism. The loss of kinetic and gravitational energy also causes accretion of the clouds and may have significant action on the gas dynamics in this region between the large scale, bar dominated flow and the central accretion to the massive black hole.

arXiv comment17 pages, 6 figures, accepted for publication in A&A

References in corpus (10)

• The Nuclear Bulge of the Galaxy. III. Large-Scale Physical Characteristics of Stars and Interstellar Matter
• Diffuse X-ray Emission in a Deep Chandra Image of the Galactic Center
• Morphology of the Interstellar Cooling Lines Detected by COBE
• Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission
• On the Origin of the Iron K Line in the Spectrum of the Galactic X-ray Background
• Origin of the Hard X-ray Emission from the Galactic Plane
• Gas Density, Stability, and Starbursts Near the Inner Lindblad Resonance of the Milky Way
• Unusual Properties of X-Ray Emission near the Galactic Center
• Nonthermal Hard X-Ray Emission from the Galactic Ridge
• A Hot Helium Plasma in the Galactic Center Region

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• Discovery of a superluminal Fe K echo at the Galactic Center: The glorious past of Sgr A* preserved by molecular clouds
• Wild at Heart:-The Particle Astrophysics of the Galactic Centre
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• The Central 300 pc of the Galaxy probed by infrared spectra of H${}_3^{+}$ and CO: part I. Predominance of warm and diffuse gas and high H${}_2$ ionization rate
• Traces of past activity in the Galactic Centre
• A Multiwavelength View of a Mass Outflow from the Galactic Center
• Study of the excess Fe XXV line emission in the central degrees of the Galactic centre using XMM-Newton data
• Embedding of FRW Cosmology in DGP Scenario with a Non-Minimally Coupled Scalar Field on the Brane