Cosmic-ray driven outflow from the galactic center and the origin of magnetized radio filaments

Abstract: Radio, X-ray and infrared observations of the inner few hundred pc of the Galactic center have highlighted two characteristics to the ISM. The cosmic ray ionization rate derived from molecular ions such as H$^+_3$, is at least two to three orders of magnitudes higher than in the Galactic disk. The other is bipolar X-ray and radio emission away from the Galactic plane. These features are consistent with a scenario in which high cosmic ray pressure drives large-scale winds away from the Galactic plane. The interaction of such a wind with stellar wind bubbles may explain the energetic nonthermal radio filaments found throughout the Galactic center. Some of the implications of this scenario is the removal of gas driven by outflowing winds, acting as a feedback to reduce the star formation rate in the central molecular zone (CMZ), and the distortion of azimuthal magnetic field lines in the CMZ to vertical direction away from the plane. The combined effects of the wind and vertical magnetic field can explain why most magnetized filaments run perpendicular to the Galactic plane. This proposed picture suggests our Milky Way nucleus has recently experienced starburst or black hole activity, as recent radio and X-ray observations indicate.

• The paper identifies cosmic rays as key drivers of galactic winds, demonstrated by elevated ionization rates and expansive radio emission bubbles.
• Detailed modeling shows that cosmic ray pressures between 500 and 5000 eV cm⁻³ reorient magnetic fields, explaining the vertical alignment of nonthermal radio filaments.
• The study highlights the feedback role of cosmic rays in regulating star formation and black hole activity, offering new pathways for future multi-scale simulations.

Cosmic-ray Driven Outflow from the Galactic Center and the Origin of Magnetized Radio Filaments

The paper presents a comprehensive analysis of the intricate interplay between cosmic rays, large-scale winds, and magnetized radio filaments in the inner region of the Milky Way. It emphasizes the significant role of cosmic rays in driving galactic outflows and their subsequent impact on the interstellar medium (ISM) and star formation rates in the central molecular zone (CMZ).

Key Observations and Interpretations

The analysis begins with observations that highlight the unique characteristics of the Galactic center, such as an elevated cosmic ray ionization rate and bipolar radio emissions. Notably, these features imply high cosmic-ray pressure driving substantial winds away from the Galactic plane. This paradigm is reinforced by observations from MeerKAT, which uncover expansive radio emission bubbles extending over hundreds of parsecs.

Cosmic-ray Driven Winds and Magnetic Fields

Cosmic rays are posited as central to these dynamics; they not only instigate galactic winds but also serve as feedback mechanisms influencing star formation and the growth of the central supermassive black hole. Their pressure modifies the orientation of magnetic fields from azimuthal to vertical, aligning with observations of nonthermal radio filaments (NRFs) primarily perpendicular to the Galactic plane. This vertical alignment suggests recent starburst or black hole activity in our galaxy's nucleus.

Theoretical Framework and Numerical Insights

Detailed modeling reveals the extent of cosmic ray influence in the Galactic center, with ionization rates surpassing those in other Galactic regions by orders of magnitude. The cosmic ray pressure required to sustain such rates varies between 500 and 5000 eV cm$^{-3}$, pointing to significant past energetic events. The study provides crucial numerical data, such as thermal X-ray plasma pressures and energy densities, reinforcing the viable existence of cosmic-ray driven winds.

The structural complexity observed in NRFs is attributable to the interaction of these winds with local stellar environments, challenging previous simpler models. The wind-driving mechanism offers a coherent explanation for the complex morphologies of NRFs discovered by recent radio telescope surveys.

Implications and Future Directions

This research underscores the cosmic rays' potent role in the self-regulation of star formation in galactic nuclei and their capability of driving substantial mass outflows. The insights glean from this study offer pathways for future exploration, particularly through the integrated examination of cosmic ray dynamics with star formation theories and magnetic field studies.

Future developments may lead to an enriched understanding of cosmic ray contributions to galaxy evolution, particularly in observing and modeling cosmic ray interactions within diverse galactic environments. Advanced observational techniques, as well as simulations capturing the multi-scale nature of these interactions, will be crucial in elucidating the comprehensive role of cosmic rays in galactic dynamics.