Statistical Properties of the Population of the Galactic Center Filaments: The Spectral Index and Equipartition Magnetic Field

Abstract: We present high-pass filtered continuum images of the inner $3.5^\circ\times2.5^\circ$ of the Galactic center at 20 cm with $6.4''$ resolution. These mosaic images are taken with MeerKAT and reveal a large number of narrow filaments, roughly an order of magnitude increase in their numbers compared to past measurements. For the first time, we carry out population studies of the spectral index and magnetic field of the entire region. The mean spectral indices of the filaments are steeper than supernova remnants (SNRs) (-0.62) with a value of $\alpha\sim-0.83$. The variation in $\alpha$ is much larger than for the SNRs, suggesting that these characteristics have a different origin. A large-scale cosmic-ray driven wind has recently been proposed to explain the origin of filaments and the large-scale 430 pc bipolar radio and X-ray structure. This favors the possibility that the large-scale bipolar radio/X-ray structure is produced by past activity of Sgr A* rather than coordinated burst of supernovae. A trend of steeper indices is also noted with increasing distance from the Galactic plane. This could be explained either by synchrotron cooling or weak shocks accelerating cosmic-ray particles in the context of the cosmic-ray driven wind. The mean magnetic field strengths along the filaments ranges from $\sim100$ to 400 $\mu$G depending on the assumed ratio of cosmic-ray protons to electrons. Given that there is a high cosmic ray pressure in the Galactic center, the large equipartition magnetic field implies that the magnetic field is weak in most of the interstellar volume of the Galactic center.

• The paper examines the statistical properties of nonthermal radio filaments, finding a mean spectral index of about -0.83.
• It employs high-pass filtering and in-band spectral analysis of MeerKAT 20 cm continuum images to isolate filament structures.
• The study estimates equipartition magnetic fields of 100–400 µG, supporting a model of intermittent strong magnetic concentrations.

Statistical Properties of Galactic Center Filaments: Spectral Indices and Magnetic Fields

The study titled "Statistical Properties of the Population of the Galactic Center Filaments: The Spectral Index and Equipartition Magnetic Field" presents an in-depth analysis of nonthermal radio filaments in the Galactic center. Using high-resolution continuum images obtained from the MeerKAT telescope, the authors have conducted the first comprehensive statistical study of the spectral indices and magnetic fields in this region.

Observations and Methodology

The authors employed high-pass filtered continuum images of the inner 3.5°×2.5° region of the Galactic center at 20 cm. The observations achieved a resolution of 6.4'', revealing a significant increase in the number of identified narrow filaments compared to previous studies. A total of 800 MHz bandwidth centered at 1.28 GHz provided the basis for these measurements. The spectral indices were calculated using in-band data, enabling precise and simultaneous measurements within the observed frequency range.

To manage the challenges of non-uniform background noise and enhance filament visibility, the authors applied a difference of Gaussians approach. This technique allowed for the subtraction of the general background, thereby isolating the filamentary structures and facilitating their spectral index analysis. The spectral index mosaic was generated with criteria ensuring reliable fits, such as a requirement for the presence of more than eight frequency bands.

Key Results

1. Spectral Index Distribution: The analysis yielded a mean spectral index of approximately -0.83 for the filaments, which is steeper than that of supernova remnants (SNRs), averaging around -0.62. This discrepancy suggests that the origins of these nonthermal features are distinct, implicating mechanisms other than traditional SNR-related processes.
2. Latitude Correlation: A notable trend was observed in which the spectral index steepens with increasing galactic latitude. Two primary mechanisms could explain this: synchrotron cooling over extended distances, indicating that higher-latitude cosmic rays are older; or the weak shocks associated with a cosmic-ray driven wind, resulting in steepened energy spectra.
3. Magnetic Field Estimation: The mean equipartition magnetic field strength along the filaments was estimated to be in the range of 100 to 400 µG. These values were derived under assumptions of equipartition between particles and fields, with variations based on the assumed electron-to-proton energy density ratio.
4. Magnetic Field Implications: The results imply that the strong magnetic fields are confined to the filaments, supporting a model where the overall interstellar magnetic field within the Galactic center is weaker. This aligns with models of intermittent magnetic energy distributions, highlighting the distinction between the dense filamentary concentrations and the broader, weaker fields in the interstellar medium (ISM).

Implications and Future Directions

The observed characteristics support the hypothesis that the Galactic center's radio bubble and nonthermal filaments might stem from historical activity associated with the central black hole, Sgr A*, rather than coordinated supernova events. The existence of a steepening spectral index with latitude supports a scenario where past energetic events, potentially from Sgr A*, influenced the cosmic ray population more substantively than SNRs.

The study contributes to understanding how energetic processes within the Galactic center influence its radio morphology and magnetic field architecture. Future research could consider the dynamic interaction processes between cosmic rays and magnetic fields within these filaments, their implications for ISM dynamics, and the global impact of past outbursts in galactic nuclei. Further improvements in observational techniques and theoretical models could refine these findings and explore the coupling between cosmic ray populations and magnetic field dynamics in such extreme environments.