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 $3.5^{\circ }\times 2.5^{\circ }$ of the Galactic center at 20 cm with $6.4^{\prime \prime }$ 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 $\sim 100$ 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.

15 pages, 9 figures, accepted for publication in Astrophysical Journal Letters

References in corpus (9)

• The 1.28 GHz MeerKAT Galactic Center Mosaic
• The Galactic Centre Chimney
• The Origin of Diffuse X-ray and $\gamma$-ray Emission from the Galactic Center Region: Cosmic Ray Particles
• Cosmic-Ray Heating of Molecular Gas in the Nuclear Disk: Low Star Formation Efficiency
• A Wide-area VLA Continuum Survey near the Galactic Center at 6 and 20 cm Wavelengths
• Infrared H3+ and CO Studies of the Galactic Core: GCIRS 3 and GCIRS 1W
• Probing Cosmic Ray Transport with Radio Synchrotron Harps in the Galactic Center
• The CARMA 3mm Survey of the Inner $0.7^{\circ }\times 0.4^{\circ }$ of the Central Molecular Zone
• Nonthermal filaments from the tidal destruction of clouds in the Galactic center

Cited by in corpus (6)

• Three-Dimensional Structure of the Central Molecular Zone
• The Population of the Galactic Center Filaments: Position Angle Distribution Reveal a Degree-scale Collimated Outflow from Sgr A* along the Galactic Plane
• Populations of Magnetized Filaments in the Intracluster Medium and the Galactic Center
• The population of Galactic centre filaments III: candidate radio and stellar sources
• Supernova-remnant origin of the Galactic-Centre filaments
• Molecular Fraction in the Galactic Center: The Central Molecular and HI Zones