Inflation of 430-parsec bipolar radio bubbles in the Galactic Centre by an energetic event (1909.05534v1)

Abstract: The Galactic Centre contains a supermassive black hole with a mass of 4 million suns within an environment that differs markedly from that of the Galactic disk. While the black hole is essentially quiescent in the broader context of active galactic nuclei, X-ray observations have provided evidence for energetic outbursts from its surroundings. Also, while the levels of star formation in the Galactic Centre have been approximately constant over the last few hundred Myr, there is evidence of elevated short-duration bursts, strongly influenced by interaction of the black hole with the enhanced gas density present within the ring-like Central Molecular Zone at Galactic longitude |l| < 0.7 degrees and latitude |b| < 0.2 degrees. The inner 200 pc region is characterized by large amounts of warm molecular gas, a high cosmic ray ionization rate, unusual gas chemistry, enhanced synchrotron emission, and a multitude of radio-emitting magnetised filaments, the origin of which has not been established. Here we report radio imaging that reveals bipolar bubbles spanning 1 degree x 3 degrees (140 parsecs x 430 parsecs), extending above and below the Galactic plane and apparently associated with the Galactic Centre. The structure is edge-brightened and bounded, with symmetry implying creation by an energetic event in the Galactic Centre. We estimate the age of the bubbles to be a few million years, with a total energy of 7 x 10^52 ergs. We postulate that the progenitor event was a major contributor to the increased cosmic-ray density in the Galactic Centre, and is in turn the principal source of the relativistic particles required to power the synchrotron emission of the radio filaments within and in the vicinity of the bubble cavities.

Inflation of 430-Parsec Bipolar Radio Bubbles in the Galactic Centre by an Energetic Event

The paper, "Inflation of 430-parsec bipolar radio bubbles in the Galactic Centre by an energetic event," presents a comprehensive analysis of radio imaging data from the Galactic Centre, unveiling a significant bipolar bubble structure traced back to an energetic event. This structure, characterized by its edge-brightened morphology, indicates an interaction with the supermassive black hole Sgr A* or the surrounding environment, possibly by a high-energy outburst or a series of outbursts, analogous to those responsible for the Fermi bubbles.

The primary observational data were obtained using the MeerKAT radio telescope, which offered high-resolution imaging. The results reveal a striking, symmetrical bipolar bubble approximately 430 parsecs in extent, oriented perpendicularly to the Galactic plane. The structural characteristics and energetics proposed suggest a progenitor event epochs ago, having imprinted effects on cosmic ray densities and synchrotron emissions in the region.

Core Findings

1. Radio Imaging and Structure: Utilizing MeerKAT radio observations, the researchers detected the bubbles with a central frequency of 1,284 MHz and angular resolution of 6 arcsec. The bipolar structure measures 1 degree by 3 degrees (140 by 430 parsecs), symmetrically aligned with the Galactic plane.
2. Energetic Origin and Synchrotron Emission: The paper proposes that synchrotron radiation, underpinning the detected non-thermal radio emissions, is powered by relativistic particles, likely hastened by cosmic ray acceleration linked to the energetic event. Estimated synchrotron cooling times span 1-2 Myr, suggesting ongoing photon emissions.
3. Temporal Estimations: Through dynamical models, the age of the radio bubbles has been approximated at around 7 Myr, aligning with the timeline of the young nuclear star cluster adjacent to Sgr A*. This temporal framework complements synchrotron lifetime estimates and reflects possible shock propagations accelerating fresh electrons.
4. Correspondence with Other Galactic Structures: The paper draws parallels with the recognized Fermi Bubbles, which are larger in extent but point towards similar energetic phenomena contributing to magnetic fields and cosmic ray populations within the Galactic Centre.

Implications and Future Directions

This work implicates remarkable interplay and feedback mechanisms between black hole-associated activities and the nuclear environment's star formation. Significantly, the paper underscores existing theories that cosmic rays and magnetic fields foster outflows driven along magnetic lines, seen in the Galactic Centre's complex radio filament structures. The correlation between filamentary structures and the boundary regions of the detected bubbles proposes a unified framework driven by a common energetic source or progression from such a source.

Moving forward, this paper advances our understanding of energetic activities in Galactic nuclei and their consequences. Further observations, particularly across varying electromagnetic spectra, would elucidate the precise mechanisms and the role of different scaling factors, such as magnetic field strength and cosmic ray density attenuation, geographically within the bubbles and beyond.

In conclusion, this paper provides compelling evidence of a significant energy outburst sculpting the observed large-scale radio structures at the Galactic Centre. It beckons continued exploration into the causal links between stellar phenomena and supermassive black hole interactions, central to Galactic evolution and dynamics.