Is magnetic field an important guiding force for gas accreting to supermassive black hole (SMBH) - for example, the one that our Milky Way Galaxy hosts? The role of magnetic field in this subject is little understood and trying to observe it has been challenging to astronomers. Researchers at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), Taiwan, led by Dr. Pei-Ying Hsieh, have obtained a good measurement by using the instruments on the James Clerk Maxwell Telescope (JCMT).

Their result provides clear evidences in showing that the orientation of the magnetic field is in alignment with the molecular torus and ionized streamers rotating with respect to Sagittarius A* - the black hole our home galaxy hosts. The findings are published in Astrophysical Journal in 2018 August (2018, ApJ, 862, 150).

Sagittarius A* (Sgr A*), being the closest SMBH in our home in the universe, the Milky Way Galaxy, has been targeted by many scientists to understand the nature of gas accretion in the past decades. Observing the gas accretion onto SMBH is critical to help us to understand how it releases tremendous energy.

The circumnuclear disk (CND) is a molecular torus rotating with respect to Sgr A*, within which are the ionized gas streamers called mini-spiral (also called Sgr A West) filling the molecular cavity. The mini-spiral is hypothesized to be originated from the inner edge of the CND. The CND, being the closest "food reservoir" of Sgr A*, is therefore critical on the understanding of the feeding of Sgr A*. However, looking for the physical evidences to connect the CND and the mini-spiral puzzles astronomers since they were discovered 35 years ago.

Intensive measurements of dynamical movements orbiting Sgr A* have been done in the past decades, but another important force - the magnetic field - is rarely probed. This is solely because the weak polarized signal generated by the magnetic field from dust emission is difficult to measure.

However, the magnetic field is expected to be important for material orbiting within and around the CND as the magnetic stress acting on the rotating disk can exert a torque to extract angular momentum from rotating gas, and thus drive gas inflows. Besides, the magnetic tension force is also possible to pull the gas back from the black hole-hell.

Taking advantage of excellent atmospheric conditions of Mauna Kea summit at 4,000 m, and large aperture size of the JCMT (15 m in diameter), the submillimeter polarization experiments were successfully obtained toward the galactic center to understand the role of magnetic field.

Tracing Magnetized Accreting Inflow
The astronomers utilized the dust polarization data obtained by the JCMT-SCUPOL instrument to image the orientation of the magnetic field. A detailed comparison with higher-resolution interferometric maps from the Submillimeter Array (SMA) reveals that the magnetic field aligns with the CND.

Moreover, the innermost observed magnetic field lines also appear to trace and align with the mini-spiral coherently. This is the first attempt to reveal the footprint of inflow linking the CND and the mini-spiral since they were discovered 35 years ago. The comparison of the model and data reinforces the key idea that the CND and the mini-spiral can be treated as a coherent inflow system.

They found that the magnetic field is dynamically significant towards the CND and the mini-spiral. This finding tells us that the magnetic field is able to guide the motion of the ionized particles originated in the CND, and produce the observed spiral pattern of the mini-spiral. The results have shown that the magnetic field is critical to explain the inflow structure and will also help to understand the inflow picture in other galaxies hosting black hole similar to Sgr A*.

Research Report: "A Magnetic Field Connecting the Galactic Center Circumnuclear Disk with Streamers and Mini-Spiral - Implications from 850-micron Polarization Data," Pei-Ying Hsieh et al., 2018 Aug. 2, Astrophysical Journal

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