Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found thousands of dense gas clumps but, surprisingly, no star formation, in a circumnuclear disk around Sagittarius A*, the 4-million-solar-mass black hole at the center of our Milky Way Galaxy.
Clumps of molecular gas overlaid on Sagittarius A* circumnuclear disk as seen by ALMA in the CS(7-6) line. Yellow circles are thin clumps that are going to be shredded by the gravitational force of supermassive black hole Sagittarius A*. Green circles are dense enough to survive the tidal shredding but are not able to form stars. Purple/pink circles have the needed density to form stars, but no star formation has been observed. Image credit: Hsieh et al. / ALMA / EOS / NAOJ / NRAO.
Every large galaxy has a central supermassive black hole that dominates and is fed by nearby molecular gas.
Since molecular gas is the material that supplies black holes and forms stars, Dr. Pei-Ying Hsieh and colleagues wanted to know how much gas is available to form stars and how much is going to feed the supermassive black hole.
The first challenge of star formation in the vicinity of the Milky Way’s center is avoiding the high tidal shear that can easily tear apart the nearby molecular clouds, preventing them from accumulating enough mass for fragmentation and core-collapse to proceed.
“The circumnuclear disk can be imagined as a factory of many doughs rotating around the supermassive black hole,” said Dr. Hsieh, an astronomer at the Joint ALMA Observatory, ESO, and the Academia Sinica Institute of Astronomy and Astrophysics.
“If the dough is too thin, it will be stretched like spaghetti by the black hole and so feed it; if the dough is dense enough, it has a chance to overcome the tidal shear and become ‘bread,’ and so a star.”
This composite image shows hot ionized gas (red) captured by Hubble and much colder molecular gas (blue and purple) captured by ALMA around Sagittarius A*’s circumnuclear disk. Image credit: Dong et al. / NASA / ESA / Hubble / Hsieh et al. / N. Lira / ALMA / EOS / NAOJ / NRAO.
The astronomers used ALMA to observe the carbon monosulfide molecule lines in the circumnuclear disk around Sagittarius A*.
Carbon monosulfide is a dense gas tracer that better samples the circumnuclear disk than carbon monoxide. This method provided a better way to constrain the gas densities and better understand what is going on in it.
The researchers found that while a significant amount of gas is available to form stars, there is no clear evidence of star formation.
The seemingly unstable clumps of molecular gas should then be marginally stabilized by other forces such as magnetic fields.
“Because the polarized signal generated by the magnetic field from dust emission is weak and difficult to measure, the magnetic field of the circumnuclear disk has not yet been probed at clump-scale (8,000 AU),” Dr. Hsieh said.
“Thanks to the high resolution and sensitivity of ALMA, we have granted the ALMA time to mosaic the magnetic field of the circumnuclear disk in future observations with ALMA.”
“We will then continue to explore the role of magnetic fields in star formation in this region.”
The findings were published in the Astrophysical Journal.
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Pei-Ying Hsieh et al. 2021. The Circumnuclear Disk Revealed by ALMA. I. Dense Clouds and Tides in the Galactic Center. ApJ 913, 94; doi: 10.3847/1538-4357/abf4cd
