Astronomers have uncovered evidence that Sagittarius A*, the 4-million-solar-mass black hole at the center of our Milky Way Galaxy, is not a sleeping monster but periodically hiccups as stars and gas clouds fall into it.
This is a composite view of X-rays, molecular gas, and warm ionized gas near the Milky Way’s center (yellow represents Hubble data, blue is Chandra data, green is ALMA data, and red is VLA data). The orange-colored features are of glowing hydrogen gas. One such feature, at the top tip of the jet is interpreted as a hydrogen cloud that has been hit by the outflowing jet. The jet scatters off the cloud into tendrils that flow northward. Farther down near the black hole are X-ray observations of superheated gas colored blue and molecular gas in green. These data are evidence that the black hole occasionally accretes stars or gas clouds, and ejects some of the superheated material along its spin axis. The graphic of a translucent, vertical white fan is added to show the suggested axis of a mini-jet from the supermassive black hole at the Galaxy’s heart. Image credit: NASA / ESA / Gerald Cecil, University of North Carolina / Joseph DePasquale, STScI.
“Sagittarius A* is dynamically variable and is currently powered down,” said Dr. Gerald Cecil, an astronomer at the University of North Carolina.
“We pieced together multiwavelength observations from a variety of telescopes that suggest the black hole burps out mini-jets every time it swallows something hefty, like a gas cloud.”
In 2013, evidence for a stubby southern jet near Sagittarius A* came from X-rays detected by NASA’s Chandra X-ray Observatory and radio waves detected by NSF’s Karl G. Jansky Very Large Array.
Dr. Cecil and colleagues were curious if there was a northern counter-jet as well.
They first looked at archival spectra of such molecules as methyl alcohol and carbon monosulfide from the Atacama Large Millimeter/Submillimeter Array (ALMA).
The ALMA data revealed an expanding, narrow linear feature in molecular gas that can be traced back at least 15 light-years to the black hole.
By connecting the dots, the researchers next found in Hubble infrared-wavelength images a glowing, inflating bubble of hot gas that aligns to the jet at a distance of at least 35 light-years from the black hole. They suggest that the black hole jet has plowed into it, inflating the bubble.
These two residual effects of the fading jet are the only visual evidence of it impacting molecular gas. As it blows through the gas the jet hits material and bends along multiple streams.
“The streams percolate out of the Milky Way’s dense gas disk,” said Dr. Alex Wagner, an astronomer at Tsukuba University.
“The jet diverges from a pencil beam into tendrils, like that of an octopus.”
This outflow creates a series of expanding bubbles that extend out to at least 500 light-years.
This larger ‘soap bubble’ structure has been mapped at various wavelengths by other telescopes.
The team next ran supercomputer models of jet outflows in a simulated Milky Way disk, which reproduced the observations.
“Like in archeology, you dig and dig to find older and older artifacts until you come upon remnants of a grand civilization,” Dr. Cecil said.
“Our central black hole clearly surged in luminosity at least 1 millionfold in the last million years. That sufficed for a jet to punch into the Galactic halo,” Dr. Wagner added.
The team’s work was published in the Astrophysical Journal.
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Gerald Cecil et al. 2021. Tracing the Milky Way’s Vestigial Nuclear Jet. ApJ 922, 254; doi: 10.3847/1538-4357/ac224f
