The supermassive black hole at the center of the Milky Way, Sagittarius A*, is currently a gentle sleeping giant, but it has not always been this way.
According to new research, about 6 million years ago Sagittarius A* was undergoing an intense quasar phase, heating up the galaxy and creating a 20,000-light-year bubble devoid of gas around the core.
The Italian-American collaboration behind the research showed that during its active period, the supermassive black hole pushed and heated almost 130 billion solar masses of gas to millions of degrees. These findings, accepted for publication in The Astrophysical Journal and available as a pre-print on arXiv, finally provide an answer to the “missing mass” of the Milky Way.
Based on several physical considerations, our galaxies should have 150 to 300 billion solar masses of visible matter – the stuff that makes us, stars, planets, capybaras, and so on. If you add all the gas, dust, and all the stars in the galaxy it would result in about 65 billion solar masses. The rest is extremely hot gas in and around the galaxy.
"We played a cosmic game of hide-and-seek. And we asked ourselves, where could the missing mass be hiding?" said lead author Fabrizio Nicastro, a research associate at the Harvard-Smithsonian Center for Astrophysics (CfA) and astrophysicist at the Italian National Institute of Astrophysics (INAF), in a statement.
"We analyzed archival X-ray observations from the XMM-Newton spacecraft and found that the missing mass is in the form of a million-degree gaseous fog permeating our galaxy. That fog absorbs X-rays from more distant background sources."
When a black hole starts feeding, it doesn’t “eat” all the surrounding material. Some of it gets heated by the tremendous gravitational forces, emitting so much energy galaxy-wide winds are created, reaching a speed of 1,000 kilometers per second (2 million miles per hour). The X-ray analysis gave a clear indication of the size of the bubble, so the researchers were able to estimate it formed over 6 million years.
This scenario is confirmed by the presence of a 6-miillion-year-old star near the core. Stars only form from cold gas, so they must have formed from the material flowing in, rather than the hot gas flying out.
"The different lines of evidence all tie together very well," says fellow Smithsonian CfA co-author Martin Elvis. "This active phase lasted for 4 to 8 million years, which is reasonable for a quasar."
Next generation X-ray observatories might provide more information about this hot fog and we might find out more clues about the last time our friendly neighborhood supermassive black hole was awake.
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