Milky Way’s center is currently a quiet place. But it wasn’t always this way – a new study published in the Astrophysical Journal Letters (arXiv.org preprint) shows that approximately six million years ago, the Galactic core blazed forth furiously.
This artist’s impression shows our Milky Way Galaxy as it may have appeared 6 million years ago during a phase of quasar-like activity. Outside of that bubble, a pervasive ‘fog’ of million-degree gas might account for the galaxy’s missing matter of 130 billion solar masses. Image credit: Mark A. Garlick / CfA.
Measurements show that our Milky Way Galaxy weighs about 1-2 trillion times as much as our Sun.
About five-sixths of that is in the form of invisible and mysterious dark matter. The remaining one-sixth of our galaxy’s heft, or 150-300 billion solar masses, is normal matter.
However, if you count up all the stars, cold and mildly photo-ionized gas and dust, you only find about 65 billion solar masses. The rest of the normal matter seems to be missing.
“We asked ourselves, where could the missing mass be hiding? We analyzed archival X-ray observations from ESA’s XMM-Newton spacecraft and found that the missing mass is in the form of a million-degree gaseous fog permeating our Galaxy,” said study lead author Dr. Fabrizio Nicastro, from the Italian National Institute of Astrophysics and the Harvard-Smithsonian Center for Astrophysics.
“That fog absorbs X-rays from more distant background sources,” he added.
Dr. Nicastro and his colleagues used the amount of absorption to calculate how much normal matter was there, and how it was distributed.
They applied computer models but learned that they couldn’t match the observations with a smooth, uniform distribution of gas.
Instead, they found that there is a ‘bubble’ in the Galactic center that extends two-thirds of the way to Earth.
Clearing out that bubble required a huge amount of energy. That energy came from the actively feeding supermassive black hole, Sagittarius A*.
While some infalling gas was swallowed by Sagittarius A*, other gas was pumped out at speeds of 2 million mph (1,000 km/sec).
Six million years later, the shock wave created by that phase of activity has crossed 20,000 light-years of space.
Meanwhile, Sagittarius A* has run out of nearby food and gone into hibernation.
This timeline is corroborated by the presence of 6-million-year-old stars near the Galactic center. Those stars formed from some of the same material that once flowed toward the black hole.
“The different lines of evidence all tie together very well. This active phase lasted for 4 to 8 million years, which is reasonable for a quasar,” said co-author Dr. Martin Elvis, from the Harvard-Smithsonian Center for Astrophysics.
The observations and associated computer models also show that the hot, million-degree gas can account for up to 130 billion solar masses of material. Thus, it just might explain where all of the Milky Way’s missing matter was hiding: it was too hot to be seen.
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F. Nicastro et al. 2016. A Distant Echo of Milky Way Central Activity closes the Galaxy’s Baryon Census. ApJ 828, L12; doi: 10.3847/2041-8205/828/1/L12
