This artist's impression shows the Milky Way as it may have appeared 6 million years ago during a "quasar" phase of activity. A wispy orange bubble extends from the galactic center out to a radius of about 20,000 light-years. Outside of that bubble, a pervasive "fog" of million-degree gas might account for the galaxy's missing matter of 130 billion solar masses. Credit: Mark A. Garlick/CfA
A supermassive black hole is currently dormant in the center of the Milky Way, and only occasionally slurps hydrogen gas in small quantities. However, this situation was not always the same.
Based on a recent study, the galaxy's core blazed furiously when hominins - the first human ancestors - walked the Earth six million years ago. The proof of this active phase was obtained during a hunt for the galaxy's missing mass.
Measurements highlight that the Milky Way galaxy weighs around 1-2 trillion times as much as the Sun. Mysterious and invisible dark matter make up for almost five-sixths of the galaxy weight.
The remaining 150-300 billion solar masses, or one-sixth of the galaxy’s heft, are made up of normal matter. The normal matter that is left is made up of electrons, protons, and neutrons.
We played a cosmic game of hide-and-seek. And we asked ourselves, where could the missing mass be hiding? 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.
Fabrizio Nicastro, Research Associate,
Harvard-Smithsonian Center for Astrophysics (CfA)
The amount of absorption was used by the astronomers to calculate the volume of normal matter that existed there, and the way in which it was distributed. Despite using computer models, the astronomers failed to match the observations with a uniform, smooth distribution of gas.
However, they went on to discover the existence of a "bubble" in the center of the galaxy that stretches two-thirds of the way to Earth.
An increased amount of energy was required to clear out that bubble. The black hole swallowed some infalling gas, while other gas was pumped out at speeds of 2 million miles per hour (1,000 km/sec).
After six million years, the shock wave developed by that phase of activity crossed 20,000 light-years of space. Meanwhile, the black hole has entered a hibernation phase and run out of accessible food.
The presence of 6-million-year-old stars close to the galactic center corroborated this timeline. The stars were developed from some of the same material that earlier traveled 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.
Martin Elvis, Harvard-Smithsonian Center for Astrophysics CfA
The associated computer models and observations reveal that the hot, million-degree gas accounts for almost 130 billion solar masses of material. This highlights the fact that the galaxy's missing matter was hiding, as it was too hot to be seen.
Additional answers could be provided by the anticipated next-generation space mission referred to as X-ray Surveyor. It would be able to observe fainter sources and map out the bubble, and also to see minute details in order to obtain more information regarding the elusive missing mass. A similar promise is offered by the European Space Agency's Athena X-ray Observatory.
These findings are available online and have been accepted for publication in The Astrophysical Journal.
The Harvard-Smithsonian Center for Astrophysics (CfA), headquartered in Cambridge, Mass., is a joint venture between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, analyze the origin, evolution and eventual destiny of the universe.