Our galaxy hasn't always been quiet a neighborhood. Scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) say that during the Miocene epoch about six million years ago, the galactic core of the Milky Way may have been so active that it became a quasar and blasted away most of the galaxy's normal matter, equivalent to hundreds of billions of Suns.

The Milky Way galaxy is estimated to be as massive as somewhere between one or two trillion Suns, of which five-sixths is dark matter that can only be detected by the gravitational field it generates. The rest is of normal matter, which should weigh in at 150 to 300 billion solar masses. But when all the visible stars, dust, and gases are added together, they only come to 65 billion solar masses.

The question is, where is the rest?

"We played a cosmic game of hide-and-seek. And we asked ourselves, where could the missing mass be hiding?" says Fabrizio Nicastro, a research associate at the CfA and astrophysicist at the Italian National Institute of Astrophysics. "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."

By calculating the amount of X-rays absorbed, the CfA team could measure how much normal matter is in this gaseous fog. They discovered that it forms a gas bubble around the center of the galaxy that extends out for two-thirds of the distance to the Solar System. This bubble is not only incredibly massive, but unbelievably hot, which means that the sort of energy that would make a supernova look like a damp squib was involved.

The source of this energy is the supermassive black hole at the galactic center, which is estimated to have a mass of 4.1 to 4.5 million solar masses. That black hole is currently inactive, having run out of "food" in its vicinity. But six million years ago things were very different according to the CfA team – the galaxy was going through a quasar phase.

Quasars as the most active objects in the Universe, punching out 100 million times the luminosity of our galaxy today. During the Milky Way's quasar phase, the black hole at its center sucked in huge amounts of matter for four to eight million years. This generated enough energy to pump gas out of the galaxy at speeds of 2 million mph (1,000 km/s).

Eventually, the center of the galaxy was depleted of gas and the black hole quieted down, but not before it had expelled 130 billion solar masses of matter at a temperature of millions of degrees, forming a shock wave that is now 40,000 light years in diameter and growing. The extremely high temperature is particularly important because it renders the gas nearly invisible to direct observation. In addition, the presence of stars at the galactic center that are only six million years old provides corroborative evidence as to the date of the event's end.

The CfA team says that future space missions, such NASA's X-ray Surveyor and ESA's Athena X-ray Observatory, could provide more data on this elusive bubble.

These results were published in The Astrophysical Journal.

Harvard-Smithsonian Center for Astrophysics
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