Hubble solves the mystery bulge at the center of the Milky Way

The Fermi Bubbles are two huge structures “burped out” by the Milky Way’s supermassive black hole and visible in X-ray and gamma-ray light. NASA’s Goddard Space Flight Center

When seen in visible light, the Milky Way appears to be a relatively flat structure. Gamma-ray emission, on the other side, shows two massive structures billowing forth from the galaxy’s bulge like a giant hourglass. These structures, known as the Fermi Bubbles, are the result of the Milky Way’s supermassive black hole gorging itself on interstellar gas in the past. Using the Hubble Space Telescope (HST), astronomers have now determined just when these structured formed.

A team lead by Rongmon Bordoloi of the Massachusetts Institute of Technology utilized distant quasars to study the structure and motion of the northern Fermi Bubble, which rises 23,000 light-years above the plane of the Milky Way and contains enough cold gas to create 2 million Sun-size stars. By monitoring the ultraviolet light from 46 quasars with HST’s Cosmic Origins Spectrograph (COS) (and adding one quasar observation with HST’s Space Telescope Imaging Spectrograph), the team determined the age of the bubble: 6 to 9 million years.

Most galaxies contain a supermassive black hole at the center, and our Milky Way is no exception. Sgr A* resides in the Milky Way’s bulge and has a mass equivalent to 4.5 million solar masses. Sgr A* is now rather quiet, quietly accreting as the galaxy matures. Quasars, on the other hand, are young, giant supermassive black holes in the nuclei of galaxies in the early universe, sucking down tremendous amounts of gas and dust that flash brightly as the material is funneled into an accretion disk before passing into the black hole. Astronomers think that, like these younger black holes, our own supermassive black hole was once more active, when the galaxy was still growing and material for accretion was more plentiful.

Sometimes, though, material doesn’t actually make it all the way into the black hole. Matter can escape along the black hole’s spin axis, exiting the area — and often the galaxy altogether — as huge outflows that span tens or hundreds of thousands of light-years. Fermi Bubbles are such outflows in the Milky Way; they were found in 2015 and called after NASA’s Fermi Gamma-Ray Telescope, which discovered them.

To learn more about the sources of these fluxes, we need to know how they move. “We have traced the outflows of other galaxies, but we have never been able to actually map the motion of the gas,” said Bordoloi in a press release announcing his group’s results. The work was also reported in The Astrophysical Journal on January 10, 2017. “The only reason we could do it here is because we are inside the Milky Way. This vantage point gives us a front-row seat to map out the kinematic structure of the Milky Way outflow.”

As the quasars’ light travels through the bubble to reach Earth, it highlights the gas in the bubble itself, allowing astronomers to determine information such as its chemical composition, temperature, and motion. The “cool” gas in the northern Fermi Bubble, which contains components like silicon and carbon, has been recorded to travel at 2 million miles per hour (3 million kph) and reach temperatures of 17,700 degrees Fahrenheit (9,800 degrees Celsius).

Such chilly gas is most likely gas from the galaxy’s disk that has been swept up by and merged into the outflow, which has temperatures of up to 18 million degrees F (nearly 10 million degrees C). It is these high temperatures that cause the gas to shine in energetic light, such as gamma rays.

Astronomers utilized the velocity of the gas — its speed and direction of travel — to turn back the clock and pinpoint when the gas began moving. This is also the last known “big meal” had by Sgr A*, who hasn’t been able to eat such a huge amount of matter since.

“What we find is that a very strong, energetic event happened 6 million to 9 million years ago,” Bordoloi explained. “It may have been a cloud of gas flowing into the black hole, which fired off jets of matter, forming the twin lobes of hot gas seen in X-ray and gamma-ray observations. Ever since then, the black hole has just been eating snacks.”