Evidence for Thousands of Black Holes Buzzing Around the Center of the Milky Way

Since the 1970s, astronomers have understood that a Supermassive Black Hole (SMBH) resides at the center of the Milky Way Galaxy. Located about 26,000 light-years from Earth between the Sagittarius and Scorpius constellations, this black hole has come to be known as Sagittarius A* (Sgr A*). Measuring 44 million km across, this object is roughly 4 million times as massive as our Sun and exerts a tremendous gravitational pull.

Since that time, astronomers have discovered that most massive galaxies have SMBHs at their core, which is what separates those that have an Active Galactic Nuclei (AGN) from those that don’t. But thanks to a recent survey conducted using NASA’s Chandra X-ray Observatory, astronomers have discovered evidence for hundreds or even thousands of black holes located near the center of the Milky Way Galaxy.

The study which described their findings was recently published in the journal Nature under the title “A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy“. The study was led by Chuck Hailey, the Pupin Professor of Physics and the Co-Director of the Columbia Astrophysics Laboratory (CAL) at Columbia University, and including members from the Instituto de Astrofísica at the Pontificia Universidad Católica de Chile and the Harvard-Smithsonian Center for Astrophysics.

The center of the Milky Way Galaxy, with X-ray binaries circled in red, other X-ray sources circled in yellow, and Sagittarius A* circled in blue at the center. Credit: NASA/CXC/Columbia University/C. Hailey et al.

Using Chandra data, the team searched for X-ray binaries containing black holes that were in the vicinity of Sgr A*. To recap, black holes are not detectable in visible light. However, black holes (or neutron stars) that are locked in close orbits with a star will pull material from their companions, which will then be accreted onto the black holes’ disks and heated up to millions of degrees.

This will result in the release of X-rays which can then be detected, hence why these systems are called “X-ray binaries”. Using Chandra data, the team sought out X-ray of sources that were located within roughly 12 light years of Sgr A*. They then selected sources with X-ray spectra similar to those of known X-ray binaries, which emit relatively large amounts of low-energy X-rays.

Using this method, they detected fourteen X-ray binaries within about three light years of Sgr A*, all of which contained stellar-mass black holes (between 5 and 30 times the mass of our Sun). Two of these sources had been identified by previous studies and were eliminated from the analysis, while the remaining twelve (circled in red in the image above) were newly-discovered.

Other sources which relatively large amounts of high energy X-rays (labeled in yellow) were believed to be binaries containing white dwarfs. Hailey and his colleagues concluded that the majority of the dozen X-ray binaries were likely to contain black holes, based on their variability and the fact that their X-ray emissions over the course of several years was different from what is expected from binaries containing neutron stars.

Artist”s impression of a black hole binary, consisting of a black hole siphoning material from its companion. Credit: ESO/L. Calçada

Given that only the brightest X-ray binaries containing black holes are likely to be detectable around Sgr A* (given its distance from Earth), Hailey and his colleagues concluded that this detection implies the existence of a much larger population. By their estimates, there could be at least 300 and as many as one thousand stellar-mass black holes present around Sgr A*.

These findings confirmed what theoretical studies on the dynamics of stars in galaxies have indicated in the past. According to these studies, a large population of stellar mass black holes (as many as 20,000) could drift inward over the course of millions of years and collect around an SMBH. However, the recent analysis conducted by Hailey and his colleagues was the first observational evidence of black holes congregating near Sgr A*.

Naturally, the authors acknowledge that there are other explanations for the X-ray emissions they detected. This includes the possibility that half of the dozen sources they observed are millisecond pulsars – very rapidly rotating neutron stars with strong magnetic fields. However, based on their observations, Hailey and his team strongly favor the black hole explanation.

In addition, a follow-up study conducted by Aleksey Generozov (et al.) of Columbia University – titled “An Overabundance of Black Hole X-Ray Binaries in the Galactic Center from Tidal Captures” – indicated that there could be as many as 10,000 to 40,000 black holes binaries at the center of our galaxy. According to this study, these binaries would be the result of companions being captured by black holes.

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.
Artist’s impression of merging binary black holes. Credit: LIGO/A. Simonnet.

In addition to revealing much about the dynamics of stars in our galaxy, this study has implications for the emerging field of gravitational wave (GW) research. Essentially, by knowing how many black holes reside at the center of galaxies (which will periodically merge with one another), astronomers will be able to better predict how many gravitational wave events are associated with them.

From this, astronomers could create predictive models about when and how GW events are likely to happen, and well as discerning what role they may play in galactic evolution. And with next-generation instruments – like the James Webb Space Telescope (JWST) and the ESA’s Advanced Telescope for High Energy Astrophysics (ATHENA) – astronomers will be able to determine exactly how many black holes reside near the center of our galaxy.

 

 

Further Reading: NASA

Weekly Space Hangout – May 6, 2016: Paul Reichert – Photography in Space!

Host: Fraser Cain (@fcain)

Special Guest:
Paul Reichert is a Photo Instructor, NASA Johnson Space Center; International Space Station Mission Lead and Astronaut Technical Imaging Instructor (LM); Project lead for crew imaging operations on the International Space Station. Imaging operations for the Multi-Purpose Crew Vehicle.

Guests:
Paul M. Sutter (pmsutter.com / @PaulMattSutter)
Morgan Rehnberg (MorganRehnberg.com / @MorganRehnberg)
Kimberly Cartier (@AstroKimCartier )
Dave Dickinson (www.astroguyz.com / @astroguyz)

Their stories this week:
Comet X1 PanSTARRS

This Week in Musk

New details on ultra-luminous x-ray sources

Three potentially habitable worlds discovered around nearby star

ExoMars Phase 2 delayed to 2020

We’ve had an abundance of news stories for the past few months, and not enough time to get to them all. So we’ve started a new system. Instead of adding all of the stories to the spreadsheet each week, we are now using a tool called Trello to submit and vote on stories we would like to see covered each week, and then Fraser will be selecting the stories from there. Here is the link to the Trello WSH page (http://bit.ly/WSHVote), which you can see without logging in. If you’d like to vote, just create a login and help us decide what to cover!

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Amazing New X-Ray Image of the Whirlpool Galaxy Shows it is Dotted with Black Holes

In any galaxy there are hundreds of X-ray binaries: systems consisting of a black hole capturing and heating material from a relatively low-mass orbiting companion star. But high-mass X-ray binaries — systems consisting of a black hole and an extremely high-mass companion star — are hard to come by. In the Milky Way there’s only one: Cygnus X-1. But 30 million light-years away in the Whirlpool galaxy, M51, there are a full 10 high-mass X-ray binaries.

Nearly a million seconds of observing time with NASA’s Chandra X-ray Observatory has revealed these specks. “This is the deepest, high-resolution exposure of the full disk of any spiral galaxy that’s ever been taken in the X-ray,” said Roy Kilgard, from Wesleyan University, at a talk presented at the American Astronomical Society meeting today in Boston. “It’s a remarkably rich data set.”

Within the image there are 450 X-ray points of light, 10 of which are likely X-ray binaries.

The Whilpool galaxy is thought to have so many X-ray binaries because it’s in the process of colliding with a smaller companion galaxy. This interaction triggers waves of star formation, creating new stars at a rate seven times faster than the Milky Way and supernova deaths at a rate 10-100 times faster. The more-massive stars simply race through their evolution in a few million years and collapse to form neutron stars or black holes quickly.

“In this image, there’s a very strong correlation between the fuzzy purple stuff, which is hot gas in the X-ray, and the fuzzy red stuff, which is hydrogen gas in the optical,” said Kilgard. “Both of these are tracing the star formation very actively. You can see it really enhanced in the northern arm that approaches the companion galaxy.”

Eight of the 10 X-ray binaries are located close to star forming regions.

Chandra is providing astronomers with an in depth look at a class of objects that has only one example in the Milky Way.

“We’re catching them at a short window in their evolutionary cycle,” said Kilgard. “The massive star that formed the black hole has died, and the massive star that is accreting material onto the black hole has not yet died. The window at which these objects are X-ray bright is really short. It’s maybe only tens of thousands of years.”

Additional information available on the Chandra website.