Milky Way’s black hole appears to be getting hungrier
UCLA astronomers announced on September 11, 2019, that, last May, they caught the supermassive black hole at the center of our Milky Way galaxy having an unusually large meal of interstellar gas and dust. They caught the feast on May 13 (although of course it happened some 25,000 years ago earlier, since the center of the galaxy is about 25,000 light-years away). What they saw was this. The black hole – called Sagittarius A*, pronounced Sagittarius A-star – became extremely bright in May 2019, growing 75 times as bright for a few hours. Yet, as of now, the researchers don’t yet understand why. Why did the area just outside the black hole’s event horizon – its point of no return – suddenly become brighter? What did it ingest, and why at that time?
Astronomer Tuan Do is lead author of new research describing this event, published September 11 in Astrophysical Journal Letters. He also produced the timelapse in the tweet below, which depicts the brightness changes at Sgr A*. Andrea Ghez, of the UCLA Galactic Center Group, is co-senior author on the new paper. She said:
We have never seen anything like this in the 24 years we have studied the supermassive black hole. It’s usually a pretty quiet, wimpy black hole on a diet. We don’t know what is driving this big feast.
In a statement, the researchers also said they:
… analyzed more than 13,000 observations of the black hole from 133 nights since 2003. The images were gathered by the W. M. Keck Observatory in Hawaii and the European Southern Observatory’s Very Large Telescope in Chile. The team found that on May 13, the area just outside the black hole’s ‘point of no return’ (so called because once matter enters, it can never escape) was twice as bright as the next-brightest observation.
They observed large changes on two other nights this year; all three of those changes were ‘unprecedented,’ Ghez said.
They said the brightness surrounding the black hole always varies somewhat, but the extreme variations in brightness observed this year left them “stunned.”
So what is going on?
In an absolute sense, the increased brightness on a few nights in 2019 can be explained by radiation from gas and dust falling into the black hole. One hypothesis about the increased activity is that when a star called S0-2 made its closest approach to the black hole during the summer 2018, it launched a large quantity of gas that reached the black hole this year. Tuan Do, the study’s lead author, said:
The first image I saw that night, the black hole was so bright I initially mistook it for the star S0-2, because I had never seen Sagittarius A* that bright. But it quickly became clear the source had to be the black hole, which was really exciting.
Another possibility involves a bizarre object known as G2, which is most likely a pair of binary stars, which made its closest approach to the black hole in 2014. It’s possible the black hole could have stripped off the outer layer of G2, Ghez said, which could help explain the increased brightness just outside the black hole.
Morris said another possibility is that the brightening corresponds to the demise of large asteroids that have been drawn in to the black hole.
Here's a timelapse of images over 2.5 hr from May from @keckobservatory of the supermassive black hole Sgr A*. The black hole is always variable, but this was the brightest we’ve seen in the infrared so far. It was probably even brighter before we started observing that night! pic.twitter.com/MwXioZ7twV
– Tuan Do (@quantumpenguin) August 11, 2019
The question for astronomers is, what does this activity mean? Is it simply an extraordinary singular event, or is it a precursor to significantly increased activity for Sgr A*? Mark Morris, UCLA professor of physics and astronomy, is another author on the paper. He said:
The big question is whether the black hole is entering a new phase – for example if the spigot has been turned up and the rate of gas falling down the black hole ‘drain’ has increased for an extended period – or whether we have just seen the fireworks from a few unusual blobs of gas falling in.
The team has continued to observe the area. They say they’ll try to settle the question based on what they see from new images. Ghez said:
We want to know how black holes grow and affect the evolution of galaxies and the universe. We want to know why the supermassive hole gets brighter and how it gets brighter.
By the way, these astronomers commented:
The black hole is some 26,000 light-years away and poses no danger to our planet. Do said the radiation would have to be 10 billion times as bright as what the astronomers detected to affect life on Earth.
Astrophysical Journal Letters also published a second article by the researchers, describing speckle holography, the technique that enabled them to extract and use very faint information from 24 years of data they recorded from near the black hole.
Ghez’s research team reported July 25 in the journal Science the most comprehensive test of Einstein’s iconic general theory of relativity near the black hole. Their conclusion that Einstein’s theory passed the test and is correct, at least for now, was based on their study of S0-2 as it made a complete orbit around the black hole.
Ghez’s team said it:
… studies more than 3,000 stars that orbit the supermassive black hole. Since 2004, the scientists have used a powerful technology that Ghez helped pioneer, called adaptive optics, which corrects the distorting effects of the Earth’s atmosphere in real time. But speckle holography enabled the researchers to improve the data from the decade before adaptive optics came into play. Reanalyzing data from those years helped the team conclude that they had not seen that level of brightness near the black hole in 24 years.
It was like doing LASIK surgery on our early images. We collected the data to answer one question and serendipitously unveiled other exciting scientific discoveries that we didn’t anticipate.
Bottom line: UCLA astronomers announced on September 11, 2019, that – in May – they caught the supermassive black hole at the center of our Milky Way galaxy having an unusually large meal of interstellar gas and dust. Why did the area just outside the black hole’s event horizon – its point of no return – suddenly become dramatically brighter? What did it ingest, and why at that time?