Here’s a Glimpse of the Jaw-Dropping Physics Surrounding Our Supermassive Black Hole

Simulation of Material Orbiting close to a Black Hole
Simulation of material orbiting close to a black hole (ESO/Gravity Consortium/L. Calçada)

Full disclosure: I wrote the press release for the University of Waterloo, whose researcher, Avery Broderick, developed the theory behind the accretion disk hotspots that have now been observed immediately surrounding our galaxy’s supermassive black hole. Read the full release on the UW website. Below is a long-form version of my article, including quotes from my interview with Broderick.

New observations of the center of our galaxy have, for the first time, revealed hotspots in the disk of chaotic gas orbiting our Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*).

Using the tremendous resolving power of the ESO’s Very Large Telescope array in Chile, astronomers used the new GRAVITY instrument to detect the “wobble” of bright patches embedded inside the accretion disk that spins with the black hole. These bright features are clocking speeds of 30 percent the speed of light.

This is the first time any feature so close to a black hole’s event horizon has been seen and, using thirteen-year-old predictions by astrophysicists, we have a good idea about what’s causing the fireworks.

“It’s mind-boggling to actually witness material orbiting a massive black hole at 30 percent of the speed of light,” said scientist Oliver Pfuhl, of the Max Planck Institute for Extraterrestrial Physics and co-investigator of the study published in the journal Astronomy & Astrophysics. “GRAVITY’s tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented detail.”

It is thought that the accretion disk surrounding a black hole is threaded with a powerful magnetic field that frequently becomes unstable and “reconnects.” Similar to the physics that drives the explosive flares in the Sun’s lower corona, these reconnection events rapidly accelerate the plasma in the disk, discharging vast quantities of radiation. These flaring events inside Sgr A*’s accretion disk create hotspots that get pulled in the direction of the material’s spin as it slowly gets digested by the black hole. The GRAVITY instrument was able to deduce that the accretion disk material is orbiting the black hole in a clockwise direction from our perspective and the accretion disk is almost face-on.

Artist’s impression of a hot accretion disk surrounding a black hole [NASA]
The original theory behind these hotspots was derived by Avery Broderick (University of Waterloo) and Avi Loeb (Harvard University) when they were both working at Harvard-Smithsonian Center for Astrophysics in the mid-2000s. In 2005 and 2006, the pair published papers that described theoretical computer models that simulated reconnection events in a black hole’s accretion disk, which caused intense heating and bright flares. The resulting hotspot would then continue to orbit with the speeding accretion disk material, cooling down and spreading out, before another instability and reconnection event would be triggered.

Their work was inspired by the detection of enigmatic bright flares erupting in the vicinity of Sgr A*. These flares were powerful and regular, occurring almost daily. At the time, a few theories were being explored—from supernovas detonating near the supermassive black hole, to asteroids straying too close to the black hole’s gravitational maw—but Broderick and Loeb decided to focus on the extreme region immediately surrounding the black hole’s event horizon.

“Avi and I thought: ‘well, if the flare timescales are close to orbital timescales around the black hole, wouldn’t it be interesting if they are actually bright features embedded in the accretion flow orbiting close to it?’,” Broderick told me.

Black holes are gravitational masters of their domain; anything that drifts too close will be blended into a superheated disk of plasma surrounding them. The matter trapped in the accretion disk then flows toward the event horizon—the point at which nothing, not even light, can escape—and consumed by the black hole via mechanisms that aren’t yet fully understood. The researchers knew that if their model was an accurate depiction of what is going on in the core of our galaxy, these hotspots could be used as visual probes to trace out structures in the accretion disk and in space-time itself.

This plot shows a comparison of the data with the hotspot model including various effects of General and Special Relativity. The continuous blue curve denotes a hot spot on a circular orbit with 1.17 times the innermost stable circular orbit, i.e. just outside the event horizon, of a 4 million solar mass black hole. The axis give the offset from the center in micro-arcseconds [MPE/GRAVITY collaboration]
It’s Sgr A*’s gravity of 4 million Suns that gives the flares a super-boost, however. “In our orbiting hotspot model, a key component of the brightening is actually caused by gravitational lensing,” added Broderick, referring to a consequence of Einstein’s general relativity, when the gravity of black holes warp space-time so much as to form lenses that can magnify the light from distant astronomical sources. “It’s like a black hole analog of a lighthouse.”

Now that GRAVITY has confirmed the existence of these hotspots, Broderick is overjoyed.

“I’m still absorbing it; it’s extremely exciting,” he said. “I’m bouncing around a little bit! The fact you can track these flares is completely new, but we predicted that you could do this.”

The GRAVITY study is led by Roberto Abuter of the European Southern Observatory (ESO), in Garching, Germany, and it describes the detection of three flares emanating from Sgr A* earlier this year. Although the hotspots cannot be fully resolved by the VLT, with the help of Broderick and Loeb’s predictions, Abuter’s team recognized the “wobble” of emissions from the flares as their associated hotspots orbited the supermassive black hole.

This discovery opens a brand-new understanding of the environment immediately surrounding Sgr A* and will complement observations made by the Event Horizon Telescope (EHT), an international collaboration of radio telescopes that are currently taking data to acquire the first image of a black hole, which is expected early next year.

Broderick hopes that these advances will help us to understand how black holes grow and consume matter, and if the predictions of general relativity break down at one of the most gravitationally extreme environments in the universe. But he’s most excited about how the first EHT image of a black hole will impact society as a whole: “It’s going to be a wonderful event, I think it will be an iconic image and it will make black holes real to a lot of people, including a lot of scientists,” he said.

Aside: In 2016, I had the incredible good fortune to visit the VLT at the ESO’s Paranal Observatory as part of the #MeetESO event. I interviewed several VLT and ALMA scientists, including Oliver Pfuhl, and helped produce the mini-documentary below:

Unexpectedly Large Black Holes and Dark Matter

The M87 black hole blasts relativistic plumes of gas 5000 ly from the centre of the galaxy (NASA)
The M87 black hole blasts relativistic plumes of gas 5000 ly from the centre of the galaxy (NASA)

I just spent 5 minutes trying to think up a title to this post. I knew what I wanted to say, but the subject is so “out there” I’m not sure if any title would be adequate. As it turns out, the title doesn’t really matter, so I opted for something more descriptive…

So what’s this about? Astronomers think they will be able to “see” a supermassive black hole in a galaxy 55 million light years away? Surely that isn’t possible. Actually, it might be.

When Very Long Baseline Interferometry is King

Back in June, I reported that radio astronomers may be able to use a future network of radio antennae as part of a very long baseline interferometry (VLBI) campaign. With enough observatories, we may be able to resolve the event horizon of the supermassive black hole lurking at the centre of the Milky Way, some 26,000 light years away from the Solar System.

The most exciting thing is that existing sub-millimeter observations of Sgr. A* (the radio source at the centre of our galaxy where the 4 million solar mass black hole lives) suggest there is some kind of active structure surrounding the black hole’s event horizon. If this is the case, a modest 7-antennae VLBI could observe dynamic flares as matter falls into the event horizon.

It would be a phenomenal scientific achievement to see a flare-up after a star is eaten by Sgr. A*, or to see the rotation of a possibly spinning black hole event horizon.

All of this may be a possibility, and through a combination of Sgr. A*’s mass and relatively close proximity to Earth, our galaxy’s supermassive black hole is predicted to have the largest apparent event horizon in the sky.

Or does it?

M87 Might be a Long Way Away, But…

As it turns out, there could be another challenger to Sgr. A*’s “largest apparent event horizon” crown. Sitting in the centre of the active galaxy called M87, 55 million light years away (that’s over 2,000 times further away than Sgr. A*), is a black hole behemoth.

M87’s supermassive black hole consumes vast amounts of matter, spewing jets of gas 5,000 light years from the core of the giant elliptical galaxy. And until now, astronomers have underestimated the size of this monster.

Karl Gebhardt (Univ. of Texas at Austin) and Thomas Jens (Max Planck Institute for Extraterrestrial Physics in Garching, Germany) took another look at M87 and weighed the galaxy by sifting through observational data with a supercomputer model. This new model accounted for the theorized halo of invisible dark matter surrounding M87. This analysis yielded a shocking result; the central supermassive black hole should have a mass of 6.4 billion Suns, double the mass of previous estimates.

Therefore, the M87 black hole is around 1,600 times more massive than our galaxy’s supermassive black hole.

A Measure for Dark Matter?

Now that the M87 black hole is much bigger than previously thought, there’s the tantalizing possibility of using the proposed VLBI to image M87’s black hole as well as Sgr. A*, as they should both have comparable event horizon dimensions when viewed from Earth.

Another possibility also comes to mind. Once an international VLBI is tested and proven to be an “event horizon telescope,” if we are able to measure the size of the M87 black hole, and its mass is confirmed to be in agreement with the Gebhardt-Jens model, perhaps we’ll have one of the first indirect methods to measure the mass of dark matter surrounding a galaxy…

Oh yes, this should be good.

UPDATE! How amiss of me, I forgot to include the best black hole tune ever:

Publication: The Black Hole Mass, Stellar Mass-to-Light Ratio, and Dark Matter Halo in M87, Karl Gebhardt et al 2009 ApJ 700 1690-1701, doi: 10.1088/0004-637X/700/2/1690.
Via: New Scientist