Artist's concept of what a future telescope might see in looking at the black hole at the heart of the galaxy M87. Illustration by Lynette Cook, courtesy of Gemini Observatory/AURA.
Editor’s note: As a student-driven publication, Reporting Texas strives for the advancement of innovative, multimedia journalism. The site also works to reveal the hidden stories of Texas. The following is the product of a joint effort with partner publication, the Austin American-Statesman, Austin’s largest daily metro newspaper.
By James Jeffrey
For Reporting Texas and the American-Statesman
Astronomers from the University of Texas have measured the most massive black hole yet discovered in a distant corner of space. Using two advanced telescopes — a giant one in Hawaii and a smaller, Texas-based companion — along with super-computers, they harnessed recent technological advances in computer software and telescopic hardware to see more of the cosmos.
Karl Gebhardt, a University of Texas astronomy professor, led the team of six studying the galaxy called M87 far beyond our own Milky Way. The data they collected, outlined this month in The Astrophysical Journal, describe a black hole there with a mass of 6.6 billion suns. The team’s key to success was the two telescopes and their complementing advanced technologies, which produced a wealth of data the supercomputers could handle.
The high-altitude Gemini North telescope in Hawaii facilitated observing deep space with less atmospheric blurring, a common problem for astronomers. The Hubble telescope orbiting Earth avoids the blurring entirely, but there is a limit to the size of telescopes that can be launched into space. Hubble is not large enough for galaxies at the distance of M87.
M87 is so far away that a ray of light, traveling at 186,282 miles a second, would take 50 million years to get there. Gemini, a $100 million scope with an 8-meter mirror, is big enough to see as far as M87 and uses adaptive optics, the latest high-tech system that compensates for variables in our shifting atmosphere. Adaptive optics combine computer software and rod actuators attached to the mirror, to measure the atmosphere and adjust the optical surface every few hundredths of a second to compensate.
“Trying to see the bottom of a shallow pond from above the water through violent waves, the light gets bent by the choppy water surface — you would only see a very blurry image of the pond floor,” said Joshua Adams, a UT-Austin astronomy graduate student who analyzed Gemini data. “If you could put another layer of water between you and the pond that had exactly the complementary shape, you could recover a smooth surface and see the pond’s bottom very well. We do the same thing with adaptive optics, except we fight the atmosphere’s motion instead of a pond’s surface.”
The combination of Gemini’s size and adaptive optics meant scientists could accurately monitor movements of stars around the black hole at the center of M87.
To pin down the black hole’s mass, the scientists also needed data about the edges of the galaxy, known as the “dark halo” and filled with dark matter. Neither can be seen, but due to forces they exert on stars, if you can track those stars, you can infer the dark matter’s mass.
This is where the smaller Harlan J. Smith telescope at UT’s McDonald Observatory in West Texas played its part. Despite a humbler 2.7-meter mirror, the scope is equipped with VIRUS-P, an integral field unit spectrograph. A spectrograph measures Doppler shift, the same effect that causes the pitch of a police car siren to be higher as the car approaches than when it’s moving away.
Measuring the Doppler shift of waves from stars gives their velocity. If you can measure their velocity, you can begin calculating the mass of dark matter, even if you can’t see it — one of the laws of physics.
Also, VIRUS-P is the world’s largest spectrograph and can monitor a huge chunk of the sky, taking in faint light from stars and aggregating wave information to form a detailed observation of what is out there.
The team combined data from the two telescopes about the central and outermost regions of the galaxy — primarily the interplay between the dark halo, the stars and the black hole. With those components considered together, plus the accuracy afforded by adaptive optics and VIRUS-P, they could accurately calculate the size of the black hole.
With something as large and complex as a black hole, results do not come quickly. Gebhardt said that after collecting the data in 2009, it took a year of data-crunching to obtain the results, even using the supercomputers at UT’s Texas Advanced Computing Center at the J.J. Pickle Research Campus.
“The TACC gets huge credit. That’s been a tremendous advantage for us with very complicated models. Five years ago I could not have done this,” he said.
The advanced technology also increases the likelihood of witnessing the edges of the black hole called the “event horizon,” the definitive property of a black hole, Gebhardt said.
The event horizon has a nightmarish quality — nothing can escape it. The M87 black hole’s event horizon is about three times the size of Pluto’s orbit. If that event horizon were close enough, it could suck in the whole of our solar system.
The data Gebhardt and his colleagues have gathered might help scientists understand how stars assemble themselves and how a galaxy is created, Gebhardt said.
Establishing the relevance of this kind of knowledge can be hard for astronomers. But Gebhardt said that when confronted with humankind’s problems and bad news, adding a sense of awe is something to get excited about, “and that is what astronomy is doing.”
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UT professor Karl Gebhardt speaks about the VIRUS-P instrument.