Out of the Abyss

Out of the Abyss
Uncommon Ground - Out of the Abyss

Einstein posited their existence, and science fiction writers have been using them as a plot device for decades, but black holes–regions in space with a gravity field so great that not even light can escape–are still among the most misunderstood phenomena in the heavens. “People imagine that black holes are somehow cosmic vacuum cleaners, sucking in whatever happens to be nearby, but they’re really just an object that has gravity, like the earth or the sun,” says Dan Homan, an associate professor of physics at Denison, who studies the mysterious objects. “Black holes are a natural part of galaxies, a product of stars living and dying, and they are a foot in the door to understanding how the universe works.”

Thanks to an ingenious new approach to scanning the skies, Homan and a team of researchers from the U.S., Germany, and the Netherlands are opening that door a little further. In May, The Astrophysical Journal Letters published early results from the team’s comparison of two important sets of data about the massive black holes at the center of distant galaxies. While there’s a black hole at the center of our own galaxy, it’s tiny–just two million times the mass of our sun–compared to the ones that Homan studies, which may be a billion times as massive. These black holes are responsible for some of the most energetic activity found in the universe, active galactic nuclei. AGN, which emit so much energy that they can appear like stars in our night sky, are believed to be the result of space material heating up as it spirals into the black hole at the galaxy’s center. Since his days as a graduate student at Brandeis University, Homan has been part of an effort to study a phenomenon found in about 10 percent of the AGN discovered to date: two streams of superheated gas, called “jets,” shooting away from black holes rather than into them, at speeds approaching that of light.

“The fundamental question is how something with the strong gravity of a black hole could produce jets that are moving away from it with such speed,” says Homan. For almost a decade, he has been part of a group that uses data from the Very Long Baseline Array, a system of 10 telescopes located from Hawaii to the U.S. Virgin Islands, to observe the energy emitted by the jets in the radio range, the low end of the electromagnetic spectrum. Last year, when NASA launched the orbiting Fermi Gamma-ray Space Telescope, which measures the northern skies every three days, Homan and his colleagues seized the opportunity to assess the energy given off by the jets at the other end of the spectrum, in the form of ultra-high frequency gamma rays. “By comparing the radio data with the new information from Fermi,” says Homan, “we’re discovering that there is some kind of relationship between the lowest and highest energy emissions from these jets.

“Black holes capture a lot of people’s imaginations,” says Homan. “They’re so different from everything we’re used to.” He points out, though, that the theories and methods behind past black-hole research have had an impact on our daily lives: the image-processing algorithms created to interpret radio images of the sky have been used in developing medical ultrasound technology, and thanks to astrophysicists’ understanding of how gravity can bend space and time, GPS satellites are able to correct for the difference in the rate that time passes on the surface of the earth vs. in the satellites’ orbits, so that we end up at Starbucks instead of several meters down the road.

Homan’s own passion for astronomy began early. “I was a victim of Carl Sagan,” he says. “I remember watching Cosmos every week when I was seven or eight years old, and I was just blown away by all that we knew about the universe and all that we didn’t know.”

Homan has invited a few Denison undergraduates to join him in his research, and Michael McCormick ‘10 was even listed as a co-author on one of the papers published in May. McCormick, whose own interest in astronomy was sparked by high school science teacher Michael Bait ‘87, says that he has learned a lot about what it takes to be a working scientist. “You have to enjoy solving puzzles, and you have to accept that sometimes you won’t get the answer,” says McCormick. “It’s very exciting, though–this is really ‘the future is now’ science, and I think that there are some important discoveries just around the corner.”

Homan is also excited by the possibilities. “Because these active galaxies are so bright and so far away, they may make nice signposts for mapping out the universe,” he says. “We’ll need to understand the jets generated by the black holes at the center of these galaxies a lot better before we can do that, though.” Homan estimates that there may be another 10 years of research ahead before the jets can be used to measure distances in the universe, but he’ll enjoy the process.

“You think you understand what is going on, and then you find something new that doesn’t really invalidate what you did before, but it means that the story is more complex,” Homan says. “Some people might find that frustrating, but for me, the capacity for the unexpected is why it’s worthwhile.”

 

The Basics of Black Hole Science

Black holes: Some black holes are formed when stars run out of fuel and collapse; others may be by-products of the Big Bang. Their gravitational pull prevents anything, even light, from escaping once it passes within a radius around the black hole known as the “event horizon.”

Active galactic nuclei: At the center of some galaxies, there is a region that emits extraordinary amounts of energy. It’s believed that this phenomenon is the result of material giving up its energy as it spirals into a central black hole. When the energy is in the form of light that we can observe with an optical telescope, like a star, the AGN may also be called a quasi-stellar object, or quasar.

Radio jets: These streams of superheated gas appear to be moving away from black holes at speeds exceeding that of light, but their composition, and even the physics of their formation, are not yet known.

Electromagnetic spectrum: The full range of electromagnetic energy, from very low frequency radio waves, to visible light, to very high frequency gamma rays. While black holes themselves cannot be seen, phenomena surrounding them have been observed across this spectrum.

Published August 2009