’Blazar’ illuminates era when stars and galaxies formed
In an article posted June 10 to the Astrophysical Journal Letters website, astrophysicists at Stanford report spotting a black hole so massive that it’s more than 10 billion times the mass of our sun. More important, this heavyweight is so far away that the scientists think it formed when the universe first began to light up with stars and galaxies, so it may provide a window into our cosmological origins.
’’In cosmology, it turns out that ’a galaxy a long time ago’ and ’far, far away’ really do go together,’’ says Associate Professor Roger Romani, who with graduate student David Sowards-Emmerd and Professor Peter Michelson of Stanford, and radio astronomer Lincoln Greenhill of the Harvard-Smithsonian Center for Astrophysics, spotted one of the oldest supermassive black holes yet found. The scientists collaborate at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford. ’’In this case, we’re looking at [a black hole] far enough away that it’s within a billion years of the origin of it all, the Big Bang.’’
The supermassive black hole sits in the center of a galaxy. A disk of stars and gas swirl around the black hole and eventually get sucked in. ’’That generates enormous amounts of power, enormous amounts of energy,’’ Romani says. ’’It’s far more efficient even than nuclear fusion. These gravity-powered sources are the most powerful sources in the universe.’’
As black holes go, this one is a messy eater. It’s Jabba the Hutt, in fact, gobbling up its galaxy so quickly that not everything is making it down its throat past the point of no return - that place, called the ’’event horizon,’’ where not even light can escape gravity’s strongest pull. The matter that doesn’t make it past the event horizon is spewing back up in the form of accelerated high-energy particles.
If a black hole amid a galaxy shoots out high-energy particles in narrow jets that just happen to be aimed at Earth, astrophysicists give the whole thing a special name - ’’blazar.’’ Amazingly, these blazars can be detected at nearly all energies, even at the high energy of gamma rays. In fact, distant blazars seem to dominate the gamma-ray sky and can obscure other objects of interest. Pulsars, spinning neutron stars nearby in our own galaxy, can also emit gamma rays, but far fewer of them are known. Romani, whose main interest is pulsars, wanted to identify and discard blazars so he could concentrate on the neutron stars.
’’I got started working on the blazars as a way of culling the wheat from the chaff,’’ Romani says. ’’But then the chaff proved just as interesting.’’
In preparation for a mission that is scheduled to launch in 2007, the co-authors have surveyed 200 blazars; eventually they hope to survey 2,000. The mission, led by Michelson, will use the Gamma Ray Large Area Space Telescope (GLAST) to study high-energy sources of radiation in the universe, such as supermassive black holes, merging neutron stars and hot streams of gas moving at nearly the speed of light. It is funded by NASA, the U.S. Department of Energy and government agencies in France, Italy, Japan and Sweden. ’’Something really new is waiting to be found in the gamma-ray sky,’’ Romani says. ’’ If we could identify all the blazars, tag the pulsars - the things that are left over, that’s where the really new discoveries will be.’’
In photographs, blazars look just like stars. So how do scientists spot them? The co-authors first identified gamma rays seen by the Energetic Gamma Ray Experiment Telescope (EGRET), a GLAST precursor initiated by Stanford physics Professor Robert Hofstadter in the 1970s and subsequently directed by Michelson.
Greenhill led the effort to obtain radio images of the blazar jet using the Very Long Baseline Array (VLBA). Funded by the National Science Foundation and operated by the National Radio Astronomy Observatory, the VLBA is essentially a radio camera. It consists of 10 dish antennas - 25 meters wide and distributed from Hawaii across the United States to St. Croix - slaved together with computers to create a composite image with a resolution Greenhill calls ’’comparable to what they would get with a single antenna about as large as a continent.’’
To find out how far away the blazar was, Romani and Sowards-Emmerd used the Hobby-Eberly Telescope (HET), an optical instrument in a remote part of Texas, to obtain spectral patterns of visible and infrared light. HET is a joint project of the University of Texas at Austin, Pennsylvania State University, Stanford, Ludwig-Maximilians-Universität München and Georg-August-Universität Göttingen.
Spectroscopy reveals signatures of elements in a galaxy’s gases. Elements such as hydrogen, nitrogen, carbon and oxygen radiate at specific energies, or equivalently at specific wavelengths. A consequence of cosmic expansion is that those wavelengths get shifted to the red part of the spectrum, or ’’red-shifted,’’ if an object is extremely far away.
The red shift corresponds to age. ’’The higher that number, the smaller the universe was when the light was emitted - hence, the earlier you’re talking about,’’ Romani explains.
The Hobby-Eberly Telescope told the researchers that the red shift of their blazar was 5.5. This high number told them this was not just some star in our backyard; it was an enormous source of energy shining from way across the universe.
’’It’s amazing to find something so interesting and unique in a relatively small survey,’’ says Sowards-Emmerd, who re-analyzed EGRET data to select the targets examined by HET and analyzed the optical data.
’’We immediately realized that a high-redshift blazar and gamma-ray source would allow us to test our understanding of relativistic radio jets and their interaction with the cosmic microwave background leftover from the Big Bang,’’ Greenhill says.
’’It’s a searchlight that’s set so far away that it illuminates matter and radiation all the way between us, between time one billion years after the Big Bang and now,’’ Romani says. ’’If you can detect it with a gamma-ray telescope, you have a handle on the birth of stars and galaxies between then and now that you never had before.’’
Scientists are currently stymied about how a black hole could have gotten so big so fast. How do you take something big enough to hold 1,000 solar systems and as heavy as all of the stars in our Milky Way galaxy put together, and quickly crunch-collapse it?
Scientists think the universe formed 13.7 billion years ago with the Big Bang. The distance of the blazar indicates it formed a billion years after that.
’’What’s interesting about a billion years after the Big Bang is that this marks the end of the ’Dark Age,’’’ Romani says. ’’The universe first formed with an enormous flash of light and heat - that’s the Big Bang - and then cooled off. And everything’s dark for about a billion years. And toward the end of that period, the first stars and black holes and galaxies start collapsing and forming and turning on. We talk about that as the end of the Dark Age. So it’s very interesting, and this is one of the big pushes in cosmology, to find objects back in the tail end of the Dark Age, when things are first lighting up, and then to use those to figure out how everything we have in the universe formed.’’
In the next year, the scientists hope to use the VLBA to take a better picture of the jet detected with radio waves and then observe its X-ray spectrum. This will help illuminate the matter between the supermassive black hole and Earth, clarify the black hole’s size and characterize the jet’s material as it moves away from the black hole at nearly the speed of light.
’’Studying these things gives us a window into the sort of physical processes that we can’t yet control here on Earth,’’ Romani says. ’’They’re the extremes of physics.’’
Those extremes fascinate Romani. ’’Pulsars are, I think, the most extreme objects in our universe,’’ he says. These cores of dead stars have collapsed, but not far enough to form an event horizon, so they are just short of turning into black holes. They are the densest things in the measurable universe. They have the strongest magnetic fields. Their surfaces have extremely high temperatures. They are cosmic accelerators that speed particles to the highest energies known.
So far, scientists have found only a handful of gamma-ray pulsars, and Romani is particularly excited about GLAST as a means of hunting down more in the Milky Way.
’’I’m particularly interested in ways in which you could find extreme physics out there in the cosmos and get a handle on physics of the 22nd or 23rd century by seeing what’s going on in the sky.’’
Dawn Levy | EurekAlert!