UW graduate student Chelsea MacLeod is using
astronomical surveys and statistics to get a handle
on why quasars vary in brightness. Photo: Engage
Science Speaker Series.
Chelsea MacLeod is using astronomical surveys and statistics to study the variability of quasars. The University of Washington graduate student says surveys have transformed the field.
“Surveys are giving us a broader view of the universe,” MacLeod said. “We are starting to get a clearer picture of everything that is out there.”
MacLeod gave a talk titled, “Survey Science: The Universe at Our Fingertips” last week as part of the Engage Science Speaker Series at the university. She noted that the technology has advanced greatly since the first effort, the Palomar Observatory Sky Survey (POSS), started in 1950. POSS used photographic plates, a time-consuming process.
Today’s CCD scopes can see fainter objects more clearly and work more quickly. The Sloan Digital Sky Survey, which MacLeod uses, has done 60 observations of a 300 square deegree patch of sky called "stripe 82" in 10 years, whereas POSS only made a couple of passes of the entire sky since 1950. MacLeod is excited about the next generation, the Large Synoptic Survey Telescope, which is still on the drawing boards. It will see even fainter stuff and be able to do 200 surveys in a decade.
“It will have capabilities to image the entire sky area in three nights,” said MacLeod of LSST. “It’s basically a motion picture of our universe.”
MacLeod says surveys have been a boon to asteroid hunters and have also helped astronomers discover many small, faint stars in our own neighborhood.
“Once you go faint enough, you can see that we’re just surrounded by these tiny stars that otherwise we would not have known about,” MacLeod marveled. “The majority of stars in the galaxy and probably the universe are these tiny, low-mass stars. We had no idea before these surveys that that was the case.”
MacLeod, however, is more interested in galaxies far, far away: quasars.
“They’re not very interesting in the optical,” she said. “In fact, they’re easily mistaken for stars because they’re basically like a point source of light. It’s actually why they were named quasi-stellar objects when they were discovered, because people first thought that they were just stars with really weird spectra.”
“They actually weren’t stars, they were galaxies at very large distances away that happened to be emitting crazy amounts of energy, which is what makes them so bright and look like stars,” MacLeod said. She explained that supermassive black holes at the center of quasars create the energy when they suck in mass quantities of matter.
Interestingly, the brightness is not uniform. “It turns out that most quasars are variable,” MacLeod said. “There’s something going on in all of these quasars that’s causing them to vary in brightness, and it’s unclear why this occurs.”
MacLeod is trying to find out. She’s in the process of examining more than 30,000 quasars to try to chart the variability. But she doesn’t have to watch each of them all the time. Just two measurements of each will give a value for change over time. Pile all the observations together, average them out, and you get what McLeod calls “ensemble variability.”
“Individual quasars vary by different amounts and on different time scales,” she said, “so looking at ensemble variability doesn’t really give you a clear picture as to how any individual quasar works. It only gives you an average of all of them.” The individuals, she noted, can vary depending on the mass of the black hole, the quasar’s intrinsic brightness, the wavelength of light it emits, or a host of other factors. So one must be careful when drawing conclusions from the data. But she says that the method is useful.
“Surveys have provided a good opportunity to compare theory with observation and constrain our theories for cosmology,” MacLeod concluded.
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