Although quasars have been studied for approximately 50 years, few have ever been observed to change in luminosity.
But with the publication of a paper by researchers at Yale and other universities describing a “changing quasar,” astrophysicists will get a close look at rare dynamics important to the life cycle of a quasar. That, in turn, may provide insight into how galaxies age. The paper was published in the Astrophysics Journal.
“Changing look quasars are a newly identified group of objects,” said Michael Eracleous, the study’s co-author and a professor of physics at Penn State University. “We don’t know what phenomenon causes the transformation, and one of the purposes of this paper was to explore possible explanations.”
A black hole is an astrophysical body that is so dense that gravitational attraction near the body grows to infinity. Scientists believe that a large black hole exists at the center of every galaxy larger than and including the Milky Way. In many cases, clouds of matter form around the black hole, orbiting around and then slowly falling into the body. Friction within the cloud causes the cloud to heat up and glow extremely brightly. When that happens, the black hole is called a quasar. The team identified one such quasar, SDSS J015957.64+003310.5, which, over less than a three-year period, began to glow significantly less brightly.
“A quasar is not a thing — it’s a process. It’s a period in the life of a galaxy where [the black hole] that was sitting there minding its own business gets irritated when somebody starts chucking stuff at it,” said Gordon Richards, co-author of the study and a physics professor at Drexel University, referring to how black holes react when dust clouds enter the picture. “That doesn’t last forever, so seeing an object like this is a very rare thing.”
The team first detected the quasar in September 2000 as part of a survey in which researchers were looking for quasars throughout the universe. Currently, there are fewer than a dozen known changing look quasars, said Eracleous.
After that detection, the team followed up in 2010 and 2014 by measuring the quasar’s emission spectrum — the color bands of light that it was projecting. According to Stephanie LaMassa, a postdoctoral student at the Yale Center for Astronomy and Astrophysics, those data showed a difference in the black hole’s emission spectrum between 2000 and 2010. With further examination, she found that the quasar had become less luminous between 2000 and 2004 with the main decreases in luminosity occurring by 2003. The 2014 data showed that the black hole’s emission spectrum had not changed since 2010.
“We think most galaxies have a super massive black hole in their center, but most of these black holes aren’t feeding — they’re dormant,” LaMassa said, adding that the subset that are active are active because there is enough dust around to feed quasar growth. Scientists do not know how long quasars last, but suspect that their lifetimes are related to galaxy evolution in some way, she added.
According to Eracleous, there are two possible explanations for why the apparent change may have occurred. The first is that an object may have come between earth and the quasar, obstructing the researchers’ view. The second is related to the properties of the quasar itself — through some unknown mechanism, the dust is not being sucked in as quickly, and thus possesses less energy to emit light. The team ruled out the first case as a possibility and showed that there were intrinsic changes to the quasar.
“Something intrinsically changed in the accretion disk that caused the luminosity to drop abruptly,” Eracleous said. “That is the fundamental cause of everything we saw.”
There are between 100 and 200 billion galaxies in the universe.