Astronomers can’t find any sign of the supermassive black hole at the center of the quasar named SDSS J1011+5442, and they couldn’t be happier. “This is the first time we’ve seen a quasar shut off this dramatically, this quickly,” said Penn State Postdoctoral Scholar Jessie Runnoe, who led the international team of astronomers from the Sloan Digital Sky Survey (SDSS) that is reporting this discovery today.
The black hole is still there, but the super-bright quasar it powered by swallowing the gas in its vicinity has disappeared over the past ten years, as it appears to have swallowed all the gas nearby. Runnoe’s team reports today that it was unable to detect the light-spectrum signature of the quasar, which now looks like an otherwise normal galaxy — one without a bright quasar in its core. Other Penn State scientists involved in this study are Professors of Astronomy and Astrophysics Michael Eracleous and Donald Schneider.
“The change in the emission from the gas over a dozen years was stunning,” remarked Eracleous. “The luminosity of the gas declined by over a factor of 50; if we had taken our first observations of the quasar today, we would view it as a typical galaxy with no suggestion that it hosted a supermassive black hole.” The change was so great that throughout the SDSS collaboration and within the broader astronomy community, the quasar became known as a changing-look quasar.
The key to the discovery was the SDSS’s long history of studying the sky. The survey first obtained a spectrum of the energy emitted by SDSS J1011+5442 in January 2003. This type of data reveals the properties of the gas being swallowed by the black hole. Another spectrum of the quasar was acquired in early 2015, allowing a comparison of the rate of infalling gas between the two measurements twelve years apart.
Over those twelve years, other telescopes have also been observing this changing-look quasar. By examining these additional observations, Runnoe’s team was able to narrow the period when the change occurred, finding that the quasar’s luminosity decreased by a factor of 50 in just a few years. The team’s conclusion is that the quasar has consumed all the glowing-hot gas in its immediate vicinity, leading to a rapid drop in brightness. “Essentially, it has run out of food, at least for the moment,” says Runnoe. “We were fortunate to catch it both before and after.”
So how do astronomers explain this unusual disappearing quasar? Quasars result from giant black holes at the centers of distant galaxies. These black holes are so enormous — in the case of the changing-look quasar, 50 million times the mass of the Sun — that infalling gas heats up to millions of degrees — thousands of times higher than the surface of the Sun. The hot gas glows so brightly that it is visible to us, even though it is billions of light years away.
Astronomers have three proposed explanations for the dramatic variability. One possibility is that a thick layer of dust is passing through the host galaxy, obscuring our view of the central black hole, but there is no known way that a dust cloud could have moved sufficiently rapidly to cause a 50-fold drop in brightness in just a few years. Another possibility is that the observed quasar in 2003 was but a brief temporary flare caused by the black hole ripping apart a nearby star. While this possibility has been invoked in prior cases of quasar variability, it cannot to explain the fact that the changing-look quasar had been shining for many years before shutting down.
The changing look quasar is the first major discovery of the Time-Domain Spectroscopic Survey (TDSS), one of the components of the Sloan Digital Sky Survey’s fourth phase. TDSS will continue for the next several years, promising many more surprising discoveries in the future.
“We are used to thinking of the sky as unchanging,” says Scott Anderson of the University of Washington and the Principal Investigator of TDSS. “The SDSS gives us a great opportunity to see that change as it happens. In fact, we found this quasar because we went back to study thousands of quasars seen before. This discovery was only possible because the SDSS is so deep and has continued so long.”