Astronomers spot disappearing quasar

Observations have been made of a bright quasar over a 12-year period, giving astronomers insight into a unique and dramatic process at work.

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An animation showing an artist’s conception of the changing-look quasar from 2003 to 2015. Gas falls into a central black hole, which uses up the surrounding gas. The quasar then shuts off, leaving what appears to be a ‘normal’ galaxy.
Credit: Dana Berry / SkyWorks Digital, Inc.; SDSS collaboration

Astronomers have managed to spot a bright quaser surrounding a supermassive black hole being dramatically shut off after all its gas was consumed, in unprecedented observations.

The central black hole appears to have swallowed all the gas in its vicinity, cutting off the quaser’s power supply and enabling scientists to observe a speedy and dramatic drop in brightness.

Quasars are thought to be extremely bright discs of matter that surround supermassive black holes at the centre of galaxies. The black hole in question is about 50 million times the mass of the Sun and sucks in surrounding gas, causing the gas to heat up and glow with an immense brightness that can be seen from Earth.

Astronomers from the Sloan Digital Sky Survey (SDSS) have been observing quasar SDSS J1011+5442 and have noted that, over a period of 10 years, its spectroscopic signature has disappeared, obscuring the black hole from view.

SDSS first measured a spectrum of the quasar in January 2003, which allowed them to learn more about the properties of the gas being swallowed by the black hole. Another measurement was made early in 2015, enabling the team to compare how the rate of gas consumption changed over those 12 years.

“This is the first time we’ve seen a quasar shut off this dramatically, this quickly,” says study author Jessie Runnoe of Pennsylvania State University.

“The difference was stunning and unprecedented,” says John Ruan of the University of Washington. “The hydrogen-alpha emission dropped by a factor of 50 in less than twelve years, and the quasar now looks like a normal galaxy.”

The change was so great that the quasar has now become known as a ‘changing-look quasar’.

Three different theories emerged as to the cause of the drop in hydrogen-alpha. One suggested that a thick layer of dust is obscuring the region, but it is extremely unlikely that a dust cloud could have moved quickly enough to cause the observed drop in brightness. Another suggested that the quasar spotted in 2003 was a flare caused by a black hole swallowing a star, but this could not explain how the object had been glowing for so long before it shut off.

This led the team to infer that the object is a quasar that has simply used up all its glowing hot gas. The discovery is the first major achievement of the Time-Domain Spectroscopic Survey (TDSS), which is part of SDSS.

“We are used to thinking of the sky as unchanging,” says Scott Anderson, 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.”


Front image: Artists’s conception of a quasar, glowing brightly as its central black hole feeds on surrounding gas.
Credit: NASA/ESA
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