每日科技报告 第109期 Cosmic Curiosity Reveals Ghostly Glow of Dead Quasar

张立涛

Cosmic Curiosity Reveals Ghostly Glow of Dead Quasar

Whilesorting through hundreds of galaxy images as part of the Galaxy Zoo citizenscience project two years ago, Dutch schoolteacher and volunteer astronomerHanny van Arkel stumbled upon a strange-looking object that baffledprofessional astronomers. Two years later, a team led by Yale Universityresearchers has discovered that the unique object represents a snapshot in timethat reveals surprising clues about the life cycle of black holes.

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The greenVoorwerp in the foreground remains illuminated by light emitted up to 70,000years ago by a quasar in the center of the background galaxy, which has sincedied out.

In a new study, the team hasconfirmed that the unusual object, known as Hanny's Voorwerp (Hanny's"object" in Dutch), is a large cloud of glowing gas illuminated bythe light from a quasar -- an extremely energetic galaxy with a supermassive blackhole at its center. The twist, described online in the Astrophysical JournalLetters, is that the quasar lighting up the gas has since burned out almostentirely, even though the light it emitted in the past continues to travelthrough space, illuminating the gas cloud and producing a sort of "lightecho" of the dead quasar.

"This system really is likethe Rosetta Stone of quasars," said Yale astronomer Kevin Schawinski, aco-founder of Galaxy Zoo and lead author of the study. "The amazing thingis that if it wasn't for the Voorwerp being illuminated nearby, the galaxynever would have piqued anyone's interest."

The team calculated that thelight from the dead quasar, which is the nearest known galaxy to have hosted aquasar, took up to 70,000 years to travel through space and illuminate theVoorwerp -- meaning the quasar must have shut down sometime within the past70,000 years.

Until now, it was assumed thatsupermassive black holes took millions of years to die down after reachingtheir peak energy output. However, the Voorwerp suggests that the supermassiveblack holes that fuel quasars shut down much more quickly than previouslythought. "This has huge implications for our understanding of how galaxiesand black holes co-evolve," Schawinski said.

"The time scale on whichquasars shut down their prodigious energy output is almost entirelyunknown," said Meg Urry, director of the Yale Centerfor Astronomy & Astrophysics and a co-author of the **. "That's whythe Voorwerp is such an intriguing -- and potentially critical -- case studyfor understanding the end of black hole growth in quasars."

Although the galaxy no longershines brightly in X-ray light as a quasar, it is still radiating at radiowavelengths. Whether this radio jet played a role in shutting down the centralblack hole is just one of several possibilities Schawinski and the team willinvestigate next.

"We've solved the mystery ofthe Voorwerp," he said. "But this discovery has raised a whole bunchof new questions."

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