Quasar part II

 作者:乔淡     |      日期:2019-03-07 05:10:00
By Hazel Muir An intriguing astronomical object has turned up near the edge of the visible Universe. Scientists think it is a new-born quasar – a cosmic powerhouse fuelled by a hungry black hole – swaddled in a thick cloud of dust. “People have been looking for these things for a long time,” says Colin Norman of Johns Hopkins University in Baltimore, leader of the team that made the discovery. “Because they probably represent most of the quasars at great distances, it’s exciting to have found one.” Quasars are extremely bright sources of radiation in the Universe, thought to be powered by giant black holes that feed on the stars, gas and dust that surround them. Until now, all the quasars found have been shining brightly in visible light as well as X-rays, says Norman. But since the 1980s, astronomers have predicted that there’s a second kind of quasar, dubbed type II. This would shine brightly in X-rays but be faint in visible light. Type II quasars would be very young and still hidden in cocoons of dust because they have yet to muster the power to blast their birth shrouds away. The dust would block visible light and only energetic X-rays would penetrate through. On Tuesday, Norman told a press conference in Washington DC that NASA’s orbiting Chandra X-ray Observatory has spotted one of these type II quasars for the first time. “Directly imaging the nucleus, we were only seeing it in X-rays,” says Norman. The quasar lies so far away that its light has taken around nine billion years to reach the Earth. So we see the quasar as it was when the Universe was only around a tenth of its current age. However, the transition from type II to type I quasars is continuous and not all astronomers think this is the first time the latter has been seen. “A fair claim could be that this is the first type II quasar found in X-ray surveys,” says Michael Rowan-Robinson, an astrophysicist at Imperial College, London. But he says that in other surveys he and his colleagues have already found several quasars that fit the bill. “Do we have type II quasars already? Well, yes, I think we do.” Nonetheless, everyone agrees that many more type II quasars are likely to turn up over the next few years, revealing vital clues about how galaxies form. “We’d like to find more of them and find out their distribution,” says Norman. “That would tell us not only about the formation of galaxies, but also about the formation of these monster black holes at their centres.” More at: