Image: NASA/JHU/AUI/R.Giacconi et al.
For two decades, scientists suspected that so-called type II quasars existed, and now they have finally spotted one. Quasars are among the most distant extragalactic objects known. They appear starlike but emit more energy than 100 giant galaxies and are thus believed to be powered by massive black holes.
Like conventional quasars, the type II variety are strong sources of x-rays and other radiation. They are, however, thought to be covered by clouds of gas and dust, making them hard to see in visible light. "The thinking is that these may be quasars in the early stages of their evolution," says Colin Norman of John Hopkins University, a member of the research team that made the discovery. Scientists believe that the clouds of gas and dust are eventually dispersed by the strong radiation emitted by the maturing quasar.
To make their find, Norman and his colleagues from the Space Telescope Science Institute and the European Southern Observatory compared data from optical observations made using the Very Large Telescope in Chile with data from NASA's orbiting Chandra X-ray Observatory. They looked at the "Chandra Deep Field South" (see photo), a small area of the sky about the size of the moon. Because there is very little gas and dust within the Milky Way in that part of the sky, the scientists were afforded a particularly clear view. To detect even the faintest possible sources, Chandra's long hard look in that one direction lasted for 11.5 days.
The type II quasar they spied is approximately 12,000 million light-years away from Earth."We found just what the theory was predicting," says Piero Rosati of the European Southern Observatory. "An object that has very narrow emission lines in the optical spectrum readings from the VLT but is giving off very hot X-rays in the readings from Chandra."