X-ray quasars, and a distance record

Apr 30, 2012
A Chandra X-ray Observatory image of the nearby galaxy Centaurus A, whose nucleus contains an active supermassive black hole. A new Chandra study of quasars, galaxies with extremely luminous nuclei, has discovered the most distant known X-ray quasar. Credit: NASA/CXC/CfA/R.Kraft et al.

Quasars are thought to be galaxies whose bright nuclei contain massive black holes around which disks are actively accreting matter. The accretion process releases vast amounts of energy, often including a wind, and as a result quasars are among the most powerful energy sources known. Because they are so bright, quasars can be seen even when they are very far away, and this combination of being both highly energetic and located at cosmological distances makes them appealing to astronomers trying to figure out the nature of galactic center black holes (our own Milky Way has one) and the conditions in the early universe that prompt these monsters to form.

No one knows exactly how quasars form, evolve, or generate their stupendous energies. One clue is that star formation in quasars is generally modest, in contrast to star formation in other kinds of luminous galaxies in which it can be responsible for much of the emission. One problem in sorting out the answer has been the lack of a large sample of suitably comparable objects.

CfA astronomers Markos Trichas, Paul J. Green, Tom Aldcroft, Dong-Woo Kim, Guillermo Torres, and Belinda J. Wilkes, along with a team of colleagues, used the Chandra X-ray Observatory to produce just such a suitable sample. The Chandra Multiwavelength Project (ChaMP) found over 19,000 X-ray sources across a region of the sky as large as 130 full moons. Follow up studies then yielded a set of 1242 X-ray galaxies with high signal-to-noise, wide-ranging optical and - enough data to determine the levels of star formation and cosmic distances for most objects.

The scientists conclude that the powerful quasar wind is probably what is responsible for quenching activity, a conclusion that rules out the alternate suggestion that heating by the accreting material is responsible. The team also reached several other significant conclusions, including the serendipitous discovery that one ChaMP quasar was so far away its light has been traveling for about twelve and one-half billion years -- a new distance record for X-ray quasars.

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