XMM-Newton views the remotest quasar
16 December 2000Quasars are the most luminous known objects in the Universe. They can emit 1000 times the energy of our entire Galaxy, and this prodigious luminosity originates from objects only the size of our solar system. XMM-Newton has detected the X-rays of the most distant known quasar, providing a view of the Universe when it was less than 1 billion years old.
Cosmic distances are measured in redshift. Redshift (symbol z) is the amount by which the wavelength of light from a receding object is lengthened, or moved towards the red, by either the Doppler shift or the expansion of the Universe. For example, redshifts over 4 indicate that light has been shifted more than 400%, so that ultraviolet lines appear in the red part of the spectrum. Taking a short cut: the higher the shift, the more distant and younger the object.
There is evidence that quasars (short for "quasi stellar" objects because of their star-like appearance) are fuelled by super-massive black holes feeding on their host galaxies. The first quasar discovered to be a highly luminous object was 3C 273. In 1963 its redshift was determined to be 0.16, placing it relatively nearby by today's standards. More than ten thousand quasars have subsequently been found.
A team of astronomers led by Niel Brandt of the Pennsylvania State University, USA, has started a project to determine the X-ray properties of the highest redshift quasars. To this end they need to use the new generation of X-ray observatories like XMM-Newton
"We want to push into that very early epoch, shortly after the Big Bang, to understand whether quasars then were different from those in the local Universe" says Neil Brandt. "By going back so far, you learn something about the hot early Universe. And yes, it was a challenge to go for the furthest known quasar."
Mobilising its great X-ray collecting power, XMM-Newton therefore targeted SDSS 1044-0125, the remotest of all known quasars. Its 5.80 redshift corresponds to an object present "only" a billion years after the Big Bang (current estimates put the age of the Universe at between 12 and 15 billion years.)
Brandt's team, which included Shai Kaspi and Donald Schneider of Penn State, Xiaohui Fan of the Institute for Advanced Study (who as a graduate student led the discovery of the quasar in the SDSS data), and Michael Strauss and James Gunn of Princeton University, worked closely with XMM-Newton scientists Matteo Guainazzi and Jean Clavel (of the mission's VILSPA Science Operations Centre at Villafranca, Spain) to interpret the X-ray data.
During an 8-hour long observation at the end of May, the observatory's EPIC-pn camera registered this quasar's X-ray emission. Surprisingly, only about 30 X-ray photons were collected when about ten times as much radiation had been expected. Such a low number of photons does not allow for detailed spectral analysis, but the flux, and hence the luminosity, could be calculated.
Given that the intergalactic medium out to a redshift of 5.80 is insufficient to occult this X-ray radiation, two possible explanations can be advanced for this low X-ray emission. On the one hand, it could be absorbed locally by gas in the galaxy surrounding the quasar. Such absorption is well known in objects such as "broad absorption line" quasars.
SDSS 1044-0125 might, on the other hand, be a 'precursor quasar'. "Its enormous black hole - 3 billion times the mass of our Sun - would be undergoing an exceptionally strong accretion process" explains Brandt. "The accretion would be so rapid that even the X-rays being generated by the immense temperatures are dragged back into the hole. This so-called 'Super-Eddington' accretion might explain the formation of such a massive black hole less than a billion years after the Big Bang".
For the moment, this X-ray study concerns only one object and other extremely high redshift quasars need to be observed to draw general conclusions. In some cases the X-ray emission may be stronger and will bring insights into the hot early Universe. But if flux levels remain low, real answers may have to wait until the arrival of even more powerful X-ray observatories such as ESA's proposed XMM-Newton successor XEUS, which is currently under study.
Main author Niel Brandt of the Pennsylvania State University PA, USA, as well as all the collaborators mentioned above.