BY DR EMILY BALDWIN
Posted: 07 January, 2009
In an on-going X-ray survey, NASA’s Swift spacecraft is revealing that nearby active galaxies are more alive than those located halfway across the Universe.
Understanding the differences between near and far active galaxies will help clarify the relationship between a galaxy and its central black hole. "There's a lot we don't know about the workings of supermassive black holes," says Richard Mushotzky of NASA's Goddard Space Flight Center. An active galaxy hosts an active galactic nucleus, a compact region at the centre of a galaxy that emits high energy radiation, believed to be a result of the accretion of mass by a supermassive black hole at the centre of the galaxy, more than a million times the mass of the Sun. "Some of these feeding black holes are the most luminous objects in the Universe. Yet we don't know why the massive black hole in our own Galaxy and similar objects are so dim."
Left: a typical red, 'dead' galaxy and right: a blue galaxy exhibiting signs of star-forming activity. Both galaxies were imaged using the two metre Kitt Peak telescope. Image: NASA/Swift/NOAO/Michael Koss (Univ. of Maryland) and Richard Mushotzky.
As well as hunting down gamma-ray bursts, Swift’s Burst Alert Telescope (BAT) scans the sky in X-rays, charting about half of the entire sky every day. "Now we have cumulative exposures for most of the sky that exceed 10 weeks," says Mushotzky.
Nearly a decade ago, surveys with NASA's Chandra X-Ray Observatory and ESA's XMM-Newton showed that active galaxies some seven billion light years away were mostly massive red-coloured, dead, galaxies in normal environments. Looking much closer to home, within about 600 million light years, the BAT survey has so far revealed that active galaxies fall mid-way between blue star-forming galaxies and older red galaxies. Many of the BAT galaxies have spiral or irregular forms and have a normal mass, and more than 30 percent are colliding with other galaxies. "This is roughly in line with theories that mergers shake up a galaxy and 'feed the beast' by allowing fresh gas to fall toward the black hole," says Mushotzky.
Swift's Hard X-ray Survey offers the first unbiased census of active galactic nuclei in decades. Dense clouds of dust and gas, illustrated here, can obscure less energetic radiation from an active galaxy's central black hole. High-energy X-rays, however, easily pass through. Image: ESA/NASA/AVO/Paolo Padovani.
Because an active galaxy's core is often obscured by thick clouds of dust and gas that block ultraviolet, optical and low-energy ‘soft’ X-ray light, astronomers have never been too sure if they were observing the whole active galactic nuclei. Furthermore, both dust near the central black hole and in a galaxy’s star-forming region are visible in the infrared, potentially obscuring the view to the central engine of the galaxy's core. But Swift's hard X-ray survey offers an unbiased census of active galactic nuclei, since ‘hard’ X-rays – those with energies between 14,000 and 195,000 electron volts – can penetrate this galactic ‘gunk’ and allow a clear view.
The results of the survey so far have lead astronomers to believe that all big galaxies have a massive central black hole, but less than 10 percent of these are active today. Active galaxies are thought to be responsible for about 20 percent of all energy radiated over the life of the Universe, and are thought to have had a strong influence on the way structure evolved in the cosmos.
This progress report of Swift's findings to date was presented at the American Astronomical Society meeting, held in California this week.