Once again, quasars have astronomers stumped. These powerful fountains of energy, emitting astounding amounts of x-rays, visible light, and radio waves, are thought to be powered by monstrous black holes in the cores of galaxies at the edge of the observable universe. But little is known about their origin and evolution, and many mysteries remain. The latest twist: The newest targeted search for the host galaxies of remote quasars has come up all but empty, suggesting that astronomers may need to rethink their models of how quasars work.
In the past, ground-based telescopes couldn't hope to spot the faint light from a galaxy surrounding a distant, luminous quasar. But now a team led by Scott Croom of the Anglo-Australian Observatory in Australia has used the 8-meter Gemini North Telescope at Mauna Kea, Hawaii, to study nine quasars some 10 billion light-years away. Armed with sensitive infrared cameras and adaptive optics to compensate for atmospheric turbulence, Croom and colleagues expected to easily detect the hosts of these quasars. But in eight cases, they found nothing, while the ninth was small and faint, the team reported in the 1 May issue of the Astrophysical Journal.
Some maverick scientists have argued for decades that most quasars are not actually located in remote host galaxies. But there is no strong observational evidence to support this view, so the vast majority of astronomers are convinced that the standard scenario of giant black holes in galaxies billions of light-years away is correct. Despite not finding host galaxies, Croom and his team suggest that the standard scenario is right, but that the host galaxies are unexpectedly small and dim. And that, they say, means that quasar theories may have to be revised.
But it may yet be possible to reconcile Croom's finding with the current thinking on quasars, says Jill Bechtold of the University of Arizona, Tucson, who is using one of the 6-meter Magellan telescopes in Chile to carry out a similar search for quasar host galaxies. "Galaxies grow by merging with each other," she says, "so you expect them to be small in the very early universe" as Croom's team reports. It's conceivable that black holes at the cores of these inconspicuous galaxies turn into the extremely luminous black holes envisioned by the current theory, Bechtold says, but how that might happen is unclear.