October 24, 2012
Distant Quasar Shows No Sign Of Feeding On Galaxy Of Stars
Lee Rannals for redOrbit.com — Your Universe Online
While looking at one of the most distant quasars in the universe, astronomers were surprised to not see an underlying host galaxy of stars feeding it.
NASA said the best explanation is that the galaxy is shrouded in so much dust that the stars are completely hidden everywhere. As stars aged and burned out in the early universe, they filled interstellar space with dust as they lost their atmosphere.
The quasar dates back to an early time in the universe's history, but was known to contain large amounts of dust from previous sub-millimeter observations. What researchers were surprised to see is how completely the dust is shrouding starlight within the galaxy.
Quasars are the brilliant cores of galaxies where in falling material fuels a super-massive black hole. This black hole is so engorged that some of the energy escapes as powerful blasts of radiation from the surrounding disk of accreting material.
This light appears as a jet-like feature, and if it beams in the Earth's direction, the "accretion disk" and jet surrounding the super-massive black hole can appear as a quasar that can outshine its surrounding galaxy a hundred or a thousand times.
The astronomers believe that the black hole is devouring the equivalent mass of a few suns per year. NASA said it may have been munching on a more voracious rate earlier at a rate of three billion solar masses in just a few hundred million years.
"If you want to hide the stars with dust, you need to make lots of short-lived massive stars earlier on that lose their mass at the end of their lifetime. You need to do this very quickly, so supernovae and other stellar mass-loss channels can fill the environment with dust very quickly," Rogier Windhorst of Arizona State University (ASU), Tempe, Arizona, said in a press release.
Matt Mechtley, also of ASU, said that "you also have to be forming them throughout the galaxy to spread the dust throughout the galaxy."
Astronomers first identified the quasar in the Sloan Digital Sky Survey (SDSS). NASA said only a handful of these ultra-luminous quasars were found by the SDSS in about one quarter of the whole sky.
The team performed follow-up observations later on, but did not show how and where dust was distributed, or where star-clusters might be visible through the dust. Most nearby galaxies still have some regions where stars or star-clusters poke through the dust.
They used the Hubble Space Telescope to subtract light from the quasar image and look for the glow of surrounding stars. Once the quasar was removed, no significant underlying starlight was detected.
"It is remarkable that Hubble didn't find any of the underlying galaxy," Windhorst said in the press release. "The underlying galaxy is everywhere much fainter than expected, and therefore must be in a very dusty environment throughout. It's one of the most rip-roaring forest fires in the universe. It´s creating so much smoke that you're not seeing any starlight, anywhere. The forest fire is complete, not a tree is spared."
Mechtley said that because they didn't see the stars, they can rule out that the galaxy that hosts this quasar is a normal galaxy.
"It´s among the dustiest galaxies in the universe, and the dust is so widely distributed that not even a single clump of stars is peeking through," Mechtley said in the release. "We're very close to a plausible detection, in the sense that if we had gone a factor of two deeper we might have detected some light from its young stars, even in such a dusty galaxy."
Windhorst said that NASA's James Webb Telescope will be able to help the astronomers even more.
"The Webb telescope is designed to make a definitive detection of this," he said. "We will get solid detections of the stars with Webb's better sensitivity to longer wavelengths of light, which will better probe the dusty regions in these young galaxies."
The research is published in the Astrophysical Journal Letters.