22Jun2012

The galaxies pictured here have so much dust surrounding them that the brilliant light from their quasars cannot be seen in these images NASA's Hubble Space Telescope. Quasars are the brilliant beacons of light that are powered by black holes feasting on captured material, and in the process, heating some of the matter to millions of degrees. Image credit: NASA/ESA/Yale

When it comes to the early Universe, apparently black holes and their resultant quasars expanded in small steps rather than giant leaps. According to new observations taken by the Hubble , most quasars were formed gradually. These mega-watt light sources are fueled by black holes consuming material and run a temperature of up to millions of degrees. The very brightest of these are found in galaxies distorted by collisions – where the maelstrom of materials is eagerly seized by voracious black holes.  Astronomers are now taking a closer look at “normal” spiral galaxies and finding a host of faint quasars which appear to have been triggered by black holes ingesting smaller fuel sources – such as a satellite galaxy or a cloud of gas. These findings came through a census of 30 quasar host galaxies taken with the Hubble and Spitzer Space Telescopes. Out of the survey, 26 of the host galaxies show no signs of having been part of a galaxy interaction.

They had no unusual shapes and only one of the group had evidence of galaxy collision event. All of the survey candidates were also roughly the same age – about 8 to 12 billion years old – formed at the peak of black hole growth.

The study, led by Kevin Schawinski of Yale University, upholds previous observations which show the enlargement of a majority of massive black holes in the early Universe were fueled by small events over a period of time – not huge steps which occurred quickly.

“Quasars that are products of galaxy collisions are very bright,” Schawinski said. “The objects we looked at in this study are the more typical quasars. They’re a lot less luminous. The brilliant quasars born of galaxy mergers get all the attention because they are so bright and their host galaxies are so messed up. But the typical bread-and-butter quasars are actually where most of the black-hole growth is happening. They are the norm, and they don’t need the drama of a collision to shine.”

Schawinski’s science paper has been accepted for publication in a letter to the Monthly Notices of the Royal Astronomical Society. In his work, Schawinski examined galaxies observed by the Spitzer and Hubble telescopes in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey.

However, they weren’t just a random choice. In infrared images taken by Spitzer, 30 dusty galaxies about the same mass as the Milky Way glowed more brightly than the rest – a clue that the dust was covering up quasar activity at visible wavelengths. Thanks to Spitzer’s capabilities, dust poses no problem and infrared light imaging allowed Schawinski to study their structures in detail.

An Artist's interpretation of a Quasar

But, Spitzer wasn’t the only camera in action. Schawinski also employed near-infrared images taken with the Hubble Space Telescope‘s Wide Field Camera 3. These well-resolved images provided for careful analysis of the galaxy’s shapes – shapes which would have been distorted if they had been involved in a merger event. Of these, only one showed evidence of an interaction. So what’s going on with quasars? Just what is providing their power?

Whatever is responsible is apparently below the detection capability of Hubble. “I think it’s a combination of processes, such as random stirring of gas, supernovae blasts, swallowing of small bodies, and streams of gas and stars feeding material into the nucleus,” Schawinski said.

When it comes to a quasars, it doesn’t take much gas to ignite them. “There’s more than enough gas within a few light-years from the center of our Milky Way to turn it into a quasar,” Schawinski explained. “It just doesn’t happen. But it could happen if one of those small clouds of gas ran into the black hole.

Random motions and stirrings inside the galaxy would channel gas into the black hole. Ten billion years ago, those random motions were more common and there was more gas to go around. Small galaxies also were more abundant and were swallowed up by larger galaxies.”

The studies aren’t ending here, though. The galaxies in Schawinski’s study are prime targets for NASA’s upcoming James Webb Space Telescope, a large infrared observatory scheduled to launch later this decade. “To get to the heart of what kinds of events are powering the quasars in these galaxies, we need the Webb telescope. Hubble and Spitzer have been the trailblazers for finding them.”

Original Story Source: JPL/NASA News Release.

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