Mystery Surrounding Brightest Cosmic Objects Said Solved
Examples of merging galaxies containing a heavily obscured growing supermassive black hole observed by the Hubble Space Telescope. The top panels show nearby galaxies at ~500 million light-years from us, while the more distant galaxies in the bottom panels are ~6.5 billion light years away, when the Universe was half its current age.
Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)/ E. Treister and K. Teramura (IfA, University of Hawaii)

Quasars are regions around giant black holes where matter is gobbled up and light is flung back into space. The origin of these blazing objects has been unclear, but a new study found they likely form when two massive galaxies collide.

Not just any large black hole becomes a quasar ? the distinction is reserved for those supermassive black holes that are growing. As they feed on swirling matter around them, friction and heat release copious amounts of radiation that make quasars some of the brightest objects in the universe.

So in order to have a quasar, a galaxy must contain a sufficient store of mass in its center ready to be eaten by the supermassive black hole there. And the best way to get this supply of food concentrated in the galactic center is a merger between two large, gas-rich galaxies, the study confirmed.

This concept was first suggested by University of Hawaii astronomer David Sanders in 1988. But it wasn't until now that observational evidence of distant galaxies from the early universe confirms this.

Sanders and a team of astronomers led by Ezequiel Treister, also of the University of Hawaii, combined data from the Hubble, Chandra and Spitzer space telescopes of far-away galaxies and searched for signs of quasars that were obscured by gas and dust around them. The telescopes, which scan the universe in optical, X-ray, and infrared light, respectively, allowed the astronomers to cut through some of the haze.

The astronomers found a sizeable sample of these distant hidden quasars, and calculated how many obscured quasars there were at various distances, which represent various epochs in the universe's history, because the farther they looked, the longer that light took to reach Earth.

The researchers think that newborn quasars are often hidden from view, but that over time the surrounding material falls into the black hole, leaving the quasar visible. That would suggest that obscured quasars are more often farther away ? earlier in time ? but more recently many of them have transitioned to their visible phase. And their observations confirmed this.

"We made a simple model in which every galaxy merger generates a quasar that is first obscured and then un-obscured," Treister told "The agreement is just remarkable. That does indicate that pretty much every galaxy merger generates a quasar."

Supermassive black holes are thought to reside in the center of most galaxies, including our own Milky Way. But many of these have not undergone mergers with other galaxies, and so lack enough spare gas waiting to be gobbled. Other galaxies may once have hosted quasars, but the quasar's food ran out and it died down into a relatively sedate central black hole.

The Milky Way doesn't seem to have undergone any major collisions with other large galaxies in the past. However, our neighbor galaxy Andromeda is supposed to slam into the Milky Way in the distant future.

"When the Milky Way collides with Andromeda, if at that time there's enough gas available, that gas will probably end up in the center around the black hole, making a quasar," Treister said [more colliding galaxies images].

The scientists report their findings in the March 26 issue of the journal Science.

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