COURTESY JOSHUA BARNES / UNIVERSITY OF HAWAII A gas-rich galaxy collides with a giant galaxy, producing a quasar, in this computer simulation by Joshua Barnes.

UH duo ties quasar light, galaxy gas

Two University of Hawaii astronomers have discovered what makes some quasars shine.

"Only if there is enough gas around a massive black hole can we detect it, and it becomes very luminous," said Hai Fu, graduate student in the Institute for Astronomy. "We call it a quasar."

Using data from the Hubble Space Telescope and telescopes on Mauna Kea, Fu and astronomy professor Alan Stockton found that black holes are feeding on material from a colliding gas-rich galaxy.

"It was kind of unexpected," Fu said.

The astronomers' findings were published in the Aug. 1 issue of the Astrophysical Journal Letters.

Quasars, lodged in the centers of giant galaxies, contain massive black holes surrounded by gas. As gas falls into the black hole, it spins so fast that its temperature rises until it is hot enough to radiate up to a trillion times the power of the sun, the astronomers reported.

Fu said he and Stockton set out to learn why the outer region of quasars have little metal heavier than hydrogen or helium when stars and other material in the surrounding giant galaxy contain heavy elements such as carbon and oxygen.

The only explanation they could come up with is that the gas came from outside the giant galaxy, from a collision with a smaller, gas-rich galaxy, Fu said.

He said that when the universe started, it had only hydrogen and helium, so the gas falling into black holes is "just original gas from the (early) universe." Heavier elements such as carbon and oxygen were produced in star formation, he said.

COURTESY A. SIMONNET / SONOMA STATE UNIVERSITY This is an artist's conception of the heart of a quasar, a massive black hole that sucks in a vortex of gas. Hawaii astronomers found that quasars shine when a giant galaxy with a large black hole collides with a gas-rich galaxy that feeds the black hole.

He said they determined that the gas was low in elements heavier than hydrogen and helium, and Fu checked measurements other astronomers made of gas close to the black hole.

"For most quasars this gas is highly enriched in the heavy elements -- more so even than our sun," he said. But for quasars showing extended ionized gas, he said, the gas close to the black hole had fewer heavy elements than the sun.

In a sample of similar quasars, he said, all of those with extended gas were lower in heavy elements than the sun, and those without extended gas were higher in heavy metals than the sun.

"In the case of the objects with the extended gas, we know that neither the gas that we see close to the black hole nor at great distances can come from the main galaxy in which the black hole is embedded," he said.

Studies show any gas native to these massive galaxies is rich with heavy elements, he said. So gas seen near the black hole and at great distances must come from the outside, from a common origin, Stockton said.

This most likely would happen through a collision of a small gas-loaded galaxy with a larger galaxy hosting a black hole, driving gas to the center of the larger galaxy where the black hole can capture it, he said.