Wednesday, October 31, 2007

Massive Black Hole Smashes Record


Using two NASA satellites, astronomers have discovered the heftiest known black hole to orbit a star. The new black hole, with a mass 24 to 33 times that of our Sun, is more massive than scientists expected for a black hole that formed from a dying star.

The newly discovered object belongs to the category of "stellar-mass" black holes. Formed in the death throes of massive stars, they are smaller than the monster black holes found in galactic cores. The previous record holder for largest stellar-mass black hole is a 16-solar-mass black hole in the galaxy M33, announced on October 17.

"We weren’t expecting to find a stellar-mass black hole this massive," says Andrea Prestwich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of the discovery paper in the November 1 Astrophysical Journal Letters. "It seems likely that black holes that form from dying stars can be much larger than we had realized."

The black hole is located in the nearby dwarf galaxy IC 10, 1.8 million light-years from Earth in the constellation Cassiopeia. Prestwich’s team could measure the black hole’s mass because it has an orbiting companion: a hot, highly evolved star. The star is ejecting gas in the form of a wind. Some of this material spirals toward the black hole, heats up, and gives off powerful X-rays before crossing the point of no return.

In November 2006, Prestwich and her colleagues observed the dwarf galaxy with NASA’s Chandra X-ray Observatory. The group discovered that the galaxy’s brightest X-ray source, IC 10 X-1, exhibits sharp changes in X-ray brightness. Such behavior suggests a star periodically passing in front of a companion black hole and blocking the X-rays, creating an eclipse. In late November, NASA’s Swift satellite confirmed the eclipses and revealed details about the star’s orbit. The star in IC 10 X-1 appears to orbit in a plane that lies nearly edge-on to Earth’s line of sight, The Swift observations, as well as observations from the Gemini Telescope in Hawaii, told Prestwich and her group how fast the two stars go around each other. Calculations showed that the companion black hole has a mass of at least 24 Suns.

There are still some uncertainties in the black hole’s mass estimate, but as Prestwich notes, "Future optical observations will provide a final check. Any refinements in the IC 10 X-1 measurement are likely to increase the black hole’s mass rather than reduce it."

The black hole’s large mass is surprising because massive stars generate powerful winds that blow off a large fraction of the star’s mass before it explodes. Calculations suggest massive stars in our galaxy leave behind black holes no heavier than about 15 to 20 Suns.

The IC 10 X-1 black hole has gained mass since its birth by gobbling up gas from its companion star, but the rate is so slow that the black hole would have gained no more than 1 or 2 solar masses. "This black hole was born fat; it didn’t grow fat," says astrophysicist Richard Mushotzky of NASA Goddard Space Flight Center in Greenbelt, Md., who is not a member of the discovery team.

The progenitor star probably started its life with 60 or more solar masses. Like its host galaxy, it was probably deficient in elements heavier than hydrogen and helium. In massive, luminous stars with a high fraction of heavy elements, the extra electrons of elements such as carbon and oxygen "feel" the outward pressure of light and are thus more susceptible to being swept away in stellar winds. But with its low fraction of heavy elements, the IC 10 X-1 progenitor shed comparatively little mass before it exploded, so it could leave behind a heavier black hole.

"Massive stars in our galaxy today are probably not producing very heavy stellar-mass black holes like this one," says coauthor Roy Kilgard of Wesleyan University in Middletown, Conn. "But there could be millions of heavy stellar-mass black holes lurking out there that were produced early in the Milky Way’s history, before it had a chance to build up heavy elements."

Hubble Spies Shells Of Sparkling Stars Around Quasar


New images taken with NASA's Hubble Space Telescope -- part of a research project led by UC Riverside's Gabriela Canalizo -- have revealed the wild side of an elliptical galaxy, nearly two billion light-years away, that previously had been considered mild-mannered.

The Hubble photos show shells of stars around a bright quasar, known as MC2 1635+119, which dominates the center of the galaxy. The presence of the shells is an indication of a titanic clash with another galaxy in the relatively recent past.

The collision, which is funneling gas into the galaxy's center, is feeding a supermassive black hole. The accretion onto the black hole is the quasar's energy-source.

"This observation supports the idea that some quasars are born from interactions between galaxies," said Canalizo, an assistant professor of astrophysics in the Department of Physics and Astronomy, and a member of the Institute of Geophysics and Planetary Physics. "It also provides more evidence that mergers are crucial for triggering quasars. Most quasars were active in the early universe, which was smaller, so galaxies collided more frequently.

"Astronomers have long speculated that quasars are fueled by interactions that bring an inflow of gas to the black holes in the centers of galaxies. Since this quasar is relatively nearby, it is a great laboratory for studying how more distant quasars are turned on."

Canalizo explained that the period of time when the central black hole of a galaxy is actively accreting material as a quasar is believed to be an essential phase in the evolution of most galaxies.

"For many decades now, there has been much debate regarding whether galaxy mergers or collisions are responsible for fueling their central black holes and turning them into quasars," she said.

Discovered nearly 50 years ago, quasars are among the brightest objects in the universe. They reside in the centers of galaxies and are powered by supermassive black holes.

Previous studies of the MC2 1635+119 galaxy with ground-based telescopes showed a normal-looking elliptical containing an older population of stars. It took the razor-sharp vision of Hubble's Advanced Camera for Surveys and the spectroscopic acuity of the W.M. Keck Observatory in Hawaii to uncover the faint, thin shells.

The new Hubble observations reveal at least five inner shells and additional debris traveling away from the galaxy's center. The shells, which sparkle with stars, resemble ripples forming in a pond when a stone is tossed in. They formed when a galaxy was shredded by tidal forces during the collision. Some of the galaxy's stars were swept up in the elliptical galaxy's gravitational field, creating the outward-moving shells. The farthest shell is about 40,000 light-years away from the center.

"This is the most spectacular shell galaxy seen at this distance," said team member Francois Schweizer of the Carnegie Observatories in Pasadena, California.

Computer simulations estimate that the encounter happened 1.7 billion years ago. The merger itself occurred over a few hundred million years and stoked a flurry of star birth. Spectroscopic data from Keck reveal that many of the stars in the galaxy are 1.4 billion years old, consistent with the age of the merger.

The shell stars are mixing with the stars in the galaxy as they travel outward. Eventually, the shells will dissipate and the stars will be scattered throughout the galaxy.

"This could be a transitory phase, common to most ellipticals, that lasts only 100 million to a billion years," Canalizo said. "So, seeing these shells tells us that the encounter occurred in the relatively recent past. Hubble caught the shells at the right time."

Canalizo and her team have yet to determine the type of merger responsible for the shells and the quasar activity. Their evidence, however, points to two possible collision scenarios.

"The shells' formation and the current quasar activity may have been triggered by an interaction between two large galaxies or between a large galaxy and a smaller galaxy," explained team member Nicola Bennert of UCR, who did all of the data processing and quantitative measurements, as well as a large fraction of the analysis. "We need high-resolution spectroscopic observations of the quasar host galaxy to determine the type of merger."

The quasar is part of an Advanced Camera for Surveys study of five galaxies, all roughly 2 billion light-years away, that are known to harbor quasars. According to Canalizo, the other four galaxies analyzed also display evidence of encounters. Her team also is using Hubble's Wide Field Planetary Camera 2 to sample 14 more galaxies with quasars.

"We want to know whether most quasars at current epochs begin their lives as mergers, or whether they simply occur in old ellipticals to which nothing very interesting has happened recently," Canalizo said.