viernes, 1 de junio de 2007

Loner Black Holes Lurk in Cosmic Voids

This slice of the universe shows the distribution of voids (blue circles) and their galaxies (blue circles). Scientists found void galaxies harbor actively accreting black holes, like the one shown in the image (right). Credit: John Parejko, Danny Pan, Anca Constantin/Drexel University

Monster black holes can be loners, lurking in cosmic voids where stellar food is somewhat unavailable and neighbors seem nonexistent, a team of scientists announced.

The dark leviathans have adapted to their recluse lifestyles by growing at slower rates than their city-dwelling relatives.

The results shed light on the formation and evolution of both supermassive black holes-weighing millions to hundreds of millions times that of the Sun-and galaxies. In recent years astronomers have found evidence that black holes and their host galaxies can interact and affect one another's evolution.

Cosmic voids

The cosmic voids are regions containing few galaxies and span hundreds of millions of light-years across, filling up half the universe. Only 5 percent of all galaxies reside in these bubble-like regions, with 95 percent of galaxies packed together in clusters, akin to celestial cities.

Anca Constantin of Drexel University in Philadelphia and her colleagues studied more than 1,000 void galaxies within a 700-million light-year slice of the universe using the Sloan Digital Survey (SDSS-II), finding that supermassive black holes are just as common in void galaxies as they are in so-called walls, which are tightly packed groups of galaxies that form a sort of wall structure.

"Interestingly, we see actively accreting black holes in all phases of evolution in these sparse regions," said Constantin, who presented the research here this week at a meeting of the American Astronomical Society.

Black holes are thought to begin their lives as voracious feeders, sucking in, or accreting, nearby material and continuing to bulk up. "They grow and grow and at some point they either get lazy or they just run out of fuel," Constantin told

Finding black holes all along this growth continuum "means that the black hole growth process is quite similar in what could be compared to the most reclusive country sides and in the crowded urban regions of the universe," Constantin said.

Different lifestyles

However, Constantin and her team did find some subtle differences between black holes within the "rural" and "city" galaxies. They found more black holes at earlier stages in their evolutionary process within void galaxies, meaning the black holes were still in the active-feeding stages of their life cycle.

In crowded galactic regions, they identified more supermassive black holes at later stages in their evolutionary process, suggesting "cities" are conducive for faster accretion rates.

"The void galaxy black holes might take longer to reach the mature, low accretion rate phase, which might explain why the most massive, lazy black holes are less frequent in voids," Constantin said.

It could be that galaxies in "void" areas have less food around. However, recently scientists found that void galaxies support higher star-formation rates, which would suggest there's more gas and dust around-the same food that fuels black hole growth.

"This is strange given that these reclusive galaxies are forming stars at higher rates than their counterparts in denser regions," said co-researcher Fiona Hoyle, an astronomer at Widener University in Delaware. "This means there is plenty of fuel, but it is not efficiently channeled toward the central engine."

So perhaps interactions between galaxies could explain the different accretion rates, the scientists said. Interactions among galaxies can act to "knock" some of the gas and dust into the nuclear region where it's free fare for a central black hole. In less populated regions of the universe, such galactic get-togethers are much less frequent, which could lead to less fuel for black holes.

"These interactions are not as frequent in voids, so the 'feeding' of the black hole is slower," Constantin said.