HONOLULU—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 SPACE.com.
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.
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