In a strange
reversal, astronomers have detected a massive
black hole but can find no traces of the surrounding galaxy that should be
feeding it.
At the center of
most large galaxies, our own Milky
Way included, are extremely dense black holes that have masses hundreds of
millions times that of the Sun.
Called quasars,
these massive black holes are the most radiant objects in the universe,
outshining even the brightest galaxies. While the black holes themselves are
undetectable, friction and heat from the swirling matter they ingest emit huge
amounts of radiation that can be detected by radio telescopes.
To maintain
their fierce brightness, however, quasars must feed off the very galaxies they
live within. That is why the discovery of a galaxy-less quasar is so surprising.
Using images
from the Hubble Space Telescope and
spectroscopy from the Very
Large Telescope (VLT) in northern Chile, an international team of
astronomers selected 20 quasars located at moderate distances from Earth to
study the properties of their host galaxies.
In 19 of them,
the astronomers found encircling galaxies as predicted. But when they looked at
HE0450-2958, a quasar located some 5
billion light-years away, they didn't find any sign of a galaxy.
"We must therefore
conclude that, contrary to our expectations, this bright quasar is not
surrounded by a massive galaxy," said Pierre Magain, an astronomer from the
University of Liege in Belgium and lead author of a new study documenting the
finding.
Quasars are relatively
small compared to the galaxies they outshine. They are only about the size of
our solar system, but they can emit up to 100 times as much radiation as an
entire galaxy.
While the
intense brilliance of quasars make them visible from clear across the universe,
it also makes detection of their host galaxies difficult because light and
radiation from the galaxies get lost in the glare of the quasars.
In the late
1990s, mathematical "deconvolution" algorithms were developed that could be
applied to images after they were transmitted to Earth and which were capable
of separating light from quasar's from that of their host galaxies. Since then,
astronomers have shown that nearly all quasars are encircled by a host galaxy.
Instead of a
galaxy, the researchers detected a cloud of ionized gas about 2,500 light years
in size near HE0450-2958. Dubbed "the blob," the researchers believe this gas
cloud is what's feeding the black hole, allowing it to become a quasar. The
researchers estimate that the quasar is siphoning off about one Sun's worth of
mass each year from blob to satisfy its ravenous appetite.
Adding to the
mystery is the detection of a deeply distorted galaxy located 50,000 light
years away from the quasar. This so-called "companion" galaxy appears to be an
extremely active stellar nursery, birthing new stars at a rapid rate, and it is
also brighter in the infra-red spectrum than most galaxies.
The combination
of these three factors--distorted shape, high rate of star production and ultra
infra-red luminosity--suggests to the researchers that the companion galaxy
suffered a cosmic collision about 100 million years ago, possibly with the
galaxy-less quasar.
Such a collision
would have stirred up dust and gas and enhanced the formation of stars, said
Gèraldine Letawe, a member of the research team also from University of Liege
in Belgium. Heat from the seething young stars, combined with dust and gas
warmed up by the collision might be responsible for the galaxy's intense
infra-red glow.
To explain the
strange cosmic setup they've discovered, the researchers have come up with
various hypotheses, all of them equally strange:
- It's possible that
the quasar does have an
encircling galaxy, but that it is too small and too faint to be detected.
If a host galaxy does exist, then it would have be to either six times
fainter than typical host galaxies or have a radius smaller than 300 light
years. Most quasar host galaxies range between 6,000 and 50,000 light
years across.
- The
quasar may not have always been galaxy-less, but the collision with the
companion galaxy may have somehow caused the quasar's galaxy to disappear
completely. The researchers note, however, that it is "hard to imagine how the complete disruption
of a galaxy could happen."
- The blob could be gas
stolen by a slow-moving black hole as it traveled through the disc of a
spiral galaxy.
- Perhaps the most intriguing
theory is that the quasar is encircled by a galaxy made up almost entirely
of dark matter, a theoretical substance which is thought to make up 25
percent of the matter in the universe but which cannot be directly
detected using current technologies.
Computer
simulations of galaxy collisions might be able to determine if the first two
options are plausible, Letawe told SPACE.com in an email interview.
It may be
possible to verify whether the galaxy is made up of dark matter by scanning the
space around the quasar for evidence of gravity lensing, a phenomenon whereby a
massive celestial object warps the fabric of space-time so much that light from
distant objects is bent around it.
Another way to test for the
presence of a dark matter galaxy is to look for gases that appear to be moving
as if drawn by the gravity of some unseen object, Letawe said.
The discovery is
documented in the September 15 issue of the journal Nature.
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