An eerie green cloud lurking around
a nearby galaxy has puzzled scientists since it was discovered last year. New
observations reveal that the cloud's ghoulish appearance may have to do with
radiation streaming from a black hole inside the galaxy.
The cloud was first discovered by
Dutch schoolteacher Hanny van Arkel, who volunteered to sift through
astronomical data as part of the Galaxy Zoo project. Van Arkel came across an
image of a huge, irregular-shaped greenish-yellowish cloud near the spiral
galaxy IC2497. The unusual object became known as "Hanny's Voorwerp,"
Dutch for "Hanny's thing."
The cloud itself is devoid of stars,
so astronomers were unsure what was generating the green glow. Some suggested
that radiation from a
massive black hole in the center of the galaxy (called a quasar) could be
the culprit, but until now no such black hole radiation had been visible.
"People had looked with X-ray
telescopes and had not detected any quasar or black hole activity," said
researcher Tom Oosterloo, an astronomer at the Netherlands Institute for Radio
Astronomy. "But our new observations, in very high resolution, show that
there is some activity. It's not spectacularly strong, but there is an active
black hole."
Oosterloo and colleagues, led by
Mike Garrett of the Netherlands Institute for Radio Astronomy and including van
Arkel, used the Westerbork Synthesis Radio Telescope in the Netherlands and an array in the European Very Long Baseline Interferometry Network to
observe Hanny's Voorwerp and IC2497.
The team found that a jet of highly
energetic particles was indeed emanating from the galaxy, pointing directly at
the Voorwerp. Because the jet was not directed toward Earth, it was difficult
to detect.
The researchers think particles in
this jet clear a path through the dense gas and dust around the galaxy,
allowing radiation from the black hole to pour into a gas cloud partially
encircling the galaxy. The optical and ultraviolet emission from the black hole
excites particles in the gas cloud, which is mostly made of hydrogen with some
oxygen and other elements. The heated-up gas emits the greenish light we see.
"In a way it's a cosmic
accident," Oosterloo told SPACE.com. "This black hole activity
happens a lot, and many galaxies have
this cloud, but it happens very rarely that the black hole jet hits the
cloud. It's not the only such object known, but it's quite an interesting
case."
The gas cloud around the spiral
galaxy likely came to be when a smaller galaxy flew by IC2497. The larger
galaxy's gravitation pull would have tugged off some of the other's gas,
keeping a blanket of material near it long after the smaller galaxy had moved
away.
Van Arkel said she was happy to have
gained new understanding of her voorwerp. "I'm happy we are making
progress," she said. "Apparently the more we learn about the
Voorwerp, the more intriguing it becomes."
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