In the
first detection of its kind, astronomers have found a cosmic "ghost"
lurking around a distant supermassive black hole.
Scientists
think that the discovery, made with NASA's Chandra
X-ray Observatory, is evidence of a huge eruption produced by the black
hole and could give astronomers a valuable opportunity to observe phenomena
that occurred when the universe was very young.
The X-ray
ghost, so-called because a diffuse X-ray source has remained after other
radiation from the outburst has died away, is in the Chandra Deep Field-North,
one of the deepest X-ray images ever taken.
The source,
dubbed HDF 130, is more than 10 billion light-years away and existed at a time
3 billion years after the
Big Bang, when galaxies and black holes were forming at a high rate. HDF
130 has a cigar-like shape that extends for some 2.2 million light-years. (A
light-year is the distance that light travels in a year, about 6 trillion miles
or 10 trillion kilometers.)
"We'd
seen this fuzzy object a few years ago, but didn't realize until now that we
were seeing a ghost," said team member Andy Fabian of the Cambridge University in the United Kingdom. "It's not out there to haunt us, rather it's
telling us something — in this case what was happening in this galaxy billions
of year ago."
Fabian and
colleagues think the X-ray glow from HDF 130 is evidence for a powerful
outburst from its central black hole in the form of jets of energetic particles
traveling at almost the speed of light.
The power
contained in the black
hole eruption was likely to be considerable, equivalent to about a billion
supernovas.
When the
eruption was ongoing, it produced prodigious amounts of radio and X-radiation,
but after several million years, the radio signal faded from view as the
electrons radiated away their energy.
However,
less energetic electrons can still produce X-rays by interacting with the
pervasive sea of photons remaining from the Big Bang — the cosmic background
radiation. This process produces an extended X-ray source that lasts for
another 30 million years or so.
"This
ghost tells us about the black hole's eruption long after it has died,"
said team member Scott Chapman, also of Cambridge University. "This means
we don't have to catch the black holes in the act to witness the big impact
they have."
This is the
first X-ray ghost ever seen after the demise of radio-bright jets. Astronomers
have observed extensive X-ray emission with a similar origin, but only from
galaxies with radio emission on large scales, signifying continued eruptions.
In HDF 130,
only a point source is detected in radio images, coinciding with the massive
elliptical galaxy seen in its optical image. This radio source indicates the
presence of a growing supermassive black hole.
"This
result hints that the X-ray sky should be littered with such ghosts, especially
if black hole eruptions are as common as we think they are in the early universe,"
said team member Caitlin Casey, also of Cambridge.
advertisement
