Hyperactive blackhole

lA new study from NASA's Chandra X-ray Observatory
tells scientists how often the biggest black holes have
been active over the last few billion years. This
discovery clarifies how supermassive black holes grow
and could have implications for how the giant black hole
at the center of the Milky Way will behave in the future.
Most galaxies, including our own, are thought to contain
supermassive black holes at their centers, with masses
ranging from millions to billions of times the mass of
the Sun. For reasons not entirely understood,
astronomers have found that these black holes exhibit a
wide variety of activity levels: from dormant to just
lethargic to practically hyper.
The most lively supermassive black holes produce what
are called "active galactic nuclei," or AGN, by pulling in
large quantities of gas. This gas is heated as it falls in
and glows brightly in X-ray light.
"We've found that only about one percent of galaxies
with masses similar to the Milky Way contain
supermassive black holes in their most active phase,"
said Daryl Haggard of the University of Washington in
Seattle, WA, and Northwestern University in Evanston, IL,
who led the study. "Trying to figure out how many of
these black holes are active at any time is important for
understanding how black holes grow within galaxies and
how this growth is affected by their environment."
This study involves a survey called the Chandra
Multiwavelength Project, or ChaMP, which covers 30
square degrees on the sky, the largest sky area of any
Chandra survey to date. Combining Chandra's X-ray
images with optical images from the Sloan Digital Sky
Survey, about 100,000 galaxies were analyzed. Out of
those, about 1,600 were X-ray bright, signaling possible
AGN activity.
Only galaxies out to 1.6 billion light years from Earth
could be meaningfully compared to the Milky Way,
although galaxies as far away as 6.3 billion light years
were also studied. Primarily isolated or "field" galaxies
were included, not galaxies in clusters or groups.
"This is the first direct determination of the fraction of
field galaxies in the local Universe that contain active
supermassive black holes," said co-author Paul Green of
the Harvard-Smithsonian Center for Astrophysics in
Cambridge, MA. "We want to know how often these giant
black holes flare up, since that's when they go through
a major growth spurt."
A key goal of astronomers is to understand how AGN
activity has affected the growth of galaxies. A striking
correlation between the mass of the giant black holes
and the mass of the central regions of their host galaxy
suggests that the growth of supermassive black holes
and their host galaxies are strongly linked. Determining
the AGN fraction in the local Universe is crucial for
helping to model this parallel growth.
One result from this study is that the fraction of galaxies
containing AGN depends on the mass of the galaxy. The
most massive galaxies are the most likely to host AGN,
whereas galaxies that are only about a tenth as massive
as the Milky Way have about a ten times smaller chance
of containing an AGN.
Another result is that a gradual decrease in the AGN
fraction is seen with cosmic time since the Big Bang,
confirming work done by others. This implies that either
the fuel supply or the fueling mechanism for the black
holes is changing with time.
The study also has important implications for
understanding how the neighborhoods of galaxies
affects the growth of their black holes, because the AGN
fraction for field galaxies was found to be
indistinguishable from that for galaxies in dense
clusters.
"It seems that really active black holes are rare but not
antisocial," said Haggard. "This has been a surprise to
some, but might provide important clues about how the
environment affects black hole growth."
It is possible that the AGN fraction has been evolving
with cosmic time in both clusters and in the field, but at
different rates. If the AGN fraction in clusters started out
higher than for field galaxies -- as some results have
hinted -- but then decreased more rapidly, at some point
the cluster fraction would be about equal to the field
fraction. This may explain what is being seen in the
local Universe.
The Milky Way contains a supermassive black hole
known as Sagittarius A* (Sgr A*, for short). Even though
astronomers have witnessed some activity from Sgr A*
using Chandra and other telescopes over the years, it
has been at a very low level. If the Milky Way follows the
trends seen in the ChaMP survey, Sgr A* should be
about a billion times brighter in X-rays for roughly 1% of
the remaining lifetime of the Sun. Such activity is likely
to have been much more common in the distant past.
If Sgr A* did become an AGN it wouldn't be a threat to
life here on Earth due to our Solar Systems remote
location in an outer spiral arm of the Milky Way, but it
would give a spectacular show at X-ray and radio
wavelengths. However, any planets that are much closer
to the center of the Galaxy, or directly in the line of fire,
would receive large and potentially damaging amounts
of radiation.
Most astronomers today believe that one of the plausible
reasons we have yet to detect intelligent life in the
universe is due to the deadly effects not from AGNs, but
because of local supernova explosions that wipe out all
life in a given region of a galaxy.
While there is, on average, only one supernova per
galaxy per century, there is something on the order of
100 billion galaxies in the observable Universe. Taking
10 billion years for the age of the Universe (it's actually
13.7 billion, but stars didn't form for the first few
hundred million), Dr. Richard Mushotzky of the NASA
Goddard Space Flight Center, derived a figure of 1 billion
supernovae per year, or 30 supernovae per second in the
observable Universe!
Astronomers think supernova explosions closer than 100
light years from Earth would be catastrophic, but the
effects of events further away are unclear and would
depend on how powerful the supernova is.
Casey Kazan via Chandra X-ray Center
Image at top of page: In this newly discovered type of
AGN, the disk and torus surrounding the black hole are
so deeply obscured by gas and dust that no visible light
escapes, making them very difficult to detect. This
illustration shows the scene from a more distant
perspective than does the other image. Click on image
for high-res version. Image credit: Aurore Simonnet,
Sonoma State University.