-
- ATLANTA -- The new discoveries
of
super-massive black holes at the center of three nearby elliptical
galaxies
are adding credence to the argument that black holes may be
the crystal
seeds around which galaxies form.
-
- The detection of the black
holes, and analysis of their
characteristics suggest an answer to the
riddle of which came first, the
black hole or the galaxy, said
University of Michigan astronomer Douglas
Richstone, who leads the team
of astronomers that found the objects. Richstone
announced the
discoveries here Thursday at the meeting of the American
Astronomical
Society.
-
- Quasars, or quasi-stellar objects, are fancy, superluminious
energetic bodies, thought to be super-massive black holes at the heart
of huge active galactic nuclei.
-
- The three new black holes support an earlier argument
that the mass of black holes in the centers of galaxies are strongly
correlated
with the overall mass of the galaxies they are in, he
said.
-
- "Somehow, these black holes, when they determine
their
mass, they know the mass of the galaxy they're sitting in, or when
the
galaxy is forming it knows the mass of a black hole that it is forming
around or that it appears in. These are mutually regulated in some
way,"
he said.
-
- The strong tie naturally gives rise to the question
about
which came first, but the newly discovered objects, coupled with
the work
of another group of scientists appears to point to the black
hole as kicking
off galaxy formation, Richstone said.
-
- Richstone and his
colleagues found the black holes each
of which has the mass of 50
million to 100 million suns in the hearts
of normal elliptical
galaxies. Two of the galaxies, NGC 4697 and NGC 4473
are about 50
million light years from Earth in the Virgo galaxy cluster.
The third,
called NGC 821, is 100 million light years away in the direction
of the
constellation Ares.
-
- The three new objects bring the total number of
confirmed
super-massive black holes to 20.
-
- The astronomers used
observations from the Hubble
Space
Telescope and the MDM
telescope on
Kitt Peak near Tucson, Ariz. to search for the black holes.
They then
processed their information through a computer model that can
detect
abrupt changes in star-velocity patterns. Stars accelerate and change
direction very rapidly moving hundreds of miles every second as they
spiral into black holes. By measuring the velocity of the swirling stars,
the scientists are able to calculate the mass of the black holes at the
galactic center.
-
- So far, it seems that all galaxies with bulges in the
center
have, at their hearts, massive black holes that drive them, Richstone
said.
-
- Interestingly, the astronomers found that black hole
mass is
strongly correlated with the overall mass of its host galaxy. This
finding is consistent with what was observed with previously discovered
black holes, Richstone said. It is difficult to determine what causes this
correlation, but that should be an area of future research, he
said.
-
- What
is more, new work by another team of scientists
suggests that the
super-massive black holes that lie at galactic cores
are actually
retired quasars. Quasars, or quasi-stellar objects, are fancy,
superluminious energetic bodies, thought to be super-massive black holes
at the heart of huge active galactic nuclei. They thrived in their heyday
when the universe was a young billion years old.
-
- Andrew Wilson, of the
University of Maryland, presented
the results of his team's research
simultaneously with those of the black-hole-census
group.
-
- The quasar team used
the Very
Large Array and the Very
Long
Baseline Array of radio telescopes to conduct detailed radio
observations
of 100 nearby galaxies. The astronomers targeted galaxies
that had been
noticed to contain certain ionized gases. They were
aiming to settle an
uncertainty about how such gasses were formed. One
theory postulated that
ultraviolet radiation from hot stars stripped
electrons from the gas. The
other idea suggested that the ionizing
radiation came from massive black
holes accreting surrounding
gas.
-
- In
their first round of observations, about a third of
the galaxies showed
a unique radio signature that had previously only been
seen in quasars.
When scientists took a more-detailed look at these quasi-quasar
galactic sources they found much to their admitted surprise that every
one of the galaxies had the signature of a black hole at its center: an
extremely intense, but compact radio source, just like quasars
emit.
-
- "An accreting super-massive black hole is the only
object
we know of that can generate these extremely bright, but very small,
radio sources," Neil Nagar, a quasar-team member from the University
of Maryland, said in a prepared statement.
-
- Nagar and Wilson concluded that
the black holes in the
center of the galaxies must be the dying
remnants of quasars that are accreting
material at a much slower rate
than they did in their youthful vigor in
the early epochs of the
universe.
-
- The finding puts punch into the Richstone's argument
that black
holes may actually be the kernels that form galaxies because
it fixes
the age of the central object in galaxies to times before galaxies
were
beginning to form stars.
-
- "These massive black holes now seen in centers of
galaxies are relics of these quasars," Richstone said. The black
holes,
therefore, should have been present "at the height of the
quasar epoch
when the universe was about one billion years
old."
-
- They would then have been in a position to influence
the
formation of galaxies, which are believed to have happened later, he
said. Their tremendous mass and gravitational potential would make black
holes are the dominant energetic force in galaxies that were busy forming
stars, he said. "It is, I think, likely that these black holes play
a key role in the control of the formation of stars in the galaxy that
is then forming and in the evolution of that galaxy."
|
