Shedding New
Light On the Brightest Objects in the Universe
July 24, 2013 — Quasars are among the brightest,
oldest, most distant, and most powerful objects in the universe. Powered by
massive black holes at the center of most known galaxies, quasars can emit
enormous amounts of energy, up to a thousand times the total output of the
hundreds of billions of stars in our entire Milky Way.
Dartmouth astrophysicists Ryan Hickox and Kevin Hainline and colleagues have
a paper scheduled for publication in The Astrophysical Journal, detailing
discoveries based upon observations of 10 quasars. They documented the immense
power of quasar radiation, which reaches out for many thousands of light years
to the limits of the quasar's galaxy.
"For the first time, we are able to see the actual extent to which
these quasars and their black holes can affect their galaxies, and we see that
it is limited only by the amount of gas in the galaxy," says Hainline, a
Dartmouth postdoctoral research associate. "The radiation excites gas all
the way to the margins of the galaxy and stops only when it runs out of
gas."
The radiation released by a quasar covers the entire electromagnetic
spectrum, from radio waves and microwaves at the low-frequency end through
infrared, ultraviolet, and X-rays, to high-frequency gamma rays. A central
black hole, also called an active galactic nucleus, may grow by swallowing
material from the surrounding interstellar gas, releasing energy in the
process. This leads to the creation of a quasar, emitting radiation that
illuminates the gas present throughout the galaxy.
"If you take this powerful, bright radiation source in the center of
the galaxy and blast the gas with its radiation, it will get excited in just
the same way the neon gets excited in neon lamps, producing light," says
Hickox, an assistant professor in the Department of Physics and Astronomy at
Dartmouth. "The gas will produce very specific frequencies of light that
only a quasar can produce. This light functioned as a tracer that we were able
to use to follow the gas excited by the black hole out to large
distances."
Quasars are small compared to a galaxy, like a grain of sand on a beach, but
the power of their radiation can extend to the galactic boundaries and beyond.
The illumination of gas can have a profound effect, since gas that is lit up
and heated by the quasar is less able to collapse under its own gravity and
form new stars. Thus, the tiny central black hole and its quasar can slow down
star formation in the entire galaxy and influence how the galaxy grows and
changes over time.
"This is exciting because we know from a number of different
independent arguments that these quasars have a profound effect on the galaxies
in which they live," Hickox says. "There is a lot of controversy
about how they actually influence the galaxy, but now we have one aspect of the
interaction that can extend on the scale of the entire galaxy. Nobody had seen
this before."
Hickox, Hainline, and their co-authors based their conclusions on
observations made with the Southern African Large Telescope (SALT), the largest
optical telescope in the southern hemisphere. Dartmouth is a partner in SALT,
giving faculty and students access to the instrument. The observations were
performed using spectroscopy, in which light is broken down into its component
wavelengths. "For this particular kind of experiment, it is among the best
telescopes in the world," says Hickox.
They also used data from NASA's Wide-field Infrared Survey Explorer (WISE)
-- a space telescope that imaged the whole sky in the infrared. The scientists
used observations in infrared light because they give a particularly reliable
measure of the total energy output by the quasar.
Source: http://www.sciencedaily.com/releases/2013/07/130724200605.htm
