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Measuring the Background Light of the Universe
Wed, 10 May 2006 -
The Universe is filled with a diffuse glow of radiation coming
from all the stars and galaxies. This cosmic fog is actually
hard to detect because we have much brighter objects nearby that
can wash it out; like how the city lights obscure the stars at
night. One way to measure this radiation is by using the
radiation from quasars, which are extremely bright and distant.
The high-energy radiation from the quasars loses energy as it
passes through this background radiation, and this can be
measured.
Full article
Artist's impression of the Extragalactic Background Light
emission and absorption. Image credit: HESS Collaboration. Click
to enlarge
All throughout space, a cosmic background light shimmers. Stars,
galaxies - all kinds of sources - contribute to it; the light is
their leftovers, in fact. Now, astrophysicists have discovered
that this light is hardly as intense as anyone had guessed. The
researchers used two distant quasars as "probes", and recorded
their gamma spectra using the H.E.S.S. telescopes in Namibia.
These spectra turned out to be just a bit reddened; the
background light seemed to only lightly obfuscate the quasars'
radiation. These observations do not just shed light on the
background light - but on topics as great as the birth and
development of galaxies (Nature, April 20, 2006).
Stars, galaxies, quasars, and many other objects contribute to
the fog of radiation in the universe. It permeates all of
intergalactic space; it is the "leftover" light that all these
objects emit. Extragalactic background light - EBL - covers up
epochs worth of stellar activity, from the time the first stars
were created to the present. Scientists have been trying for a
long time to measure this emission. Doing that directly is not
easy, however, and extremely inaccurate, because Earth's
atmosphere, the Solar System, and the Milky Way send out
radiation which gets in the way of observing weak EBL.
One way out of this problem is observing quasars - the cosmic
energy factories which have a huge black hole in their middle.
These "gravity traps" swallow up gas around them and spit some
of it back as plasma, accelerated to nearly the speed of light.
It is radiation bundled out of protons, electrons, and
electromagnetic waves. Often, it can be hundreds of times wider
than its mother galaxy. If this "quasar spray" heads in the
direction of Earth, the radiation can appear quite strong -
astronomers call this a "blazar".
The two objects which H.E.S.S. researchers observed are both
blazars. How to use them as probes? They send out very energetic
gamma light particles, which lose strength on their way to Earth
when they hit EBL photons. This causes the original blazar gamma
spectrum to redden - like when the Sun nears the horizon at dusk
and the Earth's atmosphere disperses more of the blue part of
the sunlight than the red. The thicker the atmosphere, the
redder the sun. Reddening depends on the thickness of the
medium. This fact is the key to investigating the composition of
EBL.
Luigi Costamante of the Max Planck Institute for Nuclear Physics
in Heidelberg says "the main problem is that energy distribution
in quasars can take many different forms. Until now, we could
not really say whether any observed spectrum looks red because
it truly had a strong reddening, or if it was that way from the
beginning."
This problem has been solved thanks to the gamma spectra of two
quasars -- H 2356-309 and 1ES 1101-232. These objects are more
distant than any sources observed until now. The sensitivity of
the H.E.S.S. telescope made it possible to investigate them. It
turns out that EBL's intensity is not strong enough to redden
quasar light; the spectra are too blue, and contain too many
higher-energy gamma rays.
H.E.S.S. data has allowed the scientists to derive the maximum
intensity of the diffused light. It is near the lowest limit
resulting from the sum of the light of single galaxies visible
in an optical telescope. That answers a question that has
puzzled astronomers for years: is diffuse light created above
all by the radiation from the first stars? The H.E.S.S. results
seem to eliminate this possibility. There is also little room
for contributions from other sources, like normal galaxies.
Looking more closely at intergalactic space gives new
perspectives on investigating gamma rays outside our own galaxy.
Original Source: Max
Planck Society
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