First direct detection of ionized intergalactic medium at highredshifts is announced (Forwarded)

U.S. Naval Observatory
Washington, D.C.

Information Contact:
Geoff Chester, USNO Public Affairs Office
(202) 762-1438

Dr. Roopesh Ojha, USNO
(202) 762-0607

FOR RELEASE: 9:20 AM PST, Tuesday January 9, 2007

FIRST DIRECT DETECTION OF IONIZED INTERGALACTIC MEDIUM AT HIGH REDSHIFTS
IS ANNOUNCED

An international team of astronomers may now have the first direct
evidence for a turbulent, ionized intergalactic medium at redshift values
of z2. The research leading to this finding was done by Drs. Roopesh Ojha
(U.S. Naval Observatory), Tapio Pursimo (Nordic Optical Telescope), David
L. Jauncey and Jim E. Lovell (Australia Telescope National Facility),
Jean-Pierre Macquart (National Radio Astronomy Observatory/Caltech) and
Michael Dutka (University of Maryland), and will be presented today at the
American Astronomical Society meeting in Seattle, Washington. This result
is a first step towards being able to understand the possible
inhomogeneity -- "lumpiness" -- of the intergalactic medium (IGM) at high
redshifts, and to trace how the IGM has changed over time.

"This technique gives astronomers an important new tool for probing the
ionized gas in the intergalactic medium", says Dr. Ojha. "While current
observational techniques are able to study the neutral gas at higher
redshifts, we now have the ability to study the ionized gas which is
postulated to be much more widespread than neutral gas at these redshifts.
Thus we may have a new way to test current theories about the nature of
the IGM."

The space between stars in a galaxy is not empty; think of it as a thin
"soup" of gas and dust. Known as the interstellar medium (ISM), this soup
is known to be both lumpy and turbulent, a sort of "cosmic minestrone".

The space between galaxies, the intergalactic medium, is much emptier.
Nonetheless, it too is a "soup", although more of a "watery consommé" when
compared to the "minestrone" of the ISM. At least that's what the IGM
appears to be like in the local, nearby universe. At redshifts above 2 --
that is, at a time when the Universe's volume was about three percent of
what it is at present -- astronomers have not had a way to probe its
nature.

But now Ojha et al. have used the ISM -- the "minestrone" in our Galaxy --
as a special kind of filter to distinguish between distant compact radio
sources -- quasars -- on the basis of their angular size. And above a
redshift of 2, the angular size of the sources appears to increase. Ojha
et al. argue that this apparent increase in size is not due to the
intrinsic nature of the sources or some trick of cosmology, but rather
that it is the result of the quasars' radio emission being scattered by a
turbulent and ionized IGM.

How does one use the interstellar medium as a filter? In the 1980s,
certain extragalactic radio sources, the quasars, were found to vary on
extremely short time scales, their radio flux strength rising and falling
drastically in just a few hours. After a bit of sleuthing, this "intraday
variability" was traced to the ISM, which was causing the quasars to
scintillate, or "twinkle", just as Earth's atmosphere causes stars to
twinkle. Radio telescope sky surveys then followed to establish how
widespread these rapidly scintillating sources were.

The largest of these surveys to date, MASIV (Micro-Arcsecond
Scintillation-Induced Variability), was carried out using the National
Science Foundation's Very Large Array radio telescope at Socorro, New
Mexico, by Jim Lovell and colleagues. MASIV detected 500 flat-spectrum
radio sources at a frequency of 5 GHz: 56% of these showed scintillation.

Ojha et al. obtained redshifts for 190 of the 500 sources in the MASIV
survey: 150 came from the literature, while 40 were new measurements made
with the 2.6-m Nordic Optical Telescope in La Palma, Spain. The entire
dataset was then used to look for any difference in the redshift
distribution of scintillating and non-scintillating sources. The
researchers found, to a 98% confidence level, a redshift dependence on the
scintillators, with a striking deficit of scintillating sources above z~2.

Scintillation occurs only if the radio sources are below a certain angular
size, which is around 100 millionths of an arcsecond -- a level of detail
that would allow us to see an ant in Albuquerque, New Mexico while sitting
in downtown Sydney, Australia! This is almost 10 times finer than the
angular size that astronomers can currently observe by other techniques.
The drop-off at z~2 implies that, at higher redshifts, the sources are
either intrinsically or apparently larger in angular size.

While an intrinsically larger angular size at higher redshifts cannot be
completely ruled out, there are a number of reasons why this is not very
likely. Current theories of galaxy formation tell us that quasars should
appear smaller at higher redshifts. Also, the sources for the MASIV survey
were selected in a manner that tended to bias the survey towards finding
smaller sources at high redshifts.

The most likely explanation of these results is that an ionized IGM is
diffusing the radio emissions from the quasars, causing them to appear
larger than the angular size below which they can twinkle. A terrestrial
analogy would be the way fog or heavy snowfall diffuses the headlights of
distant oncoming cars at night. At z2 the Universe was much smaller than
it is today, so the intergalactic medium must have been much denser,
overall, than it is now. Hot young stars, radio galaxies, and quasars, all
present in abundance at this epoch, would have been "ionization factories"
pumping out both ionizing radiation and charged particles.

"We believe, therefore, that we're seeing the effect of the radio signals
being scattered by the ionized intergalactic medium," said Dr. Jauncey.

"But to confirm this result, we really need to get more redshifts,
particularly for the weaker sources, so that we can rule out any possible
selection effects," said Dr. Pursimo.

Given enough radio sources of the right kind, the scientists say, they
might be able to trace how the scattering properties of the intergalactic
medium changed through time at higher redshifts. The degree of
inhomogeneity, or lumpiness, in the intergalactic medium could be
established by taking optical spectra along the lines of sight to quasars
from the survey.

IMAGE CAPTIONS:

[Image 1:
http://www.usno.navy.mil/pao/Digipix/scint_cartoon.jpg (70KB)]
Schematic diagram of IGM broadening of z2 quasar's apparent size. Credit:
Dr. Jean-Pierre Macquart.

[Image 2:
http://www.not.iac.es/general/photos...s/IMG_3097.JPG (418KB)]
The Nordic Optical Telescope, La Palma, Spain. Credit: Nordic Optical
Telescope, Jyri Näränen