(draft text to be used for the OSU press release)

Cosmic magnifying lenses distorts the view of the most distant galaxies 

EMBARGOED UNTIL: 12:45 pm US PST, Wednesday January 12, 2011 

Gravitational lensing occurs when light from a distant object is distorted by a
massive object that is in the foreground. Astronomers have started to apply
this concept in a new way to determine the number of very distant galaxies and
and to measure dark matter in the universe. Though recent progress has been
made in extending the use of gravitational lensing, a Letter published in
Nature on January 12, 2011, by Stuart Wyithe (U. Melbourne), Haojing Yan (OSU),
Rogier Windhorst (ASU), and Shude Mao (Jodrell Bank, UK, and NAOC, China) makes
the case that the tool may be even more necessary than originally thought when
looking at distant galaxies. 

   Albert Einstein showed that gravity will cause light to bend. The effect is
normally extremely small, but when light passes close to a very massive object
-- a "gravitational lens" as astronomers would call it -- the bending of the
light rays becomes more easily noticeable. When light from a very distant
galaxy passes a massive object much closer to us, it can detour around this 
foreground "lens" and forms a larger and brighter image of the distant galaxy.
In this case, the foreground object works as a "natural telescope" and makes
the distant galaxy more easily detectable to us.

   This effect is not new to astronomers, and in fact a few massive clusters of 
galaxies have been used as such "natural telescopes" to search for very distant
galaxies. Individual galaxies, while much less massive than galaxy clusters,
can also work as lenses as well if they are heavy and dense enough. "What we
have found is," says Yan, "that when we look back into the very distant
universe, the incidence of gravitational lensing by individual foreground 
galaxies might be higher than what people had thought. This is good, because it
gives us a better chance to detect faint galaxies that would not be seen if
there were no lensing. On the other hand, this effect boosts up the number of
detectable distant galaxies and their fluxes, and we should take this into
account if we are to obtain an undistorted census of distant galaxies in the
early universe."

   When we look back to when the universe was young, we are seeing extremely
early objects that are very far away. The farther away the object, the more
the foreground universe there is to look through, which means the greater the
chance that there will be something heavy in the foreground as lens to magnify
the background image. With its new Wide Field Camera 3 (WFC3), Hubble Space
Telescope (HST) has begun to see some of the brightest galaxies at redshifts of
8--10, which correspond to a very early stage of the universe when it was only
650--480 million years old. The future James Webb Space Telescope (JWST) will
not only enable us to detect many more, fainter galaxies at these redshifts but
also galaxies at even higher redshifts.

  In their Nature Letter, Yan and his colleagues suggest that our measurement
of the flux and count of very distant galaxies is likely to be significantly
distorted by the magnification through individual foreground galaxies. The
exact ratio of magnified and unmagnified distant galaxies depends on a number of
factors which astronomers are currently not certain about. A big uncertainty
is the intrinsic brightness of very distant galaxies as a whole. "We do have a
reasonable knowledge about this value at lower redshifts, but its value at
redshifts of 8--10 and higher still needs to be found out." Yan explains, "We
have explored a large number of possibilities, and we find that the fraction of
lensed galaxies ranges from a few to a few tens percent. In extreme cases, 
all what we will be able to see at redshift of 20 with the future JWST would be
lens-magnified objects. And the possibilities of such high incidence of lensing
is what we need to watch out for."

   In fact, there might already have been some evidence that we are seeing such
high incidence of lensing events in the recent deep near-IR surveys by the HST
WFC3. In a separate study one year ago, Yan and his colleagues used these data
and found a large number of candidate galaxies at redshifts of 8--10. "A
puzzling thing back then was that about 20-30% of our candidate distant
galaxies are found around foreground galaxies, and this cannot be simply
attributed to alignment-by-chance," says Yan, "Now with our new study, this
begins to make sense -- gravitational lensing happens around foreground
galaxies, and naturally a high rate of lensing will result in a high rate of
concentration of distant galaxies around foreground galaxies. In our
calculation, we show that the observed rate of concentration is consistent with
the expectation from our model."

   However, Yan cautions that such evidence is still only tentative. The more
decisive evidence would have to come from the shapes of those candidate lensed
distant galaxies. Gravitational lensing not only magnifies a background galaxy
but also forms multiple, arc-like images of it around the foreground lens.
While HST is a very powerful telescope, it is still not powerful enough to see
the multiplicity or to tell the difference of the shape at these extreme
distances. "Only JWST can ultimately do this," Yan says. "Let's hope it will
get finished -- and with the designed resolution and sensitivity. This will not
just be about the proof of the high incidence of gravitational lensing of the
distant galaxies, but about how we are going to obtain an accurate picture of
the early universe."

(Modified from the text provided by Nikki Cassis, Media and Public Relations,
School of Earth and Space Exploration, Arizona State University.)

Photo and Figure credits: NASA, ESA, Z. Levay and A. Feild (STScI)

Press release text and images are on: http://www.asu.edu/clas/hst/www/nature11/