Cambridge, MA– Many examples are known where a galaxy acts as a
gravitational lens, producing multiple images on the sky of a more distant
object like a bright quasar hidden behind it. But there has been a
persistent mystery for over 20 years: Einstein’s general theory of
relativity predicts there should be an odd number of images, yet almost all
observed lenses have only 2 or 4 known images. Now, astronomer Joshua Winn
of the Harvard-Smithsonian Center for Astrophysics (CfA) and two former CfA
colleagues, David Rusin (now at the University of Pennsylvania) and
Christopher Kochanek (The Ohio State University), have identified a third,
central image of a lensed quasar. Radio observations of the system known as
PMN J1632-0033 in the constellation Ophiuchus uncovered a faint central
image, which can be used to investigate the properties of the lensing galaxy
and the supermassive black hole expected to lie at its center.

“Finding this central image is interesting in its own right, but is even
more important for what it can tell us about the lensing galaxy. This offers
us a new tool for studying galaxies so far away that, even to the Hubble
Space Telescope, they’re just faint smudges,” said Winn.

Quasars are extremely distant and bright objects believed to be powered by
supermassive black holes. They shine brightly by converting the
gravitational energy of matter falling into the black hole into light and
other types of radiation, such as radio waves.

In gravitational lensing, light rays from a quasar which pass close to a
galaxy are bent by the galaxy’s gravitational field, much as they would be
bent when passing through a glass lens. The denser the center of a galaxy,
and the stronger its gravity, the fainter the central image will be. Yet
this central image, whose light has passed closest to the middle of the
lensing galaxy, can tell us much about that galaxy’s core. That opportunity
makes finding such central images particularly desirable.

In the system PMN J1632-0033, a radio-loud quasar at redshift z=3.42 (a
distance of about 11.5 billion light-years) is being lensed by an elliptical
galaxy at redshift z~1 (about 8 billion light-years away). Two images of the
quasar were known to exist, and a third, very faint radio source was
suspected to be the central image. However, that third source was right on
top of the lensing galaxy, and so might have been intrinsic to the lensing
galaxy itself.

By observing the radio “color,” or spectrum, of all three images using the
National Science Foundation’s Very Large Array and Very Long Baseline Array,
Winn and his colleagues provided compelling evidence that the third source
is indeed the quasar’s central image. Its spectrum is essentially identical
to the other two images, except at low frequencies where some of the radio
energy was absorbed by the lensing galaxy.

The geometry and properties of the quasar’s three images already are telling
us about the core of the lensing galaxy. For example, its central black hole
weighs less than 200 million solar masses. Also, its surface density (amount
of matter as projected against the plane of the sky) at the location of the
central image is more than 20,000 solar masses per square parsec. (For
comparison, the surface density of the Milky Way near our sun is about 50
solar masses per square parsec.) Both figures for the lensing galaxy agree
with expectations based on detailed observations of galaxies hundreds of
times closer to the Earth.

“Almost all of our knowledge about galaxy centers comes from studying very
nearby galaxies. The remarkable thing about central images is that you can
get similar information about the cores of galaxies hundreds of times
farther away, and billions of years younger than our neighboring galaxies,”
said Winn.

This research is available online at http://arxiv.org/abs/astro-ph/0312136
and will be published in the February 12, 2004 issue of the journal Nature.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics is a joint collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory. CfA scientists, organized
into six research divisions, study the origin, evolution and ultimate fate
of the universe.