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EMBARGOED UNTIL: 12 Noon (EDT) May 6, 1999

CONTACT: Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone: 410-338-4514)
(E-Mail: villard@stsci.edu)

Jayanne English
Space Telescope Science Institute, Baltimore, MD
(Phone: 410-338-4352)
(E-Mail: jenglish@stsci.edu)


PHOTO NO.: STScI-PRC99-16

RING AROUND A GALAXY

Space Telescope Science Institute astronomers are giving the
public chances to decide where to aim NASA's Hubble Space
Telescope. Guided by 8,000 Internet voters, Hubble has already been
used to take a close-up, multi-color picture of the most popular
object from a list of candidates, the extraordinary "polar-ring"
galaxy NGC 4650A.

Located about 130 million light-years away, NGC 4650A is one of
only 100 known polar-ring galaxies. Their unusual disk-ring structure
is not yet understood fully. One possibility is that polar rings are
the remnants of colossal collisions between two galaxies sometime in
the distant past, probably at least 1 billion years ago. What is left
of one galaxy has become the rotating inner disk of old red stars in
the center. Meanwhile, another smaller galaxy which ventured too close
was probably severely damaged or destroyed. During the collision the
gas from the smaller galaxy would have been stripped off and captured
by the larger galaxy, forming a new ring of dust, gas, and stars,
which orbit around the inner galaxy almost at right angles to the old
disk. This is the polar ring which we see almost edge-on in Hubble's
Wide Field Planetary Camera 2 image of NGC 4560A, created using 3
different color filters (which transmit blue, green, and near-infrared
light).

This HST image clearly distinguishes structures in this
extraordinary galaxy that only recently were discovered in images made
using large ground-based telescopes. It also displays features that
were previously unknown. The image confirms that the bright central
concentration of light, which appears to be slightly orange in this
image, has a completely smooth, regular appearance, indicating that it
is a dense system composed of older stars and containing little gas or
dust. We believe this was once a typical medium-sized galaxy that has
been altered, probably by the process that made the complex polar
ring. The central dark lanes are due to blockage of light by clouds
of gas and dust in the ring, located between us and the inner
galaxy. These are the sites of star formation in most galaxies, and
NGC 4605A is no exception. The bright bluish clumps, which are
especially prominent in the outer parts of the ring, are regions
containing luminous young stars, examples of stellar rebirth from the
remnants of an ancient galactic disaster. The polar ring appears to
be highly distorted. No regular spiral pattern stands out in the main
part of the ring, and the presence of young stars below the main ring
on one side and above on the other shows that the ring is warped and
does not lie in one plane. Determining the typical ages of the stars
in the polar ring is an initial goal of our Polar Ring Science Team
that can provide a clue to the evolution of this unusual galaxy.

Because the polar ring extends far into the halo of NGC 4650A, it
provides a unique opportunity to map "dark matter," which is
thought to surround most disk galaxies. It is called dark matter
because it doesn't emit light, but only reveals itself through its
gravitational effect on the stars and gas in the galaxy. Dark matter
explains why disk stars in most spiral galaxies move at constant
speeds around their galactic cores, regardless of their distances from
the center axes of rotation. This motion is unlike the planets
orbiting around our Sun, which move more slowly the farther they are
away from the Sun, in response to the weakening of the Sun's
gravitational pull with increasing distance. A constant speed requires
extra gravitational pull and therefore implies the existence of unseen
material in most galaxies that is supplying additional gravity. In NGC
4650A, both the old, rotating disk and the dark matter surrounding
this galaxy pull on its polar ring. The alignment of the ring along
the pole of the inner disk's rotation allows scientists to probe this
combination of tugs and thus the distribution of dark matter.

The name NGC 4650A means that this galaxy is the brighter of a
pair of galaxies numbered 4650 in the "New General Catalogue of
Nebulae and Clusters of Stars," known to astronomers as the "NGC."
This catalog was published by J.L. Dreyer in 1888. It is mainly based
on observations obtained by William and John Herschel as they looked
at the skies with their telescopes while Caroline Herschel checked and
reduced the celestial positions of interesting objects. In only 100
years we have gone from astronomers peering at the sky through
telescopes on the ground to using electronic cameras on robotic
telescopes in space!

The HST exposures were acquired by the Hubble Heritage Team,
consisting of Keith Noll, Howard Bond, Carol Christian, Jayanne
English, Lisa Frattare, Forrest Hamilton, Anne Kinney and Zolt
Levay, and guest collaborators Jay Gallagher (University of
Wisconsin-Madison), Lynn Matthews (National Radio Astronomy
Observatory-Charlottesville), and Linda Sparke (University of
Wisconsin-Madison). These data were available in the public
archive on the release date (May 6, 1999) of the image produced
by the Hubble Heritage Team. The guest astronomers, plus
Anne Kinney, form the Polar Ring Science Team. They played a
role in producing the observing proposal and are providing a
preliminary analysis of the data.

Image Credit: The Hubble Heritage Team (AURA/STScI/NASA)