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. During the collision the gas from a smaller galaxy would have been stripped off and captured by a larger galaxy, forming a new ring of dust, gas, and stars, which orbit around the inner galaxy almost at right angles to the larger galaxy's disk. This is the vertical 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).

The ring appears to be highly distorted and the presence of bluish, 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. The typical ages of the stars in the polar ring may provide a clue to the evolution of this unusual galaxy. Because the polar ring extends far into the halo of NGC 4650A, it also provides a unique opportunity to map "dark matter" which is thought to surround most disk galaxies.

The HST exposures were acquired by the Hubble Heritage Team and guest astronomers Jay Gallagher (University of Wisconsin-Madison), Lynn Matthews (National Radio Astronomy Observatory-Charlottesville), and Linda Sparke (University of Wisconsin-Madison).

Credit: NASA and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: J. Gallagher (U. Wisconsin-Madison)

See the full caption for more information about the structure in this galaxy and about dark matter.