NGC 2915: The Darkest Spiral Galaxy

(1996) AJ 111, 1551

The accompanying picture shows NGC 2915, which Australian, Canadian, and American astronomers have shown to be the darkest known disk galaxy. While in the optically visible light of stars, (here displayed in yellow) NGC 2915 appears to be a faint dwarf galaxy, in the radio emission of neutral hydrogen gas (here displayed in blue) NGC 2915 appears much larger, and has spiral arms. The team of astronomers, lead by Gerhardt Meurer at Johns Hopkins University, analyzed the rotation of the galaxy and showed that NGC 2915 is composed mostly of "dark matter". Dark matter is extremely difficult to detect since it does not emit any radiation. Its presence can only be inferred from the gravitational influence of its mass. The ratio of the total mass of the galaxy to the optically visible light is nearly eighty (relative to the mass and light of the sun), the largest such ratio found for a disk galaxy. NGC 2915 may be a galaxy that has largely failed to form, except at its very center where the gravity of the dark matter traps interstellar gas until enough builds up to form stars. The results of Meurer's group will be published in the April 1996 issue of the "Astronomical Journal".
The optical portion of the picture was obtained with 3.9m Anglo-Australian Telescope (AAT), while the radio image was obtained with the Australia Telescope Compact Array (ATCA). Further details can be obtained from:

Gerhardt R. Meurer	office: Bloomberg Building, rm# 524
Johns Hopkins University	URL: http://www.pha.jhu.edu/~meurer
Department of Physics and Astronomy	e-mail: meurer@pha.jhu.edu
Baltimore, MD, 21218, U.S.A.	phone: +1 (410) 516-5154
U.S.A.	fax: +1 (410) 516-5096

HI distribution (blue) of NGC 2915 overlaid onto the optical image (yellow)

HI distribution of NGC 2915 obtained with the ATCA

• For information and pictures of other galaxies see: ATCA Observations of HI in Galaxies