Hubble image sheds light on darkness within galaxies

enlarge   Spiral galaxy pair

May 11, 2000
Web posted at: 12:33 PM EDT (1633 GMT)

By Richard Stenger
CNN Interactive Writer

BALTIMORE, Maryland (CNN) -- The chance alignment of two spiral galaxies offers a rare glimpse of elusive galactic dark material, according to Hubble Space Telescope astronomers, who released a striking image of the pair on Thursday.

The unusual shadow show has helped resolve a controversy over the role of interstellar dust in the composition of galaxies, a Hubble scientist said.

Facing our direction, the galaxy in the foreground seems unusually dark because its interstellar dust is silhouetted against the much larger star assemblage in the background.

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Some 140 million light-years from Earth, the silhouetting reveals the location of dust clouds in the smaller galaxy and the amount of light they absorb, according to Hubble researchers.

As the spiral arms of the closer galaxy extend outward, they seem to change from dark to bright as their backdrop changes from the second galaxy to deep space.

Near the center of the first galaxy, interstellar dust and gas have generated bright blue stars in a pinwheel shape. Interstellar dust only accompanies such bright young stars in many galaxies. But the foreground one contains additional dark material sections unassociated with such stars, Hubble astronomers said.

The galactic alignment provided scientists with a chance to measure more interstellar dust than in the past, helping debunk a traditional scientific view on the makeup of galaxies, said William Keel, the principal investigator for the project.

In the past astronomers thought that galaxies must have large amounts of hidden dark matter because the visible matter could not account for the mass needed to hold them together.

But the Hubble image supports a newer idea: Galaxies have enough dust to hide the light of half or more of their stars. This circumstance "means galaxies need less dark matter for the total mass for stars to hold their orbits, said Keel, a University of Alabama physicist.

Keel and other Hubble Heritage Team researchers composed the montage of the galaxy pair, NGC3314, from images taken by the Hubble orbiter in April 1999 and March 2000.

The more recent observations recorded a bright flash not seen in the earlier ones. The Hubble team suspects it was a strong nova or supernova. Click here to see a brief animated movie starring the bright flash.

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