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Monday, 11 February, 2002, 09:05 GMT
Spiral galaxy winds up astronomers
Galaxy NGC 4622
Spiral arms could be due to a galactic collision
By BBC News Online science editor Dr David Whitehouse

The beautiful but strange galaxy NGC 4622 is confounding astronomers as it appears to break all the rules about how galaxies should rotate.

All so-called spiral galaxies seem to rotate in such a way that the spiral arms are winding up, even though their galactic arms do not become crowded together because stars move in and out of them all the time.

This is the density wave theory that explains why the often beautiful stellar arms of a spiral galaxy are long-lived features and do not become smeared out.

But the galaxy called NGC 4622 appears to be rotating in the opposite direction to that expected throwing astronomers into confusion.

Clockwise galaxy

NGC 4622 resides 111 million light-years away in the constellation Centaurus.

The new pictures were taken in May 2001 with the Hubble Space Telescope's Wide Field Planetary Camera 2.

NGC 4622 puzzles astronomers because the inner part of the galaxy suggests a clockwise rotation. But according to the directions of the galaxy's outer spiral arms the rotation should be anti-clockwise.

A recent analysis of the galaxy, presented at an astronomical conference last month, concluded that: "This unexpected result requires a completely new theoretical explanation."

Merger of galaxies

Astronomers hope that they can explain the strange galaxy's spiral arms as the result of a galactic collision.

They suspect that in the past NGC 4622 has interacted with another galaxy - its two lopsided outer arms suggesting that something disturbed it.

The Hubble image suggests that NGC 4622 consumed a small companion galaxy.

Observations of the galaxy's core does provide some indication that a merger did take place.

Researchers stress that these observations need checking and that if the galactic merger hypothesis does not stand up then we may have to have a fundamental rethink about galaxies.

See also:

24 Jul 01 | Sci/Tech
Stellar winds shake distant star
17 Oct 00 | Sci/Tech
Heart of the Milky Way
28 Jun 99 | Sci/Tech
Gemini images rival space telescope
15 Feb 02 | Sci/Tech
Telescope snaps 'perfect spiral'
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