Hubble Zooms in on Unusual NGC 922

An international team of astronomers using the Wide Field Camera 3 and the Wide Field and Planetary Camera 2 aboard NASA’s Hubble Space Telescope has taken the most detailed image so far of the spiral galaxy NGC 922.

The barred spiral galaxy NGC 922 is located in the southern constellation of Fornax at a distance of about 150 million light years.

The new Hubble picture reveals that the NGC 922’s spiral arms are disrupted, a stream of stars extends out towards the top of the image, and a bright ring of nebulae encircles the core.

The galaxy’s current unusual form is a result of a cosmic bullseye some 330 million years ago.

A smaller galaxy, labeled 2MASXI J0224301-244443, plunged right through the heart of NGC 922 and shot out the other side. In wide-field views of the galaxy, the small interloper can be still be seen shooting away from the scene of the crash.

As the small galaxy passed through the middle of NGC 922, it set up ripples that disrupted the clouds of gas, and triggered the formation of new stars whose radiation then lit up the remaining gas. The bright pink color of the resulting nebulae is a characteristic sign of this process, and it is caused by excited hydrogen gas. This process of excitation and emission of light by gases is similar to that in neon signs.

In theory, if two galaxies are aligned just right, with the small one passing through the center of the larger one, the ring of nebulae should form a perfect circle, but more often the two galaxies are slightly off kilter, leading to a circle that, like this one, is noticeably brighter on one side than the other.

These objects, called collisional ring galaxies, are relatively rare in our cosmic neighborhood. Although galaxy collisions and mergers are commonplace, the precise alignment and ratio of sizes necessary to form a ring like this is not, and the ring-like phenomenon is also thought to be relatively short-lived.