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Interacting Ring-shaped Galaxies

06 November 2008

Credit: NASA, ESA, and M. Livio (STScI)

The Hubble Space telescope (HST) is back in operation, and it aimed its Wide Field Planetary Camera 2 (WFPC2) at a bizarre pair of gravitationally interacting galaxies, known as Arp 147. For about 4 weeks, HST was offline, in a safe protective mode, due to a malfunction that occurred in late September. The image was taken on 27 October, a couple of days after the Hubble repair, and it demonstrates that the WFPC2 is operating normally.

The galaxy to the left is relatively undisturbed, showing only a smooth ring of starlight. The plane of this galaxy is very near to the line of sight. Therefore, it is said to be edge-on. The galaxy to the right, resembling a zero, exhibits a clumpy, blue ring of active star formation.

The blue ring is believed to have formed after the edge-on galaxy passed through the ring-shaped galaxy. Just as ripples form when a pebble is thrown into a pond, a propagating density wave was generated at the point of impact and spread outward. When this density wave hit material in the target galaxy that was moving inward, due to the gravitational tug of the two galaxies, shocks and dense gas were produced, stimulating star formation.

The dusty reddish knot at the lower left of the blue ring probably highlights the location of the original nucleus of the target galaxy.

Arp 147 is the 147th entry of the Arp Atlas of Peculiar Galaxies, compiled by astronomer Halton Arp. This picture was produced from WFPC2 images, taken in three different filters. The blue, visible-light, and infrared filters are represented by the colors blue, green, and red, respectively. Arp 147 lies in the constellation Cetus, the Sea Monster, and it is over 400 million light-years distant.

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