The NASA/ESA Hubble Space Telescope has returned to operation and, as if to prove the case, has taken a snapshot of the fascinating galaxy pair Arp 147. The science operations were resumed on October 25 , 2008, four weeks after a problem with the science data formatter prompted the spacecraft to enter its safe mode.
Just two days after the orbiting observatory was brought back online, Hubble aimed its prime working camera, the Wide Field Planetary Camera 2 (WFPC2), at the particularly intriguing target that’s comprised of a pair of gravitationally interacting galaxies named Arp 147. The image demo’d that the camera is working exactly as it was before going offline — it scored a ‘perfect 10’ for performance and beauty. The chance alignment of the two galaxies certainly ensured a perfect performance. The left-most galaxy is relatively undisturbed, apart from a smooth ring of starlight. The galaxy appears nearly edge-on to the camera’s line of sight. The right-most galaxy exhibits a clumpy, blue ring of intense star formation. The blue ring was formed after the galaxy on the left met and then passed through the galaxy on the right. Just as a pebble thrown into a pond creates an outwardly moving circular wave, or ripples, an outwardly propagating ring of higher density was generated at the point of impact of the two galaxies. As this excess density collided with outer material that was moving inwards due to the gravitational pull 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 marks the location of the original nucleus of the galaxy that was hit.
Arp 147 appears in the Arp Atlas of Peculiar Galaxies, compiled by Halton Arp in the 1960s and published in 1966. This picture was assembled from WFPC2 images taken with three separate filters. The colors blue, green, and red represent the blue, visible-light, and infrared filters respectively. The galaxy pair was photographed on October 27-28,2008. Arp 147 lies in the constellation of Cetus, more than 400 million light-years away from Earth.
