NGC 7318 (also known as UGC 12099/UGC 12100 or HCG 92d/b) are a pair of collidinggalaxies about 300 million light-years away in the Constellation Pegasus. They are members of the famous Stephan’s Quintet.
NGC 7318B has two optical arms emanating from the eastern part of the main body. Since these arms are similar morphologically to the tidal tails of merging galaxies such as NGC 4038/9, it is considered that NGC 7318B itself is a major merger with a retrograde orbit. In order to study the emission-line activity in the tidal arms of NGC 7318B, scientists took CCD narrow-band (Ha ON and OFF) images and then found a large-scale arc in Ha emission which traces closely the arms.
Credit: NASA, ESA, and the Hubble SM4 ERO Team