Celestial hemisphere:  Northern  ·  Constellation: Pegasus (Peg)
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UGC 12914 & 12915, Gary Imm
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UGC 12914 & 12915

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UGC 12914 & 12915, Gary Imm
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UGC 12914 & 12915

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Description

This object is a pair of colliding galaxies located 180 million light years away in the constellation of Pegasus at a declination of +23 degrees. This object is the unusual case of a head-on collision between two gas-rich spiral galaxies. I am not sure why this object was not included in the Arp catalog.

The larger galaxy on the right is UGC 12914, 2.5 arc-minutes long in our apparent view which translates to a diameter of 130,000 light years. The galaxy on the left is UGC 12915, 1.6 arc-minutes long with a diameter of 80,000 light years.

Both galaxies are disturbed from their collision and have yellow cores, oddly shaped dark dust lanes, star clusters sprinkled throughout, and interesting star streams. The most outstanding feature to me is the bright blue hook-shaped arc of bright star clusters which occurs similarily in both galaxies. The right galaxy is especially interesting - it is hard to tell from our viewing angle how much of the disk has been disturbed out of plane.

This beautiful object has the nickname of the Taffy Galaxies (named after the candy) - don't get me started on this one. I hate both nicknames and taffy. But, in this case the nickname is interesting and appropriate. The "taffy" shape is named after an unusually strong bridge of hydrogen gas emitting huge quantities of radio waves, connecting these two galaxies in their post-collison state. The bridge looks interesting in the papers I have seen, and does look like taffy being pulled. But I am still not a fan of nicknames based on features not seen in visible light (like the War and Peace Nebula). The nicknames don't make sense to 99% of the people who see the object.

Though we can't see the radio bridge, we can see evidence of the collision. From the paper "The 'Taffy' Galaxies UGC 12914/5" by Condon, Helou, Sanders and Soifer - "A unique characteristic of nearly direct face-on collisions is the formation of large-scale rings, and the elliptical ring surrounding UGC 12914 may have been produced in this way. Simple models indicate that ring formation requires a fairly concentrated intruder galaxy (UGC 12915) penetrating a victim galaxy (UGC 12914) of comparable mass with 45 degrees of normal incidence and with an impact parameter less than 15% of the victim's outer radius, conditions all met by the UGC 12914/5 system. If the impact parameter is too large, a tidal hook will be formed instead of a ring; this may explain the hook extending to the northwest of UGC 19215."

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