Arp 155

Arp 155 (also known as NGC 3656, UGC 6403, VV22a, and PGC 34989) is a peculiar galaxy located at a distance of 135 million light-years in the constellation of Ursa Major. It was discovered on the 14th of April 1789 by William Herschel. In the famous Atlas of Peculiar Galaxies that Halton Arp compiled, Arp 155 is part of the “Interior absorption” class.

Arp 155 is the result of the merger of two galaxies, and based on its distance is nearly 70,000 light-years across. The merging process created shells around Arp 155 and two faint tidal tails.

One of the tidal tails points in a northwest direction towards a dwarf galaxy. The other points to the northeast but is not as well defined as the other. On the southern part of Arp 155, a condensation connected with a ring-like feature is present, and is associated with a bright shell.

In addition, Arp 155 has a dark lane across its minor axis, similar to that observed in Centaurus A. Observations revealed the presence of atomic hydrogen at the centre of Arp 155, with a disk morphology. It extends for 22,800 light-years. On top of that, the rotation axis of the nucleus is perpendicular to the rest of the galaxy. The latter is a feature created by the merging process. 

Moreover, analysis of observational data showed that the current star formation of Arp 155 is 1.1 solar masses. This is similar to the current star-forming rate of our Milky Way (i.e., one solar mass per year).

Finally, two supernovae have been discovered in Arp 155. These are SN 1963K (unknown type) and SN 1973C (unknown type).

Image: Composite optical image of Arp 155. It was created with data from Hubble Space Telescope (606 nm, luminosity) and the Sloan Digital Sky Survey (SDSS) using filters centred at 477 nm (G-band, blue), 623 nm (R-band, green), and 762 nm (I-band, red).  

Image Credit: Hubble/SDSS. Acknowledgement: Solomon Hendrix