This is an image of a rarely photographed couple of galaxies, NGC 4618 and NGC 4625 in Canes Venatici, collectively known as Arp 23 in the Arp Catalogue of Peculiar Galaxies. The trio is formed with the very small, hardly visible NGC 4625A situated SE from NGC 4625, NGC 4625A being a low surface brightness galaxy (looking like a faint smudge close to NGC 4625).

The two brighter galaxies have an interesting story behind them and I realized that, serendipitously, I have pointed my telescope towards a very interesting couple. NGC 4625 and NGC 4618 both are single arm spirals. Single arm spirals are rare (and it was the reason for the inclusion in the Arp catalogue). Until 2004, it was thought that NGC 4625 was a dwarf galaxy, but UV data from the GALEX (Galaxy Evolution Explorer ) telescope showed that it actually has spiral arms in UV light that are 4 times more extended than the size of the optical disk. These arms were considered absent before this data, since they were not present in the images taken before,  in the visible spectrum. So the galaxy was considered to be limited to the smaller bright core.

This UV emission is a sign of recent star formation and offers an opportunity to study the physics of star formation in conditions similar to those present when our own galaxy was appearing (A. Gil de Paz et al., The Astrophysical Journal 2005). The hypothesis was that NGC 4618 interacted gravitationally with NGC 4625 and "stole" one of its arms. This does not explain how come NGC 4618 has one arm too. A possible interaction with NGC 4625A was also advanced. The interaction took place probably in the past since there is a 10 million l.y. distance between the two bright ones, 4618 and 4625.

I could only find one image on the Internet showing the faint, blue arms of NGC 4625 in visible light, by Martin Winder and processed by Dietmar Hager. The other image of these arms is in false color, based on the UV data produced by GALEX.