Arp 4
Arp 4 is an irregular galaxy in the constellation of Cetus. Its velocity with respect to the cosmic microwave background is, which corresponds to a Hubble distance of. Additionally, one non-redshift measurement gives a similar distance estimate of. It was discovered by Dutch-Canadian astronomer Sidney van den Bergh in 1959, who listed it in the David Dunlap Observatory Catalogue as dwarf galaxy DDO 14.
Arp 4 is the fourth object in Halton Arp's Atlas of Peculiar Galaxies, and one of six Arp objects in the 'Low Surface Brightness Galaxies' section. The other five low surface brightness galaxies are Arp 1, Arp 2, Arp 3, Arp 5, and Arp 6.
Arp 4 has a possible active galactic nucleus, i.e. it has a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars.
Arp 4 and the galaxy PGC 6629 are visually very close to each other, but the alignment is purely optical, as the Hubble distance to PGC 6629 is, which is about 10 times further away than Arp 4.