ARP GALAXY PROGRAM
In 1966 Dr. Halton C. Arp cataloged 338 galaxies in his Atlas of Peculiar Galaxies from photographs from the 200-inch Palomar telescope. The entrants contain peculiar galaxies, interacting pairs and larger groups. A peculiar galaxy is an irregular galaxy that has an abnormal shape (neither elliptical, spiral, nor lenticular) and/or has another unusual characteristic, like jets of gas spewing from the nucleus, unusual amounts of dust, low surface brightness, etc. They are probably formed as galaxies collide or have companion galaxies that influence them.
This program contains 72 galaxies from the Arp catalog having a mean surface brightness less than 24 (less than 23.9 Mag/arc-second^2). The definition of surface brightness is the measure of an object's brightness per unit area. Typically surface brightness is expressed as stellar magnitudes per square arc-second.