Jim's Corner Blog

Introducing the Arp Atlas of Peculiar Galaxies

In 1966, astronomer Halton Arp published a catalog of 338 galaxies entitled Atlas of Peculiar Galaxies. The main goal of the catalog was to present astro-photographs of many different kinds of peculiar structures of galaxies found among the various sky surveys. The reason why galaxies formed into spiral or elliptical shapes was not well understood, and he hoped that by publishing the atlas would stimulate the astronomy community into further study. He perceived peculiar galaxies as small “experiments” that astronomers could use to understand the physical processes that distort spiral or elliptical galaxies. With this atlas, astronomers had a focus group of peculiar galaxies that could be studied in detail. The atlas is a sampling of peculiar galaxies in the sky, each providing examples of the different phenomena as observed in galaxies.

Dr. Halton Arp received his Bachelors degree from Harvard College in 1949 and his Ph.D. From the California Institute of Technology in 1953. For 28 years he was staff astronomer at both the Mt. Palomar and Mt. Wilson observatories. It was during his tenure that he produced Atlas of Peculiar Galaxies. Arp is also famous for questioning the validity of Doppler redshift as a sole determinator for
extreme distances and that the assumption that high red shift objects have to be very far away, upon which the Big Bang theory and all current cosmology thinking is based.

The peculiar galaxies in the atlas are sorted based on their appearance, because little was known in 1966 about the physical processes that caused the various shapes. The atlas was published with a rational order:

Objects 1–101 are individual peculiar spiral galaxies or spiral galaxies that apparently have small companions.
Objects 102–145 are elliptical and elliptical-like galaxies.
Objects 146–268 are individual or groups of galaxies with neither elliptical nor spiral shapes.
Objects 269–327 are double galaxies.
Objects 332–338 are galaxies that simply do not fit into any of the above categories.

Most of the peculiar galaxies are best known by their Messier, NGC, IC, or other designations, with only a handful of galaxies identifiable by their Arp numbers. Some Arp catalog objects are well known: for example M82 the Cigar Galaxy is Arp 337, and M51 the Whirlpool galaxy is Arp 85.