Let me introduce… The names of galaxies

The names of astronomical objects, and therefore of galaxies, are generally composed of letters and numbers, only the most renowned of them have a proper noun. The letters refer to the catalogues in which they are listed, while the numbers indicate the object’s entry in the catalogue. This is why a galaxy can have multiple names, for example, the Andromeda Galaxy is also known as M31 or NGC 224.
Some of the most common catalogues are:

  • M (Messier): A catalogue compiled by Charles Messier and several colleagues in the eighteenth century. In this catalogue there are many of the brightest and most remarkable objects, including nebulae and star clusters.
  • NGC/IC (New General Catalogue) / (Index Catalogue): The catalogue, compiled by JLE Dreyer from the 1860s-1880s, includes —in addition to star clusters and nebulae— about 10,000 of the most important galaxies and the first collection of astronomical photographs. Until recently, almost all the known galaxies belonged to this catalogue.
  • Arp: In 1966, Halton Arp  published the Atlas of Peculiar Galaxies, which contains 338 galaxies in total. The main purpose of the catalogue was to present photographically examples of different types of unusual galaxies’ structures. It was therefore a tool to facilitate the work of understanding what determines the form of elliptical or spiral galaxies.
  • UGC (Uppsala General Catalogue): It contains data for 12,921 galaxies north of declination = -2° 30’. The catalogue was published in 1973 by Peter Nilsson, classifying objects by location, size, orientation, and magnitude from Palomar Sky Survey photographs.

Other names, instead, refer to a survey name and the object’s coordinates. The digits, therefore, indicate the right ascension and declination (RA+/-DEC) or (α+/-δ) —either for epoch 1950 or 2000.
In this case some of the most common catalogues are: 

  • PKS: Radio sources from the Parkes radio telescope (i.e. PKS 0521-36).
  • IRAS: Infrared Astronomical Satellite (i.e. IRAS 09104+4109).

All the other data (whether they are numbers, letters, initials or abbreviation) are explained in the introductory part of each catalogue.
You can find the catalogue listing the beautiful Galactic Rose here.

Contacted by Shannon
Image Credit: NASA, ESA, and the Hubble Heritage Team.