The galaxy alphabet

Astronomers have written a lot about galaxies over the past few centuries .... a LOT. We've created innumerable names and terms and acronyms to describe and classify and sort galaxies into useful groups.

Can I come up with at least one item of galaxy jargon starting with each letter of the alphabet? As a challenge, I won't look anything up in books or websites, but try only to remember things stuffed somewhere deep in my brain.

Here we go!

A

AGN stands for Active Galactic Nucleus (or Nuclei), a phenomenon caused by the accretion of gas into a supermassive black hole at the center of a galaxy.

E+A galaxies have shapes like ellipticals (the "E"), but spectra showing strong absorption lines in the Balmer series (like "A" stars); a peculiar combination of young stars in an elliptical.

B

Bar A linear feature running through the center of some spiral galaxies, from which spiral arms extend.

B125823+088509 If the first letter in a galaxy's position, given as a series of numbers for Right Ascension and then Declination, is "B", then the position is given for the equinox 1950.0 (aka B1950). If the first letter is "J", then the coordinates are those for the equinox 2000.0 (aka J2000).

C

c stands for "concentration index", a measure of the distribution of light in a galaxy.

cD galaxies are the giant ellipticals sometimes found near the centers of rich galaxy clusters, showing extended envelopes of light.

3C is the "Third Cambridge Catalog of Radio Sources," which turned out to be rich in active galaxies. Many quasars are known by their 3C names, such as 3C48 and 3C273.

D

cD (see entry above)

disk is the flattened system of gas and dust in spiral galaxies. Most stars form in this disk, and therefore move in circular orbits in the plane when they are young. As they age, their orbits grow less circular and somewhat inclined to the plane, forming a thick disk.

E

E stands for "Elliptical", one of the two major classes of galaxy.

F

G

H

Hyperleda is one of the largest recent galaxy catalogs.

I

I stands for "Irregular", meaning a galaxy which lacks a coherent structure (neither spiral nor elliptical). The Magellanic Clouds are sometimes described as Irregulars.

IC is the Index Catalog, a supplement to the New General Catalog (see entry for NGC).

IRAS is used as the base of names for galaxies detected by the IRAS infrared satellite.

J

J125823+088509 If the first letter in a galaxy's position, given as a series of numbers for Right Ascension and then Declination, is "J", then the position is given for the equinox 2000.0 (aka J2000). If the first letter is "B", then the coordinates are those for the equinox 1950.0 (aka B1950).

K

K-corrections are changes in the colors of distant galaxies due to redshift.

L

L is used to denote the luminosity of galaxies. In particular, L_* is the value of luminosity at the knee of the galaxy luminosity function -- the point at which the number of galaxies starts to drop precipitously.

M

M is used to denote the mass of galaxies. It often has subscripts to denote the mass of particular components, such as M_* to mean "the mass of the stellar component."

MCG is the Morphological Catalog of Galaxies, based on visual inspection of the Palomar Sky Survey plates by astronomers in the Soviet Union.

N

N-type galaxies are spirals with very bright, star-like nuclei. This term isn't used much any more; instead, N galaxies are simply stuffed into the AGN category.

NGC is the New General Catalog, one of the first large catalogs of non-stellar objects -- including many galaxies. Compiled by John Louis Dreyer in the late nineteenth century, many of the objects were first noticed and described by William Herschel.

O

P

PCG is the Palomar Compact Galaxies catalogue, a collection of small groups of galaxies which appear on Palomar Sky Survey images.

PGC is the Principal Galaxies Catalogue, a catalog of over 70,000 galaxies first published in 1988 (and since updated).

Q

Quasar stands for "quasi-stellar radio source". Some active galaxies were first discovered as strong radio sources. When their optical counterparts appeared at first to be stars, astronomers were puzzled -- how could stars emit so much energy in the radio? We've since realized that the "stars" are really just the very luminous nuclei at the center of active galaxies, outshining everything else.

R

S

S is for Spiral, one of the two main classes of galaxies. It is often followed by other letters to denote a particular sub-class, such as Sa or SBc.

Seyfert galaxies are named after Carl Seyfert, who wrote a classic paper (ApJ 97, 28, 1943) describing peculiar galaxies with very bright, star-like nuclei. These, like N galaxies, are now a sub-group of AGN.

T

T is an index used to describe quantitatively the position of a galaxy in along the tuning-fork sequence. The range runs from T = -5 for ellipticals of type E0, past T = 0 at the change to spirals, to T = 6 at type Sc and T = 10 for irregulars.

U

U is one of the trio of letters (UVW) used to describe the motions of stars in our Milky Way Galaxy. The positive "U" coordinate runs from our Sun toward the center of the Milky Way.

UGC is the Uppsala Galaxy Catalog, published in 1973, based on visual inspection of the Palomar Sky Survey photographic plates by astronomers in Sweden.

V

V is one of the trio of letters (UVW) used to describe the motions of stars in our Milky Way Galaxy. The positive "V" coordinate runs tangential to the center of the Milky Way, in the direction of rotation of the disk.

W

W is one of the trio of letters (UVW) used to describe the motions of stars in our Milky Way Galaxy. The positive "W" coordinate runs positive toward the North Galactic Pole.

X

X is one of the trio of letters (XYZ) used to describe the positions of stars in our Milky Way Galaxy. The positive "X" coordinate runs from our Sun toward the center of the Milky Way.

Y

Y is one of the trio of letters (XYZ) used to describe the positions of stars in our Milky Way Galaxy. The positive "Y" coordinate runs tangential to the center of the Milky Way, in the direction of rotation of the disk.

Z

Z is one of the trio of letters (XYZ) used to describe the positions of stars in our Milky Way Galaxy. The positive "Z" coordinate runs positive toward the North Galactic Pole.