Galaxy Properties (a thumbnail sketch)

As a fiducial number, let's look at the Milky Way. The Milky Way has

Spiral galaxies

About 3/4 of big galaxies in the field are spirals.

Spiral galaxy scale lengths run from ~ 1 kpc (dwarfs) to ~ 50 kpc (giant LSBs like Malin 1)

They have absolute magnitudes from -16 to -23 (that's a factor of ~ 1000 in luminosity)

Masses from 109 - few x 1012 Msun

So in these terms, the Milky Way is a big spiral galaxy, but not the most extreme.

Elliptical galaxies

About 20% of big field galaxies are ellipticals

Ellipticals have a wide range of properties. There are

E's (normal ellipticals)
cD's (massive bright ellipticals at the centers of galaxy clusters)
dE's (dwarf ellipticals)
dSph's (dwarf spheroidals)
We'll talk about these more down the road...

The size of an elliptical galaxy is characterized not by a scale length, but by its effective radii, the radius which encloses half the light. (For comparison, the effective radius in an exponential disk is 1.7 times the scale length...)

Sizes run from a few tenths of a kpc (dE's) to tens of kpc (cD's)

Absolute magnitudes from -10 (dSph's) to -25 (cD's) (and that's a factor of 106 in luminosity)

Masses go from 107 Msun (dSph's) to 1013 - 1014 Msun (cD's)

In fact, the elliptical sequence is not a continuous sequence physically. cD's, E's, dE's, and dSph's are all structurally, kinematically, and physically different objects. It's not just a question of scale!

Irregular galaxies

Irregulars make up a few percent of the field galaxy population.

Generally (but not always!) smaller galaxies: sizes of a few kpc

Absolute magnitues of -13 to -20

Masses of 108 to 1010 Msun


The Morphology - Density Relationship

Dressler (1980) showed that the galaxy morphology was linked to the environment: spirals prefer low density environments ("field galaxies" or "loose groups"), while ellipticals prefer high density environments ("clusters"):




The Luminosity Function of Galaxies

Define the luminosity function as the number of galaxies per unit luminosity (or magnitude) bin. This has been characterized by the Schecter function:

Rough numbers for the parameters:
  • M* ~ -21 (in B magnitudes)
  • L* ~ 2x1010 Lsun
  • alpha ~ -1 to -1.5 or so



Observed Luminosity Function (SDSS DR 6, All Galaxies):



In the field, the faintest galaxies are typically blue star forming dwarf galaxies. The brightest galaxies tend to be red ellipticals, but big bright spirals exist, too.

Observed Luminosity Function (SDSS DR 6, red galaxies vs blue galaxies):





But there are significant variations of the luminosity function depending on galaxy type and environment -- clusters (top) versus field (bottom). Note the rise of the dwarf ellipticals (dE's) in the cluster environment:



From Helmut Jerjen's Luminosity Function Generator