1 : Preliminaries   6 :   Dynamics I 11 : Star Formation  16 : Cosmology
2 : Morphology   7 :   Ellipticals 12 : Interactions 17 : Structure Growth 
3 : Surveys 8 :   Dynamics II 13 : Groups & Clusters  18 : Galaxy Formation 
4 : Lum. Functions  9 :   Gas & Dust   14 : Nuclei & BHs 19 : Reionization & IGM  
5 : Spirals 10 : Populations    15 : AGNs & Quasars 20 : Dark Matter





(1) Introduction

Galaxies come in a huge range of luminosity and mass : ~106 (MB -7.5 to -22.5).

Look at any galaxy cluster, and you see a wide range of galaxy luminosities [image]
The Luminosity Function specifies the relative number of galaxies at each luminosity.

The Luminosity function contains information about :

Although this information is (badly) convolved, nevertheless :


(2) Brief History


(3) The Schechter Function

In 1974 Press and Schechter calculated the mass distribution of clumps emerging from the young universe, and in 1976 Paul Schechter applied this function to fit the luminosity distribution of galaxies in Abell clusters [image]. The fit turned out to be excellent, though the reasons why are still not well understood (see sec 7).


(4) Methods of Evaluating Luminosity Functions

Cluster and field samples require quite different approaches:

(a) Cluster Samples

Since all cluster galaxies are at the same distance: Complications arise principally from trying to eliminate fore/back-ground field galaxy contamination:

(b) Field Samples

In general, deriving LFs for the field is more difficult than for clusters:
Many methods have been developed, here is the simplest:


(5) Different LFs for Different Hubble Types

Early work showed :

Recently, things are becoming clearer :

More specifically, broken down by type, we have the following LFs :

LFs for the Field and Virgo are illustrated here: [image].
Clearly, full sample LFs :


(6) Different LFs for Different Environments

It seems the LFs of galaxies in clusters can be different from galaxies in the field.
In general, cluster LFs :

We can now understand much of this :

Analysis of the SDSS shows similar results, but cast in terms of the red and blue sequences [image]

See Topic 13 § 7 for a discussion of the physical origin of the morphology-density relation.


(7) Physical Origin of the Luminosity Function

Why does the galaxy luminosity function have the form that it does?
A complete understanding of this is not yet possible, but here are the ingredients:
Making galaxies involves at least two things Here is a very brief account: