The 2dFGRS Percolation-Inferred Galaxy Group ( 2PIGG ) catalogue of \sim 29000 objects is used to study the luminous content of galaxy systems of various sizes .
Mock galaxy catalogues constructed from cosmological simulations are used to gauge the accuracy with which intrinsic group properties can be recovered .
It is found that a Schechter function is a reasonable fit to the galaxy luminosity functions in groups of different mass in the real data , and that the characteristic luminosity L _ { * } is larger for more massive groups .
However , the mock data show that the shape of the recovered luminosity function is expected to differ from the true shape , and this must be allowed for when interpreting the data .
Luminosity function results are presented in both the b _ { J } and r _ { F } wavebands .
The variation of halo mass-to-light ratio , \Upsilon , with group size is studied in both these wavebands .
A robust trend of increasing \Upsilon with increasing group luminosity is found in the 2PIGG data .
Going from groups with b _ { J } luminosities equal to 10 ^ { 10 } { \it h } ^ { -2 } { L _ { \odot } } to those 100 times more luminous , the typical b _ { J } -band mass-to-light ratio increases by a factor of 5 , whereas the r _ { F } -band mass-to-light ratio grows by a factor of 3.5 .
These trends agree well with the predictions of the simulations which also predict a minimum in the mass-to-light ratio on a scale roughly corresponding to the Local Group .
Our data indicate that if such a minimum exists , then it must occur at L \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 10 % ^ { 10 } { \it h } ^ { -2 } { L _ { \odot } } , below the range accurately probed by the 2PIGG catalogue .
According to the mock data , the b _ { J } mass-to-light ratios of the largest groups are expected to be approximately 1.1 times the global value .
Assuming that this correction applies to the real data , the mean b _ { J } luminosity density of the Universe yields an estimate of \Omega _ { m } = 0.26 \pm 0.03 .
Various possible sources of systematic error are considered , with the conclusion that these can affect the estimate of \Omega _ { m } by up to 20 % .