The paper presents the most accurate estimate to date of the abundance of galaxies of different luminosities. This "luminosity function" is the most basic statistic characterizing the galaxy population. Our result was derived from the 2dF Galaxy Redshift Survey (2dFGRS). This is currently the largest redshift survey ever completed, with more than 220,000 unique galaxy redshifts. For the first time, we were able to pin down the form and shape of the luminosity function, for both faint and bright galaxies, with random errors that are smaller than the systematic errors, over more than three orders of magnitude in luminosity. This is well beyond what any previous survey had been able to achieve. We also presented the "selection function" of the 2dFGRS, which is an essential input into measurements of the clustering of the galaxies in the survey.

The luminosity function of galaxies is the most basic descriptor of the galaxy population. An accurate estimate of it is of fundamental importance for evolutionary studies of galaxies and is a primary constraint on models of galaxy formation. The characterization of the angular and radial selection function of a survey is also essential for robust measurements of galaxy clustering.

The luminosity function provides a census of the local galaxy population. It tells us how many galaxies of a given brightness there are per unit volume in the local universe. Our estimates show that for every galaxy like the Milky Way, there are many more fainter galaxies and relatively fewer brighter ones. To quantify exactly how many requires measuring many thousands of galaxy distances and these are estimated from their redshift. The "2dF Galaxy Redshift Survey," an Anglo-Australian collaboration, has recently measured 220,000 galaxy redshifts. It is the largest complete survey of the galaxy population ever undertaken. Explaining the luminosity function is a rite of passage for theoretical models of galaxy formation, since it encodes the physical processes that determine how galaxies are made. Do the models predict the right number of faint galaxies? Do they reproduce the dramatic decline in abundance of galaxies with increasing luminosity? In this paper we succeeded in making the most precise measurement to date of this fundamental property of the local galaxy population.

I became a member of the 2dFGRS team upon starting my Ph.D. at the University of Durham (UK), and thereafter became involved in the statistical analysis of the survey. The research presented in this paper is part of my Ph.D. thesis, which also includes several clustering analyses of the 2dFGRS. Naturally all this research would not have been possible without the very close collaboration we have within the 2dFGRS Team and the efforts of the staff at the Anglo-Australian Telescope and the builders of the 2dF spectrograph.

Dr. Peder Norberg, Zwicky Fellow
Institut für Astronomie 
Hönggerberg Campus
ETH Zürich
Zürich, Switzerland