The 2dF Galaxy Redshift Survey: Galaxy luminosity functions per spectral type

We calculate the optical bJ luminosity function (LF) of the 2dF Galaxy Redshift Survey (2dFGRS) for different subsets defined by their spectral properties. These spectrally selected subsets are defined using a new parameter, η, which is a linear combination of the first two projections derived from a Principal Component Analysis. This parameter η identifies the average emission-and absorption-line strength in the galaxy rest frame spectrum, and hence is a useful indicator of the present star formation. We use a total of 75000 galaxies in our calculations, chosen from a sample of high signal-to-noise ratio, low-redshift galaxies observed before 2001 January. We find that there is a systematic steepening of the faint-end slope (α) as one moves from passive (α = -0.54) to active (α = -1.50) star-forming galaxies, and that there is also a corresponding faintening of the rest frame characteristic magnitude M* - 5 log10(h) (from -19.6 to -19.2). We also show that the Schechter function provides a poor fit to the quiescent (Type 1) LF for very faint galaxies [MbJ -5 log10(h) fainter than -16.0], perhaps suggesting the presence of a significant dwarf population. The LFs presented here give a precise confirmation of the trends seen previously in a much smaller preliminary 2dFGRS sample, and in other surveys. We also present a new procedure for determining self-consistent k-corrections, and investigate possible fibre-aperture biases.