The 2dF Galaxy Redshift Survey: the number and luminosity density of galaxies

We present the bivariate brightness distribution (BBD) for the 2dF Galaxy R edshift Survey (2dFGRS) based on a preliminary subsample of 45 000 galaxies . The BBD is an extension of the galaxy luminosity function, incorporating surface brightness information. It allows the measurement of the local lumi nosity density, j(B), and of the galaxy luminosity and surface brightness d istributions, while accounting for surface brightness selection biases. The recovered 2dFGRS BBD shows a strong luminosity-surface brightness relat ion [M(B)proportional to (2.4-(+1.5)(0.5))mu (e)], providing a new constrai nt for galaxy formation models. In terms of the number density, we find tha t the peak of the galaxy population lies at M(B)greater than or equal to -1 6.0 mag. Within the well-defined selection limits (-24 <M-B< -16.0 mag, 18. 0 < mu (e)< 24.5 mag arcsec(-2)) the contribution towards the luminosity de nsity is dominated by conventional giant galaxies (i.e., 90 per cent of the luminosity density is contained within -22.5 <M < -17.5, 18.0 < mu (e)< 23 .0). The luminosity-density peak lies away from the selection boundaries, i mplying that the 2dFGRS is complete in terms of sampling the local luminosi ty density, and that luminous low surface brightness galaxies are rare. The final value we derive for the local luminosity density, inclusive of surfa ce brightness corrections, is j(B)=2.49+/-0.20x10(8) h(100) L. Mpc(-3). Rep resentative Schechter function parameters are M* = -19.75+/-0.05, phi* = 2. 02+/-0.02x10(-2) and alpha = -1.09+/-0.03. Finally, we note that extending the conventional methodology to incorporate surface brightness selection ef fects has resulted in an increase in the luminosity density of similar to 3 7 per cent. Hence surface brightness selection effects would appear to expl ain much of the discrepancy between previous estimates of the local luminos ity density.