The Millennium Galaxy Catalogue: a census of local compact galaxies

Abstract

We use the Millennium Galaxy Catalogue (MGC) to study the effect of compact galaxies on the local field galaxy luminosity function. Here we observationally define as `compact' galaxies that are too small to be reliably distinguished from stars using a standard star-galaxy separation technique. In particular, we estimate the fraction of galaxies that are misclassified as stars due to their compactness. We have spectroscopically identified all objects to B_MGC = 20 mag in a 1.14 deg^2 sub-region of the MGC, regardless of morphology. From these data we develop a model of the high surface brightness incompleteness and estimate that ~1 per cent of galaxies with B_MGC < 20 mag are misclassified as stars, with an upper limit of 2.3 per cent at 95 per cent confidence. However, since the missing galaxies are preferentially sub-L* their effect on the faint end of the luminosity function is substantially amplified: we find that they contribute ~6 per cent to the total luminosity function in the range -17 < M_B < -14 mag, which raises the faint end slope alpha by 0.03^{+0.02}_{-0.01}. Their contribution to the total B-band luminosity density is ~2 per cent. Roughly half of the missing galaxies have already been recovered through spectroscopy of morphologically stellar targets selected mainly by colour. We find that the missing galaxies mostly consist of intrinsically small, blue, star-forming, sub-L* objects. In combination with the recent results of Driver et al. (2005) we have now demonstrated that the MGC is free from both high and low surface brightness selection bias for giant galaxies (M_B <~ -17 mag). Dwarf galaxies, on the other hand, are significantly affected by these selection effects. To gain a complete view of the dwarf population will require both deeper and higher resolution surveys.