The Millennium Galaxy Catalogue: The luminosity functions of bulges and discs and their implied stellar mass densities

Authors: Simon P. Driver, Paul D. Allen, Jochen Liske, Alister W. Graham

Abstract: We derive the luminosity functions of elliptical galaxies, galaxy bulges, galaxy pseudo-bulges and galaxy discs from our structural catalogue of 10,095 galaxies. In addition we compute their associated luminosity and stellar mass densities. We show that spheroidal systems (elliptical galaxies and the bulges of disc galax- ies) exhibit a strong color bimodality indicating two distinct types of spheroid which are separated by a core color of (u - r) ~ 2 mag. We argue that the similarity of the red elliptical and the red bulge luminosity functions supports our previous arguments that they share a common origin and surprisingly find that the same follows for the blue ellipticals and blue bulges, tha latter which we refer to as pseudo-bulges. In terms of the stellar mass budget we find that 57 per cent is currently in the form of discs, 39 per cent in the form of red spheroids (12 per cent ellipticals, 27 per cent bulges) and the remainder is in the form of blue spheroidal systems (2 per cent blue ellipticals and 2 per cent pseudo-bulges). In terms of galaxy formation we argue that our data on galaxy components strongly supports the notion of a two-stage formation process (spheroid first, disc later) but with the additional complexity of secular evolution occurring in quiescent discs giving rise to two distinct bulge types: genuine classical spheroids and pseudo-bulges. We therefore advocate that there are three significant structures underpinning galaxy evolution: classical bulges (old); pseudo-bulges (young) and discs (intermediate). The luminous nearby galaxy population is a mixture of these three structural types. The nature of the blue elliptical galaxies remains unclear but one possibility is that these constitute recently collapsed structures supporting the notion of mass-dependent spheroid formation with redshift.