The Millennium Galaxy Catalogue: The Luminosity Functions of Bulges and Disks and Their Implied Stellar Mass Densities

Abstract

We derive the luminosity functions of elliptical galaxies, galaxy bulges, galaxy pseudobulges, and galaxy disks from our structural catalog of 10,095 galaxies. In addition, we compute their associated luminosity densities and stellar mass densities. We show that spheroidal systems (elliptical galaxies and the bulges of disk galaxies) exhibit a strong color bimodality indicating two distinct types of spheroid that 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, the latter of which we refer to as pseudobulges. In terms of the stellar mass budget we find that 58% ± 6% is currently in the form of disks, 39% ± 6% in the form of red spheroids (13% ± 4% ellipticals, 26% ± 4% bulges), and the remainder is in the form of blue spheroidal systems (~1.5% blue ellipticals and ~1.5% pseudobulges). 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, disk later) but with the additional complexity of secular evolution occurring in quiescent disks, thus giving rise to two distinct bulge types: genuine bulges and pseudobulges. We therefore advocate that there are three significant structures underpinning galaxy evolution: classical spheroids (old), pseudobulges (young), and disks (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.