THE NUMBER, LUMINOSITY AND MASS DENSITY OF SPIRAL GALAXIES AS A FUNCTION OF SURFACE BRIGHTNESS

I present analytic expressions for the relative number, luminosity and mass density of disc galaxies as a function of surface brightness. Th ese surface brightness distributions are asymmetric, with long tails t o lower surface brightnesses. This asymmetry induces systematic errors in most determinations of the galaxy luminosity function. Galaxies of low surface brightness exist in large numbers, but the additional con tribution to the integrated luminosity density is modest, probably 10- 30 per cent.