The baryonic mass function of galaxies

Abstract

In the Big Bang about 5% of the mass that was created was in the form of normal baryonic matter (neutrons and protons). Of this about 10% ended up in galaxies in the form of stars or of gas (that can be in molecules, can be atomic, or can be ionised). In this work, we measure the baryonic mass function of galaxies, which describes how the baryonic mass is distributed {\it within} galaxies of different types (e.g. spiral or elliptical) and of different sizes. This can provide useful constraints on our current cosmology, convolved with our understanding of how galaxies form. This work relies on various large astronomical surveys, for example the optical Sloan Digital Sky Survey (to observe stars) and the HIPASS radio survey (to observe atomic gas). We then perform an integral over our mass function to determine the cosmological density in baryons in galaxies: $Ω_{\rm b, gal} = 0.0035$. Most of these baryons are in stars: $Ω_{*} = 0.0028$. Only about 20% are in gas. The error on the quantities, as determined from the range obtained between different methods, is $\sim 10$%; systematic errors may be much larger. Most ($\sim 90$%) of the baryons in the Universe are not in galaxies. They probably exist in a warm/hot intergalactic medium. Searching for direct observational evidence and deeper theoretical understanding for this will form one of the major challenges for astronomy in the next decade.