Aug 2012: Where did all the baryons go?
Graduate student, Manolis Papastergis, writes about his recent paper:
"A direct measurement of the baryonic mass function of galaxies & implications for the galactic baryon fraction" by Manolis Papastergis, Andrea Cattaneo, Shan Huang, Riccardo Giovanelli, Martha P. Haynes
We know that the cosmological ratio of "regular" baryonic matter to dark matter in the universe is about 1/5. Calculating this same ratio ('baryon fraction') in present day galaxies is much more challenging, but holds important clues on how galaxies form and evolve. Stellar mass is one way to measure a galaxy's baryon content. Low mass, dwarf galaxies typically have stellar-to-total mass ratios of 1/500. Is this because low-mass halos are truly very bad at retaining baryonic material over cosmic time, or are they extremely inefficient at turning their gas into stars?
In this article (Papastergis+ 2012) we aim to measure the baryonic content of galaxies, including both their stellar and their atomic gas components to answer this. Using optical data from the Sloan Digital Sky Survey (SDSS) and 21cm emission-line data from Cornell's ALFALFA survey, and combining this with the distribution of dark matter halo masses inferred from N-body simulations, we can obtain an average relation between the galactic baryon fraction and the host halo mass.
The figure shows the result: that atomic gas becomes more and more dominant as we move to smaller halos, but overall low-mass halos become worse and worse at retaining their share of baryonic material (the gold line takes into account only the stars in galaxies while the gray shaded area includes the atomic gas, as well).
This result raises the interesting question - how was the baryon deficit created? The usual explanation, blowout of gas by supernova explosions, doesn't seem to work for the lowest-mass galaxies: they don't have a big enough stellar population to expel the necessary amount of gas without astonishingly efficient supernova blowout or galactic outflows surpassing the most extreme observational cases known.