Vol. 626
In section 4. Extragalactic astronomy

Peak star formation efficiency and no missing baryons in massive spirals

by L. Posti, F. Fraternali, and A. Marasco 2019, A&A, 626, A56 alt

We know that the fraction of baryons in the total matter of the Universe is 17%. But what is the proportion of baryons and dark matter in galaxies? Numerical simulations provide the mass function of dark matter halos, and observations provide the luminosity function of galaxies. Through abundance matching, it is possible to associate the most-massive dark halos to the most luminous galaxies, and progressively following down the mass and luminosity distribution, to associate galaxies to halos. It results that only a small fraction of cosmic baryons are in galaxies, most of the baryons are in cosmic filaments. It is then believed that at maximum (for a galaxy such as the Milky Way) the baryon fraction is 3-4%, and even lower at small and high galaxy masses. The authors have tested this using a sample of nearby star-forming galaxies, from dwarfs (stellar masses M* of 10^7 solar masses) to high-mass spirals (M* of 10^11 solar masses) with HI rotation curves and near-infrared photometry. Contrary to previous results, they find that the star-formation efficiency is a monotonically increasing function of M* with no sign of a decline at high masses, and that the most massive spirals (M* of 1-3 10^11 solar masses) have a mass fraction between stars and dark halo of 5-17%. In other words, they almost reach the universal baryon fraction. Moreover, there is no evidence of mass quenching of the star formation in these massive galaxies.