Competitive accretion and the baryonic fraction in galaxies

The baryonic fraction of galaxies is observed to vary with the mass of their dark matter (DM) halo. Low-mass galaxies have low baryonic fractions that increase to a maximum for masses near 10¹² M_☉, and decrease thereafter with increasing galaxy mass. This trend is generally attributed to the action of feedback from star formation at the low end and of active galactic nuclei at the high-mass end. An alternative is that the baryonic fraction is at least partially due to the ability of galaxies to competitively accrete gas in a group or clustered environment. Most galaxies in a group including those of lower masses orbit the cluster centre at significant speeds and hence their accretion is limited by a Bondi–Hoyle-type process,  M˙_acc ∝ M_DM². In contrast, the few high-mass galaxies reside in the core of the cluster and accrete in a tidal accretion process, M˙_acc ∝ M_DM^2/3. These two mechanisms result in a baryonic mass fraction that increases as M_DM at low masses and decreases as M_DM^-1/3 at high masses. This model predicts that lower mass haloes in small-N groups should have higher baryonic fractions relative to those in large clusters.