Competitive Accretion and the Baryonic Fraction in Galaxies

Abstract

The baryonic fraction of galaxies is observed to vary with the mass of its dark matter (DM) halo. Low-mass galaxies have low baryonic fractions which increase to a maximum for masses near $10^{12}\ M_{\odot}$, and decreases 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, $\dot{M}_{acc} \propto M_{DM}^2$. In contrast, the few high-mass galaxies reside in the core of the cluster and accrete in a tidal accretion process, $\dot{M}_{acc} \propto M_{DM}^{2/3}$. 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 halos in small-N groups should have higher baryonic fractions relative to those in large clusters.