The abundance of dark galaxies

We show that gas in a large fraction of low-mass dark matter haloes may form Toomre-stable discs, if angular momentum is conserved when the gas contracts. Such haloes would be stable to star formation and therefore remain dark. This may potentially explain deviation between the predicted and the observed faint-end slope of the luminosity function and the discrepancy between the predicted and observed number of dwarf satellites in the Local Group. The above mechanism does not require a strong variation of the baryon fraction with the virial mass of the dark halo. We show that model fits to rotation curves are also consistent with this hypothesis: none of the observed galaxies lies in the region of parameter space forbidden by the Toomre stability criterion.