Abundance of field galaxies

We present new measurements of the abundance of galaxies with a given circular velocity in the Local Volume: a region centred on the Milky Way Galaxy and extending to distance ∼10 Mpc. The sample of ∼750 mostly dwarf galaxies provides a unique opportunity to study the abundance and properties of galaxies down to absolute magnitudes M B ≈ −10 and virial masses $M_{\rm vir}= 10^{9}{\,\mathrm{M}_{\odot }}$ . We find that the standard Λ cold dark matter (ΛCDM) model gives remarkably accurate estimates for the velocity function of galaxies with circular velocities V ≳ 70 kms−1 and corresponding virial masses $M_{\rm vir}\gtrsim 5\times 10^{10}{\,\mathrm{M}_{\odot }}$ , but it badly fails by overpredicting ∼5 times the abundance of large dwarfs with velocities V = 30–40 kms−1. The warm dark matter (WDM) models cannot explain the data either, regardless of mass of WDM particle. Just as in previous observational studies, we find a shallow asymptotic slope dN/dlog V ∝ V α, α ≈ −1 of the velocity function, which is inconsistent with the standard ΛCDM model that predicts the slope α = −3. Though reminiscent to the known overabundance of satellite problem, the overabundance of field galaxies is a much more difficult problem. For the standard ΛCDM model to survive, in the 10 Mpc radius of the Milky Way there should be 1000 not yet detected galaxies with virial mass $M_{\rm vir}\approx 10^{10}{\,\mathrm{M}_{\odot }}$ , extremely low surface brightness and no detectable H i gas. So far none of this type of galaxies have been discovered.