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 similar to 10 Mpc. The sample of similar to 750 mostly dwarf galaxies provides a unique opportunity to study the abundance and properties of galaxies down to absolute magnitudes MB approximate to -10 and virial masses M-vir = 10(9)M(circle dot). We find that the standard Lambda cold dark matter (Lambda CDM) model gives remarkably accurate estimates for the velocity function of galaxies with circular velocities V greater than or similar to 70 kms(-1) and corresponding virial masses M-vir greater than or similar to 5 x 10(10)M circle dot, but it badly fails by overpredicting similar to 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 alpha V-alpha, alpha approximate to -1 of the velocity function, which is inconsistent with the standard Lambda CDM model that predicts the slope alpha=-3. Though reminiscent to the known overabundance of satellite problem, the overabundance of field galaxies is a much more difficult problem. For the standard Lambda CDM model to survive, in the 10 Mpc radius of the Milky Way there should be 1000 not yet detected galaxies with virial mass M-vir approximate to 10(10)M(circle dot), extremely low surface brightness and no detectable H I gas. So far none of this type of galaxies have been discovered.