Abundance of Field Galaxies

TL;DR

This study measures galaxy abundance in the Local Volume, revealing discrepancies with LCDM and WDM models, especially for dwarf galaxies, highlighting a significant overabundance problem in current cosmological theories.

Contribution

First detailed measurement of galaxy velocity function in the Local Volume down to dwarf galaxy scales, challenging standard dark matter models.

Findings

LCDM accurately predicts galaxy abundance for V>60 km/s but overestimates for V=30-50 km/s.

WDM models cannot reconcile the observed velocity function.

The observed shallow slope of the velocity function conflicts with LCDM predictions.

Abstract

We present new measurements of the abundance of galaxies with a given circular velocity in the Local Volume: a region centered on the Milky Way Galaxy and extending to distance 10Mpc. The sample of 750 mostly dwarf galaxies provides a unique opportunity to study the abundance and properties of galaxies down to absolute magnitudes MB= -10, and virial masses Mvir= 1e9Msun. We find that the standard LCDM model gives remarkably accurate estimates for the velocity function of galaxies with circular velocities V>60kms and corresponding virial masses Mvir> 3e10Msun, but it badly fails by over-predicting 5 times the abundance of large dwarfs with velocities V= 30-50kms. The Warm Dark Matter models cannot explain the data either, regardless of mass of the WDM particle. Just as in previous observational studies, we find a shallow asymptotic slope dN/dlog V = V**alpha, alpha =-1 of the velocity function, which is inconsistent with the standard LCDM model that predicts the slope alpha =-3. Though reminiscent to the known overabundance of satellites problem, the overabundance of field galaxies is a much more difficult problem. For the LCDM model to survive, in the 10Mpc radius of the Milky Way there should be 1000 dark galaxies with virial mass Mvir= 1e10Msun, extremely low surface brightness and no detectable HI gas. So far none of this type of galaxies have been discovered.