The emptiness of voids: yet another over-abundance problem for the LCDM model

TL;DR

This paper examines the discrepancy between observed voids and dwarf galaxy counts in the Local Volume and the predictions of the standard LCDM cosmological model, highlighting a significant overabundance problem.

Contribution

It provides a detailed comparison of observed void sizes and dwarf galaxy counts with high-resolution LCDM simulations, revealing a severe overabundance issue for small haloes.

Findings

LCDM predicts ten times more dwarf haloes than observed.

Void sizes in observations match high-resolution simulations only for haloes with Vc > 35-45 km/s.

Many observed dwarf galaxies have HI velocities below LCDM predictions.

Abstract

We study the luminosity function, the peculiar velocities, and the sizes of voids in the Local Volume (LV) in observational samples of galaxies which contain galaxies down to MB=-10-12 within the distance 4-8Mpc. Comparing the results with the predictions of the standard cosmological LCDM model, we find that the theory faces a sever problem: it predicts a factor of ten more dwarf haloes as compared with the observed number of dwarf galaxies. In the LV we identify voids with sizes ranging from 1 to 4.5Mpc and compare the observational distribution of void sizes with the voids in very high resolution simulations of the LCDM model with WMAP1 and WMAP3 parameters. The theoretical void function matches the observations remarkably well only if we use haloes with circular velocities Vc larger than 40-45 km/s (Mvir=(1-2)e10msun) for models with with sigma8 = 0.9 and Vc>35km/s (Mvir=(6-8)e9msun) for sigma8 = 0.75. We exclude the possibility that in the LCDM model haloes with circular velocities < 35km/s can host galaxies as bright as MB=-12: there are too many small haloes in the LCDM model resulting in voids being too small as compared with the observations. The problem is that many of the observed dwarf galaxies have HI rotational velocities below 25km/s that strictly contradicts the LCDM predictions. Thus, the LCDM model faces the same overabundance problem, which it had with the number of satellites in the LG. We also estimate the rms deviations from the Hubble flow sigmaH and find that in most of our model LV-candidates the rms peculiar velocities are consistent with observational values sigmaH=50km/s for distances less than 3Mpc and sigmaH=80km/s for distances less than 8Mpc. At distances 4 (8) Mpc, the observed overdensities of galaxies are 3.5-5.5 (1.3-1.6) -- significantly larger than typically assumed.