Journal Article

The emptiness of voids: yet another overabundance problem for the Λ cold dark matter model

Anton V. Tikhonov and Anatoly Klypin

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 395, issue 4, pages 1915-1924
Published in print June 2009 | ISSN: 0035-8711
Published online May 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14686.x
The emptiness of voids: yet another overabundance problem for the Λ cold dark matter model

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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 and to MB=−12 within the distance 4–8 Mpc. When we compare the results with the predictions of the standard cosmological λ cold dark matter (ΛCDM) model, we find that the theory faces a sever problem: it predicts a factor of 10 more dwarf haloes as compared with the observed number of dwarf galaxies. In the LV, we identify voids with sizes ranging from 1 to 4.5 Mpc and compare the observational distribution of void sizes with the voids in very high-resolution simulations of the ΛCDM model with Wilkinson Microwave Anisotropy Probe 1(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−1[Mvir= (1−2) × 1010 M] for models with σ8= 0.9 and Vc > 35 km s−1[Mvir= (6−8) × 109 M] for σ8= 0.75. We exclude the possibility that in the ΛCDM model haloes with circular velocities <35 km s−1 can host galaxies as bright as MB=−12: there are too many small haloes in the ΛCDM model resulting in voids being too small as compared with the observations. The problem is that many of the observed dwarf galaxies have H i rotational velocities below 25 km s−1 that strictly contradicts the ΛCDM predictions. Thus, the ΛCDM model faces the same overabundance problem, which it had with the number of satellites in the Local Group (LG). We also estimate the rms deviations from the Hubble flow σH for galaxies at different distances from the LG and find that in most of our model LV candidates the rms peculiar velocities are consistent with observational values: σH= 50 km s−1 for distances less than 3 Mpc and σH= 80 km s−1 for distances less than 8 Mpc. At distances 4 (8) Mpc, the observed overdensities of galaxies are 3.5–5.5 (1.3–1.6) – significantly larger than typically assumed.

Keywords: galaxies: formation; galaxies: kinematics and dynamics; galaxies: luminosity function, mass function; dark matter; large-scale structure of Universe

Journal Article.  8361 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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