The Void Phenomenon

TL;DR

The paper analyzes the void phenomenon—the pronounced emptiness of voids in the galaxy distribution—and argues that simple biased galaxy formation within CDM struggles to explain observed void emptiness without invoking suppressed galaxy formation or modifications to the mass distribution. It promotes the nearest-neighbor statistic as a robust, interpretable metric to quantify void populations, applying it to dwarfs, irregulars, and low surface brightness galaxies to derive quantitative limits on void populations. Theoretical discussion contrasts Einstein–de Sitter and low-density cosmologies, outlining potential resolutions including suppression mechanisms and warm dark matter, and emphasizes the need for more observational tests and simulations. Overall, the work positions voids as a stringent benchmark for structure formation theories and the scientific method in cosmology, highlighting a potential crisis that could drive paradigm shifts or theory refinement.

Abstract

Advances in theoretical ideas on how galaxies formed have not been strongly influenced by the advances in observations of what might be in the voids between the concentrations of ordinary optically selected galaxies. The theory and observations are maturing, and the search for a reconciliation offers a promising opportunity to improve our understanding of cosmic evolution. I comment on the development of this situation and present an update of a nearest neighbor measure of the void phenomenon that may be of use in evaluating theories of galaxy formation.

Figures (6)

• Figure 1: Map of Optical Redshift Survey galaxies (filled circles) in a slice in redshift space 600 km s$^{-1}$ deep centered on SBS 0335-052 (open box). The field is $23.5h^{-1}$ Mpc wide by $21.3h^{-1}$ Mpc high.
• Figure 2: Map of Optical Redshift Survey galaxies in a slice in redshift space. The normal to the slice points in the direction of the Virgo Cluster, from the opposite side of the Milky Way. Filled squares are elliptical galaxies, open squares S0s, crosses Sa to Sb-c, plus signs later spiral types, and filled circles dwarfs and irregulars.
• Figure 3: Map of ORS galaxies in a slice in redshift space on the same side of the Milky Way as the Virgo Cluster.
• Figure 4: Distributions of distances $D$ in redshift space of the nearest spiral neighbors of spirals (broken lines) and of dwarfs and irregulars (solid lines). The upper dotted lines show the effect of randomly shifting one in three of the dwarfs and irregulars, the lower dotted lines the effect of randomly shifting one in ten. The redshift ranges are $200 < cz < 500$ km s$^{-1}$ in the bottom plot, $500 < cz < 750$ km s$^{-1}$ in the middle, and $750 < cz < 1000$ km s$^{-1}$ in the top.
• Figure 5: Distributions of distances of the nearest spiral neighbors of spirals and of LSBSample92 (1992) LSB galaxies. The dotted lines show the effect of randomly shifting one in three of the LSB galaxies. The redshift ranges are $200 < cz < 3000$ km s$^{-1}$ in the bottom plot, $3000 < cz < 6000$ km s$^{-1}$ in the middle, and $6000 < cz < 9000$ km s$^{-1}$ in the top.