Giant Void

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.

Abstract: Despite a history that dates back at least a quarter of a century studies of voids in the large–scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void–finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of thirteen different void finders constructed using particles, haloes, and semi– analytical model galaxies extracted from a subvolume of the Millennium simulation. The study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 percent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than mB = 20 is found to be smaller than about 0.8 by all our void finding algorithms.

Abstract: We measure the r-band luminosity function (LF) of a sample of 103 void galaxies over a large range of magnitude, -21.5 5 A], have brighter M but faint-end slopes similar to those of void galaxies. In contrast, the LFs of wall galaxies with red g - r color, elliptical-like profiles, or low star formation rates have significantly shallower faint-end slopes and brighter values of M than we find for void galaxies. We conclude that the void galaxy population is dominated by faint, late-type galaxies. The shift in M* between the void and wall galaxy LFs is consistent with the shift of the mass function in voids predicted by extended Press-Schechter theory.

Abstract: Using the sample presented by Pan et al., we analyse the photometric properties of 88?794 void galaxies and compare them to galaxies that reside in higher density environments with the same absolute magnitude distribution as the void galaxies. We analysed the Sloan Digital Sky Survey Data Release 7 and found a total of 1054 dynamically distinct voids with radius larger than 10?h-1?Mpc. The voids are not empty, but are underdense, with d?/? < -0.9 in their centres. In this paper, we study the photometric properties of these void galaxies. We look at the u - r colours as an indication of star formation activity and the inverse concentration index as an indication of galaxy type. We find that void galaxies are statistically bluer than galaxies found in higher density environments with the same magnitude distribution. We examine the colours of the galaxies as a function of magnitude, dividing the galaxies into bright, medium, faint and dwarf groups, and we fit each colour distribution with a double-Gaussian model for the red and blue subpopulations. As we move from bright to dwarf galaxies, the population of red galaxies steadily decreases and the fraction of blue galaxies increases in both voids and walls; however, the fraction of blue galaxies in the voids is always higher and bluer than in the walls. We also split the void and wall galaxies into samples depending on galaxy type, as measured by the inverse concentration index. We find that late-type void galaxies are bluer than late-type wall galaxies and the same holds for early galaxies. We also find that early-type and dwarf void galaxies are blue in colour. We also study the properties of void galaxies as a function of their distance from the centre of the void. We find very little variation in the properties, such as magnitude, colour and type, of void galaxies as a function of their location in the void. The only exception is that the dwarf void galaxies may live closer to the centres of voids. As shown by Pan et al., the centres of voids have very similar density contrast and hence all void galaxies live in very similar density environments, which may explain the lack of variation of galaxy properties with location within voids.