The Void Phenomenon Revisited

The Void Phenomenon consists in the apparent discrepancy between the number of observed dwarf halos in cosmic voids and that expected from CDM simulations. We approach the problem considering the challenging prospects of detecting field dwarf systems with halo masses ≤ 10 M⊙, via their possible HI emission. A brief review of recent work is followed by preliminary results from the ALFALFA survey, which suggest the possibility, but not yet the proof, that such objects may have been already detected towards the outskirts of the Local Group. 1 The Void Problem: Where are the Dwarfs? More than 30 years ago it started to become apparent that bright galaxies are generally found in dense environments and that the vast majority of cosmic space is devoid of them. The high density regions are highlighted by clusters, arising in filamentary structures; regions between them that are underdense in galaxies brighter than ∼ L by an order of magnitude or more fit spherical volumes of diameter often exceeding 10–20 Mpc, the “voids”. This topology is now well understood as a natural evolution of matter density fluctuations progressively amplified by gravitational instability. Current computer simulations readily reproduce the main characteristics of the observed topology. Voids are not empty, either in simulations or in deep redshift surveys. Peebles (2001, 2008) has however pointed out that a discrepancy appears to exist between observations and simulations obtained within the standard Cold Dark Matter (CDM) scenario: while a significant population of low mass halos is predicted to exist by simulations, observations have failed to detect faint galaxies in the expected abundance. An extension to voids of the morphology–density relation, seen to be relevant in higher density environments, does not seem to explain the discrepancy, as surveys of low surface brightness (e.g. Thuan, Gott & Schneider 1987) and blue compact dwarf galaxies (e.g. Salzer, Hanson & Gavazzi 1990) replicate the topology of brighter galaxies. Peebles has referred to this as the “Void Phenomenon”, which appears to be of analogous nature to the “missing satellite” problem pointed out by Klypin et al. (1999) and Moore et al. (1999). The organizers of this conference have asked me to revisit this issue, possibly with the expectation that the partial results of the currently ongoing ALFALFA HI survey may be able to throw some new light, as that survey is highly sensitive to low mass, gas rich systems. In this report, an overview of key observations and high resolution simulations will be followed by results which do not solve the problem — if indeed a void problem still exists: see the paper by Tinker in these proceedings and Tinker & Conroy (2009) — but may provide interesting