Shrinking voids

Early observational and theoretical studies of voids (Joeveer et al., 1978; Gregory & Thompson, 1978; Kirshner et al., 1981) have been supplemented through more recent large galaxy redshift surveys (Colless et al., 2001; Abazajian et al., 2009) that provide provide comprehensive and complete pictures of voids, complemented by numerous void-finding techniques (e.g. El-Ad & Piran (1997); Hoyle & Vogeley (2002); Aragon-Calvo et al. (2010). Voids are an easily recognised (and, initially quite unexpected) feature of the Cosmic Web, spanning between 20 and 50h 1_{Mpc. We now know that the>1h} 1_{Mpc environment of a galaxy in a void is very}

empty relative to that of a galaxy in a dense cluster, making such objects ideal for studying galaxy evolution independent of environmental processes. The few galaxies that do exist in voids are subject to dynamics that are unique to these underdense regions due to the lack of neighbouring galaxies (Blumenthal et al., 1992; Sheth & van de Weygaert, 2004). Voids can serve as tools for constraining cosmological parameters, or for testing the accuracy of large cosmological simulations, as shown in Dekel & Rees (1994); Lavaux & Wandelt (2010); Park et al. (2012) and many other works.

In the GLSSC, a void galaxy is defined as a galaxy that is at least 4.13 h 1 _{Mpc away}

from the nearest galaxy that belongs to a tendril (and tendril galaxies themselves must be at least 4.56 h 1 _{Mpc away from the nearest filament), so they exist in the most underdense}

regions of the GAMA fields. We do not determine where voids are, or their sizes, only the galaxies that exist in very underdense regions. This is a significantly different way of defining a galaxy in a void compared to the traditional approach of detecting voids using voidfinding

algorithms (examples of which are given at the start of this chapter) that depend on locating underdensities and selecting the galaxies within them as void galaxies.

Recently, Pan et al. (2012) have released a catalogue of void galaxies obtained from the SDSS-DR7 data using a voidfinder similar to the one introduced by El-Ad & Piran (1997). In Pan et al. (2012), voids are identified within a volume limited subsample of 120606 SDSS galaxies with Mr < 20.09 mag andz < 0.107: galaxies are first determined to be within

the field or not, using the third nearest neighbour distance d₃ and the standard deviation of this distance d3. Any galaxy with d= d3+1.5 d3>6.3h

1 _{Mpc is considered to belong to}

a part of large scale structure: a filament or a cluster. These galaxies are afterwards referred to as ‘wall’ galaxies. All other galaxies are classified as ‘field galaxies’ and are removed from the sample. The remaining wall galaxies are then gridded into cells of side 5h 1_{Mpc and all}

empty cells are considered to be possible centres of voids. A sphere is grown from each cell until it is bounded by four wall galaxies, and any two spheres with more than 10% overlap are considered to belong to the same void. This process is continued iteratively until no more spheres can be grown or merged. A sphere must have a radius of at least 10h 1 _{Mpc to be}

considered a void.

As the GAMA regions overlap with the SDSS, there are some voids in the Pan et al. (2012) catalogue that lie in GAMA regions. In order to be as conservative as possible, a sample of all GAMA galaxies that lie within the inner two thirds of SDSS voids in GAMA regions is taken. This sample contains 1130 galaxies with 0.001z_0.1 andM_r< 19.77. Of these galaxies, 132 are part of the GLSSC – that is to say that they were part of the sample that the algorithm described on the preceding chapter was run. This small number is due to the fact that SDSS is a more faint survey with a lower target density. Figure 4.17 shows all galaxies in the GAMA large scale catalogue for the three equatorial fields in grey and circles the 132 galaxies that are matched to the 1130 GAMA galaxies in the SDSS voids. The circles are coloured according to the structures those galaxies are classified as being in the GLSSC: blue circles are galaxies in filaments, green circles are galaxies in tendrils, and red circles are galaxies in voids. Only 25% of the GLSSC galaxies found in SDSS voids are also galaxies that are identified to be in voids in the GLSSC, with the vast majority (65%) considered to be in tendrils, and a further 11% associated with filaments. For a subsample of 79 galaxies with M_r < 20.06 mag, 15% are filament galaxies, 61% in tendrils and 24% in voids. This highlights the importance of using a deep survey with a high target density when investigating the properties of void galaxies,

Filaments Tendrils Voids

M_r< 19.77 11% 64% 25%

Mr< 20.06 15% 61% 24%

Table 4.2:Percentage of galaxies, grouped by large scale environment, matched with galaxies in voids located in GAMA fields, for two different absolute magnitude cuts. It is interesting to note that when moving from the brighter to the dimmer magnitude cut, many tendrils are reclassified as filaments, but the fraction of voids remains largely the same.

as voids defined in a shallow survey may contain additional undetected galaxies. Despite the differences in void galaxy definition, it is clear that regions considered to be empty in the SDSS do in fact contain galaxies.

The implication of these results is that voids are largely susceptible to the diagnostics of observational surveys used to detect them, and that it is not always advisable to use surveys with a shallow magnitude limit for searching for these structures – and less so for characterising the properties of galaxies that reside in them. Having said that, there is no evidence that there will not be a future survey that revisits the GAMA regions with a much deeper magnitude limit and finds that the voids presented in this work are just as full of more faint, less massive objects. The fact that the excess line correlation function for voids in Figure 4.16 shows no signal does seem to indicate that the void galaxies in the GLSSC are truly free from structures, but this is not evidence that there are no more undected sources in the background. However, the fact that the fraction of void galaxies in SDSS voids remains virtually unchanged when moving from a Mr> 19.77 cut to aMr> 20.06 cut might imply that there are not many

deeper void galaxies to find.