New Insight into the Void-in-Cloud Process

Author: Hei Yin Jowett Chan

Collaborator: Masashi Chiba, Tomoaki Ishiyama

Using the analytical model for void statistics by Sheth & van de Weygaert (2004), we found a distinctive behaviour of void distribution between the CDM and WDM cosmology on scales below 1 Mpc/h, where the small-scale problems with the $\Lambda$CDM model become relevant. However, the cause of the distinctive behaviour, the void-in-cloud process, is rarely investigated by previous researches. Therefore, using a suite of high resolution simulations (Multidark, Phi-1 & Phi-0) with box sizes ranging from 1000 Mpc/h to 8 Mpc/h, we investigate the the basic properties of void and find that the standard void-in-cloud effect fails to explain properties of identified small voids in simulations. (i) The number density of small voids, with sizes smaller than 1 Mpc/h, are found to be 2 orders of magnitude more than the prediction of the analytical model, suggesting that the void-in-cloud effect is not suppressing as much voids as predicted. (ii) The measured averaged velocity profile of small voids shows that most of them are partially collapsing structures, without being completely crushed as assumed in the standard void-in-cloud scenario. (iii) Lastly, we split the void distribution into four tidal environment categories, and find that the correlation between distributions and environments can be explained by the void-in-void effect alone. The void-in-cloud effect turns out to be rather weak at influencing the abundance of small voids, even within filaments and clusters. Since a void distribution with weak void-in-cloud effect behaves similarly to the halo mass function in the WDM Universe, we conclude that the void distribution might not be as unique as predicted regarding its possibility to probe dark matter types.