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Dark Matter

The nature of dark matter is one of the key questions in modern day cosmology. The currently favoured Cold dark matter model (ΛCDM) provides a good description of the large scale structure of the Universe. Comparison of robust model predictions with empirical galaxy clustering measurements on Mpc scales supports ΛCDM for the growth of structure. On sub-Mpc scales, i.e., on galaxy and group scales, baryons and baryonic physics become critical: the kpc to Mpc range is the key scale over which dark matter halos virialize and merge, and baryons decouple, collapse and eventually form complex structures such as galaxies. In this regime, our theoretical understanding is less well-founded, in great part due to the immense complexity of the physics encountered.

Galaxy-group samples are now able to probe down to a few 1012M, with arguably the most complete being the “GAMA Galaxy Group Catalogue” (G3C, Robotham et. al. 2011). Properties of low mass galaxy groups are limited by the intrinsic lack of survey depth. This explains why for 1012M systems (Mh12 hereafter), only two well-studied examples currently exist: the Milky-Way and the Andromeda systems, both in our own neighbourhood. Galaxies in the Local Group are extensively used to probe in detail the CDM paradigm. They provide the strongest evidence against the standard ΛCDM model, from the missing satellite problem to the too big to fail problem.


Given that galaxy formation efficiency peaks around Mh12 in all standard CDM models, Mh12 is critical in testing CDM. It is therefore necessary to ensure that this limited sample of well-studied Mh12 systems is representative.

To create such a statistical sample is a central goal of WAVES. The survey depth of WAVES-Wide results in Large Magellanic Cloud-like galaxies to be close to volume limited out to z ∼ 0.2, and is specifically designed to deliver a high fidelity group catalogue (∼4k groups with 5+ members), probing to the very lowest halo masses. This sample size will allow for the intrinsic scatter in the sub-halo / stellar mass occupation statistics to be measured. WAVES-Wide will result in a proper characterisation of Mh12 groups, including assessing how representative our two best studied examples are.

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Fig. 1: Three numerical simulations of the distribution of dark matter in cold, warm or self-interacting dark matter (left to right). There are many ways in which these distributions appear different, all of which will be explored by WAVES, WAVES+VST and WAVES+LSST.