The recent detection of a 3.5 keV X-ray line from the centres of galaxies and clusters by Bulbul et al. (2014a) and Boyarsky et al. (2014a) has been interpreted as emission from the decay of 7 keV sterile neutrinos which could make up the (warm) dark
matter (WDM). As part of the COpernicus COmplexio (COCO) programme, we investigate the properties of dark matter haloes formed in a high-resolution cosmological $N$-body simulation from initial conditions similar to those expected in a universe in which the dark matter consists of 7 keV sterile neutrinos. This simulation and its cold dark matter (CDM) counterpart have $sim13.4$bn particles, each of mass $sim 10^5, h^{-1} M_odot$, providing detailed information about halo structure and evolution down to dwarf galaxy mass scales. Non-linear structure formation on small scales ($M_{200}, leq, 2 times 10^9,h^{-1},M_odot$) begins slightly later in COCO-Warm than in COCO-Cold. The halo mass function at the present day in the WDM model begins to drop below its CDM counterpart at a mass $sim 2 times 10^{9},h^{-1},M_odot$ and declines very rapidly towards lower masses so that there are five times fewer haloes of mass $M_{200}= 10^{8},h^{-1},M_odot$ in COCO-Warm than in COCO-Cold. Halo concentrations on dwarf galaxy scales are correspondingly smaller in COCO-Warm, and we provide a simple functional form that describes its evolution with redshift. The shapes of haloes are similar in the two cases, but the smallest haloes in COCO-Warm rotate slightly more slowly than their CDM counterparts.
The detection of planar structures within the satellite systems of both the Milky Way (MW) and Andromeda (M31) has been reported as being in stark contradiction to the predictions of the standard cosmological model ($Lambda$CDM). Given the ambiguity
in defining a planar configuration, it is unclear how to interpret the low incidence of the MW and M31 planes in $Lambda$CDM. We investigate the prevalence of satellite planes around galactic mass haloes identified in high resolution cosmological simulations. We find that planar structures are very common, and that ~10% of $Lambda$CDM haloes have even more prominent planes than those present in the Local Group. While ubiquitous, the planes of satellite galaxies show a large diversity in their properties. This precludes using one or two systems as small scale probes of cosmology, since a large sample of satellite systems is needed to obtain a good measure of the object-to-object variation. This very diversity has been misinterpreted as a discrepancy between the satellite planes observed in the Local Group and $Lambda$CDM predictions. In fact, ~10% of $Lambda$CDM galactic haloes have planes of satellites that are as infrequent as the MW and M31 planes. The look-elsewhere effect plays an important role in assessing the detection significance of satellite planes and accounting for it leads to overestimating the significance level by a factor of 30 and 100 for the MW and M31 systems, respectively.
