Investigating planar distributions of satellites around Local Group analogs

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2019-12

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Tippens, Rebecca Gabrielle

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Abstract

Among the various outstanding small scale challenges to ACDM Cosmology is the observation of apparently thin, kinematically coherent planes of satellites around galaxies in the Local Group and beyond. The issue remains remarkably contentious, with conflicting claims about the significance of observed planes, the efficacy of dark matter only simulations in exploring this phenomenon, and broad theoretical disagreement about the occurrence of planes of satellites even between analyses of the same suites of simulations. In this paper, we build upon existing analyses planes of satellites around z=0 Local Group analogs in the ELVIS suite of dissipationless simulations by making use of the full ELVIS merger trees. These allow us to track the kinematic coherence and evolution of "present-day" planes back through cosmic time, and weigh in on their relation to host galaxy properties and environment. Modeling our plane search on observational claims about M31, we find that comparable distributions of z=0 satellites are rare in the ELVIS simulations, but they do exist. However, their co-rotation ratios are less impressive than, for example, the apparently strong co-rotation of M31's plane of satellites, and their other properties do not hold up under further scrutiny. These planes are rarely uniquely-defined or kinematically coherent by more robust measures at z=0, and their properties vary significantly across cosmic time, even with our most generous selection criteria. We interpret these results with the aid of z=0 analogs from the FIRE Latte simulations -- both with and without the contribution of a stellar disk component. The Latte sample suggests that a stellar disk potential helps create less radially concentrated, more statistically significant present-day planes but, as with our ELVIS DMO sample, these configurations are not particularly kinematically coherent by our metrics. We therefore conclude that, to the degree that planes like M31's exist in the ELVIS simulations, they are chance alignments of satellites that do not constitute a significant challenge to CDM. Finally, we weigh in on existing arguments concerning the utility of DMO simulations in plane analyses, demonstrating that thoughtful subhalo sample selection can help systems from DMO simulations recreate the kinematic effects of a baryonic contribution in the form of a stellar disk potential.

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