We are not the 99 percent: quantifying asphericity in the distribution of Local Group satellites
Abstract
We use simulations to build an analytic probability distribution for the asphericity in the satellite distribution around Local Group (LG) type galaxies in the Lambda Cold Dark Matter (LCDM) paradigm. We use this distribution to estimate the atypicality of the satellite distributions in the LG even when the underlying simulations do not have enough systems fully resembling the LG in terms of its typical masses, separation, and kinematics. We demonstrate the method using three different simulations (Illustris1, Illustris1Dark, and ELVIS) and a number of satellites ranging from 11 to 15. Detailed results differ greatly among the simulations suggesting a strong influence of the typical DM halo mass, the number of satellites and the simulated baryonic effects. However, there are three common trends. First, at most 2 per cent of the pairs are expected to have satellite distributions with the same asphericity as the LG; second, at most 80 per cent of the pairs have a halo with a satellite distribution as aspherical as in M31; and third, at most 4 per cent of the pairs have a halo with satellite distribution as planar as in the Milky Way. These quantitative results place the LG at the level of a 3σ outlier in the LCDM paradigm. We suggest that understanding the reasons for this atypicality requires quantifying the asphericity probability distribution as a function of halo mass and largescale environment. The approach presented here can facilitate that kind of study and other comparisons between different numerical setups and choices to study satellites around LG pairs in simulations.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 August 2018
 DOI:
 10.1093/mnras/sty1349
 arXiv:
 arXiv:1805.03188
 Bibcode:
 2018MNRAS.478.5533F
 Keywords:

 dark matter;
 Local Group;
 methods: numerical;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 15 pages, 8 figures, MNRAS accepted