The Lopsidedness of Satellite Galaxy Systems in $\Lambda$CDM simulations
  The spatial distribution of satellite galaxies around pairs of galaxies in
the Sloan Digital Sky Survey (SDSS) have been found to bulge significantly
towards the respective partner. Highly anisotropic, planar distributions of
satellite galaxies are in conflict with expectations derived from cosmological
simulations. Does the lopsided distribution of satellite systems around host
galaxy pairs constitute a similar challenge to the standard model of cosmology?
We investigate whether such satellite distributions are present around stacked
pairs of hosts extracted from the $\Lambda$CDM simulations Millennium-I,
Millennium-II, ELVIS, and Illustris-1. By utilizing this set of simulations
covering different volumes, resolutions, and physics, we implicitly test
whether a lopsided signal exists for different ranges of satellite galaxy
masses, and whether the inclusion of hydrodynamical effects produces
significantly different results. All simulations display a lopsidedness similar
to the observed situation. The signal is highly significant for simulations
containing a sufficient number of hosts and resolved satellite galaxies (up to
$5\,\sigma$ for Millennium-II). We find a projected signal that is up to twice
as strong as that reported for the SDSS systems for certain opening angles
($\sim16\%$ more satellites in the direction between the pair than expected for
uniform distributions). Considering that the SDSS signal is a lower limit owing
to likely back- and foreground contamination, the $\Lambda$CDM simulations
appear to be consistent with this particular empirical property of galaxy
pairs.
