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\subsection{Simulations}  To measure infall times of satellites, we use ELVIS (Exploring the Local Volume in Simulations), a suite of cosmological zoom-in $N$-body simulations intended to model the LG \citep{GarrisonKimmel2014}.  ELVIS was run using \textsc{GADGET-3} and \textsc{GADGET-2} \citep{Springel2005e} \citep{GarrisonKimmel2014}  ina  $\Lambda$CDM cosmology based on WMAP7 \citep{Larson2011}: cosmology:  $\sigma_8=0.801$, $\omegamatter=0.266$, $\omegalambda=0.734$, $n_s=0.963$ and $h=0.71$. Within the zoom-in regions, the particle mass is $1.9\times10^5\msun$ and the Plummer-equivalent force softening is $140\pc$ physical.  ELVIS contains 48 dark-matter halos of masses similar to the MW or M31 ($\mvir=1.0-2.8\times10^{12}\msun$), with a median $\rvir\approx300\kpc$, the distance where observed dwarf galaxies show a strong transition.  Half of the halos are part of a pair that resemble the masses, distance, and relative velocity of the MW-M31 pair, while the other half are single isolated halos.  In Given  the absence lack  of systematic differences in satellite infall times for the paired versus isolated halos \citep{Wetzel2015}, we use all 48 to improve the statistics. ELVIS identifies dark-matter (sub)halos using the six-dimensional halo finder \textsc{rockstar} \citep{Behroozi2013a} and constructs merger trees using the \textsc{consistent-trees} algorithm \citep{Behroozi2013b}.  For each halo, we assign a virial mass, $\mvir$, and radius, $\rvir$, according to \citet{BryanNorman1998}.