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To measure the virial-infall times of satellites in a cosmological context, we use ELVIS (Exploring the Local Volume in Simulations), a suite of cosmological zoom-in $N$-body simulations that are targeted to modeling the LG \citep{GarrisonKimmel2014}.
ELVIS was run using \textsc{GADGET-3} and \textsc{GADGET-2} \citep{Springel2005e}, with initial conditions generated using \textsc{MUSIC} \citep{HahnAbel2011}, all with $\Lambda$CDM cosmology based on WMAP7 \citep{Larson2011}: $\sigma_8=0.801$, $\omegamatter=0.266$, $\omegalambda=0.734$, $n_s=0.963$ 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 $140\pc$ (comoving at
$z > 9$, $z>9$, physical at
$z < 9$). $z<9$).
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
$\rvir \approx 300 \kpc$. $\rvir\approx300\kpc$.
Half of these halos are
located in zoom-in regions that were selected to contain part of a pair
of halos that resemble the masses, distance, and relative velocity of the MW-M31 pair, while the other half are single isolated halos matched in masses to the paired ones.
We use all
48 halos, given the lack of strong difference in satellite
virial-infall infall times in the paired versus isolated halos \citep{Wetzel2015}.
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 that is not a subhalo, we assign a virial mass, $\mvir$, and radius, $\rvir$,
using the evolution of the virial relation from according to \citet{BryanNorman1998}.
We define a A ``subhalo''
as is a halo whose center is inside $\rvir$ of a
(more massive) host
halo.
When halo, and when a (sub)halo passes within
$\rvir$ of a host halo, the (sub)halo $\rvir$, it becomes
its a ``satellite'' and experiences ``virial infall''.
For each (sub)halo, we compute the peak mass, $\mpeak$, that it reached along the history of its primary progenitor.
In order to match subhalos to observed satellites, we assign $\mstar$ to subhalos
based on their $\mpeak$ using the relation from abundance matching in \citet{GarrisonKimmel2014}, which reproduces the observed mass function at
$\mstar < 10 ^ 9 \msun$ $\mstar<10^9\msun$ in the LG if one accounts for observational incompleteness \citep{Tollerud2008, Hargis2014}.
%While the relation between $\mstar$ and subhalo $\mpeak$ for dwarf galaxies remains highly uncertain, likely with significant scatter, in this work the relation is important \emph{only} in assigning virial-infall time distributions to satellites in a 1-dex bin of $\mstar$.
%As \citet{Wetzel2015} showed, satellite infall times generally change by $<10-20\%$ over $\sim 1$ dex in $\mstar$.
