The causal effect of environment on halo mass and concentration
  Understanding the impact of environment on the formation and evolution of
dark matter halos and galaxies is a crucial open problem. Studying statistical
correlations in large simulated populations sheds some light on these impacts,
but the causal effect of an environment on individual objects is harder to
pinpoint. Addressing this, we present a new method for resimulating a single
dark matter halo in multiple large-scale environments. In the initial
conditions, we 'splice' (i.e. insert) the Lagrangian region of a halo into
different Gaussian random fields, while enforcing consistency with the
statistical properties of $\Lambda$CDM. Applying this technique, we demonstrate
that the mass of halos is primarily determined by the density structure inside
their Lagrangian patches, while the halos' concentration is more strongly
affected by environment. The splicing approach will also allow us to study, for
example, the impact of the cosmic web on accretion processes and galaxy
quenching.
