Quadratic genetic modifications: a streamlined route to cosmological
  simulations with controlled merger history
  Recent work has studied the interplay between a galaxy's history and its
observable properties using "genetically modified" cosmological zoom
simulations. The approach systematically generates alternative histories for a
halo, while keeping its cosmological environment fixed. Applications to date
altered linear properties of the initial conditions such as the mean
overdensity of specified regions; we extend the formulation to include
quadratic features such as local variance, which determines the overall
importance of smooth accretion relative to mergers in a galaxy's history. We
introduce an efficient algorithm for this new class of modification and
demonstrate its ability to control the variance of a region in a
one-dimensional toy model. Outcomes of this work are two-fold: (i) a
clarification of the formulation of genetic modifications and (ii) a proof of
concept for quadratic modifications leading the way to a forthcoming
implementation in cosmological simulations.
