Dependency of halo concentration on mass, redshift and fossilness in
  Magneticum hydrodynamic simulations
  We study the dependency of the concentration on mass and redshift using three
large N-body cosmological hydrodynamic simulations carried out by the
Magneticum project. We constrain the slope of the mass-concentration relation
with an unprecedented mass range for hydrodynamic simulations and find a
negative trend on the mass-concentration plane and a slightly negative redshift
dependency, in agreement with observations and other numerical works. We also
show how the concentration correlates with the fossil parameter, defined as the
stellar mass ratio between the central galaxy and the most massive satellite,
in agreement with observations. We find that haloes with high fossil parameter
have systematically higher concentration and investigate the cause in two
different ways. First we study the evolution of haloes that lives unperturbed
for a long period of time, where we find that the internal region keeps
accreting satellites as the fossil parameter increases and the scale radius
decreases (which increases the concentration). We also study the dependency of
the concentration on the virial ratio and the energy term from the surface
pressure $E_s$. We conclude that fossil objects have higher concentration
because they are dynamically relaxed, with no in-fall/out-fall material and had
time to accrete their satellites.
