Scaling relations and mass bias in hydrodynamical f(R) gravity
  simulations of galaxy clusters
  We investigate the impact of chameleon-type f(R) gravity models on the
properties of galaxy clusters and groups. Our f(R) simulations follow for the
first time also the hydrodynamics of the intracluster and intragroup medium.
This allows us to assess how f(R) gravity alters the X-ray scaling relations of
clusters and how hydrostatic and dynamical mass estimates are biased when
modifications of gravity are ignored in their determination. We find that
velocity dispersions and intracluster medium temperatures are both increased by
up to 1/3 in f(R) gravity in low-mass halos, while the difference disappears in
massive objects. The mass scale of the transition depends on the background
value f_R0 of the scalar degree of freedom. These changes in temperature and
velocity dispersion alter the mass-temperature and X-ray luminosity-temperature
scaling relations and bias dynamical and hydrostatic mass estimates that do not
explicitly account for modified gravity towards higher values. Recently, a
relative enhancement of X-ray compared to weak lensing masses was found by the
Planck Collaboration (2013). We demonstrate that an explanation for this offset
may be provided by modified gravity and the associated bias effects, which
interestingly are of the required size. Finally, we find that the abundance of
subhalos at fixed cluster mass is only weakly affected by f(R) gravity.
