On the degeneracy between baryon feedback and massive neutrinos as
  probed by matter clustering and weak lensing
  Massive neutrinos, due to their free streaming, produce a suppression in the
matter power spectrum at intermediate and small scales which could be probed by
galaxy clustering and/or weak lensing observables. This effect happens at
scales that are also influenced by baryon feedback, i.e. galactic winds or
Active Galactic Nuclei (AGN) feedback, which in realistic hydrodynamic
simulations has also been shown to produce a suppression of power. Leaving
aside, for the moment, the complex issue of galaxy bias, we focus here on
matter clustering and tomographic weak lensing, we investigate the possible
degeneracy between baryon feedback and neutrinos showing that it is not likely
to degrade significantly the measurement of neutrino mass in future surveys. To
do so, we generate mock data sets and fit them using the Markov Chain Monte
Carlo (MCMC) technique and explore degeneracies between feedback parameters and
neutrino mass. We model baryon feedback through fitting functions, while
massive neutrinos are accounted for, also in the non-linear regime, using
Halofit calibrated against accurate N-body neutrino simulations. In the error
budget, we include the uncertainty in the modelling of non-linearities. For
both matter clustering and weak lensing, we always recover the input neutrino
mass within $\sim 0.25\sigma$ confidence level. Finally, we also take into
account the intrinsic alignment effect in the weak lensing mock data. Even in
this case, we are able to recover the right parameters: in particular, we find
a significant degeneracy pattern between $M_\nu$ and the intrinsic alignment
parameter $A_\mathrm{IA}$ .
