Galaxy formation in the brane world I: overview and first results
  We carry out ``full-physics'' hydrodynamical simulations of galaxy formation
in the normal-branch Dvali-Gabadadze-Porrati (nDGP) braneworld model using a
new modified version of the {\sc Arepo} code and the IllustrisTNG galaxy
formation model. We simulate two nDGP models (N5 and N1) which represent,
respectively, weak and moderate departures from GR, in boxes of sizes
$62\,h^{-1}{\rm Mpc}$ and $25\,h^{-1}{\rm Mpc}$ using $2\times512^3$ dark
matter particles and initial gas cells. This allows us to explore, for the
first time, the impact of baryonic physics on galactic scales in braneworld
models of modified gravity and to make predictions on the stellar content of
dark matter haloes and galaxy evolution through cosmic time in these models. We
find significant differences between the GR and nDGP models in the power
spectra and correlation functions of gas, stars and dark matter of up to $\sim
25$ per cent on large scales. Similar to their impact in the standard
cosmological model ($\Lambda$CDM), baryonic effects can have a significant
influence over the clustering of the overall matter distribution, with a sign
that depends on scale. Studying the degeneracy between modified gravity and
galactic feedback in these models, we find that these two physical effects on
matter clustering can be cleanly disentangled, allowing for a method to
accurately predict the matter power spectrum with baryonic effects included,
without having to run hydrodynamical simulations. Depending on the braneworld
model, we find differences compared with GR of up to $\sim15$ per cent in
galaxy properties such as the stellar-to-halo-mass ratio, galaxy stellar mass
function, gas fraction and star formation rate density. The amplitude of the
fifth force is reduced by the presence of baryons in the very inner part of
haloes, but this reduction quickly becomes negligible above $\sim0.1$ times the
halo radius.
