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We have carried out a set of cosmological hydrodynamical simulations that follow galaxy formation in f (R) modified gravity models. Our simulations employ the Illustris-TNG full physics model and a new modified gravity solver in the AREPO code. For the first time we are able to investigate the degeneracy in the matter power spectrum between the effects of f (R)-gravity and feedback from active galactic nuclei (AGN), and the imprint of modified gravity on the properties of galaxies and on the distribution of dark matter, gas and stars in the universe. f (R)-gravity has an observable effect on the neutral hydrogen power spectrum at high redshift at a level of 20%. For both the F6 and F5 models, this is significantly larger than the predicted errors for the SKA1-MID survey, making this probe a powerful test of gravity on large scales. A similar effect is present in the power spectrum of the stars at high redshift. We also show that rotationally supported disc galaxies can form in f (R)-gravity, even in the partially screened regime. Our simulations indicate that there might be more disc galaxies in F6 compared to GR, and fewer in F5. Finally, we show that the back reaction between AGN feedback and modified gravity in the matter power spectrum is not important in the F6 model but has a sizeable effect in F5.

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 ICM temperatures are both increased by up to 1/3 in f (R) gravity in low-mass haloes, 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 subhaloes at fixed cluster mass is only weakly affected by f (R) gravity.

We present a novel fitting formula for the halo concentration enhancement in chameleon f (R) gravity relative to General Relativity (GR). The formula is derived by employing a large set of N-body simulations of the Hu-Sawicki f (R) model which cover a wide range of model and cosmological parameters, resolutions and simulation box sizes. The complicated dependence of the concentration on halo mass M, redshift z, and the f (R) and cosmological parameters can be combined into a simpler form that depends only on a rescaled mass M/10 p 2 , with p 2 ≡ 1.5 log 10 |f R (z)|/(1 + z) +21.64 andf R (z) the background scalar field at z, irrespective of the f (R) model parameter. Our fitting formula can describe the concentration enhancement well for redshifts z ≤ 3, nearly 7 orders of magnitude in M/10 p 2 and five decades in halo mass. This is part of a series of works which aims to provide a general framework for selfconsistent and unbiased tests of gravity using present and upcoming galaxy cluster surveys. The fitting formula, which is the first quantitative model for the concentration enhancement due to chameleon type modified gravity, is an important part in this framework and will allow continuous exploration of the parameter space. It can also be used to model other statistics such as the matter power spectrum.

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 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−1Mpc and 25 h−1Mpc using 2 × 5123 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 ∼25 per cent on large scales. Similar to their impact in the standard cosmological model (ΛCDM), 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 ∼15 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 ∼0.1 times the halo radius.