Dr Harry Desmond

Abstract:

f ( R ) is a paradigmatic modified gravity theory that typifies extensions to General Relativity with new light degrees of freedom and hence screened fifth forces between masses. These forces produce observable signatures in galaxy morphology, caused by a violation of the weak equivalence principle due to a differential impact of screening among galaxies’ mass components. We compile statistical datasets of two morphological indicators—offsets between stars and gas in galaxies and warping of stellar disks—and use them to constrain the strength and range of a thin-shell-screened fifth force. This is achieved by applying a comprehensive set of upgrades to past work [H. Desmond et al., Phys. Rev. D 98, 064015 (2018); H. Desmond et al., Phys. Rev. D 98, 083010 (2018) ]: we construct a robust galaxy-by-galaxy Bayesian forward model for the morphological signals, including full propagation of uncertainties in the input quantities and marginalization over an empirical model describing astrophysical noise. Employing more stringent data quality cuts than previously we find no evidence for a screened fifth force of any strength Δ G / G N in the Compton wavelength range 0.3–8 Mpc, setting a 1 σ bound of Δ G / G N < 0.8 at λ C = 0.3     Mpc that strengthens to Δ G / G N < 3 × 10 − 5 at λ C = 8     Mpc . These are the tightest bounds to date beyond the Solar System by over an order of magnitude. For the Hu-Sawicki model of f ( R ) with n = 1 we require a background scalar field value f R 0 < 1.4 × 10 − 8 , forcing practically all astrophysical objects to be screened. We conclude that this model can have no relevance to astrophysics or cosmology.

Abstract:

We use the mass discrepancy–acceleration relation (the correlation between the ratio of total-to-visible mass and acceleration in galaxies; MDAR) to test the galaxy–halo connection. We analyse the MDAR using a set of 16 statistics that quantify its four most important features: shape, scatter, the presence of a ‘characteristic acceleration scale’, and the correlation of its residuals with other galaxy properties. We construct an empirical framework for the galaxy– halo connection inLCDMto generate predictions for these statistics, starting with conventional correlations (halo abundance matching;AM)and introducing more where required. Comparing to the SPARC data, we find that: (1) the approximate shape of the MDAR is readily reproduced by AM, and there is no evidence that the acceleration at which dark matter becomes negligible has less spread in the data than in AM mocks; (2) even under conservative assumptions, AM significantly overpredicts the scatter in the relation and its normalization at low acceleration, and furthermore positions dark matter too close to galaxies’ centres on average; (3) the MDAR affords 2σ evidence for an anticorrelation of galaxy size and Hubble type with halo mass or concentration at fixed stellar mass. Our analysis lays the groundwork for a bottom-up determination of the galaxy–halo connection from relations such as the MDAR, provides concrete statistical tests for specific galaxy formationmodels, and brings into sharper focus the relative evidence accorded by galaxy kinematics to LCDM and modified gravity alternatives.