Dr Harry Desmond

Abstract:

ABSTRACT In chameleon f(R)-gravity, the fifth force will lead to ‘upturns’ in galaxy rotation curves near the screening radius. The location of the upturn depends on the cosmic background value of the scalar field $\bar{f}_\mathrm{R0}$, as well as the mass, size, and environment of the galaxy. We search for this signature of modified gravity in the SPARC sample of measured rotation curves, using an MCMC technique to derive constraints on $\bar{f}_\mathrm{R0}$. Assuming NFW dark matter haloes and with $\bar{f}_\mathrm{R0}$ freely varying for each galaxy, most galaxies prefer f(R) gravity to ΛCDM, but there is a large spread of inferred $\bar{f}_\mathrm{R0}$ values, inconsistent with a single global value. Requiring instead a consistent $\bar{f}_\mathrm{R0}$ value for the whole sample, models with $\log _{10}|\bar{f}_\mathrm{R0}|\gt -6.1$ are excluded. On the other hand, models in the range $-7.5\lt \log _{10}|\bar{f}_\mathrm{R0}|\lt -6.5$ seem to be favoured with respect to ΛCDM, with a significant peak at −7. However, this signal is largely a result of galaxies for which the f(R) signal is degenerate with the core/cusp problem, and when the NFW profile is replaced with a cored halo profile, ΛCDM gives better fits than any given f(R) model. Thus, we find no convincing evidence of f(R) gravity down to the level of $|\bar{f}_\mathrm{R0}|\sim 6 \times 10^{-8}$, with the caveat that if cored halo density profiles cannot ultimately be explained within ΛCDM, a screened modified gravity theory could possibly provide an alternative solution for the core/cusp problem. However, the f(R) models studied here fall short of achieving this.

Abstract:

We present a new empirical relation between galaxy dark matter halo mass (Mhalo) and the velocity along the flat portion of the rotation curve (Vflat), derived from 120 late-type galaxies from the SPARC data base. The orthogonal scatter in this relation is comparable to the observed scatter in the baryonic Tully–Fisher relation (BTFR), indicating a tight coupling between total halo mass and galaxy kinematics at r ≪ Rvir. The small vertical scatter in the relation makes it an extremely competitive estimator of total halo mass. We demonstrate that this conclusion holds true for different priors on M*/L[3.6μ] that give a tight BTFR, but requires that the halo density profile follow DC14 rather than NFW. We provide additional relations between Mhalo and other velocity definitions at smaller galactic radii (i.e. V2.2, Veff, and Vmax) which can be useful for estimating halo masses from kinematic surveys, providing an alternative to abundance matching. Furthermore, we constrain the dark matter analogue of the radial acceleration relation and also find its scatter to be small, demonstrating the fine balance between baryons and dark matter in their contribution to galaxy kinematics.