Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

In the standard ΛCDM (Lambda cold dark matter) paradigm, dwarf galaxies are expected to be dark matter-rich, as baryonic feedback is thought to quickly drive gas out of their shallow potential wells and quench star formation at early epochs. Recent observations of local dwarfs with extremely low dark matter content appear to contradict this picture, potentially bringing the validity of the standard model into question. We use NewHorizon, a high-resolution cosmological simulation, to demonstrate that sustained stripping of dark matter, in tidal interactions between a massive galaxy and a dwarf satellite, naturally produces dwarfs that are dark matter-deficient, even though their initial dark matter fractions are normal. The process of dark matter stripping is responsible for the large scatter in the halo-to-stellar mass relation in the dwarf regime. The degree of stripping is driven by the closeness of the orbit of the dwarf around its massive companion and, in extreme cases, produces dwarfs with halo-to-stellar mass ratios as low as unity, consistent with the findings of recent observational studies. ∼30 per cent of dwarfs show some deviation from normal dark matter fractions due to dark matter stripping, with 10 per cent showing high levels of dark matter deficiency (Mhalo/M⋆ < 10). Given their close orbits, a significant fraction of dark matter-deficient dwarfs merge with their massive companions (e.g. ∼70 per cent merge over time-scales of ∼3.5 Gyr), with the dark matter-deficient population being constantly replenished by new interactions between dwarfs and massive companions. The creation of these galaxies is therefore a natural by-product of galaxy evolution and their existence is not in tension with the standard paradigm.

Abstract:

Galileons are scalar field theories which obey the Galileon symmetry $\varphi \to \varphi + b + c_\mu x^\mu$ and are capable of self-acceleration if they have an inverted sign for the kinetic term. These theories violate the Strong Equivalence Principle, such that black holes (BHs) do not couple to the Galileon field, whereas non-relativistic objects experience a fifth force with strength $\Delta G / G_{\rm N}$ relative to gravity. For galaxies falling down a gradient in the Galileon field, this results in an offset between the centre of the galaxy and its host supermassive BH. We reconstruct the local gravitational and Galileon fields through a suite of constrained N-body simulations (which we dub CSiBORG) and develop a Monte Carlo-based forward model for these offsets on a galaxy-by-galaxy basis. Using the measured offset between the optical centre and active galactic nucleus of 1916 galaxies from the literature, propagating uncertainties in the input quantities and marginalising over an empirical noise model describing astrophysical and observational noise, we constrain the Galileon coupling to be $\Delta G / G_{\rm N} < 0.16$ at $1\sigma$ confidence for Galileons with crossover scale $r_{\rm C} \gtrsim H_0^{-1}$.

Abstract:

Galileons are scalar field theories which obey the Galileon symmetry $\varphi \to \varphi + b + c_\mu x^\mu$ and are capable of self-acceleration if they have an inverted sign for the kinetic term. These theories violate the Strong Equivalence Principle, such that black holes (BHs) do not couple to the Galileon field, whereas non-relativistic objects experience a fifth force with strength $\Delta G / G_{\rm N}$ relative to gravity. For galaxies falling down a gradient in the Galileon field, this results in an offset between the centre of the galaxy and its host supermassive BH. We reconstruct the local gravitational and Galileon fields through a suite of constrained N-body simulations (which we dub CSiBORG) and develop a Monte Carlo-based forward model for these offsets on a galaxy-by-galaxy basis. Using the measured offset between the optical centre and active galactic nucleus of 1916 galaxies from the literature, propagating uncertainties in the input quantities and marginalising over an empirical noise model describing astrophysical and observational noise, we constrain the Galileon coupling to be $\Delta G / G_{\rm N} < 0.16$ at $1\sigma$ confidence for Galileons with crossover scale $r_{\rm C} \gtrsim H_0^{-1}$.

Abstract:

We perform a novel test of general relativity by measuring the gravitational constant in the Large Magellanic Cloud (LMC). The LMC contains six well-studied Cepheid variable stars in detached eclipsing binaries. Radial velocity and photometric observations enable a complete orbital solution, and precise measurements of the Cepheids' periods permit detailed stellar modelling. Both are sensitive to the strength of gravity, the former via Kepler's third law and the latter through the gravitational free-fall time. We jointly fit the observables for stellar parameters and the gravitational constant. Performing a full Markov Chain Monte Carlo analysis of the parameter space including all relevant nuisance parameters, we constrain the gravitational constant in the Large Magellanic Cloud relative to the Solar System to be ${G}_{\mathrm{LMC}}/{G}_{\mathrm{SS}}=0.9{3}_{\ensuremath{-}0.04}^{+0.05}$. We discuss the implications of this 5% measurement of Newton's constant in another galaxy for dark energy and modified gravity theories. This result excludes one Cepheid, CEP-1812, which is an outlier and needs further study: it is either a highly unusual system to which our model does not apply, or it prefers ${G}_{\mathrm{LMC}}<{G}_{\mathrm{SS}}$ at $2.6\ensuremath{\sigma}$. We also obtain new bounds on critical parameters that appear in semianalytic descriptions of stellar processes. In particular, we measure the mixing length parameter to be $\ensuremath{\alpha}=0.9{0}_{\ensuremath{-}0.26}^{+0.36}$ (when assumed to be constant across our sample), and obtain constraints on the parameters describing turbulent dissipation and convective flux.