Galaxy morphology rules out astrophysically relevant Hu-Sawicki $f(R)$ gravity

Galaxy morphology rules out astrophysically relevant Hu-Sawicki $f (R)$ gravity

Harry Desmond and Pedro G. Ferreira

Phys. Rev. D 102, 104060 – Published 20 November 2020

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 $Δ G / G_{N} < 3 \times 10^{- 5}$ 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 $f_{R 0} < 1.4 \times 10^{- 8}$ , forcing practically all astrophysical objects to be screened. We conclude that this model can have no relevance to astrophysics or cosmology.

Received 18 September 2020
Accepted 2 November 2020

DOI:https://doi.org/10.1103/PhysRevD.102.104060

Physics Subject Headings (PhySH)

Alternative gravity theories Astrophysical studies of gravity

Galaxies

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Harry Desmond ^* and Pedro G. Ferreira

Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, United Kingdom

^*harry.desmond@physics.ox.ac.uk

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 102, Iss. 10 — 15 November 2020

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
$1 σ$ constraints on $Δ G / G_{N}$ as a function of $λ_{C}$ or $f_{R 0}$ , for the gas–star offset (blue) and warp (red) analyses, and their combination (black). The horizontal dashed black line indicates the value of $Δ G / G_{N} = 1 / 3$ in $f (R)$ .
Reuse & Permissions
Figure 2
Left: distribution of measured angular gas–star offsets $r_{*}$ over the 15,634 galaxies in the gas–star offset sample. Right: distribution of measured $r$ - and $i$ -band warp magnitudes $w_{1}$ over the 4,149 galaxies in the warp sample.
Reuse & Permissions
Figure 3
Example Monte Carlo likelihoods with Gaussian mixture model fits overplotted. Left shows a typical galaxy with 3 Gaussian components while right shows a more extreme one with 11; both come from the gas–star offset sample with $Δ G / G_{N} = 1$ and $λ_{C} = 2.4 Mpc$ .
Reuse & Permissions
Figure 4
Histograms of various quantities in the warp sample at $λ_{C} = 0.4 Mpc$ (red) and $λ_{C} = 8 Mpc$ (blue). Upper left: Modal total screening potential ( $environmental + internal$ ) over the Monte Carlo model realizations. The dashed vertical lines indicate the threshold values for screening in the two models [ $χ$ ; Eq. (13)]. Note that although on average $| Φ_{tot} |$ always exceeds $χ$ for $λ_{C} = 0.4 Mpc$ this is not necessarily the case for individual realizations, which can lead to nonzero unscreened fractions. Upper right: Magnitude of fifth-force field. Lower left: Fraction of Monte Carlo realizations in which $| Φ_{tot} | < χ$ for a given galaxy. Lower right: Number of Gaussian mixture model components fitted to the $w_{1}$ likelihood [Eq. (30)].
Reuse & Permissions
Figure 5
Corner plot of the constraints on the model parameters ( $Δ G / G_{N}$ and the two noise terms) for the joint analysis of gas–star offsets and warps at $λ_{C} = 0.4 Mpc$ , corresponding to $f_{R 0} = 1.6 \times 10^{- 8}$ . The blue vertical line indicates $Δ G / G_{N} = 1 / 3$ , which is ruled out at $> 1 σ$ confidence.
Reuse & Permissions
Figure 6
As Fig. 1, but for a different set of screening scenarios. The blue and red lines show constraints from the gas–star offset and warp analyses with screening switched off, so that all test galaxies are considered unscreened and all source masses contribute to ${\vec{a}}_{5}$ . The cyan, magenta, green and gold lines show constraints with $χ$ raised or lowered by a factor of 10 from its fiducial dependence on $λ_{C}$ [Eq. (13)]. In this case the $Δ G / G_{N}$ limits are evaluated at $λ_{C} = 0.4$ , 2.4, 4.4, 6.4 and 8 Mpc (marked by stars) and connected by piecewise linear interpolation. The black solid line is reproduced from Fig. 1.
Reuse & Permissions

Physical Review D

covering particles, fields, gravitation, and cosmology