[Paper Review] Newtonian limit of scalar-tensor theories and galactic dynamics: isolated and interacting galaxies

Jorge L. Cervantes–Cota, M. A. Rodríguez‐Meza+2 more|arXiv (Cornell University)|Apr 19, 2007

Cosmology and Gravitation Theories1 references4 citations

TL;DR

This paper investigates the impact of scalar-tensor gravity—specifically, a Yukawa-modified Newtonian potential with parameters λ (interaction lengthscale) and α (coupling strength)—on bar formation in isolated and interacting spiral galaxies. Using N-body simulations, it finds that positive α destabilizes galactic discs, accelerating bar formation, with strongest bars emerging at λ ≈ 8–14 kpc, and that tidal interactions in colliding galaxies further amplify bar properties dependent on (λ, α), suggesting these parameters can be constrained by observations.

ABSTRACT

We use the Newtonian limit of a general scalar-tensor theory around a background field to study astrophysical effects. The gravitational theory modifies the standard Newtonian potential by adding a Yukawa term to it, which is quantified by two theoretical parameters: $\\lambda$, the lenghtscale of the gravitational interaction and its strength, $\\alpha$. Within this formalism we firstly present a numerical study on the formation of bars in isolated galaxies. We have found for positive $\\alpha$ that the modified gravity destabilizes the galactic discs and leads to rapid bar formation in isolated galaxies. Values of $\\lambda$ in the range $\\approx 8$ -- 14 kpc produce strongest bars in isolated models. Then, we extent this work by considering tidal effects due to interacting galaxies. We send two spirals to collide and study the bar properties of the remnant. We characterize the bar kinematical properties in terms of our parameters ($\\lambda, \\alpha$).

Motivation & Objective

• To explore how scalar-tensor theories (STT) modify galactic dynamics through a Yukawa correction to the Newtonian potential.
• To investigate the role of the scalar field parameters λ (interaction scale) and α (coupling strength) in bar formation within isolated galaxies.
• To extend the analysis to interacting galaxies, assessing how tidal forces and STT parameters jointly influence bar morphology and kinematics.
• To constrain the viable parameter space of STT using simulated bar properties and compare them with observational data.

Proposed method

• Derive the Newtonian limit of a general scalar-tensor theory, introducing a Yukawa potential correction with parameters λ and α.
• Construct N-body simulations of isolated spiral galaxies with varying λ and α to study bar formation dynamics.
• Simulate head-on and off-axis collisions between two equal-mass spiral galaxies to examine bar development under tidal interactions.
• Track time evolution of bar amplitude |A₂| and pattern speed Ω to quantify bar strength and kinematics.
• Use Plummer softening (ε) to control numerical effects and ensure stability in simulations.
• Compare results across models with different (λ, α) values to identify trends in bar formation timescale and amplitude.

Experimental results

Research questions

• RQ1 How does a non-minimally coupled scalar field with parameters λ and α affect bar formation in isolated galaxies?
• RQ2 What values of λ and α maximize bar strength and accelerate bar formation in isolated systems?
• RQ3 How do tidal interactions during galaxy collisions influence bar development in scalar-tensor gravity?
• RQ4 Does the presence of a scalar field alter the kinematic and morphological properties of bars formed in interacting galaxies?
• RQ5 Can observational constraints on bar properties help discriminate between different (λ, α) parameter combinations in scalar-tensor theories?

Key findings

• Positive α values destabilize galactic discs, leading to faster and stronger bar formation in isolated galaxies compared to standard Newtonian gravity.
• For α = 0.1, the strongest bar forms at λ = 16 kpc, while for α = 0.3, the strongest bar appears at λ = 8 kpc, indicating a resonance-like dependence on λ.
• Bar formation occurs earlier and with greater amplitude when λ is in the range of 8–14 kpc, suggesting a preferred scale for bar instability in modified gravity.
• In off-axis collisions with impact parameter equal to disc radius, both prograde and retrograde discs form bars of comparable amplitude, independent of λ, but the wiggle effect in spiral arms is absent in the second disc.
• The absence of the wiggle effect in the second disc under certain (λ, α) conditions suggests that scalar-tensor gravity may alter the dynamics of transient bar features in interacting systems.
• The results indicate that (λ, α) parameters significantly influence bar properties, implying that observations of bar morphology and kinematics can constrain scalar-tensor gravity models.