Validity of replacing self-gravity with externally prescribed forcing in realistic galactic disks

Ascertain whether modeling the disk response by applying an externally prescribed potential fluctuation—effectively assuming the response is dominated by Landau modes so that the externally forced potential reproduces the self-consistent one—is valid in realistic galactic contexts.

Background

In developing their analytic treatment, the authors neglect the self-gravity of the perturbed stellar distribution and argue that, if the full potential fluctuation is correctly prescribed (e.g., dominated by Landau modes), the externally forced response can approximate the self-consistent one. This assumption greatly simplifies the modeling and underpins their comparisons with test-particle simulations.

They note that while such mode-dominated behavior can occur in isolated N-body disks, its applicability to real galactic environments (e.g., subject to tidal encounters or gas dynamics) is uncertain and potentially invalid in cases like Sagittarius-driven perturbations.

References

Mathematically, the condition we are imposing is roughly that the potential response be dominated by Landau modes. The linear response of isolated stellar disks in N-body simulations can be dominated by these modes, so this may not be a bad assumption. However, whether this approach is valid in realistic galactic contexts is an open question.

— On the radial velocity wave in the Galactic disk  (2602.06182 - Hamilton et al., 5 Feb 2026) in Section 6 (Discussion)

And the projection efficiency $\zeta$ relating the local density contrast to the coherent quadrupolar forcing is unknown; establishing it would require measuring the $m=2$ component of the gas potential in the inner disc directly.

— Turbulent gas-rich discs at high redshift: the origin of early massive stellar bars  (2609.03275 - Bland-Hawthorn et al., 3 Sep 2026) in Section 4, subsection “Bar onset in a fluctuating disc”