$S_8$ from peculiar velocities: agreement with Planck for Tully--Fisher and supernovae, tension for the fundamental plane

Abstract

Peculiar velocity measurements constrain the parameter combination $f\sigma_8$, the product of the linear growth rate $f$ and the fluctuation amplitude $\sigma_8$. Under the approximation that $f$ is a monotonic function of $\Omega_{\rm m}$, this can be related to $S_8 \equiv \sigma_8 \sqrt{\Omega_{\rm m}/0.3}$, enabling direct comparison with weak lensing and cosmic microwave background results. We use three classes of direct-distance tracers -- the Tully--Fisher relation, the fundamental plane, and Type Ia supernovae -- to infer peculiar velocities. A unified hierarchical forward model jointly calibrates each distance indicator and a linear theory reconstruction of the local Universe. This is the first consistent Bayesian analysis to treat all three major classes of distance indicators within a common framework, enabling cross-checks of systematics across diverse galaxy populations. Combining the Tully--Fisher and Type Ia supernova samples, we obtain $S_8 = 0.798 \pm 0.035$ ($f\sigma_8 = 0.412 \pm 0.018$), in agreement with Planck and robust under the choice of galaxy bias model, with the uncertainty dominated by the variance of the 2M++ galaxy field. The fundamental plane constraints are instead unstable under the inhomogeneous Malmquist bias treatment; the quadratic extension preferred by the fundamental plane data drives their $S_8$ values lower. These findings indicate that low-redshift peculiar velocity data are concordant with the cosmic microwave background and do not reinforce the early-versus-late $S_8$ tension, though the fundamental plane results call for further scrutiny of their systematics.