Isolating the linear signal when making redshift space distortion measurements

Abstract: Constraints on the linear growth rate, $f\sigma_8$, using small scale redshift space distortion measurements have a significant statistical advantage over those made on large scales. However, these measurements need to carefully disentangle the linear and non-linear information when interpreting redshift space distortions in terms of $f\sigma_8$. It is particularly important to do this given that some previous measurements found a significant deviation from the expectation based on the $\Lambda$CDM model constrained by Planck CMB data. We construct a new emulator-based model for small scale galaxy clustering with scaling parameters for both the linear and non-linear velocities of galaxies, allowing us to isolate the linear growth rate. We train the emulator using simulations from the AbacusCosmos suite, and apply it to data from the extended Baryon Oscillation Spectroscopic Survey (eBOSS) luminous red galaxy sample. We obtain a value of $f\sigma_8(z=0.737)=0.368\pm0.041$, in 2.3-$\sigma$ tension with the Planck 2018 $\Lambda$CDM expectation, and find less dependence on the minimum measurement scale than previous analyses.