Detection of spatial correlations of Fundamental Plane residuals, and cosmological implications

The Fundamental Plane (FP) is a widely used tool to investigate the properties of early-type galaxies, and the tight relation between its parameters has spawned several cosmological applications, including its use as a distance indicator for peculiar velocity surveys and as a means to suppress intrinsic noise in cosmic size magnification measurements. Systematic trends with the large-scale structure across the FP could cause serious biases for these cosmological probes, but may also yield new insights into the early-type population. Here we report the first detection of spatial correlations among offsets in galaxy size from an FP that explicitly accounts for redshift trends, using a sample of about 95 000 elliptical galaxies from the Sloan Digital Sky Survey. We show that these offsets correlate with the density field out to at least 10 h⁻¹ Mpc at 4σ significance in a way that cannot be explained by systematic errors in galaxy size estimates. We propose a physical explanation for the correlations by dividing the sample into central, satellite, and field galaxies, identifying trends for each galaxy type separately. Central (satellite) galaxies lie on average above (below) the FP, which we argue could be due to a higher (lower) than average mass-to-light ratio. We fit a simple model to the correlations of FP residuals and use it to predict the impact on peculiar velocity power spectra, finding a contamination larger than 10 per cent for k > 0.04 h Mpc⁻¹. Moreover, cosmic magnification measurements based on an FP could be severely contaminated over a wide range of scales by the intrinsic FP correlations.