Lenses Live in Crowded Places: The Sample-Level Hubble Bias from Overdense Lens Environments

Lens galaxies are massive ellipticals, and massive ellipticals live in overdense environments. If external-convergence corrections are calibrated on random fields rather than on lens-like fields, the sample mean convergence is underestimated, and the inferred Hubble constant is biased high by the same fraction. For a plausible mean underestimate of 0.015 in convergence, the bias is 1.1 km/s/Mpc, of the same order as the statistical precision of the current benchmark sample. The tabulated relation runs from 0.37 km/s/Mpc at an underestimate of 0.005 to 2.2 at 0.03. Unlike statistical errors, this bias does not shrink as the sample grows; it is a property of the selection, and only environment-matched calibration removes it. Computed from the mass-sheet relation; not measured.