The effect of selection -- a tale of cluster mass measurement bias
  induced by correlation and projection
  Cosmology analyses using galaxy clusters by the Dark Energy Survey have
recently uncovered an issue of previously unknown selection effect affecting
weak lensing mass estimates. In this letter, we use the Illustris-TNG
simulation to demonstrate that selecting on galaxy counts induces a selection
effect because of projection and correlation between different observables. We
compute the weak-lensing-like projected mass estimations of dark matter halos
and examine their projected subhalo counts. In the 2-D projected space, halos
that are measured as more massive than truth have higher subhalo counts. Thus,
projection along the line of sight creates cluster observables that are
correlated with cluster mass measurement deviations, which in turn creates a
mass measurement bias when the clusters are selected by this correlated
observable. We demonstrate that the bias is predicted in a forward model using
the observable-mass measurement correlation.
