Galaxy Assembly Bias: A Significant Source of Systematic Error in the
  Galaxy-Halo Relationship
  It is common practice for methods that use galaxy clustering to constrain the
galaxy-halo relationship, such as the halo occupation distribution (HOD) and/or
conditional luminosity function (CLF), to assume that halo mass alone suffices
to determine a halo's resident galaxy population. Yet, the clustering strength
of cold dark matter halos depends upon halo properties in addition to mass,
such as formation time, an effect referred to as assembly bias. If galaxy
characteristics are correlated with any of these auxiliary halo properties, the
basic assumption of HOD/CLF methods is violated. We estimate the potential for
assembly bias to induce systematic errors in inferred halo occupation
statistics. We use halo abundance matching and age matching to construct
fiducial mock galaxy catalogs that exhibit assembly bias as well as additional
mock catalogs with identical HODs, but with assembly bias removed. We fit a
parameterized HOD to the projected two-point clustering of mock galaxies in
each catalog to assess the systematic errors induced by reasonable levels of
assembly bias. In the absence of assembly bias, the inferred HODs generally
describe the true underlying HODs well, validating the basic methodology.
However, in all of the cases with assembly bias, the inferred HODs have
systematic errors that are statistically significant. In most cases, these
systematic errors cannot be identified using void statistics as auxiliary
observables. We conclude that the galaxy-halo relationship inferred from galaxy
clustering should be subject to a non-negligible systematic error induced by
assembly bias. Our work suggests that efforts to model and/or constrain
assembly bias should be high priorities as it is a threatening source of
systematic error in galaxy evolution studies as well as the precision cosmology
program.
