On Detecting Halo Assembly Bias with Galaxy Populations
  The fact that the clustering of dark matter halos depends not only on their
mass, but also the formation epoch, is a prominent, albeit subtle, feature of
the cold dark matter structure formation theory, and is known as assembly bias.
At low mass scales ($\sim 10^{12}\,h^{-1}M_\odot$), early-forming halos are
predicted to be more strongly clustered than the late-forming ones. In this
study we aim to robustly detect the signature of assembly bias observationally,
making use of formation time indicators of central galaxies in low mass halos
as a proxy for the halo formation history. Weak gravitational lensing is
employed to ensure our early- and late-forming halo samples have similar
masses, and are free of contamination of satellites from more massive halos.
For the two formation time indicators used (resolved star formation history and
current specific star formation rate), we do not find convincing evidence of
assembly bias. For a pair of early- and late-forming galaxy samples with mean
mass $M_{200c} \approx 9\times10^{11}\,h^{-1}M_\odot$, the relative bias is
$1.00\pm 0.12$. We attribute the lack of detection to the possibilities that
either the current measurements of these indicators are too noisy, or they do
not correlate well with the halo formation history. Alternative proxies for the
halo formation history that should perform better are suggested for future
studies.
