Galaxy and Mass Assembly (GAMA): halo formation times and halo assembly
  bias on the cosmic web
  We present evidence for halo assembly bias as a function of geometric
environment. By classifying GAMA galaxy groups as residing in voids, sheets,
filaments or knots using a tidal tensor method, we find that low-mass haloes
that reside in knots are older than haloes of the same mass that reside in
voids. This result provides direct support to theories that link strong halo
tidal interactions with halo assembly times. The trend with geometric
environment is reversed at large halo mass, with haloes in knots being younger
than haloes of the same mass in voids. We find a clear signal of halo
downsizing - more massive haloes host galaxies that assembled their stars
earlier. This overall trend holds independently of geometric environment. We
support our analysis with an in-depth exploration of the L-Galaxies
semi-analytic model, used here to correlate several galaxy properties with
three different definitions of halo formation time. We find a complex
relationship between halo formation time and galaxy properties, with
significant scatter. We confirm that stellar mass to halo mass ratio, specific
star-formation rate and mass-weighed age are reasonable proxies of halo
formation time, especially at low halo masses. Instantaneous star-formation
rate is a poor indicator at all halo masses. Using the same semi-analytic
model, we create mock spectral observations using complex star-formation and
chemical enrichment histories, that approximately mimic GAMA's typical
signal-to-noise and wavelength range. We use these mocks to assert how well
potential proxies of halo formation time may be recovered from GAMA-like
spectroscopic data.
