Rigging dark halos: why is hierarchical galaxy formation consistent with
  the inside-out build-up of thin discs?
  State-of-the-art hydrodynamical simulations show that gas inflow through the
virial sphere of dark matter halos is focused (i.e. has a preferred inflow
direction), consistent (i.e. its orientation is steady in time) and amplified
(i.e. the amplitude of its advected specific angular momentum increases with
time). We explain this to be a consequence of the dynamics of the cosmic web
within the neighbourhood of the halo, which produces steady, angular momentum
rich, filamentary inflow of cold gas. On large scales, the dynamics within
neighbouring patches drives matter out of the surrounding voids, into walls and
filaments before it finally gets accreted onto virialised dark matter halos. As
these walls/filaments constitute the boundaries of asymmetric voids, they
acquire a net transverse motion, which explains the angular momentum rich
nature of the later infall which comes from further away. We conjecture that
this large-scale driven consistency explains why cold flows are so efficient at
building up high redshift thin discs from the inside out.
