Co-planar streams, pancakes, and angular-momentum exchange in high-z
  disc galaxies
  We study the feeding of massive galaxies at high redshift through streams
from the cosmic web using the Mare Nostrum hydro-cosmological simulation. Our
statistical sample consists of 350 dark-matter haloes of ~10^12 Msun at z =
2.5. We find that ~70% of the influx into the virial radius Rv is in narrow
streams covering 10% of the virial shell. On average 64% of the stream influx
is in one stream, and 95% is in three dominant streams. The streams that feed a
massive halo tend to lie in a plane that extends from half to a few Rv,
hereafter "the stream plane" (SP). The streams are typically embedded in a thin
sheet of low-entropy gas, a Zel'dovich pancake, which carries ~20% of the
influx into Rv. The filaments-in-a-plane configuration about the massive haloes
at the nodes of the cosmic web differs from the large- scale structure of the
web where the filaments mark the intersections of slanted sheets. The stream
plane is only weakly aligned with the angular momentum (AM) near Rv, consistent
with the fact that typically 80% of the AM is carried by one dominant stream.
The galactic disc plane shows a weak tendency to be perpendicular to the
large-scale SP, consistent with tidal-torque theory. Most interesting, the
direction of the disc AM is only weakly correlated with the AM direction at Rv.
This indicates a significant AM exchange at the interphase between streams and
disc in the greater environment of the disc inside an "AM sphere" of radius
~0.3Rv . The required large torques are expected based on the perturbed
morphology and kinematics within this interaction sphere. This AM exchange may
or may not require a major modification of the standard disc modeling based on
AM conservation, depending on the extent to which the amplitude of the disc AM
is affected, which is yet to be studied.
