The Origin of Disks and Spheroids in Simulated Galaxies
  In the simplest scenario, disk galaxies form predominantly in halos with high
angular momentum and quiet recent assembly history, whereas spheroids are the
slowly-rotating remnants of repeated merging events. We explore these
assumptions using one hundred systems with halo masses similar to that of the
Milky Way, identified in a series of cosmological gasdynamical simulations
GIMIC. At z=0, the simulated galaxies exhibit a wide variety of morphologies,
from dispersion-dominated spheroids to pure disk galaxies. Surprisingly, these
morphological features are very poorly correlated with their halo properties:
disks form in halos with high and low net spin, and mergers play a negligible
role in the formation of spheroid stars, most of which form in-situ. More
important to morphology is the coherent alignment of the angular momentum of
baryons that accrete over time to form a galaxy. Spheroids tend to form when
the spin of newly-accreted gas is misaligned with that of the extant galaxy,
leading to the episodic formation of stars with different kinematics that
cancel out the net rotation of the system. Disks, on the other hand, form out
of gas that flows in with similar angular momentum to that of earlier-accreted
material. Gas accretion from a hot corona thus favours disk formation, whereas
gas that flows "cold", often along separate, misaligned filaments, favours the
formation of spheroids. In this scenario, most spheroids consist of
superpositions of stellar components with distinct kinematics, age, and
metallicity, an arrangement that might survive to the present day given the
paucity of major mergers. Since angular momentum is acquired largely at
turnaround, morphology is imprinted early by the interplay of the tidal field
and the shape of the material destined to form the galaxy.
