Globular Cluster Formation from Colliding Substructure
  We investigate a scenario where the formation of Globular Clusters (GCs) is
triggered by high-speed collisions between infalling atomic-cooling subhalos
during the assembly of the main galaxy host, a special dynamical mode of star
formation that operates at high gas pressures and is intimately tied to LCDM
hierarchical galaxy assembly. The proposed mechanism would give origin to
"naked" globulars, as colliding dark matter subhalos and their stars will
simply pass through one another while the warm gas within them clashes at
highly supersonic speed and decouples from the collisionless component, in a
process reminiscent of the Bullet galaxy cluster. We find that the resulting
shock-compressed layer cools on a timescale that is typically shorter than the
crossing time, first by atomic line emission and then via fine-structure
metal-line emission, and is subject to gravitational instability and
fragmentation. Through a combination of kinetic theory approximation and
high-resolution $N$-body simulations, we show that this model may produce: (a)
a GC number-halo mass relation that is linear down to dwarf galaxy scales and
agrees with the trend observed over five orders of magnitude in galaxy mass;
(b) a population of old globulars with a median age of 12 Gyr and an age spread
similar to that observed; (c) a spatial distribution that is biased relative to
the overall mass profile of the host; and (d) a bimodal metallicity
distribution with a spread similar to that observed in massive galaxies.
