The role of mergers and halo spin in shaping galaxy morphology
  Mergers and the spin of the dark matter halo are factors traditionally
believed to determine the morphology of galaxies within a $\Lambda$CDM
cosmology. We study this hypothesis by considering approximately 18,000 central
galaxies at $z=0$ with stellar masses $M_{\ast} = 10^{9}-10^{12} \, {\rm
M}_{\odot}$ selected from the Illustris cosmological hydrodynamic simulation.
The fraction of accreted stars -- which measures the importance of massive,
recent and dry mergers -- increases steeply with galaxy stellar mass, from less
than 5 per cent in dwarfs to 80 per cent in the most massive objects, and the
impact of mergers on galaxy morphology increases accordingly. For galaxies with
$M_{\ast} \gtrsim 10^{11} \, {\rm M}_{\odot}$, mergers have the expected
effect: if gas-poor they promote the formation of spheroidal galaxies, whereas
gas-rich mergers favour the formation and survivability of massive discs. This
trend, however, breaks at lower masses. For objects with $M_{\ast} \lesssim
10^{11} \, {\rm M}_{\odot}$, mergers do not seem to play any significant role
in determining the morphology, with accreted stellar fractions and mean merger
gas fractions that are indistinguishable between spheroidal and disc-dominated
galaxies. On the other hand, halo spin correlates with morphology primarily in
the least massive objects in the sample ($M_{\ast} \lesssim 10^{10} \, {\rm
M}_{\odot}$), but only weakly for galaxies above that mass. Our results support
a scenario where (1) mergers play a dominant role in shaping the morphology of
massive galaxies, (2) halo spin is important for the morphology of dwarfs, and
(3) the morphology of medium-sized galaxies -- including the Milky Way -- shows
little dependence on galaxy assembly history or halo spin, at least when these
two factors are considered individually.
