Globular cluster numbers in dark matter haloes in a dual formation
  scenario: an empirical model within EMERGE
  We present an empirical model for the number of globular clusters (GCs) in
galaxies based on recent data showing a tight relationship between dark matter
halo virial masses and GC numbers. While a simple base model forming GCs in
low-mass haloes reproduces this relation, we show that a second formation
pathway for GCs is needed to account for observed younger GC populations. We
confirm previous works that reported the observed linear correlation as being a
consequence of hierarchical merging and its insensitivity to the exact GC
formation processes at higher virial masses, even for a dual formation
scenario. We find that the scatter of the linear relation is strongly
correlated with the relative amount of smooth accretion: the more dark matter
is smoothly accreted, the fewer GCs a halo has compared to other haloes of the
same mass. This scatter is smaller than that introduced by halo mass
measurements, indicating that the number of GCs in a galaxy is a good tracer
for its dark matter mass. Smooth accretion is also the reason for a lower
average dark matter mass per GC in low-mass haloes. Finally, we successfully
reproduce the observed general trend of GCs being old and the tendency of more
massive haloes hosting older GC systems. Including the second GC formation
mechanism through gas-rich mergers leads to a more realistic variety of GC age
distributions and also introduces an age inversion in the halo virial mass
range $\log M_\mathrm{vir}/\mathrm{M}_\odot = 11{-}13$.
