Subhalo Demographics in the Illustris Simulation: Effects of Baryons and
  Halo-to-Halo Variation
  We study the abundance of subhaloes in the hydrodynamical cosmological
simulation Illustris, which includes both baryons and dark matter in a
$\Lambda$CDM volume 106.5 Mpc a side. We compare Illustris to its dark
matter-only (DMO) analogue, Illustris-Dark, and quantify the effects of
baryonic processes on the demographics of subhaloes in the host mass range
$10^{11}$ to $3 \times 10^{14} \msun$. We focus on both the evolved ($z=0$)
subhalo cumulative mass functions (SHMF) and the statistics of subhaloes ever
accreted, i.e. infall subhalo mass function. We quantify the variance in
subhalo abundance at fixed host mass and investigate the physical reasons
responsible for such scatter. We find that in Illustris, baryonic physics
impacts both the infall and $z=0$ subhalo abundance by tilting the DMO function
and suppressing the abundance of low-mass subhaloes. The breaking of
self-similarity in the subhalo abundance at $z=0$ is enhanced by the inclusion
of baryonic physics. The non-monotonic alteration of the evolved subhalo
abundances can be explained by the modification of the concentration--mass
relation of Illustris hosts compared to Illustris-Dark.Interestingly, the
baryonic implementation in Illustris does not lead to an increase in the
halo-to-halo variation compared to Illustris-Dark. In both cases, the
normalized intrinsic scatter today is larger for Milky Way-like haloes than for
cluster-sized objects. For Milky Way-like haloes, it increases from about eight
per cent at infall to about 25 per cent at the current epoch. In both runs,
haloes of fixed mass formed later host more subhaloes than early formers.
