The galaxy size to halo spin relation of disk galaxies in cosmological
  hydrodynamical simulations
  In the standard disk galaxy formation model, the sizes of galactic disks are
tightly related to the spin parameters $\lambda$ of their dark matter haloes.
The model has been wildly adopted by various semi-analytic galaxy formation
models which have been extremely successful to interpret a large body of
observational data. However, the size-$\lambda$ correlation was rarely seen in
most modern hydrodynamical simulations of galaxy formation. In this short
paper, we make use of 4 sets of large hydrodynamical simulations to explore the
size-spin parameter relation with a large sample of simulated disk galaxies,
and compare it with a popular disk galaxy formation model of Mo et al. (1998).
Intriguingly, galactic sizes correlate strongly with spin parameters of their
dark matter haloes in the simulations developed by the IllustrisTNG
collaborations, albeit the relation does not always agree with prediction of
MMW98 model over all stellar mass range we examined. There is also a size-spin
correlation for the Milky way analogies in the EAGLE simulations, while it is
relatively weaker than that of the IllustrisTNG counterparts. For the dwarfs in
the simulations from the EAGLE collaboration, there is NULL correlation. We
conclude that either the detailed subgrid physics or hydrodynamics solvers
account for the size-spin relation, which will be explored in our future work.
