  We present CoSANG (Coupling Semi-Analytic and N-body Galaxies), a new hybrid
model for cosmological dark matter and stellar halo simulations. In this
approach a collisionless model (Gadget3) is used for gravitational interactions
while a coupled semi-analytic model (SAGE) calculates baryonic effects at each
time-step. This live self-consistent interaction at each time-step is the key
difference between CoSANG and traditional semi-analytic models that are mainly
used for post-processing. By accounting for the gravitational effect of the
baryons, CoSANG can overcome some of the deficiencies of pure N-body
simulations, while being less computationally expensive than hydrodynamic
simulations. Moreover CoSANG can produce stellar halo populations via tagging
tracer dark matter particles. We demonstrate the performance and dynamical
accuracy of this approach using both controlled test simulations and a set of
three cosmological zoom-in simulations of Milky Way (MW) mass halos. We
simulate each target halo both without the coupling (dark matter only,
hereafter DMO) and with the coupling (CoSANG). We compare the internal
structure (subhalo distribution, shape and orientation) of the halos. The
following changes are observed in the CoSANG model compared to the DMO model:
1) the total number of subhaloes close to the center of the halo is reduced, 2)
the $V_{\mathrm{max}}$ distribution peaks at a lower value and lies below the
DMO model, 3) the axis ratio is smaller. The difference between DMO and CoSANG
simulations is more significant in early forming halos.
