  We construct several Milky Way-like galaxy models containing a gas halo (as
well as gaseous and stellar disks, a dark matter halo, and a stellar bulge)
following either an isothermal or an NFW density profile with varying mass and
initial spin. In addition, galactic winds associated with star formation are
tested in some of the simulations. We evolve these isolated galaxy models using
the GADGET-3 $N$-body/hydrodynamic simulation code, paying particular attention
to the effects of the gas halo on the evolution. We find that the evolution of
the models is strongly affected by the adopted gas halo component. The model
without a gas halo shows an increasing star formation rate (SFR) at the
beginning of the simulation for some hundreds of millions of years and then a
continuously decreasing rate to the end of the run at 3 Gyr. On the other hand,
the SFRs in the models with a gas halo emerge to be either relatively flat
throughout the simulations or increasing over a gigayear and then decreasing to
the end. The models with the more centrally concentrated NFW gas halo show
overall higher SFRs than those with the isothermal gas halo of the equal mass.
The gas accretion from the halo onto the disk also occurs more in the models
with the NFW gas halo, however, this is shown to take place mostly in the inner
part of the disk and not to contribute significantly to the star formation
unless the gas halo has very high density at the central part. The rotation of
a gas halo is found to make SFR lower in the model. The SFRs in the runs
including galactic winds are found to be lower than the same runs but without
winds. We conclude that the effects of a hot gaseous halo on the evolution of
galaxies are generally too significant to be simply ignored, and expect that
more hydrodynamical processes in galaxies could be understood through numerical
simulations employing both gas disk and gas halo components.
