  We inspect the coupled dependence of physical parameters of the SDSS galaxies
on the small-scale (distance to and morphology of the nearest neighbor galaxy)
and the large-scale (background density smoothed over 20 nearby galaxies)
environments. The impacts of interaction on galaxy properties are detected at
least out to the neighbor separation corresponding to the virial radius of
galaxies, which is typically between 200 and 400 h^{-1}kpc for the galaxies in
our sample. To detect these long-range interaction effects it is crucial to
divide galaxy interactions into four cases dividing the morphology of target
and neighbor galaxies into early and late types. We show that there are two
characteristic neighbor-separation scales where the galaxy interactions cause
abrupt changes in the properties of galaxies. The first scale is the virial
radius of the nearest neighbor galaxy r_{vir,nei}. Many physical parameters
start to deviate from those of extremely isolated galaxies at the projected
neighbor separation r_p of about r_{vir,nei}. The second scale is at r_p
\approx 0.05 r_{vir,nei} = 10--20 h^{-1} kpc, and is the scale at which the
galaxies in pairs start to merge. We find that late-type neighbors enhance the
star formation activity of galaxies while early-type neighbors reduce it, and
that these effects occur within r_{vir,nei}. The hot halo gas and cold disk gas
must be participating in the interactions at separations less than the virial
radius of the galaxy plus dark halo system. Our results also show that the role
of the large-scale density in determining galaxy properties is minimal once
luminosity and morphology are fixed. We propose that the weak residual
dependence of galaxy properties on the large-scale density is due to the
dependence of the halo gas property on the large-scale density.

Astrophysics