  The paper introduces the spirality character of the almost fiber part for a
closed essentially immersed subsurface of a closed orientable aspherical
3-manifold, which generalizes an invariant due to Rubinstein and Wang. The
subsurface is virtually embedded if and only if the almost fiber part is
aspiral, and in this case, the subsurface is virtually a leaf of a taut
foliation. Besides other consequences, examples are exhibited that
non-geometric 3-manifolds with no Seifert fibered pieces may contain
essentially immersed but not virtually embedded closed subsurfaces.
