We present a sample of 54 disk galaxies which have well developed extraplanar structures. We selected them using visual inspections from the color images of the Sloan Digital Sky Survey. Since the sizes of the extraplanar structures are comparable to the disks, they are considered as prominent stellar halos rather than large bulges. A single S´ersic profile fitted to the surface brightness along the minor-axis of the disk shows a luminosity excess in the central regions for the majority of sample galaxies. This central excess is considered to be caused by the central bulge component. The mean S´ersic index of the single component model is 1.1 ± 0.9. A double S´ersic profile model that employs n = 1 for the inner region, and varying n for the outer region, provides a better fit than the single S´ersic profile model. For a small fraction of galaxies, a S´ersic profile fitted with n = 4 for the inner region gives similar results. There is a weak tendency of increasing n with increasing luminosity and central velocity dispersion, but there is no dependence on the local background density.

Gravitational interactions — mergers and y-by encounters — between galaxies play a key role as the drivers of their evolution. Here we perform a cosmological N-body simulation using the tree-particle-mesh code GOTPM, and attempt to separate out the effects of mergers and y-bys between dark matter halos. Once close pair halos are identified by the halo finding algorithm PSB, they are classified into mergers (E12 < 0) and y-by encounters (E12 > 0) based on the total energy (E12) between two halos. The y-by and merger fractions as functions of redshift, halo masses, and ambient environments are calculated and the result shows the following: (1) Among Milky-way sized halos (0:33-2:0 X 1012h-1M⊙), 5:37±0:03% have experienced major y-bys and 7.98±0.04% have undergone major mergers since z ~ 1; (2) Among dwarf halos (0:1 - 0.33 X 1012h-1M⊙), 6.42 ± 0.02% went through major y-bys and 9.51 ± 0.03% experienced major mergers since z ~ 1; (3) Milky-way sized halos in the cluster environment experienced fly-bys (mergers) 4-11(1.5 - 1.7) times more frequently than those in the field since z ~ 1; and (4) Approaching z = 0, the y-by fraction decreases sharply with the merger fraction remaining constant, implying that the empirical pair/merger fractions (that decrease from z ~ 1) are in fact driven by the fly-bys, not by the mergers themselves.