2009ApJ...692..955M

We report the discovery of a giant, looplike stellar structure around the edge-on spiral galaxy NGC 4013. This arcing feature extends 6' (∼26 kpc in projected distance) northeast from the center and 3' (≃12 kpc) from the disk plane; likely related features are also apparent on the southwest side of the disk, extending to 4' (∼17 kpc). The detection of this low surface brightness (µ_R= 27.0^+0.3_–0.2 mag/arcsec2) structure is independently confirmed in three separate datasets from three different telescopes. Although its true three-dimensional geometry is unknown, the sky-projected morphology of this structure displays a match with the theoretical predictions for the edge-on, projected view of a stellar tidal stream of a dwarf satellite moving in a low inclined (≃25°), nearly circular orbit. Using the recent model of the Monoceros tidal stream in the Milky Way by Peñarrubia and colleagues as a template, we find that the progenitor system may have been a galaxy with an initial mass 6x10⁸ M_☉, whose current position and final fate are unknown. According to this simulation, the tidal stream may be approximately ∼2.8 Gyr of age. Our results demonstrate that NGC 4013, previously considered a prototypical isolated disk galaxy in spite of having one of the most prominent H I warps detected thus far, may have in fact suffered a recent minor merger. This discovery highlights that undisturbed disks at high surface brightness levels in the optical but warped in H I maps may in fact reveal complex signatures of recent accretion events in deep photometric surveys.