DISCOVERY OF A GIANT STELLAR TIDAL STREAM AROUND THE DISK GALAXY NGC 4013

We report the discovery of a giant, looplike stellar structure around the edge-on spiral galaxy NGC 4013. This arcing feature extends 6' (similar to 26 kpc in projected distance) northeast from the center and 3' (similar or equal to 12 kpc) from the disk plane; likely related features are also apparent on the southwest side of the disk, extending to 4' (similar to 17 kpc). The detection of this low surface brightness (mu(R) = 27.0(-0.2)(+0.3) mag arcsec(-2)) 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 (similar or equal to 25 degrees), nearly circular orbit. Using the recent model of the Monoceros tidal stream in the Milky Way by Penarrubia and colleagues as a template, we find that the progenitor system may have been a galaxy with an initial mass 6 x 10(8) M-circle dot, whose current position and final fate are unknown. According to this simulation, the tidal stream may be approximately similar to 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 HI 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 HI maps may in fact reveal complex signatures of recent accretion events in deep photometric surveys.