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We report the discovery of a giant stellar tidal stream in the halo of NGC4631, a nearby edge-on spiral galaxy interacting with the spiral NGC 4656, indeep images taken with a 40-cm aperture robotic telescope. The stream has twocomponents: a bridge-like feature extended between NGC 4631 and NGC 4656(stream_SE) and an overdensity with extended features on the opposite side ofthe NGC 4631 disk (stream_NW). Together, these features extend more than 85 kpcand display a clear (g-r) colour gradient. The orientation of stream_SErelative to the orientations of NGC 4631 and NGC 4656 is not consistent with anorigin from interaction between these two spirals, and is more likely debrisfrom a satellite encounter. The stellar tidal features can be qualitativelyreproduced in an N-body model of the tidal disruption of a single, massivedwarf satellite on a moderately eccentric orbit (e=0.6) around NGC 4631 over$\sim$ 3.5 Gyr, with a dynamical mass ratio (m1:m2) of ~40. Both modelling andinferences from the morphology of the streams indicate these are not associatedwith the complex HI tidal features observed between both spirals, which likelyoriginate from a more recent, gas-rich accretion event. The detailed structureof stream_NW suggests it may contain the progenitor of the stream, in agreementwith the N-body model. In addition, stream_NW is roughly aligned with two veryfaint dwarf spheroidal candidates. The system of dwarf galaxies and the tidalstream around NGC 4631 can provide an additional interesting case for exploringthe anisotropy distribution of satellite galaxies recently reported in LocalGroup spiral galaxies by means of future follow-up observations.

Discovery of a stellar tidal stream around the Whale galaxy, NGC 4631