Discovery of a stellar tidal stream around the Whale galaxy, NGC 4631
  We report the discovery of a giant stellar tidal stream in the halo of NGC
4631, a nearby edge-on spiral galaxy interacting with the spiral NGC 4656, in
deep images taken with a 40-cm aperture robotic telescope. The stream has two
components: a bridge-like feature extended between NGC 4631 and NGC 4656
(stream_SE) and an overdensity with extended features on the opposite side of
the NGC 4631 disk (stream_NW). Together, these features extend more than 85 kpc
and display a clear (g-r) colour gradient. The orientation of stream_SE
relative to the orientations of NGC 4631 and NGC 4656 is not consistent with an
origin from interaction between these two spirals, and is more likely debris
from a satellite encounter. The stellar tidal features can be qualitatively
reproduced in an N-body model of the tidal disruption of a single, massive
dwarf 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 and
inferences from the morphology of the streams indicate these are not associated
with the complex HI tidal features observed between both spirals, which likely
originate from a more recent, gas-rich accretion event. The detailed structure
of stream_NW suggests it may contain the progenitor of the stream, in agreement
with the N-body model. In addition, stream_NW is roughly aligned with two very
faint dwarf spheroidal candidates. The system of dwarf galaxies and the tidal
stream around NGC 4631 can provide an additional interesting case for exploring
the anisotropy distribution of satellite galaxies recently reported in Local
Group spiral galaxies by means of future follow-up observations.
