Stellar Tidal Streams in Spiral Galaxies of the Local Volume: A Pilot
  Survey with Modest Aperture Telescopes
  [Abridged] Within the hierarchical framework for galaxy formation, minor
merging and tidal interactions are expected to shape all large galaxies to the
present day. As a consequence, most seemingly normal disk galaxies should be
surrounded by spatially extended stellar 'tidal features' of low surface
brightness. As part of a pilot survey for such interaction signatures, we have
carried out ultra deep, wide field imaging of 8 isolated spiral galaxies in the
Local Volume, with data taken at small (D=0.1-0.5m) robotic telescopes that
provide exquisite surface brightness sensitivity (mu_V)~28.5$ mag/arcsec^2).
This initial observational effort has led to the discovery of six previously
undetected extensive (to ~30 kpc) stellar structures in the halos surrounding
these galaxies, likely debris from tidally disrupted satellites. In addition,
we confirm and clarify several enormous stellar over-densities previously
reported in the literature, but never before interpreted as tidal streams. Even
this pilot sample of galaxies exhibits strikingly diverse morphological
characteristics of these extended stellar features: great circle-like features
that resemble the Sagittarius stream surrounding the Milky Way, remote shells
and giant clouds of presumed tidal debris far beyond the main stelar body, as
well as jet-like features emerging from galactic disks. A qualitative
comparison with available simulations set in a Lambda-Cold Dark Matter
cosmology shows that the extraordinary variety of stellar morphologies detected
in this pilot survey matches that seen in those simulations. The common
existence of these tidal features around 'normal' disk galaxies and the
morphological match to the simulations constitutes new evidence that these
theoretical models also apply to a large number of other Milky Way-mass disk
galaxies in the Local Volume.
