Morphological evidence for a past minor merger in the Seyfert galaxy NGC
  1068
  Deep optical imaging with both Hyper Suprime-Cam and Suprime-Cam on the 8.2 m
Subaru Telescope reveals a number of outer faint structures around the
archetypical Seyfert galaxy NGC 1068 (M 77). We find three ultra diffuse
objects (UDOs) around NGC 1068. Since these UDOs are located within the
projected distance of 45 kpc from the center of NGC 1068, they appear to be
associated with NGC 1068. Hereafter, we call them UDO-SW, UDO-NE, and UDO-SE
where UDO = Ultra Diffuse Object, SW = south west, NE = north west, and SE =
south east; note that UDO-SE was already found in the SDSS Stripe 82 data.
Among them, both UDO-NE and UDO-SW appear to show a loop or stream structure
around the main body of NGC 1068, providing evidence for the physical
connection to NGC 1068. We consider that UDO-SE may be a tidal dwarf galaxy. We
also find another UDO-like object that is 2 magnitudes fainter and smaller by a
factor of 3 to 5 than those of the three UDOs. This object may belong to a
class of low surface brightness galaxy. Since this object is located along the
line connecting UDO-NE and UDO-SW, it is suggested that this object is related
to the past interaction event that formed the loop by UDO-NE and UDO-SW, thus
implying the physical connection to NGC 1068. Another newly-discovered feature
is an asymmetric outer one-arm structure emanated from the western edge of the
outermost disk of NGC 1068 together with a ripple-like structure at the
opposite side. These structures are expected to arise in a late phase of a
minor merger according to published numerical simulations of minor mergers. All
these lines of evidence show that NGC 1068 experienced a minor merger several
billions years ago. We then discuss the minor-merger driven triggering of
nuclear activity in the case of NGC 1068.
