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Deep optical imaging with both Hyper Suprime-Cam and Suprime-Cam on the 8.2 mSubaru Telescope reveals a number of outer faint structures around thearchetypical Seyfert galaxy NGC 1068 (M 77). We find three ultra diffuseobjects (UDOs) around NGC 1068. Since these UDOs are located within theprojected distance of 45 kpc from the center of NGC 1068, they appear to beassociated with NGC 1068. Hereafter, we call them UDO-SW, UDO-NE, and UDO-SEwhere 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 structurearound the main body of NGC 1068, providing evidence for the physicalconnection to NGC 1068. We consider that UDO-SE may be a tidal dwarf galaxy. Wealso find another UDO-like object that is 2 magnitudes fainter and smaller by afactor of 3 to 5 than those of the three UDOs. This object may belong to aclass of low surface brightness galaxy. Since this object is located along theline connecting UDO-NE and UDO-SW, it is suggested that this object is relatedto the past interaction event that formed the loop by UDO-NE and UDO-SW, thusimplying the physical connection to NGC 1068. Another newly-discovered featureis an asymmetric outer one-arm structure emanated from the western edge of theoutermost disk of NGC 1068 together with a ripple-like structure at theopposite side. These structures are expected to arise in a late phase of aminor merger according to published numerical simulations of minor mergers. Allthese lines of evidence show that NGC 1068 experienced a minor merger severalbillions years ago. We then discuss the minor-merger driven triggering ofnuclear activity in the case of NGC 1068.

Morphological evidence for a past minor merger in the Seyfert galaxy NGC 1068