The ATLAS3D project - IX. The merger origin of a fast- and a slow-rotating early-type galaxy revealed with deep optical imaging: first results

Abstract

The mass assembly of galaxies leaves imprints in their outskirts, such as\nshells and tidal tails. The frequency and properties of such fine structures\ndepend on the main acting mechanisms - secular evolution, minor or major\nmergers - and on the age of the last substantial accretion event. We use this\nto constrain the mass assembly history of two apparently relaxed nearby\nEarly-Type Galaxies (ETGs) selected from the Atlas-3D sample, NGC 680 and NGC\n5557. Our ultra deep optical images obtained with MegaCam on the\nCanada-France-Hawaii Telescope reach 29 mag/arcsec^2 in the g-band. They reveal\nvery low-surface brightness (LSB) filamentary structures around these\nellipticals. Among them, a gigantic 160 kpc long tail East of NGC 5557 hosts\ngas-rich star-forming objects. NGC 680 exhibits two major diffuse plumes\napparently connected to extended HI tails, as well as a series of arcs and\nshells. Comparing the outer stellar and gaseous morphology of the two\nellipticals with that predicted from models of colliding galaxies, we argue\nthat the LSB features are tidal debris, the star-forming objects near NGC 5557,\nlong lived Tidal Dwarf Galaxies and that each of the two ETGs was assembled\nduring a relatively recent, major wet merger, which likely occurred at a\nredshift below z = 0.5. The inner kinematics of NGC 680 is typical for fast\nrotators which make the bulk of nearby ETGs in the Atlas-3D sample. NGC 5557\nbelongs to the poorly populated class of massive, round, slow rotators that are\npredicted by semi-analytic models and cosmological simulations to be the\nend-product of a complex mass accretion history, involving ancient major\nmergers and more recent minor mergers. Our observations suggest that under\nspecific circumstances a single binary merger may dominate the formation\nhistory of such objects and thus that at least some massive ETGs may form at\nrelatively low redshift (abridged).\n