Loops formed by tidal tails as fossil records of a major merger
  Many haloes of nearby disc galaxies contain faint and extended features,
including loops, which are often interpreted as relics of satellite infall in
the main galaxy's potential well. In most cases, however, the residual nucleus
of the satellite is not seen, although it is predicted by numerical
simulations. We test whether such faint and extended features can be associated
to gas-rich, major mergers, which may also lead to disc rebuilding and thus be
a corner stone for the formation of spiral galaxies. Using the TreeSPH code
GADGET-2, we model the formation of an almost bulge-less galaxy similar to NGC
5907 (B/T $\le$ 0.2) after a gas-rich major merger. We indeed find that 3:1
major mergers can form features similar to the loops found in many galactic
haloes, including in NGC 5907, and can reproduce an extended thin disc, a
bulge, as well as the pronounced warp of the gaseous disc. Even though it
remains difficult to fully cover the large volume of free parameters, the
present modelling of the loops in NGC 5907 proves that they could well be the
result of a major merger. It has many advantages over the satellite infall
scenario; e.g., it solves the problem of the visibility of the satellite
remnant, and it may explain some additional features in the NGC 5907 halo, as
well as some gas properties of this system. For orbital parameters derived from
cosmological simulations, the loops in NGC 5907 can be reproduced by major
mergers (3:1 to 5:1) and possibly by intermediate mergers (5:1 to 12:1). The
major merger scenario thus challenges the minor merger one and could explain
many properties that haloes of spiral galaxies have in common, including their
red colours and the presence of faint extended features.
