Loops formed by tidal tails as fossil records of a major merger

摘要

Context. 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. Aims. 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. Our goal is to test whether the major merger scenario can provide a good model for a particularly difficult case, that of NGC?5907, and to compare to the scenario of a satellite infall. Methods. Using the TreeSPH code GADGET-2, we model the formation of an almost bulge-less galaxy similar to NGC?5907 (B/T???? 0.2) after a gas-rich major merger. First, we trace tidal tail particles captured by the galaxy gravitational potential to verify whether they can form loops similar to those discovered in the galactic haloes. Results. 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. Relatively small bulge fractions can be reproduced by a large gas fraction in the progenitors, as well as appropriate orbital parameters. Conclusions. 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.