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

摘要

Many haloes of nearby disc galaxies contain faint and extended features,including loops, which are often interpreted as relics of satellite infall inthe main galaxy's potential well. In most cases, however, the residual nucleusof the satellite is not seen, although it is predicted by numericalsimulations. We test whether such faint and extended features can be associatedto gas-rich, major mergers, which may also lead to disc rebuilding and thus bea corner stone for the formation of spiral galaxies. Using the TreeSPH codeGADGET-2, we model the formation of an almost bulge-less galaxy similar to NGC5907 (B/T $\le$ 0.2) after a gas-rich major merger. We indeed find that 3:1major mergers can form features similar to the loops found in many galactichaloes, including in NGC 5907, and can reproduce an extended thin disc, abulge, as well as the pronounced warp of the gaseous disc. Even though itremains difficult to fully cover the large volume of free parameters, thepresent modelling of the loops in NGC 5907 proves that they could well be theresult of a major merger. It has many advantages over the satellite infallscenario; e.g., it solves the problem of the visibility of the satelliteremnant, and it may explain some additional features in the NGC 5907 halo, aswell as some gas properties of this system. For orbital parameters derived fromcosmological simulations, the loops in NGC 5907 can be reproduced by majormergers (3:1 to 5:1) and possibly by intermediate mergers (5:1 to 12:1). Themajor merger scenario thus challenges the minor merger one and could explainmany properties that haloes of spiral galaxies have in common, including theirred colours and the presence of faint extended features.