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

Abstract

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.

Metallicity and Age of the Stellar Stream Around the Disk Galaxy NGC 5907

“…The stellar stream in NGC 5907 has been previously modeled with a satellite galaxy accretion event (Martínez-Delgado et al 2008, total mass ratio 1:4000) and with a gasrich major galaxy merger (Wang et al 2012, total mass ratio Figure 7) through the data (black) and extended PSF background models (blue and red colors) crossing the plane of NGC 5907 and along the stellar stream. The slices are plotted as a function of distance from the galaxy midplane.…”

“…Martínez-Delgado et al (2008) also produced a simple N-body model that mimicked the observed loopy stellar stream as a fossil of the tidal disruption of a single satellite in a merger event (total mass ratio of 1:4000), rejecting the hypothesis of multiple merger events in the halo of NGC 5907. In contrast, Wang et al (2012) reported a model that reproduced the general structure of the stellar stream around NGC5907 in a major merger scenario. Wang et al (2012) suggested that the colors of the stream may be used to distinguish between a minor satellite accretion event and a major merger origin of the stellar stream in NGC5907 (the stream is too faint to be observed spectroscopically).…”

“…In contrast, Wang et al (2012) reported a model that reproduced the general structure of the stellar stream around NGC5907 in a major merger scenario. Wang et al (2012) suggested that the colors of the stream may be used to distinguish between a minor satellite accretion event and a major merger origin of the stellar stream in NGC5907 (the stream is too faint to be observed spectroscopically). They used both (1) the color-inferred iron abundance in the disk outskirts (affected also by stars in the inner halo) and found that the R−I color from Zheng et al (1999) is similar to that of massive disk galaxies instead of dwarf satellite galaxies, and (2) the mass-metallicity relation in which, for example, the low-mass Sagittarius satellite galaxy (with low [Fe/H]) would produce a bluer color than what is observed.…”

Imaging of NGC 5907's stellar stream

“…Martínez-Delgado et al (2008) modeled the stellar stream with a satellite companion having a stellar mass of ≈ 3.5× 10 8 M . In contrast, Wang et al (2012) modeled the stream with a major merger (mass ratio 1:3 -1:12). While our relatively high metallicity value suggests a major merger origin of the stream, it is possible that the metallicity is consistent with a satellite accretion event such as the Sagittarius Stream (Chou et al 2007) or the giant stream in M31 (Kalirai et al 2006).…”

The Milky Way and other spiral galaxies

“…There are now many simulations of disk formation via gas-rich mergers, including in the cosmological context [36]. They naturally produce an important thick disk component that is made of material re-accreted by the newly re-formed galaxy [15,38].…”