The NGC 4013 tale: a pseudo-bulged, late-type spiral shaped by a major merger

Many spiral galaxy haloes show stellar streams with various morphologies when observed with deep images. The origin of these tidal features is discussed, either coming from a satellite infall or caused by residuals of an ancient, gas-rich major merger. By modelling the formation of the peculiar features observed in the NGC 4013 halo, we investigate their origin. By using GADGET -2 with implemented gas cooling, star formation, and feedback, we have modelled the overall NGC 4013 galaxy and its associated halo features. A gas-rich major merger occurring 2.7-4.6 Gyr ago succeeds in reproducing the NGC 4013 galaxy properties, including all the faint stellar features, strong gas warp, boxy-shaped halo and vertical 3.6 mum luminosity distribution. High gas fractions in the progenitors are sufficient to reproduce the observed thin and thick discs, with a small bulge fraction, as observed. A major merger is able to reproduce the overall NGC 4013 system, including the warp strength, the red colour and the high stellar mass density of the loop, while a minor merger model cannot. Because the gas-rich model suffices to create a pseudo-bulge with a small fraction of the light, NGC 4013 is perhaps the archetype of a late-type galaxy formed by a relatively recent merger. Then late type, pseudo-bulge spirals are not mandatorily made through secular evolution, and the NGC 4013 properties also illustrate that strong warps in isolated galaxies may well occur at a late phase of a gas-rich major merger.Comment: 11 pages,9 figures,accepted for publication in MNRA

The vast thin plane of M31 co-rotating dwarfs: an additional fossil signature of the M31 merger and of its considerable impact in the whole Local Group

The recent discovery by Ibata et al. (2013) of a vast thin disk of satellites (VTDS) around M31 offers a new challenge for the understanding of the Local Group properties. This comes in addition to the unexpected proximity of the Magellanic Clouds (MCs) to the Milky Way (MW), and to another vast polar structure (VPOS), which is almost perpendicular to our Galaxy disk. We find that the VTDS plane is coinciding with several stellar, tidally-induced streams in the outskirts of M31, and, that its velocity distribution is consistent with that of the Giant Stream (GS). This is suggestive of a common physical mechanism, likely linked to merger tidal interactions, knowing that a similar argument may apply to the VPOS at the MW location. Furthermore, the VTDS is pointing towards the MW, being almost perpendicular to the MW disk, as the VPOS is. We compare these properties to the modelling of M31 as an ancient, gas-rich major merger, which has been successfully used to predict the M31 substructures and the GS origin. We find that without fine tuning, the induced tidal tails are lying in the VTDS plane, providing a single and common origin for many stellar streams and for the vast stellar structures surrounding both the MW and M31. The model also reproduces quite accurately positions and velocities of the VTDS dSphs. Our conjecture leads to a novel interpretation of the Local Group past history, as a gigantic tidal tail due to the M31 ancient merger is expected to send material towards the MW, including the MCs. Such a link between M31 and the MW is expected to be quite exceptional, though it may be in qualitative agreement with the reported rareness of MW-MCs systems in nearby galaxies.Comment: Accepted for publication in MNRAS, 8 pages, 3 figure