The milky way, an exceptionally quiet galaxy: Implications for the formation of spiral galaxies (original) (raw)
The Milky Way has been generally considered to be representative of the numerous spiral galaxies inhabiting the local Universe, thus providing general and perhaps the most detailed constraints on numerical models of galaxy formation. We compare both the Milky Way and M31 galaxies to local external disk galaxies within the same mass range, using their locations in the planes drawn by Vf lat versus MK (the “Tully-Fisher” relation), jdisk (angular momentum) and the average Fe abundance, [Fe/H], of stars in the outskirts of the galaxy. These relations are thought to be the imprints of the dynamical, star-formation, and accretion history of disk galaxies. We compare the best established Tully-Fisher relations and reconcile their slopes and zero points in the plane MK-Vf lat. We then compare the properties of local spirals from a representative sample to those of the Milky Way and M31 considering how these two galaxies would appear if observed at larger distances. We find, for all relationships, that the Milky Way is systematically offset by ∼ 1 σ or more from the distribution of comparable local galaxies: specifically, it shows a too small stellar mass, angular momentum, disk radius and outskirts stars [Fe/H] ratio at a given Vf lat, the latter being taken as a proxy for the total mass. In contrast with the Milky Way, M31 lies well within the mean of the fundamental relationships. On the basis of their locations in the (MK , Vf lat and R d) volume, the fraction of spirals like the Milky Way is 7 ±1%, while M31 appears to be a “typical” spiral. As with M31, the bulk of local spirals show evidence for a formation history shaped mainly by merging. The Milky Way appears to have had an exceptionally quiet formation history and had escaped any significant merger over the last ∼10 Gyrs which may explain why its angular momentum, stellar mass and [Fe/H](outskirts) are two to three times smaller than those of other local spirals. We conclude that the standard scenario of secular evolution driven by the accretion of gas and disk instabilities is generally unable to reproduce the properties of most (if not all) spiral galaxies, which are well represented by M31. However, the relatively recent proposal explaining the evolution of spiral galaxies through merging (the so-called “spiral rebuilding” scenario of Hammer et al. 2005) is consistent with the properties of both distant galaxies (e.g., stellar mass assembly through episodic IR luminous burst phases driven predominately by mergers) as well as to those of their descendants – the local spirals.