The Milky Way, an Exceptionally Quiet Galaxy: Implications for the Formation of Spiral Galaxies

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 V_flat versus M_K (the Tully-Fisher relation), j_disk (angular momentum), and the average Fe abundance, [Fe/H], of stars in the galaxy outskirts. We find, for all relationships, that the Milky Way is systematically offset by ~1 σ, showing a significant deficiency in stellar mass, angular momentum, disk radius, and [Fe/H] in the stars in its outskirts at a given V_flat. On the basis of their location in the (M_K, V_flat, and R_d) volume, the fraction of spirals like the Milky Way is 7% ± 1%, while M31 appears to be a "typical" spiral. Our galaxy appears to have escaped any significant merger over the last ~10 Gyr, which may explain why it is deficient by a factor of 2-3 in stellar mass, angular momentum, and outskirt metallicity, thus unrepresentative of the typical spiral. As with M31, most local spirals show evidence of a history shaped mainly by relatively recent merging. 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. However, the so-called spiral-rebuilding scenario proposed two years ago by Hammer et al. is consistent with the properties of both distant galaxies and of their descendants, the local spirals.