Rapid filamentary accretion as the origin of extended thin discs

Galactic outflows driven by stellar feedback are crucial for explaining the inefficiency of star formation in galaxies. Although strong feedback can promote the formation of galactic discs by limiting star formation at early times and removing low angular momentum (AM) gas, it is not understood how the same feedback can result in diverse objects such as elliptical galaxies or razor thin spiral galaxies. We investigate this problem using cosmological zoom-in simulations of two galaxies forming within 10¹² M_⊙ haloes with almost identical mass accretion histories and halo spin parameters. However, the two resulting galaxies end up with very different bulge-to-disc ratios at z = 0. At z > 1.5, the two galaxies feature a surface density of star formation Σ_SFR ≃ 10 M_⊙ yr^-1 kpc^-2, leading to strong outflows. After the last starburst episode, both galaxies feature a dramatic gaseous disc growth from 1 to 5 kpc during 1 Gyr, a decisive event we dub 'the Grand Twirl'. After this event, the evolutionary tracks diverge strongly, with one galaxy ending up as a bulge-dominated galaxy, whereas the other ends up as a disc-dominated galaxy. The origins of this dichotomy are the AM of the accreted gas, and whether it adds constructively to the initial disc angular momentum. The build-up of this extended disc leads to a rapid lowering of Σ_SFR by over two orders of magnitude with Σ_SFR ≲ 0.1 M_⊙ yr^-1 kpc^-2, in remarkable agreement with what is derived from Milky Way stellar populations. As a consequence, supernovae explosions are spread out and cannot launch galactic outflows anymore, allowing for the persistence of a thin, gently star-forming, extended disc.