Why stellar feedback promotes disc formation in simulated galaxies

We study how feedback influences baryon infall on to galaxies using cosmological, zoom-in simulations of haloes with present mass M_vir = 6.9 × 10¹¹ to 1.7 × 10¹² M_⊙. Starting at z = 4 from identical initial conditions, implementations of weak and strong stellar feedback produce bulge- and disc-dominated galaxies, respectively. Strong feedback favours disc formation: (1) because conversion of gas into stars is suppressed at early times, as required by abundance matching arguments, resulting in flat star formation histories and higher gas fractions; (2) because 50 per cent of the stars form in situ from recycled disc gas with angular momentum only weakly related to that of the z = 0 dark halo; (3) because late-time gas accretion is typically an order of magnitude stronger and has higher specific angular momentum, with recycled gas dominating over primordial infall; (4) because 25-30 per cent of the total accreted gas is ejected entirely before z ∼ 1, removing primarily low angular momentum material which enriches the nearby intergalactic medium. Most recycled gas roughly conserves its angular momentum, but material ejected for long times and to large radii can gain significant angular momentum before re-accretion. These processes lower galaxy formation efficiency in addition to promoting disc formation.