The rates and modes of gas accretion on to galaxies and their gaseous
  haloes
  (Abridged) We study the rate at which gas accretes onto galaxies and haloes
and investigate whether the accreted gas was shocked to high temperatures
before reaching a galaxy. For this purpose we use a suite of large
cosmological, hydrodynamical simulations from the OWLS project. We improve on
previous work by considering a wider range of halo masses and redshifts, by
distinguishing accretion onto haloes and galaxies, by including important
feedback processes, and by comparing simulations with different physics. The
specific rate of gas accretion onto haloes is, like that for dark matter, only
weakly dependent on halo mass. For halo masses Mhalo>>10^11 Msun it is
relatively insensitive to feedback processes. In contrast, accretion rates onto
galaxies are determined by radiative cooling and by outflows driven by
supernovae and active galactic nuclei. Galactic winds increase the halo mass at
which the central galaxies grow the fastest by about two orders of magnitude to
Mhalo~10^12 Msun. Gas accretion is bimodal, with maximum past temperatures
either of order the virial temperature or <~10^5 K. The fraction of gas
accreted on to haloes in the hot mode is insensitive to feedback and metal-line
cooling. It increases with decreasing redshift, but is mostly determined by
halo mass, increasing gradually from less than 10% for ~10^11 Msun to greater
than 90% at 10^13 Msun. In contrast, for accretion onto galaxies the cold mode
is always significant and the relative contributions of the two accretion modes
are more sensitive to feedback and metal-line cooling. The majority of stars
present in any mass halo at any redshift were formed from gas accreted in the
cold mode, although the hot mode contributes typically over 10% for
Mhalo>~10^11 Msun. Galaxies, but not necessarily their gaseous haloes, are
predominantly fed by gas that did not experience an accretion shock when it
entered the host halo.
