  We investigate the baryon fraction in dark matter haloes formed in
non-radiative gas-dynamical simulations of the LambdaCDM cosmogony. By
combining a realisation of the Millennium Simulation (Springel et al.) with a
simulation of a smaller volume focussing on dwarf haloes, our study spans five
decades in halo mass, from 10^10 Msun/h to 10^15 Msun/h. We find that the
baryon fraction within the halo virial radius is typically 90% of the cosmic
mean, with an rms scatter of 6%, independently of redshift and of halo mass
down to the smallest resolved haloes. Our results show that, contrary to the
proposal of Mo et al. (2005), pre-virialisation gravitational heating is unable
to prevent the collapse of gas within galactic and proto-galactic haloes, and
confirm the need for non-gravitational feedback in order to reduce the
efficiency of gas cooling and star formation in dwarf galaxy haloes.
Simulations including a simple photoheating model (where a gas temperature
floor of T_{floor} = 2x10^4 K is imposed from z=11) confirm earlier suggestions
that photoheating can only prevent the collapse of baryons in systems with
virial temperatures T_{200} < ~2.2 T_{floor} ~ 4.4x10^4 K (corresponding to a
virial mass of M_{200} ~ 10^10 Msun/h and a circular velocity of V_{200} ~ 35
km/s). Photoheating may thus help regulate the formation of dwarf spheroidals
and other galaxies at the extreme faint-end of the luminosity function, but it
cannot, on its own, reconcile the abundance of sub-L* galaxies with the vast
number of dwarf haloes expected in the LambdaCDM cosmogony. The lack of
evolution or mass dependence seen in the baryon fraction augurs well for X-ray
cluster studies that assume a universal and non-evolving baryon fraction to
place constraints on cosmological parameters.
