Galaxies-Intergalactic Medium Interaction Calculation --I. Galaxy
  formation as a function of large-scale environment
  [Abridged] We present the first results of hydrodynamical simulations that
follow the formation of galaxies to z=0 in spherical regions of radius ~20
Mpc/h drawn from the Millennium Simulation. The regions have overdensities that
deviate by (-2, -1, 0, +1, +2)sigma from the cosmic mean, where sigma is the
rms mass fluctuation on a scale of ~20Mpc/h at z=1.5. The simulations have mass
resolution of up to 10^6 Msun/h, cover the entire range of large-scale
environments and allow extrapolation of statistics to the entire 500 (Mpc/h)^3
Millennium volume. They include gas cooling, photoheating from an ionising
background, SNe feedback and winds, but no AGN. We find that the specific SFR
density at z <~ 10 varies systematically from region to region by up to an
order of magnitude, but the global value, averaged over all volumes, reproduces
observational data. Massive, compact galaxies, similar to those observed in the
GOODS fields, form in the overdense regions as early as z=6, but do not appear
in the underdense regions until z~3. These environmental variations are not
caused by a dependence of the star formation properties on environment, but
rather by a strong variation of the halo mass function from one environment to
another, with more massive haloes forming preferentially in the denser regions.
At all epochs, stars form most efficiently in haloes of circular velocity ~ 250
km/s. However, the star formation history exhibits a form of "downsizing" (even
in the absence of AGN): the stars comprising massive galaxies at z=0 have
mostly formed by z=1-2, whilst those comprising smaller galaxies typically form
at later times. However, additional feedback is required to limit star
formation in massive galaxies at late times.
