Hydrodynamical simulations and semi-analytic models of galaxy formation:
  two sides of the same coin
  In this work we develop a new method to turn a state-of-the-art
hydrodynamical cosmological simulation of galaxy formation (HYD) into a simple
semi-analytic model (SAM). This is achieved by summarizing the efficiencies of
accretion, cooling, star formation, and feedback given by the HYD, as functions
of the halo mass and redshift. Surprisingly, by turning the HYD into a SAM, we
conserve the mass of individual galaxies, with deviations at the level of 0.1
dex, on an object-by-object basis. This is true for all redshifts, and for the
mass of stars and gas components, although the agreement reaches 0.2 dex for
satellite galaxies at low redshift. We show that the same level of accuracy is
obtained even in case the SAM uses only one phase of gas within each galaxy.
Moreover, we demonstrate that the formation history of one massive galaxy
provides sufficient information for the SAM to reproduce the population of
galaxies within the entire cosmological box. The reasons for the small scatter
between the HYD and SAM galaxies are: a) The efficiencies are matched as
functions of the halo mass and redshift, meaning that the evolution within
merger-trees agrees on average. b) For a given galaxy, efficiencies fluctuate
around the mean value on time scales of 0.2-2 Gyr. c) The various mass
components of galaxies are obtained by integrating the efficiencies over time,
averaging out these fluctuations. We compare the efficiencies found here to
standard SAM recipes and find that they often deviate significantly. For
example, here the HYD shows smooth accretion that is less effective for low
mass haloes, and is always composed of hot or dilute gas; cooling is less
effective at high redshift; and star formation changes only mildly with cosmic
time. The method developed here can be applied in general to any HYD, and can
thus serve as a common language for both HYDs and SAM (Abridged).
