Abstract
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, with no significant systematics. 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, smooth accretion 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 SAMs.
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