Hydrodynamical simulations and semi-analytic models of galaxy formation: two sides of the same coin

Abstract

In this work we develop a new method to turn a state-of-the-art\nhydrodynamical cosmological simulation of galaxy formation (HYD) into a simple\nsemi-analytic model (SAM). This is achieved by summarizing the efficiencies of\naccretion, cooling, star formation, and feedback given by the HYD, as functions\nof the halo mass and redshift. Surprisingly, by turning the HYD into a SAM, we\nconserve the mass of individual galaxies, with deviations at the level of 0.1\ndex, on an object-by-object basis. This is true for all redshifts, and for the\nmass of stars and gas components, although the agreement reaches 0.2 dex for\nsatellite galaxies at low redshift. We show that the same level of accuracy is\nobtained even in case the SAM uses only one phase of gas within each galaxy.\nMoreover, we demonstrate that the formation history of one massive galaxy\nprovides sufficient information for the SAM to reproduce the population of\ngalaxies within the entire cosmological box. The reasons for the small scatter\nbetween the HYD and SAM galaxies are: a) The efficiencies are matched as\nfunctions of the halo mass and redshift, meaning that the evolution within\nmerger-trees agrees on average. b) For a given galaxy, efficiencies fluctuate\naround the mean value on time scales of 0.2-2 Gyr. c) The various mass\ncomponents of galaxies are obtained by integrating the efficiencies over time,\naveraging out these fluctuations. We compare the efficiencies found here to\nstandard SAM recipes and find that they often deviate significantly. For\nexample, here the HYD shows smooth accretion that is less effective for low\nmass haloes, and is always composed of hot or dilute gas; cooling is less\neffective at high redshift; and star formation changes only mildly with cosmic\ntime. The method developed here can be applied in general to any HYD, and can\nthus serve as a common language for both HYDs and SAM (Abridged).\n