EGG: hatching a mock Universe from empirical prescriptions
  This paper introduces EGG, the Empirical Galaxy Generator, a tool designed
within the ASTRODEEP collaboration to generate mock galaxy catalogs for deep
fields with realistic fluxes and simple morphologies. The simulation procedure
is based exclusively on empirical prescriptions -- rather than first principles
-- to provide the most accurate match with observations at 0<z<7. In
particular, we consider that galaxies can be either quiescent or star-forming,
and use their stellar mass (M*) and redshift (z) as the fundamental properties
from which all the other observables can be statistically derived. Drawing z
and M* from the observed galaxy stellar mass functions, we associate a star
formation rate (SFR) to each galaxy from the tight SFR-M* main sequence, while
dust attenuation, optical colors and morphologies (including bulge-to-total
ratios, sizes and aspect ratios) are obtained from empirical relations that we
establish from the high quality Hubble and Herschel observations available in
the CANDELS fields. Random scatter is introduced in each step to reproduce the
observed distributions of each parameter. Based on these observables, a
panchromatic spectral energy distribution (SED) is selected for each galaxy and
synthetic photometry is produced by integrating the redshifted SED in common
broad-band filters. Finally, the mock galaxies are placed on the sky at random
positions with a fixed angular two-point correlation function to implement
basic clustering. The resulting flux catalogs reproduce accurately the observed
number counts in all broad bands from the ultraviolet up to the sub-millimeter,
and can be directly fed to image simulators such as Skymaker. The images can
then be used to test source extraction softwares and image-based techniques
such as stacking. EGG is open-source, and is made available to the community
together with a set of pre-generated catalogs and images.
