The current version of the GALICS catalog is available at http://vo.iucaa.ernet.in/galics

Researchers who are involved in observational programs, and who are willing to get mock catalogs that are customized to their needs should contact the author of this article.

A brief introduction to the simple modelling of hierarchical galaxy formation

In the early 1990’s, several authors proposed attempts to model galaxy formation in a cosmological context, from initial density fluctuations to the observed features of galaxies. These attempts were made possible by a slow but steady progress in the undertanding of the role of (cold) dark matter, and of the processes that rule the physics of baryons. In these early papers, previous works were linked together to produce a consistent scenario, with some variants, that was able to predict a lot of properties, and to be tested against observations.

The implementation of this scenario was made possible through semi-analytic calculations that can be run quite easily on a computer. It starts from the modelling of the power spectrum of (dark matter) density fluctuations at some given age after recombination. Then the properties of dark matter halos are easily computed from two numbers: the (linearly extrapolated to z=0) density contrast and the halo mass. More specifically, the collapse redshift, the virial radius, the circular velocity, and the density profile after collapse can be computed. The Press-Schecher formalism gives the number density of halos of mass M at a collapse redshift z, as well as (in its extended form) the probabilty density that matter elements that are in a given halo at a given redshift can be incorporated in a more massive halos at a lower redshift. All this is sufficient to draw Monte-Carlo realizations of this process, and to construct merging history trees of dark matter halos, with all their properties.

Of course, these merging history trees of dark matter halos, and the properties of the latter can be measured on large simulations of cosmological volumes. In any case, it is necessary to get this description of the history of the halos as a first step.

Then the description of the baryons can be modelled. Simple, but physically motivated recipes are introduced to model the following processes: gas heating during the collapse of dark matter halos (and possibly after heating by radiations), gas cooling in the potential wells of dark matter halos, conservation of angular momentum, rotational or pressure equilibrium, star formation, stellar evolution, stellar feedback (energy, gas, and heavy elements) to the interstellar and/or intergalactic medium, interaction between galaxies, gas stripping, major and minor merging of galaxies and the formation of spheroids.

This gives the merging history tree of galaxies within the merging history tree of dark matter halos.

Once the star formation rate and metal content histories of each galaxy is computed along its merging history tree, the multiwavelength spectrophotometric properties can be derived. A quick estimate of the dust content of each galaxy (based on its gas metallicity) and simple transfer can be added in order to compute (UV-optical) extinction and (IR-submm) emission.

At the end of the computation, the statistical distribution of quantities such as galaxy masses, gas contents, luminosities, radii, morphologies, etc. can be predicted at any time/redshift.