Linking haloes to galaxies: how many halo properties are needed?

Recent studies emphasize that an empirical relation between the stellar mass of galaxies and the mass of their host dark matter subhaloes can predict the clustering of galaxies and its evolution with cosmic time. In this paper we study the various assumptions made by this methodology using a semi-analytical model (SAM). To this end, we randomly swap between the locations of model galaxies within a narrow range of subhalo mass (Minfall). We find that shuffled samples of galaxies have different auto-correlation functions in comparison with the original model galaxies. This difference is significant even if central and satellite galaxies are allowed to follow a different relation between Minfall and stellar mass, and can reach a factor of ∼2 for massive galaxies at redshift zero. We analyse three features within SAMs that contribute to this effect. (a) The relation between stellar mass and subhalo mass evolves with redshift for central galaxies, affecting satellite galaxies at the time of infall. (b) In addition, the stellar mass of galaxies falling into groups and clusters at high redshift is different from the mass of central galaxies at the same time. (c) The stellar mass growth for satellite galaxies after infall can be significant and depends on the infall redshift and the group mass. All of the above ingredients modify the stellar mass of satellite galaxies in a way that is more complicated than a dependence on the subhalo mass only. By using two different SAMs, we show that the above is true for differing models of galaxy evolution, and that the effect is sensitive to the treatment of dynamical friction and stripping of gas in satellite galaxies. We find that by using the FOF group mass at redshift zero in addition to Minfall, an empirical model is able to accurately reproduce the clustering properties of galaxies. On the other hand, using the infall redshift as a second parameter does not yield as good results because it is less correlated with stellar mass. Our analysis indicates that environmental processes that affect galaxy evolution are important for properly modelling the clustering and abundance of galaxies.