Tracking the orbit of unresolved subhalos for semi-analytic models

Abstract

We present an updated model for the evolution of the orbits of "orphangalaxies" to be used in the SAG semi-analytical model of galaxy formation andevolution. In cosmological simulations, orphan galaxies are those satellitegalaxies for which, due to limited mass resolution, halo finders lose track oftheir dark matter subhalos and can no longer be distinguished as self-boundoverdensities within the larger host system. Since the evolution of orphansdepends strongly on the orbit they describe within their host halo, a propertreatment of their evolution is crucial in predicting the distribution ofsubhalos and satellite galaxies. The model proposed takes into account thedynamical friction drag, mass loss by tidal stripping and a proximity mergercriterion, also it is simple enough to be inexpensive from a computationalpoint of view. To calibrate this model, we apply it onto a dark matter onlysimulation and compare the results with a high resolution simulation,considering the halo mass function and the two-point correlation function asconstraints. We show that while the halo mass function fails to put tightconstraints on the dynamical friction, the addition of clustering informationhelps to better define the parameters of the model related to the spatialdistribution of subhalos. Using the model with the best fit parameters allowsus to reproduce the halo mass function to a precision better than 5 per cent,and the two point correlation function at a precision better than 10 per cent.