Assessing colour-dependent occupation statistics inferred from galaxy group catalogues

Abstract

We investigate the ability of current implementations of galaxy group finders\nto recover colour-dependent halo occupation statistics. To test the fidelity of\ngroup catalogue inferred statistics, we run three different group finders used\nin the literature over a mock that includes galaxy colours in a realistic\nmanner. Overall, the resulting mock group catalogues are remarkably similar,\nand most colour-dependent statistics are recovered with reasonable accuracy.\nHowever, it is also clear that certain systematic errors arise as a consequence\nof correlated errors in group membership determination, central/satellite\ndesignation, and halo mass assignment. We introduce a new statistic, the halo\ntransition probability (HTP), which captures the combined impact of all these\nerrors. As a rule of thumb, errors tend to equalize the properties of distinct\ngalaxy populations (i.e. red vs. blue galaxies or centrals vs. satellites), and\nto result in inferred occupation statistics that are more accurate for red\ngalaxies than for blue galaxies. A statistic that is particularly poorly\nrecovered from the group catalogues is the red fraction of central galaxies as\nfunction of halo mass. Group finders do a good job in recovering galactic\nconformity, but also have a tendency to introduce weak conformity when none is\npresent. We conclude that proper inference of colour-dependent statistics from\ngroup catalogues is best achieved using forward modelling (i.e., running group\nfinders over mock data), or by implementing a correction scheme based on the\nHTP, as long as the latter is not too strongly model-dependent.\n