Detection of galaxy assembly bias

Assembly bias describes the finding that the clustering of dark matter haloes depends on halo formation time at fixed halo mass. In this paper, we analyse the influence of assembly bias on galaxy clustering using both semi-analytic galaxy formation models (SAMs) and observational data. At fixed stellar mass, SAMs predict that the clustering of central galaxies depends on the specific star formation rate (sSFR), with more passive galaxies having a higher clustering amplitude. We find similar trends using the Sloan Digital Sky Survey (SDSS) group catalogues, and verify that these are not affected by possible biases due to the group finding algorithm. Low-mass central galaxies reside in narrow bins of halo mass, so the observed trends of higher clustering amplitude for galaxies with lower sSFR is not driven by variations of the parent halo mass. We argue that the clustering dependence on sSFR represent a direct detection of assembly bias. In addition, contrary to what expected based on clustering of dark matter haloes, we find that low-mass central galaxies in SAMs with larger host halo mass have a lower clustering amplitude than their counter-parts residing in lower mass haloes. This results from the fact that, at fixed stellar mass, assembly bias has a stronger influence on clustering than the dependence on the parent halo mass.