Understanding the relationship between galaxies hosting active galactic nuclei (AGN) and the dark matter halos in which they reside is key to constraining how black-hole fueling is triggered and regulated. Previous efforts have relied on simple halo
mass estimates inferred from clustering, weak gravitational lensing, or halo occupation distribution modeling. In practice, these approaches remain uncertain because AGN, no matter how they are identified, potentially live a wide range of halo masses with an occupation function whose general shape and normalization are poorly known. In this work, we show that better constraints can be achieved through a rigorous comparison of the clustering, lensing, and cross-correlation signals of AGN hosts to a fiducial stellar-to-halo mass relation (SHMR) derived for all galaxies. Our technique exploits the fact that the global SHMR can be measured with much higher accuracy than any statistic derived from AGN samples alone. Using 382 moderate luminosity X-ray AGN at z<1 from the COSMOS field, we report the first measurements of weak gravitational lensing from an X-ray selected sample. Comparing this signal to predictions from the global SHMR, we find that, contrary to previous results, most X-ray AGN do not live in medium size groups ---nearly half reside in relatively low mass halos with Mh~10^12.5 Msun. The AGN occupation function is well described by the same form derived for all galaxies but with a lower normalization---the fraction of halos with AGN in our sample is a few percent. By highlighting the relatively normal way in which moderate luminosity X-ray AGN hosts occupy halos, our results suggest that the environmental signature of distinct fueling modes for luminous QSOs compared to moderate luminosity X-ray AGN is less obvious than previously claimed.
The observed stellar mass function (SMF) is very different to the halo mass function predicted by Lambda-CDM, and it is widely accepted that this is due to energy feedback from supernovae and black holes. However, the strength and form of this feedba
ck is not understood. In this paper, we use the phenomenological model GALFORM to explore how galaxy formation depends on the strength and halo mass dependence of feedback. We focus on expulsion models in which the wind mass loading, beta, is proportional to 1/vdisk^n, with n=0,1,2 and contrast these models with the successful Bower et al. 2008 model (B8W7). A crucial development is that our code explicitly accounts for the recapture of expelled gas as the systems halo mass (and thus gravitational potential) increases. We find that a model with modest wind speed but high mass loading matches the flat portion of the SMF. When combined with AGN feedback, the model provides a good description of the observed SMF above 10^9 h^-1 Msol. However, in the expulsion models, the brightest galaxies are assembled more recently than in B8W7, and the specific star formation rates of galaxies decrease strongly with decreasing stellar mass. The expulsion models also tend to have a cosmic star formation density that is dominated by lower mass galaxies at z=1-3, and dominated high mass galaxies at low redshift. These trends are in conflict with observational data, but the comparison highlights some deficiencies of the B8W7 model also. The experiments in this paper give us important physical insight to the impact of the feedback process on the formation histories of galaxies, but the strong mass dependence of feedback adopted in B8W7 still appears to provide the most promising description of the observed universe.
