The AGN-galaxy-halo connection: the distribution of AGN host halo masses to z=2.5

It is widely reported, based on clustering measurements of observed active galactic nucleus (AGN) samples, that AGNs reside in similar mass host dark matter haloes across the bulk of cosmic time, with log M/M-circle dot similar to 12.5-13.0 to z similar to 2.5. We show that this is due in part to the AGN fraction in galaxies rising with increasing stellar mass, combined with AGN observational selection effects that exacerbate this trend. Here, we use AGN specific accretion rate distribution functions determined as a function of stellar mass and redshift for star-forming and quiescent galaxies separately, combined with the latest galaxy-halo connection models, to determine the parent and subhalo mass distribution function of AGNs to various observational limits. We find that while the median (sub)halo mass of AGNs, approximate to 10(12) M-circle dot, is fairly constant with luminosity, specific accretion rate, and redshift, the full halo mass distribution function is broad, spanning several orders of magnitude. We show that widely used methods to infer a typical dark matter halo mass based on an observed AGN clustering amplitude can result in biased, systematically high host halo masses. While the AGN satellite fraction rises with increasing parent halo mass, we find that the central galaxy is often not an AGN. Our results elucidate the physical causes for the apparent uniformity of AGN host haloes across cosmic time and underscore the importance of accounting for AGN selection biases when interpreting observational AGN clustering results. We further show that AGN clustering is most easily interpreted in terms of the relative bias to galaxy samples, not from absolute bias measurements alone.

Automated summary

The study finds that AGNs reside in similar mass host dark matter haloes across cosmic time, primarily due to the rising AGN fraction in galaxies with increasing stellar mass and the exacerbating effect of AGN selection biases. While the median (sub)halo mass of AGNs remains fairly constant, the full halo mass distribution function spans several orders of magnitude. Additionally, interpreting AGN clustering in terms of relative bias to galaxy samples is more informative than absolute bias measurements alone.