Black hole mass, host galaxy mass, and dark matter halos: Testing the environmental connection

¹ Instituto de Fisica de Cantabria (CSIC-Universidad de Cantabria), Avenida de los Castros, 39005, Santander, Spain
² Department of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
³ Institute for Astronomy & Astrophysics, National Observatory of Athens, V. Paulou & I. Metaxa, 11532, Athens, Greece

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 February 2026
Accepted: 22 April 2026

Abstract

We investigate the relation between supermassive black holes (SMBHs), their host galaxies, and large-scale dark-matter halos by combining broad-line X-ray active galactic nuclei (AGNs) from the XMM–XXL and Stripe 82X surveys with galaxies from VIPERS and SDSS/Stripe 82. Building on the homogeneous host-galaxy catalogue developed in Paper I, we test whether AGNs with a given black-hole mass, M_BH, occupy the same or different large-scale environments as non-AGN galaxies with statistically indistinguishable host properties. We characterised the empirical M_BH − M_★ distribution of the AGN sample. A shallow scaling between M_BH and stellar mass, M_★, is present, but with a large intrinsic scatter influenced by flux-limited selection and virial-mass uncertainties. The ratio M_BH/M_★ declines with increasing M_★ over the sampled range. Overmassive and undermassive AGN subsets, defined relative to this empirical trend, exhibit distinct median host properties consistent with partially non-synchronous SMBH and M_★ growth. We then selected AGNs in two M_BH intervals, 8.0 ≤ log(M_BH/M_⊙) < 8.5 and 8.5 ≤ log(M_BH/M_⊙) < 9.0, and constructed galaxy control samples matched in M_★, SFR, and sSFR (SFR/M_★) using a multivariate nearest-neighbour procedure. Using AGN–galaxy cross-correlation functions, we inferred characteristic halo masses for AGNs and matched galaxies in each bin. The AGNs with 8.0 ≤ log(M_BH/M_⊙) < 8.5 occupy halos statistically indistinguishable from those of their controls, indicating no detectable environmental dependence at these masses once host properties are controlled. In the higher-mass bin, 8.5 ≤ log(M_BH/M_⊙) < 9.0, we find a mild indication that AGNs may reside in more massive halos than the matched non-AGN galaxies. The inferred difference is ∼0.4 dex but remains formally consistent within the uncertainties. If confirmed with larger samples, this may indicate that halo-scale processes become increasingly relevant only at the highest M_BH, while at lower masses AGN environments remain indistinguishable from those of inactive galaxies with similar host properties.