2025ApJ...986...61L

We study the impact of environmental effects on the measurement of the Hubble constant (H₀) from gravitational-wave (GW) observations of binary black hole mergers residing in active galactic nuclei (AGNs) near the central supermassive black hole. Using the potential hierarchical triple merger candidate GW190514-GW190521 in AGN J124942.3+344929 with its potential electromagnetic counterpart ZTF19abanrhr as a multimessenger case study, we demonstrate that environmental effects can be negligible for mergers at approximately tens to hundreds of Schwarzschild radii from the supermassive black hole. We find ${H}_{0}=40.{9}_{-8.9}^{+19.3}\,{\rm{km}}\,{{\rm{s}}}^{-1}\,{{\rm{Mpc}}}^{-1}$ (median and 68% credible interval) under a flat prior and flat ΛCDM cosmology. Incorporating GW170817 prior information improves constraints to ${H}_{0}=68.{8}_{-6.0}^{+7.7}\,{\rm{km}}\,{{\rm{s}}}^{-1}\,{{\rm{Mpc}}}^{-1}$. We suggest that, in general, AGN environments could serve as viable laboratories for cosmological studies from GW observations where environmental effects remain below detection thresholds.