Active galactic nuclei with massive black holes have closer galactic neighbors

k-nearest-neighbor statistics of an unbiased sample at z ∼ 0.03

¹ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA
² Leibniz-Institut fur Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
³ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 77845, USA
⁴ Leiden Observatory, PO Box 9513 2300 RA Leiden, The Netherlands
⁵ Eureka Scientific, 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
⁶ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul, Santiago 7820436, Chile
⁷ Korea Astronomy and Space Science Institute, 776, Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea
⁸ Yale Center for Astronomy & Astrophysics and Department of Physics, Yale University, P.O. Box 208120 New Haven, CT 06520-8120, USA
⁹ Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹⁰ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
¹¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy
¹² INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
¹³ Departamento de Física, Universidad Técnica Federico Santa María, Vicuña Mackenna, 3939 San Joaquín, Santiago de Chile, Chile
¹⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
¹⁵ Dipartimento di Matematica e Fisica, Università di Roma 3, Via della Vasca Navale, 84, 00146 Roma, RM, Italy

^⋆ Corresponding author: mpowell@aip.de

Received: 25 April 2025
Accepted: 25 June 2025

Abstract

Context. The large-scale environments of active galactic nuclei (AGNs) reveal important information on the growth and evolution of supermassive black holes (SMBHs). Previous AGN clustering measurements using two-point correlation functions have hinted that AGNs with massive black holes preferentially reside in denser cosmic regions than AGNs with less massive SMBHs. At the same time, little to no dependence on the accretion rate has been found; however, the significance of such trends has been limited.

Aims. Here, we present kth-nearest-neighbor (kNN) statistics of 2MASS galaxies around AGNs from the Swift/BAT AGN Spectroscopic survey. These statistics have been shown to contribute additional higher order clustering information on the cosmic density field.

Methods. By calculating the distances to the nearest seven galaxy neighbors in angular separation to each AGN within two redshift ranges (0.01 < z < 0.03 and 0.03 < z < 0.06), we compared their cumulative distribution functions to that of a randomly distributed sample to demonstrate the sensitivity of this method to the clustering of AGNs. We also split the AGNs into bins of bolometric luminosity, black hole mass, and Eddington ratio (while controlling for redshift) to search for trends between kNN statistics and fundamental AGN properties.

Results. We find that AGNs with massive SMBHs have significantly closer neighbors than AGNs with less massive SMBHs (at the 99.98% confidence level), especially in our lower redshift range. We find less significant trends with luminosity or Eddington ratio. By comparing our results to empirical SMBH-galaxy-halo models implemented in N-body simulations, we show that small-scale kNN trends with black hole mass may go beyond stellar mass dependencies.

Conclusions. This suggests that massive SMBHs in the local universe reside in more massive dark matter halos and denser regions of the cosmic web, which may indicate that environment is important for the growth of SMBHs, bolstering prior conclusions based on correlation functions.