Enhanced active galactic nucleus activity in overdense galactic environments at 2 < z < 4

¹ Department of Physics and Astronomy, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
² Gemini Observatory, NSF NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA
³ Space Telescope Science Institute, Baltimore, MD 21218, USA
⁴ Department of Physics and Astronomy, Texas A & M University, College Station, TX 77843-4242, USA
⁵ INAF-Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129 Bologna, Italy
⁶ University of Hawai’i, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁷ Department of Astronomy, Tsinghua University, Beijing 100084, China
⁸ INAF – Osservatorio astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
⁹ INAF-IASF Milano, Via Alfonso Corti 12, 20133 Milano, Italy
¹⁰ University of Bologna – Department of Physics and Astronomy “Augusto Righi” (DIFA), Via Gobetti 93/2, 40129 Bologna, Italy
¹¹ SISSA, Via Bonomea 265, I-34136 Trieste, Italy
¹² IFPU – Institute for fundamental physics of the Universe, Via Beirut 2, 34014 Trieste, Italy

^⋆ Corresponding author: eashah@ucdavis.edu

Received: 30 August 2024
Accepted: 25 August 2025

Abstract

We carried out a study on the relationship between galaxy environments and their active galactic nucleus (AGN) activity at high redshifts (2.0 < z < 4.0). Specifically, we studied the AGN fraction in galaxies residing in a range of environments at these redshifts, from field galaxies to the densest regions of highly overdense peaks in the GOODS-S extragalactic field. Utilizing the extensive photometric and spectroscopic observations in this field, we measured the local overdensities (σ_δ) and global overdensities across a broad a range of environments, including those in massive (M_tot ≥ 10^14.8 M_⊙) protostructures. We employed a multiwavelength AGN catalog consisting of AGNs in nine different categories. Our analysis shows a higher AGN fraction (10.9_−2.3^+3.6%) for galaxies in the highest local overdensity regions compared to the AGN fraction (1.9_−0.3^+0.4%) in the corresponding coeval field galaxies (a ∼4σ difference). This trend of increasing AGN fraction in denser environments relative to the field is present in all redshift bins. We also find this trend to be consistent across all five AGN categories that have a sufficient number of AGNs to make a meaningful comparison: the mid-infrared (MIR) spectral energy distribution (SED), MIR color, X-ray luminosity, X-ray-luminosity-to-radio-luminosity-ratio, and optical-spectroscopy. Our results also demonstrate a clear trend for higher (∼4×) AGN fractions in denser local overdensity environments for a given stellar mass. Additionally, we observe the same trend (though at a lower significance) with the global environment of galaxies, measured using a metric based on the projected distance of galaxies from their nearest massive (M_tot > 10^12.8 M_⊙) overdense (σ_δ > 5.0) peak, normalized with respect to the size of the peak. These findings indicate that the prevalence of AGN activity is highly dependent on the environment a host galaxy resides in, even at early times in the formation history of the Universe.