Connecting current and future dual AGN searches to LISA and PTA gravitational wave detections

TL;DR

This work builds a forward-modeling framework that links dual AGN demographics to massive black hole mergers and their gravitational-wave signatures. Using the large-volume ASTRID simulation, the authors generate DAGN catalogs tailored to current (COSMOS-Web, DESI) and future (AXIS, Roman) surveys, validating them against observed dual fractions, separations, and host-galaxy properties. They quantify how many DAGN evolve into MBH mergers and the share that produce LISA-detectable events or contribute to the PTA stochastic background, highlighting environmental and redshift dependencies. The results provide concrete, multi-messenger targets and establish a cosmological connection between DAGN populations and the multi-messenger SMBH binary landscape, informing coordinated EM and GW search strategies.

Abstract

Dual active galactic nuclei (DAGN) mark an observable stage of massive black hole (MBH) pairing in galaxy mergers and are precursors to the MBH binaries that generate low-frequency gravitational waves. Using the large-volume ASTRID cosmological simulation, we construct DAGN catalogs matched to current (COSMOS-Web, DESI) and forthcoming (AXIS, Roman) searches. With realistic selection functions applied, ASTRID reproduces observed dual fractions, separations, and host-galaxy properties across redshifts. We predict a substantial population of small-separation (<5 kpc) duals that current surveys fail to capture, indicating that the apparent paucity of sub-kpc systems in COSMOS-Web is driven primarily by selection effects rather than a physical deficit. By following each simulated dual forward in time, we show that dual AGN are robust tracers of MBH mergers: ~30-70% coalesce within $\lesssim 1$ Gyr, and 20-60% of these mergers produce gravitational-wave signals detectable by LISA. Duals accessible to AXIS and Roman are the progenitors of ~10% of low-redshift LISA events and ~30% of the PTA-band stochastic background. Massive green-valley galaxies with moderate-luminosity AGN, together with massive star-forming hosts containing bright quasars at $z>1$, emerge as the most likely environments for imminent MBH binaries. These results provide a unified cosmological framework linking dual AGN demographics, MBH binary formation, and gravitational-wave emission, and they identify concrete, high-priority targets for coordinated electromagnetic and GW searches in upcoming multi-messenger surveys.

Figures (16)

• Figure 1: Redshift and projected separation of light-cone samples in each mock dual AGN catalog. We mock four observational samples: the COSMOS-Web sample LiJunyao2025ApJ...986..101L and the DESI sample Dadiani2025 are mocking existing dual AGN candidates from recent observations; the AXIS (blue) and Roman (grey) mocks target two future observations with the potential to discover a large population of dual AGN. We also label the (optimistic) estimation for the number of dual AGN candidates discoverable by AXIS and Roman.
• Figure 2: Redshift and bolometric luminosity of the primary AGN of the four mock samples. The black arrows and text label the DESI dual AGN categories in each redshift range.
• Figure 3: Upper:$z=0.5$ gas density colored by temperature in a slice of $250\ {\rm Mpc}/h\times 150\ {\rm Mpc}/h$. Duals within a slab of $20\ {\rm Mpc/h}$ in thickness are marked. For COSMOS and AXIS, we only present the mock duals in the squares (COSMOS volume: 0.26 ${\rm deg}^2$; AXIS-Deep volume: 0.16 ${\rm deg}^{2}$; AXIS-Intermediate volume: 2.5 ${\rm deg}^{2}$; ) Lower: each panel is 60 ckpc$/h$ per side. The projected separation (in physical units) of each dual is labeled.
• Figure 4: Top: Dual AGN fraction as a function of redshift for dual AGN mock catalogs created from ASTRID, targeting the COSMOS-Web sample LiJunyao2025ApJ...986..101L, SDSS+HSC sample Silverman2020 and DESI AGN sample Dadiani2025. For the COSMOS-Web mock we also show the $2\sigma$ uncertainty due to cosmic variance and Poisson fluctuation from 200 light-cone realizations (red shaded). The dual AGN fraction in each observed sample is shown as circles in the corresponding colors. We also show the dual fraction quoted in Perna2025AA...696A..59P at $z=3.5$ and Koss2012 at $z\sim 0$. Notably, the high dual AGN fraction measured by Perna2025AA...696A..59P is marginally outside the $2\sigma$ of the simulated sample distribution with similar luminosities. Bottom: predicted dual AGN fraction for upcoming observations AXIS and Roman.
• Figure 5: Comparison between the DESI-Mock sample from ASTRID (red lines) and the observed DESI dual AGN candidates (grey shaded) across five redshift bins. We show comparisons of the projected separation (top panels), bolometric luminosities (middle panels), and total star-formation rate of the dual AGN host galaxies (bottom panels).