Colin J. Burke

The black hole occupation fraction (BHOF) is a powerful but intrinsically layered diagnostic of massive black hole (BH) seeding and subsequent galaxy assembly. We measure the total, central, and wandering BHOF in the ASTRID cosmological hydrodynamical simulation from z = 5 to z = 0 in galaxies with stellar masses ranging from 10 ⁷ to 10 ¹² M _⊙ . For the full population, the total BHOF remains close to unity across most stellar masses and redshifts, reflecting efficient seeding in eligible halos. At low stellar masses, the central BHOF declines toward the present day, while the wandering BHOF rises, indicating that mergers deposit BHs at off-center locations in galaxies. Compared to these full-population BHOFs, the heavy-seed BHOFs are substantially smaller at the low-mass end. The active BHOFs are smaller at all stellar masses, especially at late times, demonstrating that active galactic nuclei (AGN)-selected samples trace duty cycles rather than intrinsic BH occupation. At z = 0, primary (central) galaxies have higher BHOFs than satellites, and star-forming low-mass galaxies preferentially host wandering rather than central BHs. Our results show that the BHOF, when decomposed by BH location, seeding history, accretion state, and larger-scale galactic environment, encodes a rich fossil record of BH origins and dynamics.

The distribution of systems in the black hole (BH) mass–stellar mass ( M _BH – M _⋆ ) plane encodes both the integrated growth of galaxies and their central BHs, and the processes that shape their evolution. Using the ASTRID and TNG300 cosmological simulations, we track massive BHs from cosmic noon ( z = 2) to z = 0.5. TNG300 repositions BHs to the centers of their host galaxies at every time step, while ASTRID instead advances them under the resolved gravitational forces plus a subgrid dynamical friction model, allowing BHs to wander off-center when orbital decay is inefficient. We follow the BHs rather than their original host galaxies, thereby capturing central BHs, BHs in satellites, and wandering BHs. We find that central BHs in both simulations evolve along a tight, nearly redshift-invariant M _BH – M _⋆ relation that is broadly consistent with local empirical constraints. Departures from this relation trace distinct evolutionary channels in which mergers play a key role. Major galaxy mergers drive BH–BH coalescence that dominates the growth of the most massive central BHs. These BHs subsequently quench their hosts through active galactic nucleus (AGN) kinetic feedback. Minor mergers tidally strip satellites to lower M _⋆ at nearly fixed M _BH , producing weakly accreting, overmassive central BHs in gas-poor systems. In ASTRID, satellite accretion and inefficient dynamical friction generate wandering BHs that are undermassive relative to their new hosts and experience minimal accretion or merger-driven growth. A BH’s location in the M _BH – M _⋆ and specific BH accretion rate-specific star formation rate planes is, therefore, a fossil record of its dynamical, accretion, merger, and feedback history.

Changing-look active galactic nuclei (CL-AGN) are AGN that transition between Seyfert types, challenging AGN unification models. Most CL-AGN have been identified via repeat spectroscopy, making it difficult to determine the duration and magnitude of the CL-AGN transition. As such, the physical mechanisms behind this transition are still unknown. We use synthetic photometry in combination with Zwicky Transient Facility (ZTF) light-curve data to develop a new criterion to identify photometric CL-AGN transitions based on changes in g -band magnitude and g − r color. We find that a CL-AGN criterion of ∣Δ g ∣ > 0.4 mag and ∣Δ( g − r )∣ > 0.2 mag recovers a photometric transition in 9 . 6 − 3.4 + 4.9 % of CL-AGN hosts over the six-year ZTF survey, including a candidate repeating changing-look event in SDSS J084957.78+274728.9. Using simulated AGN light curves, we estimate the false positive rate among the simulated Seyferts to be 1 . 6 − 0.17 + 0.19 % . We find that the rate of similar flares is 1 . 2 − 0.50 + 0.87 % among Type 1 Seyferts and ≤0.39% among Type 2 Seyferts over six years. Photometric CL-AGN transitions last between 21 and 560 days, with a median duration of 360 days, consistent with the thermal or orbital timescales for AGN disks. We do not detect a correlation between black hole mass and transition duration, likely due to the small sample of detected photometric transitions. This method can be applied to the upcoming Legacy Survey of Space and Time to identify CL-AGN candidates and test theories of their origins.

Several local ( z ≲ 0.2) metal-poor dwarf active galactic nuclei (AGNs) have remarkably similar properties to those of high-redshift little red dots (LRDs), and are proposed to be local analogs of LRDs. We use long-term photometric and spectroscopic observations of three local LRDs spanning ∼20 yr to measure variability in their rest-frame optical continua and broad H α emission lines. Using Zwicky Transient Facility light curves over a rest-frame ∼5 yr baseline, the r -band intrinsic rms variability is ( 9 × 1 0 − 5 ) − 9 × 10 - 5 + 0.014 mag (J1022+0841), 0.025 ± 0.004 mag (J1025+1402) and 0.020 ± 0.005 mag (J1047+0739), indicating low intrinsic variability (σ ). These variability amplitudes are much lower than those for dwarf AGNs and more massive quasars. We constrain the broad H α variability to be less than a few percent, without significant profile changes, over a rest-frame baseline of ∼15 yr in J1025+1402 and J1047+0739, respectively. Local LRDs have exceptionally large H α equivalent widths and H α /H β ratios that far exceed the Case B recombination value, in contrast with normal broad-line AGNs. In the context of recent theoretical models of LRDs as dense gas-enshrouded massive black holes with super-Eddington accretion, our results suggest that the photosphere emission is long-term stable and the broad Balmer lines do not arise primarily by photoionization. This scenario is consistent with the lack of variability, large H α /H β ratios, little dust extinction, and expected high gas density. Virial black hole mass estimates using broad H α assuming photoionization are therefore highly questionable for LRDs.

The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) will monitor tens of millions of active galactic nuclei (AGNs) for a period of 10 yr with an average cadence of 3 days in six broad photometric bands. This unprecedented data set will enable robust characterizations of AGN UV/optical variability across a wide range of AGN physical properties. However, existing tools for modeling AGN light curves are not yet capable of fully leveraging the volume, cadence, and multiband nature of LSST data. We present EzTaoX , a scalable light-curve modeling tool designed to take advantage of LSST’s multiband observations to simultaneously characterize AGN UV/optical stochastic variability and measure interband time delays. EzTaoX achieves a speed increase of ∼10 ² –10 ⁴ × on CPUs over current tools with similar capabilities, while maintaining equal or better accuracy in recovering simulated variability properties. This performance gain enables continuum time-delay measurements for all AGNs discovered by LSST—both in the Wide Fast Deep survey and the Deep Drilling Fields—thereby opening new opportunities to probe AGN accretion-flow geometries. In addition, EzTaoX ’s multiband capability allows for robust characterization of AGN stochastic variability down to hourly timescales, facilitating the identification of accreting low-mass AGNs—such as those residing in dwarf galaxies—through their distinctive variability signatures.

Between the groundbreaking detections of stellar-mass black holes by LIGO/Virgo/KAGRA and JWST's revelation of a surprisingly abundant population of supermassive black holes, one crucial missing link remains: the elusive intermediate-mass black holes (IMBHs). IMBHs represent a key phase in the hierarchical growth of black holes, yet they have persistently evaded detection. Traditional methods, effective for both actively accreting and quiescent black holes, have largely failed to uncover this hidden population. Here, we argue that novel observational strategies--particularly time-domain variability studies of active galactic nuclei (AGN) and tidal disruption events--provide a promising path forward. Finding IMBHs will resolve critical gaps in our understanding of black hole formation and the various mechanisms driving their subsequent growth. The upcoming Vera C. Rubin Observatory, with its unprecedented capacity to monitor the dynamic sky, stands to revolutionize our ability to detect these long-sought IMBHs, shedding new light on the assembly history of black holes across cosmic time.

We study the black hole mass–host galaxy stellar mass relation, M _BH – M _* , for a sample of 706 z ≲ 1.5 and i ≲ 24 optically variable active galactic nuclei (AGNs) in three Dark Energy Survey (DES) Deep Fields: C3, X3, E2, which partially cover Chandra Deep Field-South, XMM Large Scale Structure survey, and European Large Area ISO Survey, respectively. The parent sample was identified by optical variability from the DES supernova survey program imaging. Using publicly available spectra and photometric catalogs, we consolidate their spectroscopic redshifts, estimate their black hole masses using broad line widths and luminosities, and obtain improved stellar masses using spectral energy distribution fitting from X-ray to mid-infrared wavelengths. Our results confirm previous work from Hyper-Suprime Camera imaging that variability searches with deep, high-precision photometry can reliably identify AGNs in low-mass galaxies up to z ∼ 1. However, we find that the hosted black holes are more massive than predicted by the local AGN relation, fixing host galaxy stellar mass. Instead, z ∼ 0.1–1.5 variability-selected AGNs lie in between the M _BH – M _* relation for local inactive early-type galaxies and local active galaxies. This result agrees with most previous studies of the M _BH – M _* relation for AGNs at similar redshifts, regardless of the selection technique. We demonstrate that studies of variability-selected AGN provide critical insights into the low-mass end of the M _BH – M _* relation, shedding light on the occupation fraction of that provides constraints on early black hole seeding mechanisms and self-regulated feedback processes during their growth and coevolution with their hosts.

We study the coevolution of black holes (BHs) and their host galaxies in the ASTRID and TNG300 cosmological simulations and the DARK SAGE semianalytic model (SAM), focusing on the evolution of the BH mass–stellar mass (M _BH–M _*) relation. Due to differences in the adopted subgrid modeling of BH seeding, dynamics, and feedback, the models differ in their predicted redshift evolution of the M _BH–M _* relation. We find that it is the interplay between the star formation rate (SFR) and the black hole accretion rate (BHAR) that drives the evolution of the mean relation. We define a quantity R , the ratio between the specific BHAR and SFR (i.e., R ≡ sBHAR/sSFR), and demonstrate that it is R that governs the evolution of individual sources in the M _BH–M _* plane. The efficiency of BH growth versus stellar mass growth in the sSFR–sBHAR plane reflects the partitioning of gas between fueling star formation versus BH accretion. This partitioning depends on the implementation of BH dynamics and the nature of how black hole feedback quenches galaxies. In the cosmological simulations (ASTRID and TNG300), the BHAR and SFR are intrinsically linked, resulting in a tight M _BH–M _* correlation, while the DARK SAGE SAM produces a significantly larger scatter. We discuss these results in the context of recently discovered overmassive BHs and massive quenched galaxies at high redshift by the James Webb Space Telescope.