We present a sample of accreting supermassive black holes (SMBHs) in dwarf galaxies at . We identify dwarf galaxies in the NEWFIRM Medium Band Survey with stellar masses of that have spectroscopic redshifts from the DEEP2 survey and lie within the region covered by deep (flux limit of ) archival Chandra X-ray data. From our sample of 605 dwarf galaxies, 10 exhibit X-ray emission consistent with that arising from active galactic nucleus (AGN) activity. If black-hole mass scales roughly with stellar mass, then we expect that these AGNs are powered by SMBHs with masses of and typical Eddington ratios of . Furthermore, we find an AGN fraction consistent with extrapolations of other searches of for and . Our AGN fraction is in good agreement with a semi-analytic model, suggesting that, as we search larger volumes, we may use comparisons between observed AGN fractions and models to understand seeding mechanisms in the early universe.

Supermassive black hole (SMBH) binaries with masses of ∼10⁸–10⁹ M _⊙ are expected to dominate the contribution to the as-yet undetected gravitational wave background (GWB) signal at the nanohertz frequencies accessible to pulsar timing arrays. We currently lack firm empirical constraints on the amplitude of the GWB due to the dearth of confirmed SMBH binaries in the required mass range. Using Hubble Space Telescope/Wide Field Camera 3 images, we have discovered a z ∼ 0.2 quasar hosted in a merger remnant with two closely separated (0.″13 or ∼430 pc) continuum cores at the heart of the galaxy SDSS J1010+1413. The two cores are spatially coincident with two powerful [O iii]-emitting point sources with quasar-like luminosities (L _AGN ∼ 5 × 10⁴⁶ erg s⁻¹), suggesting the presence of a bound SMBH system, each with M _BH > 4 × 10⁸ M _⊙. We place an upper limit on the merging timescale of the SMBH pair of 2.5 billion years, roughly the universe lookback time at z ∼ 0.2. There is likely a population of quasar binaries similar to SDSS J1010+1413 that contribute to a stochastic GWB that should be detected in the next several years. If the GWB is not detected this could indicate that SMBHs merge only over extremely long timescales, remaining as close separation binaries for many Hubble times, the so-called “final-parsec problem.”

The nanohertz gravitational wave background (GWB) is believed to be dominated by GW emission from supermassive black hole binaries (SMBHBs). Observations of several dual-active galactic nuclei (AGN) strongly suggest a link between AGN and SMBHBs, given that these dual-AGN systems will eventually form bound binary pairs. Here we develop an exploratory SMBHB population model based on empirically constrained quasar populations, allowing us to decompose the GWB amplitude into an underlying distribution of SMBH masses, SMBHB number density, and volume enclosing the GWB. Our approach also allows us to self-consistently predict the number of local SMBHB systems from the GWB amplitude. Interestingly, we find the local number density of SMBHBs implied by the common-process signal in the NANOGrav 12.5-yr data set to be roughly five times larger than previously predicted by other models. We also find that at most ∼25% of SMBHBs can be associated with quasars. Furthermore, our quasar-based approach predicts ≳95% of the GWB signal comes from z ≲ 2.5, and that SMBHBs contributing to the GWB have masses ≳10⁸ M _⊙. We also explore how different empirical galaxy–black hole scaling relations affect the local number density of GW sources, and find that relations predicting more massive black holes decrease the local number density of SMBHBs. Overall, our results point to the important role that a measurement of the GWB will play in directly constraining the cosmic population of SMBHBs, as well as their connections to quasars and galaxy mergers.

The detection of the stochastic gravitational wave background (SGWB) is essential for understanding black hole populations, especially for supermassive black hole binaries. The recent promising results from various pulsar timing array (PTA) collaborations allude to an imminent detection. In this paper, we investigate the relative astrometric gravitational wave detection method, which can contribute to SGWB studies in the microhertz range. We consider the Roman Space Telescope and Gaia as candidates and quantitatively discuss the survey sensitivity in both the frequency and spatial domains. We emphasize the importance of survey specific constraints on performance estimates by considering mean field of view (FoV) signal subtraction and angular power spectrum binning. We conclude that if the SGWB is at a similar level as in PTA estimates, both Roman and Gaia have the potential to detect this frequency-domain power excess. However, both Roman and Gaia are subject to FoV limitations, and are unlikely to be sensitive to the spatial pattern of the SGWB.

Gravitational wave (GW) detections have considerably enriched our understanding of the universe. To date, all GW events from individual sources have been found by interferometer-type detectors. In this paper, we study a GW detection technique based on astrometric solutions from photometric surveys and demonstrate that it offers a highly flexible frequency range that can uniquely complement existing detection methods. From repeated point-source astrometric measurements, periodic GW-induced deflections can be extracted and wave parameters inferred. We emphasize that this method can be applied widely to any photometric surveys relying on relative astrometric measurements, in addition to surveys designed to measure absolute astrometry, such as Gaia. We illustrate how high-cadence observations of the galactic bulge, such as offered by the Roman Space Telescope’s Exoplanet MicroLensing (EML) survey, have the potential to be a potent GW probe with a complementary frequency range to Gaia, pulsar timing arrays, and the Laser Interferometer Space Antenna. We calculate that the Roman EML survey is sensitive to GWs with frequencies ranging from $7.7\times {10}^{-8}\mathrm{Hz}$ to $5.6\times {10}^{-4}\mathrm{Hz}$, which opens up a unique GW observing window for supermassive black hole binaries and their waveform evolution. While the detection threshold assuming the currently expected performance proves too high for detecting individual GWs in light of the expected supermassive black hole binary population distribution, we show that binaries with chirp mass ${\mathcal{M}}_c>{10}^{8.3}{M}_{\odot }$ out to 100 Mpc can be detected if the telescope is able to achieve an astrometric accuracy of 0.11 mas. To confidently detect binaries with ${\mathcal{M}}_c>{10}^7{M}_{\odot }$ out to 50 Mpc, a factor of 100 sensitivity improvement is required. We propose several improvement strategies, including recovering the mean astrometric deflection and increasing astrometric accuracy, number of observed stars, field-of-view size, and observational cadence. We also discuss how other existing and planned photometric surveys could contribute to detecting GWs via astrometry.