The observation of GW170817 in both gravitational and electromagnetic waves provides a number of unique tests of general relativity. One question we can answer with this event is: Do large-wavelength gravitational waves and short-frequency photons experience the same number of spacetime dimensions? In models that include additional non-compact spacetime dimensions, as the gravitational waves propagate, they "leak" into the extra dimensions, leading to a reduction in the amplitude of the observed gravitational waves, and a commensurate systematic error in the inferred distance to the gravitational wave source. Electromagnetic waves would remain unaffected. We compare the inferred distance to GW170817 from the observation of gravitational waves, d GW L , with the inferred distance to the electromagnetic counterpart NGC 4993, d EM L . We constrain d GW L = (d EM L /Mpc) γ with γ = 1.01 +0.04 −0.05 (for the SHoES value of H 0 ) or γ = 0.99 +0.03 −0.05 (for the Planck value of H 0 ), where all values are MAP and minimal 68% credible intervals. These constraints imply that gravitational waves propagate in D = 3 + 1 spacetime dimensions, as expected in general relativity. In particular, we find that D = 4.02 +0.07 −0.10 (SHoES) and D = 3.98 +0.07 −0.09 (Planck). Furthermore, we place limits on the screening scale for theories with D > 4 spacetime dimensions, finding that the screening scale must be greater than ∼ 20 Mpc. We also place a lower limit on the lifetime of the graviton of t > 4.50 × 10 8 yr.

We present a sample of accreting supermassive black holes (SMBHs) in dwarf galaxies at z < 1. We identify dwarf galaxies in the NEWFIRM Medium Band Survey with stellar masses M < 3 × 10 9 M that have spectroscopic redshifts from the DEEP2 survey and lie within the region covered by deep (flux limit of ∼ 5 × 10 −17 − 6 × 10 −16 erg cm −2 s −1 ) archival Chandra X-ray data. From our sample of 605 dwarf galaxies, 10 exhibit X-ray emission consistent with that arising from AGN activity. If black hole mass scales roughly with stellar mass, then we expect that these AGN are powered by SMBHs with masses of ∼ 10 5 − 10 6 M and typical Eddington ratios ∼ 5%. Furthermore, we find an AGN fraction consistent with extrapolations of other searches of ∼ 0.6−3% for 10 9 M ≤ M ≤ 3×10 9 M and 0.1 < z < 0.6. 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. Subject headings: d False Detection Probability for each band -see description in Section 2.2 e All Bands in which the source is significant f Probability that the X-ray Luminosity observed could be due to an HXMB -see Section 3.2 for a full description * ≥ 3σ sources

Supermassive black hole (SMBH) binaries with masses of ∼ 10 8 -10 9 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 (PTA). 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 HST/WFC3 images, we have discovered a z ∼ 0.2 quasar hosted in a merger remnant with two closely separated (0.13 or ∼430pc) continuum cores at the heart of the galaxy SDSSJ1010+1413. The two cores are spatially coincident with two powerful [OIII]-emitting point sources with quasar-like luminosities (L AGN ∼ 5 × 10 46 erg s −1 ), suggesting the presence of a bound SMBH system, each with M BH > 4 × 10 8 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 SDSSJ1010+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'.

Gravitational wave (GW) detections have considerably enriched our understanding of the universe. To date, all the known events were observed via direct detection. In this paper, we study a GW detection technique based on astrometric observation and demonstrate that it offers a highly flexible frequency range that can uniquely complement existing detection methods. Using repeated point-source astrometric measurements, periodic GW-induced deflections can be extracted and wave parameters inferred. 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 complementary frequency range to Gaia, pulsar timing arrays (PTAs), and the Laser Interferometer Space Antenna (LISA). We calculate that the Roman EML survey is sensitive to GWs with frequencies ranging from 7.7 × 10 −8 Hz to 5.6 × 10 −4 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 Mc > 10 7.6 M out to 10 Mpc can be detected if the telescope is able to achieve an astrometric accuracy of 0.11 mas. To confidently detect binaries with Mc > 10 7 M 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.

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