Discovery of a Close-separation Binary Quasar at the Heart of a z ∼ 0.2 Merging Galaxy and Its Implications for Low-frequency Gravitational Waves

Supermassive black hole (SMBH) binaries with masses of ∼10⁸-10⁹ M _o 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 _o. 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".