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

Abstract

Supermassive black hole (SMBH) binaries with masses of ~10^8--10^9 Msun 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 ~ 5x10^46 erg/s, suggesting the presence of a bound SMBH system, each with M_BH > 4x10^8 Msun. 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'.

Accepted to ApJL (in press), 9 pages, 5 figures

References in corpus (13)

• A Catalog of Quasar Properties from SDSS DR7
• An Observational Determination of the Bolometric Quasar Luminosity Function
• The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays
• Kiloparsec-scale outflows are prevalent among luminous AGN: outflows and feedback in the context of the overall AGN population
• A Compact Supermassive Binary Black Hole System
• A synthesis model for AGN evolution: supermassive black holes growth and feedback modes
• The Local Nanohertz Gravitational-Wave Landscape From Supermassive Black Hole Binaries
• Reliable Identification of Compton-thick Quasars at z~2: Spitzer Mid-IR spectroscopy of HDF-oMD49
• Interactions between multiple supermassive black holes in galactic nuclei: a solution to the final parsec problem
• Sizes and Kinematics of Extended Narrow-Line Regions in Luminous Obscured AGN Selected by Broadband Images
• Constraining the Solution to the Last Parsec Problem with Pulsar Timing
• Extended X-ray Emission From a Quasar-Driven Superbubble
• Probing supermassive black hole mergers and stalling with pulsar timing arrays

Cited by in corpus (5)

• Spitzer Observations of the Predicted Eddington Flare from Blazar OJ 287
• NGC6240: A triple nucleus system in the advanced or final state of merging
• Dual supermassive black holes at close separation revealed by the Hyper Suprime-Cam Subaru Strategic Program
• Double-peak emission line galaxies in the SDSS catalogue. A minor merger sequence
• The most massive galaxies with large depleted cores: structural parameter relations and black hole masses