# 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

## TL;DR

This paper reports the discovery of a close-separation binary quasar at the center of a merging galaxy at z~0.2, providing empirical evidence relevant to gravitational wave background predictions and SMBH merger timescales.

## Contribution

It presents the first direct imaging of a close SMBH binary in a merging galaxy at low redshift, constraining SMBH merger timescales and implications for gravitational wave detection.

## Key findings

- Discovered a binary quasar with two SMBHs separated by ~430 pc.
- Estimated the SMBH pair's merging timescale to be less than 2.5 billion years.
- Suggests a population of similar binaries could contribute to the gravitational wave background.

## 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'.