Binary Supermassive Black Holes Orbiting Dark Matter Solitons: From the Dual AGN in UGC4211 to NanoHertz Gravitational Waves

Abstract

We explore orbital implications of the Supermassive Black Hole (SMBH) binary in UGC4211 for the energy spectrum of stochastic gravitational wave background (SGWB), measured with pulsar timing. The SMBH binary in UGC4211 has a projected separation of $\sim 230$pc and relative velocity of $\sim 150$km/s along the line of sight. It orbits with a disk of gas and stars, with a total mass $\sim 1.7\times 10^9{M}_{\odot }$ that is several times larger than the combined SMBHs plus the observed gas and stars. The unseen mass can be naturally explained by a soliton of wave dark matter present within the SMBH orbit. Such a scenario is encouraging as during galaxy merger, the two precursor galactic solitons are expected to combine to generate a new soliton and hence bind the two initial SMBHs efficiently. Generalizing this scenario to the cosmological population of SMBH binaries, we show the SGWB spectrum produced by late-stage inspiraling is modified preferentially at low frequency by the presence of soliton. Finally, we demonstrate that the NANOGrav and EPTA data can be well-fit in this scenario, favoring $\{m_a,f_a\}\sim \{10^{-21.7}\mathrm{eV},10^{15.5}\mathrm{GeV}\}$ and $\{10^{-20.5}\mathrm{eV},10^{16.8}\mathrm{GeV}\}$ respectively when the UGC4211 data and the constraints from dwarf galaxies are also combined.

Fig. 2 and Fig. 3 updated. Dicusssions added

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