Progenitor of the recoiling super-massive black hole RBH-1 identified using HST/JWST imaging

Abstract

Using a combination of \textit{Hubble Space Telescope} and \textit{James Webb Space Telescope} imaging, a runaway supermassive black hole (RBH-1) was recently identified with an inferred velocity of $954^{+110}_{-126}\,\mathrm{km\,s^{-1}}$, likely ejected from a compact star-forming galaxy (denoted as GX) at $z \approx 0.96$. Assuming the runaway black hole was the outcome of the gravitational-wave-driven merger of two black holes, we use its measured runaway velocity together with gravitational-wave recoil predictions from numerical relativity and black hole perturbation theory to constrain the mass ratio and spin configuration of the progenitor SMBHs that overcame the final-parsec problem and merged $\sim 70$~Myr ago. We find that the progenitor binary must have been precessing, with a mass ratio $m_1/m_2\lesssim 6$, and that the more massive SMBH must have possessed a high spin (dimensionless spin magnitude $\sim 0.75$) in order to generate a recoil of this magnitude. This has important astrophysical implications as similar SMBH mergers can be an interesting source population for the upcoming LISA mission with signal-to-noise ratios $\gtrsim$ 1000. Furthermore, the progenitor SMBH properties imply that GX was likely formed through a major, gas-rich (``wet'') merger between two galaxies of comparable mass, with a mass ratio $\lesssim 4$.

Figures (8)

• Figure 1: We show the merger scenarios that cannot produce RBH-1 speed of $954^{+110}_{-126}\,\mathrm{km\,s^{-1}}$ (vertical shaded gray region). These include non-spinning SMBH mergers, non-precessing SMBH mergers, and precessing SMBH mergers with mass ratios $q \in [5,20]$. The only scenario consistent with the large kick is for near equal-mass ratio systems with spin-precession (i.e., the individual SMBH spins are misaligned with the orbital angular momentum). We use the gwModel for this plot.
• Figure 2: We show the progenitor SMBH mass ratio $q (=m_1/m_2)$ and the dimensionless spin magnitudes $|\chi_{1,2}|$ that are consistent with the inferred RBH-1 speed of $954^{+110}_{-126}\,\mathrm{km\,s^{-1}}$, under the assumption that the progenitor SMBHs were in a precessing configuration. We use three different recoil-kick models calibrated against NR (and/or BHPT) simulations: gwModel (blue), HLZ (orange), and NRSur (green). We show the priors in gray for comparison. There is a clear preference for more symmetric systems with strongly spinning primary BH.
• Figure 3: Similar to Fig. \ref{['fig:progenitor']} but we show the inferred spin angles of the progenitor SMBHs with the orbital angular momentum of binary: $\theta_1\ (=\cos^{-1}[\hat{L}\cdot\hat{S}_1])$, $\theta_2$ and the effective precession parameter $\chi_{\rm p}$ (see Eq. \ref{['eq:chi_p']}) obtained from the recoil kick models HLZ and NRSur. We do not show the gwModel case because it only provides results marginalized over spin angles. We show the priors in gray. There is a strong preference for spin-precession in the system, and a weak measurement of the primary's spin angle.
• Figure 4: Same as Fig. \ref{['fig:progenitor']} but using astrophysically-motivated priors from two different accretion scenarios in gaseous environments Lousto:2017uavLousto:2012suZlochower:2015wga: hot accretion (crimson) and cold accretion (dark blue), while the default model is shown again for comparison. We use the NRSur recoil kick model for this plot.
• Figure 5: We show the progenitor SMBH mass ratio $q$, the dimensionless spin magnitude of the larger progenitor SMBH $|\chi_{1}|$ and the resulting remnant SMBH dimensionless spin magnitude $\chi_{\rm SMBH}$ that are consistent with the inferred runaway speeds of $954^{+110}_{-126}\,\mathrm{km\,s^{-1}}$ and $1310^{+21}_{-21}\,\mathrm{km\,s^{-1}}$ for the RBH-1 (blue) and the quasar 3C 186 (brown), respectively, under the assumption that the progenitor SMBHs were in a precessing configuration. We use gwModel as our recoil-kick prescription. In all cases, the priors are shown in gray.