First Light And Reionization Epoch Simulations (FLARES) -- XIX: Supermassive black hole mergers in the early Universe and their environmental dependence

TL;DR

This study uses FLARES, a suite of high-resolution cosmological zoom-in simulations, to characterize supermassive black hole (SMBH) mergers at $5 \lesssim z \lesssim 10$ across diverse environments. It finds primary SMBHs to have a broad mass distribution up to $\sim 10^{8}~M_{\odot}$ while secondaries cluster near seed masses, with mass ratios spanning $\sim 10^{-4}$ to 1 and most mergers occurring at $z \lesssim 7$. The environmental overdensity drives higher merger rates and heavier SMBHs, with the merger density scaling as $n_{\rm merger} = 10^{-3.81} (1 + \delta)^{4.78}$ and a dense region hosting up to ~500× more mergers. For LISA, about 0.030 per year is expected to detect $z \ge 4.69$ SMBH mergers with ${\rm SNR} \ge 10$, though predictions are sensitive to merger time delays, seed mass, and spins. The work highlights the need for improved SMBH seeding and dynamical modeling to refine GW forecasts and fully exploit LISA’s potential to probe SMBH growth in the early Universe.

Abstract

The upcoming space-based gravitational wave (GW) observatory, LISA, is expected to detect GW signals from supermassive black hole (SMBH) mergers occurring at high redshifts. However, understanding the origin and growth of SMBHs in the early Universe remains an open problem in astrophysics. In this work, we utilize the First Light And Reionization Epoch Simulations (FLARES), a suite of cosmological hydrodynamical zoom-in simulations, to study SMBH mergers at $5 \lesssim z \lesssim 10$ across a wide range of environments. Most mergers in FLARES involve secondary SMBHs near the seed mass ($m_{\rm seed} \approx 1.5 \times 10^{5}~{\rm M}_{\odot}$) while primary SMBHs span up to $10^{9}~{\rm M}_{\odot}$, resulting in mass ratios from $q \sim 10^{-4}$ to $1$, with a peak at $q \sim 1$. The number of mergers increases rapidly towards lower redshifts, and the comoving total number density scales with overdensity as $n_{\rm merger} = 10^{-3.81} (1 + δ)^{4.78}$. Denser regions host more massive mergers, with higher merger redshifts and lower mass ratios. Within the FLARES redshift range, LISA is expected to detect mergers with $10^{5} \lesssim M_{\rm tot}/{\rm M}_{\odot} \lesssim 10^{8}$ and $q \gtrsim 10^{-2}$, corresponding to a detection rate of $0.030~{\rm yr}^{-1}$ for events with signal-to-noise ratio ${\rm SNR} \geq 10$. Our study demonstrates the sensitivity of GW predictions at high redshifts to SMBH seed models and merger time delays, highlighting the need for improved modeling in future cosmological simulations to maximize LISA's scientific return.

Figures (13)

• Figure 1: Primary ($M_{1}$) and secondary ($M_{2}$) SMBH masses of all SMBH mergers in FLARES. Each dot represents an SMBH merger and it is colour-coded by the redshift of the event, $z_{\rm merger}$. The solid lines show the mass ratios ($M_{2}$:$M_{1}$) from 1:1 to 1:1000. The mass range below the SMBH seed mass ($10^5 h^{-1}{\rm M}_{\sun}$) is marked by a grey colour. The top and right sub-panels show the histograms of $M_{1}$ and $M_{2}$, respectively. The inset panel displays the histogram of $z_{\rm merger}$. The majority of the SMBH mergers occur at $z \la 7$ and have their $M_{2}$ close to the seed mass.
• Figure 2: Composite mass functions of SMBH mergers for the primary SMBH ($M_1$, left), the secondary SMBH ($M_2$, middle), and the SMBH binary ($M_{\rm tot} = M_1 + M_2$, right). Different merger redshift ranges are plotted using different colours, as specified in the legend. The redshift bins are chosen such that each spans an equal time interval of $200$ Myr. The total mass functions, which account for SMBH mergers across all redshifts in FLARES, are shown as black curves. For comparison, the composite black hole mass function -- including all SMBHs, not just those that have experienced mergers -- at $z = 5$ is plotted as a gray line in each panel. The secondary SMBHs are dominated by those with masses close to the seed mass, particularly at high redshifts.
• Figure 3: Composite distributions of the SMBH merger mass ratio $q = M_2 / M_1$. The panels, from left to right, show distributions with different cuts in $M_2$: $M_{2} \geq m_{\rm seed} = 1.48 \times 10^{5}~{\rm M}_{\sun}$, $M_{2} \geq 10^{6}~{\rm M}_{\sun}$, and $M_{2} \geq 10^{7}~{\rm M}_{\sun}$. In each panel, the total distribution is shown in black, while the distributions of SMBH mergers at different $z_{\rm merger}$ ranges are plotted in different colours, as specified in the legend. For all SMBH mergers in FLARES (black composite histogram in the left-hand panel), the mass ratios span a wide range, from ${\sim}10^{-4}$ to $1$.
• Figure 4: Composite distribution of merger redshifts. As indicated in the legend, the distribution of all mergers (i.e. $M_{2} \geq m_{\rm seed}$) is plotted in black, while the distributions of SMBH mergers with different $M_{2}$ cuts are shown in different colours. The merger number density increases rapidly as redshift decreases.
• Figure 5: Tests on the impact of the SMBH seed mass using Region 08. The fiducial run with $m_{\rm seed} = 10^{5} ~ h^{-1}{\rm M}_{\sun}$ is plotted in blue, while the test run with a lower SMBH seed mass (i.e. $m_{\rm seed} = 10^{4} ~ h^{-1}{\rm M}_{\sun}$) is shown in red. Left-hand panel: The secondary SMBH mass versus the primary SMBH mass of all mergers in Region 08. The diagonal solid line marks the mass ratio of $1:1$, and the two horizontal dashed lines indicate the seed masses. The right and top sub-panels compare the histograms from two runs. Middle panel: The histograms of the mass ratios for all merger events. Right-hand panel: The histograms of the merger redshifts from the two runs. Compared to the fiducial run, the test run with a lower SMBH seed mass exhibits fewer SMBH mergers, fails to produce massive SMBHs (i.e. $M_{\rm BH} \ga 10^{6}~{\rm M}_{\sun}$), has fewer SMBH mergers at low redshifts, and has more mergers with high mass ratios.