Compact object mergers and
X-ray binaries at Cosmic Dawn
Embedded Files
Fig. 1. Co-moving merger rate density of Pop III BBHs as a function of redshift in our fiducial model (using the LOG1 initial condition model for Pop III binary stars, see Sec. 2.3 in Costa et al., 2023). The results for mergers in galaxy fields, NSCs, and combined are shown with the thin solid, dotted, and thick solid curves, respectively. The rate density of NSC infall is also shown with the dash-dotted curve. For comparison, the other curves show the results for other populations of BBH mergers from Franciolini et al. (2022). The local rate inferred from observations is shown with the triangle (Abbott et al., 2021).
Fig. 2. Detection rates of Pop III BBH mergers in galaxy fields (diamonds) and NSCs (circles, triangles, and squares) by ET with SNR > 8 for 18 models combining 3 IMFs (delineated by the power-law slope 𝛼), 2 models for initial binary statistics (close/left versus wide/right), and 3 models for NSCs (obs, low, and full).
Fig. 3. Intrinsic primary mass distributions of Pop III BBH mergers in galaxy fields (solid) and NSCs (dotted), initial BBHs (dashed), BBHs that fall into NSCs (dash-dotted), and progenitor stars (shaded) in the fiducial model. The primary mass distribution inferred from the 69 confident BBH mergers in GWTC-3 (Edelman et al. 2023) is shown with the thick solid curve.
Binary black hole mergers from the first stars: roles played by two evolution channels (2024, MNRAS, 534, 1634)
Binary black hole mergers from the first stars: roles played by two evolution channels (2024, MNRAS, 534, 1634)
The gravitational wave (GW) signal from binary black hole (BBH) mergers is a promising probe of population III (Pop III) stars, which are more efficient at producing massive black holes (BHs) than their population I/II (Pop I/II) counterparts. To fully unleash the power of the GW probe, one important step is to understand the relative importance and unique features of different evolution channels. We implement two channels, i.e., isolated binary stellar evolution (IBSE) and nuclear star cluster-dynamical hardening (NSC-DH), in the semi-analytical model A-SLOTH (Hartwig et al., 2022) for early star/galaxy formation. This framework is combined with binary population results from the code SEVN (Costa et al., 2023) to predict the BBH merger properties under various assumptions on Pop III initial mass function (IMF), binary statistics and high-z NSCs.
We find that the NSC-DH channel can be as efficient as the IBSE channel in producing Pop~III BBH mergers at z<9 (Fig. 1), which contributes 8-95% of Pop III BBH mergers across cosmic history, with higher contributions achieved by initially wider binary stars, more top-heavy IMFs, and more abundant high-z NSCs. The stochastic GW background (SGWB) produced by Pop III BBH mergers has a peak value of 1-8*10^-11 around observer-frame frequencies 10-100 Hz, which can be a non-negligible (~2-32%) component in the total SGWB below 10 Hz. The estimated detection rates of Pop~III BBH mergers by the Einstein Telescope (ET) are ~6-230 and ~30-1230 events per year for the NSC-DH and IBSE channels, respectively (Fig. 2). BBH mergers in NSCs are more massive than those from IBSE (Fig. 3), so they dominate the Pop III SGWB below ~20 Hz in most cases (Fig. 4). Besides, the detection rate of Pop III BBH mergers involving at least one intermediate-mass BH (IMBH) above 100 Msun by the Einstein Telescope is 0.5-200/yr in NSCs (Fig. 5) but remains below 0.1/yr for IBSE.
Fig. 4. SGWB energy density spectrum contributed by Pop III BBH mergers in the fiducial model. The annotations are identical to those in Fig. 1. The contributions of other populations of BBH mergers are taken from Bvera et al. (2022). The shaded regions show the sensitivities of two gravitational wave detectors. The triangle shows the total SGWB energy density estimated by population analysis through GWTC-3 (Abbott et al. 2023).
Fig. 5. Same as Fig. 2 but for the detection rates of Pop III BBH mergers including at least one IMBH (>100 Msun) in NSCs. The detection rates of such mergers in galaxy fields remain below 0.1/yr and are therefore not shown here.
The BPS catalogs involved here are also used by Mestichelli et al. (2025) to study Pop III and Pop II compact object mergers in star clusters.
Including this work, a brief review of Pop III BBH mergers is provided in the blue book "The Science of the Einstein Telescope" as Section 3.7 (Page 170-177).
This work was reported by Universe Today.
X-ray luminosity per unit SFR of Be-XRBs as a function of metallicity in our fiducial model (thin curves with data points) in comparison with previous theoretical predictions for other types of HMXBs (powered by Roche lobe overflow and spherical winds, gray thick curves) and observations of nearby galaxies (shaded region).
Population synthesis of Be X-ray binaries: metallicity dependence of X-ray outputs (2024, MNRAS, 527, 5023)
Population synthesis of Be X-ray binaries: metallicity dependence of X-ray outputs (2024, MNRAS, 527, 5023)
The X-ray output from Be X-ray binaries (Be-XRBs), powered by accretion of compact objects from decretion disks around rapidly rotating O/B stars, has not been investigated systematically in previous binary population synthesis (BPS) studies, despite that Be-XRBs are the largest class of observed high-mass XRBs (HMXBs) and are expected to play even more important roles in metal-poor galaxies at high redshifts.
To fill this gap, we build a physically-motivated model for Be-XRBs based on recent hydrodynamic simulations and observations of Be stars which, combined with appropriate initial conditions and binary evolution parameters, is able to reproduce the observed population of Be-XRBs in the Small Magellanic Cloud. We derive the X-ray output from Be-XRBs as a function of metallicity in the (absolute) metallicity range 𝑍 ∈ [0.0001 , 0.03]. The simulated Be-XRBs can explain a non-negligible fraction (>30%) of the total X-ray output from HMXBs observed in nearby galaxies for 𝑍 ~ 0.0003 − 0.02. A similar fraction of observed ultra-luminous (>10^39 erg/s ) X-ray sources can also be explained by the simulated Be-XRBs. The X-ray luminosity per unit star formation rate (SFR) from Be-XRBs in our fiducial model increases by a factor of 8 from Z=0.02 to Z=0.0003, which is similar to the trend seen in observations for all types of HMXBs. We conclude that Be-XRBs are potentially important X-ray sources that deserve greater attention in BPS of XRBs.
We study Population III (Pop III) binary remnant mergers in nuclear star clusters (NSCs) with a semi-analytical approach for early structure formation based on halo merger trees to follow the dynamics of Pop III binary (compact object) remnants in their host galaxies during cosmic structure formation, and construct the population of Pop III binary remnants that fall into NSCs by dynamical friction of field stars. The subsequent evolution within NSCs is then derived from three-body encounters and gravitational-wave (GW) emission.
We find that on average 7.5% of Pop III binary remnants will fall into the centres (< 3 pc) of galaxies that can host NSCs with masses above 10^5 Msun. About 5-50% of these binaries will merge at z>0 in NSCs, including those with very large initial separations (up to 1~pc). The merger rate density (MRD) peaks at z~5-7 with ~0.4-10 /(yr Gpc^3), comparable to the MRDs found in the binary stellar evolution channel. We predict a promising detection rate 170-3000 /yr for planned 3rd-generation GW detectors such as the Einstein Telescope that can reach z~10.
Low-mass (<10^6 Msun) NSCs host most (>90%) of our mergers, which mainly consist of black holes (BHs) with masses ~40-85 Msun, similar to the most massive BHs found in LIGO events. Particularly, our model can produce events like GW190521 involving BHs in the standard mass gap for pulsational pair-instability supernovae with a MRD~0.01-0.09 /(yr Gpc^3) at z~1, consistent with that inferred by LIGO.
Joint distribution of primary and secondary masses, where the 57 events detected by LIGO (from the 3-OGC) are plotted as data points with 90% confidence intervals. We highlight with triangles the events involving massive BHs that are most likely to have Pop III progenitors, as well as the special event GW190814 that may be a NS-BH merger.
Co-moving rate densities of PopIII BHB mergers similar to GW190521 from different evolution channels and environmental parameters (curves of different styles) based on our updated binary statistics, in comparison with the rate inferred from observation (shaded region).
The origin of the special BBH merger GW190521 reported by the LIGO collaboration is still in debate. This event involves "unusual" BH masses in the mass gap of (pulsational) pair-instability supernovae (PPISNe) from the standard stellar evolution models. Pop III stars are promising progenitors of the BHs found in GW190521, because Pop III stars, with small sizes and little mass loss, are likely to retain most of their hydrogen envelopes until the pre-supernova (SN) stage, avoid the PPISN regime and form BHs in the mass gap. We find that Pop III BBHs can naturally explain the observation, especially for the BBH evolution channel via dynamical hardening in nuclear star clusters.
Our Pop III scenario is further proved to be able to reproduce the observed merger rate density of GW190521-like events by more detailed modelling of the galactic dynamics and internal evolution of Pop III binary BHs (2021, MNRAS, 506, 5451).
Gravitational waves from Pop III binary black holes formed by dynamical capture (2020, MNRAS, 495, 2475)
Gravitational waves from Pop III binary black holes formed by dynamical capture (2020, MNRAS, 495, 2475)
The detection of gravitational waves (GWs) from merging compact objects such as black holes (BHs) has opened a new observable window in astrophysics. The GW window can perfectly complement the electromagnetic (EM) window to constrain theories of star and BH formation, especially for the first stars and their remanent which are difficult to detect with EM signals.
In this work, we use cosmological hydrodynamic simulations to predict the GW signals of binary BHs originated from the first stars formed by dynamical capture. We have found promising detection rates (~1-100 events per year) for future planned GW detectors such as the Einstein Telescope.
Waveforms of the GWs from our simulated binary BHs (gray solid curves), on top of the sensitivity curves of select GW detectors. Typical waveforms for mergers of direct collapse black holes (DCBHs) and DCBH-Pop III hybrid mergers are also shown with the red and orange curves.
Page updated
Google Sites
Report abuse