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Register a new userOrbital evolution of binary black holes in active galactic nucleus disks: a disk channel for binary black hole mergers?
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Ya-Ping Li
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We perform a series of high-resolution 2D hydrodynamical simulations of equal-mass binary black holes (BBHs) embedded in active galactic nucleus (AGN) accretion disks to study whether these binaries can be driven to merger by the surrounding gas. We find that the gravitational softening adopted for the BBH has a profound impact on this result. When the softening is less than ten percent of the binary separation, we show that, in agreement with recent simulations of isolated equal-mass binaries, prograde BBHs expand in time rather than contract. Eventually, however, the binary separation becomes large enough that the tidal force of the central AGN disrupts them. Only when the softening is relatively large do we find that prograde BBHs harden. We determine through detailed analysis of the binary torque, that this dichotomy is due to a loss of spiral structure in the circum-single disks orbiting each BH when the softening is a significant fraction of the binary separation. Properly resolving these spirals -- both with high resolution and small softening -- results in a significant source of binary angular momentum. Only for retrograde BBHs do we find consistent hardening, regardless of softening, as these BBHs lack the important spiral structure in their circum-single disks. This suggests that the gas-driven inspiral of retrograde binaries can produce a population of compact BBHs in the gravitational-wave-emitting regime in AGN disks, which may contribute a large fraction to the observed BBH mergers.
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Using the Binary Population and Spectral Synthesis code BPASS, we have calculated the rates, timescales and mass distributions for binary black hole mergers as a function of metallicity. We consider these in the context of the recently reported 1st LIGO event detection. We find that the event has a very low probability of arising from a stellar population with initial metallicity mass fraction above Z=0.010 (Z>0.5Zsun). Binary black hole merger events with the reported masses are most likely in populations below 0.008 (Z<0.4Zsun). Events of this kind can occur at all stellar population ages from ~3 Myr up to the age of the universe, but constitute only 0.1 to 0.4 per cent of binary BH mergers between metallicities of Z=0.001 to 0.008. However at metallicity Z=0.0001, 26 per cent of binary BH mergers would be expected to have the reported masses. At this metallicity the progenitor merger times can be close to ~10Gyr and rotationally-mixed stars evolving through quasi-homogeneous evolution, due to mass transfer in a binary, dominate the rate. The masses inferred for the black holes in the binary progenitor of GW,150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.
We aim to study the progenitor properties and expected rates of the two lowest-mass binary black hole (BH) mergers, GW 151226 and GW 170608, detected within the first two Advanced LIGO-Virgo runs, in the context of the isolated binary-evolution scenario. We use the public MESA code, which we adapted to include BH formation and unstable mass transfer developed during a common-envelope (CE) phase. Using more than 60000 binary simulations, we explore a wide parameter space for initial stellar masses, separations, metallicities, and mass-transfer efficiencies. We obtain the expected distributions for the chirp mass, mass ratio, and merger time delay by accounting for the initial stellar binary distributions. Our simulations show that, while the progenitors we obtain are compatible over the entire range of explored metallicities, they show a strong dependence on the initial masses of the stars, according to stellar winds. All the progenitors follow a similar evolutionary path, starting from binaries with initial separations in the $30-200~R_odot$ range, experiencing a stable mass transfer interaction before the formation of the first BH, and a second unstable mass-transfer episode leading to a CE ejection that occurs either when the secondary star crosses the Hertzsprung gap or when it is burning He in its core. The CE phase plays a fundamental role in the considered low-mass range: only progenitors experiencing such an unstable mass-transfer phase are able to merge in less than a Hubble time. We find integrated merger-rate densities in the range $0.2-5.0~{rm yr}^{-1}~{rm Gpc}^{-3}$ in the local Universe for the highest mass-transfer efficiencies explored. The highest rate densities lead to detection rates of $1.2-3.3~{rm yr}^{-1}$, being compatible with the observed rates. A high CE-efficiency scenario with $alpha_{rm CE}=2.0$ is favored when comparing with observations. ABRIDGED.
The astrophysical origin of gravitational wave (GW) transients is a timely open question in the wake of discoveries by LIGO/Virgo. In active galactic nuclei (AGNs), binaries form and evolve efficiently by interaction with a dense population of stars and the gaseous AGN disk. Previous studies have shown that stellar-mass black hole (BH) mergers in such environments can explain the merger rate and the number of suspected hierarchical mergers observed by LIGO/Virgo. The binary eccentricity distribution can provide further information to distinguish between astrophysical models. Here we derive the eccentricity distribution of BH mergers in AGN disks. We find that eccentricity is mainly due to binary-single (BS) interactions, which lead to most BH mergers in AGN disks having a significant eccentricity at $0.01,mathrm{Hz}$, detectable by LISA. If BS interactions occur in isotropic-3D directions, then $8$--$30%$ of the mergers in AGN disks will have eccentricities at $10,mathrm{Hz}$ above $e_{10,rm Hz}gtrsim 0.03$, detectable by LIGO/Virgo/KAGRA, while $5$--$17%$ of mergers have $e_{10,rm Hz}geq 0.3$. On the other hand, if BS interactions are confined to the AGN-disk plane due to torques from the disk, with 1-20 intermediate binary states during each interaction, or if BHs can migrate to $lesssim10^{-3},mathrm{pc}$ from the central supermassive black hole, then $10$--$70%$ of the mergers will be highly eccentric ($e_{10,rm Hz} geq 0.3$), consistent with the possible high eccentricity in GW190521.
We review the main physical processes that lead to the formation of stellar binary black holes (BBHs) and to their merger. BBHs can form from the isolated evolution of massive binary stars. The physics of core-collapse supernovae and the process of common envelope are two of the main sources of uncertainty about this formation channel. Alternatively, two black holes can form a binary by dynamical encounters in a dense star cluster. The dynamical formation channel leaves several imprints on the mass, spin and orbital properties of BBHs.
Gravitational wave interferometers have proved the existence of a new class of binary black holes (BBHs) weighting tens of solar masses and they have provided the first reliable measurement of the rate of coalescing black holes (BHs) in the local universe. On another side, long gamma-ray bursts (GRBs) detected with gamma-ray satellites are believed to be associated with the birth of stellar mass BHs, providing a measure of the rate of these events across the history of the universe, thanks to the measure of their cosmological redshift. These two types of sources, which are subject to different detection biases and involve BHs born in different environments with potentially different characteristics, provide complementary information on the birth rate of stellar BHs. We compare here the birth rates of BHs found in BBH mergers and in long GRBs. We construct a simple model which makes reasonable assumptions on the history of GRB formation, and which takes into account some major uncertainties, like the beaming angle of GRBs or the delay between the formation of BBHs and their coalescence. We use this model to evaluate the ratio of the number of stellar mass BHs formed in BBH mergers to those formed in GRBs. We find that in our reference model the birth rate of stellar BHs in BBH mergers represents from few percent to 100% of the rate of long GRBs and that comparable birth rates are favored by models with moderate beaming angles. We briefly discuss this result in view of our understanding of the progenitors of GRBs and BBH mergers, and we emphasize that this ratio, which will be better constrained in the coming years, can be directly compared with the prediction of stellar evolution models if a single model is used to produce GRBs and of BBH mergers with the same assumptions.
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