ABSTRACT
Coalescing massive black hole binaries (MBHBs) of $10^{4-7}~\rm M_{\odot }$, forming in the aftermath of galaxy mergers, are primary targets of the space mission LISA, the Laser ...Interferometer Space Antenna. An assessment of LISA detection prospects requires an estimate of the abundance and properties of MBHBs that form and evolve during the assembly of cosmic structures. To this aim, we employ a semi-analytic model to follow the co-evolution of MBHBs within their host galaxies. We identify three major evolutionary channels driving the binaries to coalescence: two standard paths along which the binary evolution is driven by interactions with the stellar and/or gaseous environment, and a novel channel where MBHB coalescence occurs during the interaction with a third black hole. For each channel, we follow the orbital evolution of MBHBs with physically motivated models that include a self-consistent treatment of the orbital eccentricity. We find that LISA will detect between ≈25 and ≈75 events per year depending on the seed model. We show that triple-induced coalescences can range from a few detected events up to ${\sim } 30{{\ \rm per\ cent}}$ of the total detected mergers. Moreover, even if the standard gas/stars-driven evolutionary channels should fail and MBHBs were to stall, triple interactions would still occur as a result of the hierarchical nature of galaxy formation, resulting in about ≈10 to ≈20 LISA detections per year. Remarkably, triple interactions among the black holes can produce coalescing binaries with large eccentricities (≳ 0.9) upon entrance into the LISA band. This eccentricity will remain significant (∼0.1) also at merger, requiring suitable templates for parameter estimation.
Abstract
The dynamics of black hole (BH) seeds in high-redshift galaxies is key to understand their ability to grow via accretion and to pair in close binaries during galactic mergers. To properly ...follow the dynamics of BHs we develop a physically motivated model to capture unresolved dynamical friction from stars, dark matter, and gas. We first validate the model and then we use it to investigate the dynamics of seed BHs born at z ∼ 9 in dwarf proto-galaxies. We perform a suite of zoom cosmological simulations with spatial resolution as high as 10 pc and with a stellar and dark matter mass resolution of $2\times 10^3 \, \, $ and $2\times 10^5 \, \, \mathrm{ M}_{\odot }$, respectively. We first explore the dynamics of a seed BH in the galaxy where it is born and show that it is highly erratic if the seed mass is less than $10^5\, \, \mathrm{ M}_{\odot }$. The dynamics is dominated by the stellar component, whose distribution is irregular and patchy, thus inducing stochasticity in the orbits: the BH may be anywhere in the proto-galaxy. When this dwarf merges into a larger galaxy, it is paramount to simulate the process with very high spatial and mass resolution in order to correctly account for the stripping of the stellar envelope of the satellite BH. The outcome of the encounter could be either a tight binary or, at least temporary, a wandering BH, leading to multiple BHs in a galaxy, each inherited from a different merger.
ABSTRACT
We estimate the amplitude of the nano-Hz stochastic gravitational wave background (GWB) resulting from an unresolved population of inspiralling massive black hole binaries (MBHBs). To this ...aim, we use the L-Galaxies semi-analytical model applied on top of the Millennium merger trees. The dynamical evolution of MBHBs includes dynamical friction, stellar and gas binary hardening, and gravitational wave (GW) feedback. At the frequencies proved by the Pulsar Timing Array experiments, our model predicts an amplitude of ${\sim }1.2 \times 10^{-15}$ at ${\sim }3 \times 10^{-8}\, \rm Hz$ in agreement with current estimations. The contribution to the background comes primarily from equal-mass binaries with chirp masses above $\rm 10^{8}\, M_{\odot }$. We then consider the recently detected common red noise in NANOGrav, PPTA, and EPTA data, working under the hypothesis that it is indeed a stochastic GWB coming from MBHBs. By boosting the massive black hole growth via gas accretion, we show that our model can produce a signal with an amplitude $A\approx (2\!-\!3) \times 10^{-15}$. There are, however, difficulties in predicting this background level without mismatching key observational constraints such as the quasar bolometric luminosity functions or the local black hole mass function. This highlights how current and forthcoming GW observations can, for the first time, confront galaxy and black hole evolution models.
ABSTRACT
Parsec-scale massive black hole binaries (MBHBs) are expected to form in hierarchical models of structure formation. Even though different observational strategies have been designed to ...detect these systems, a theoretical study is a further guide for their search and identification. In this work, we investigate the hosts properties and the electromagnetic signatures of massive black holes gravitationally bound on parsec-scales with primary mass $\rm {\gt }\, 10^7\, M_{\odot }$. For that, we construct a full-sky light-cone by the use of the semi-analytical model L-Galaxies in which physically motivated prescriptions for the formation and evolution of MBHBs have been included. Our predictions show that the large majority of the MBHBs are placed either in spiral galaxies with a classical bulge structure or in elliptical galaxies. Besides, the scaling relations followed by MBHBs are indistinguishable from the ones of single massive black holes. We find that the occupation fraction of parsec-scale MBHBs reaches up to ${\sim }\, 50{{\ \rm per\ cent}}$ in galaxies with $\rm M_{stellar}\, {\gt }\, 10^{11}\, M_{\odot }$ and drops below 10 per cent for $\rm M_{stellar}\, {\lt }\, 10^{11}\, M_{\odot }$. Our model anticipates that the majority of parsec-scale MBHBs are unequal mass systems and lie at $z\, {\sim }\, 0.5$, with ${\sim }\, 20$ objects per $\rm deg^2$ in the sky. However, most of these systems are inactive, and only $\rm {1-0.1}$ objects per $\rm deg^2$ have an electromagnetic counterpart with a bolometric luminosity in excess of 1043 erg s−1. Very luminous phases of parsec-scale MBHBs are more common at $z\, {\gt }\, 1$, but the number of binaries per $\rm deg^2$ is ${\lesssim }\, 0.01$ at $\rm L_{\rm bol}\, {\gt }\, 10^{45}\,\rm erg\,s^{-1}$.
In current stellar evolutionary models, the occurrence of pair-instability supernovae implies a lack of stellar black holes (BHs) with masses between about 60, 120 M , resulting in the presence of an ...upper-mass gap in the BH mass distribution. In this Letter, we propose a simple approach to describe BHs beyond the pair-instability gap by convolving the initial mass function and star formation rate with the metallicity evolution across cosmic time. Under the ansatz that the underlying physics of binary formation does not change beyond the gap, we then construct the cosmic population of merging BH binaries. The detection rate of BH binaries with both mass components above the gap is found to range between 0.4, 7 yr−1 for LIGO/Virgo at design sensitivity and 10, 460 yr−1 for third-generation ground-based detectors, considering the most pessimistic and optimistic scenarios. The Laser Interferometer Space Antenna (LISA) can individually detect these binaries up to thousands of years from coalescence. The number of events merging in less than four years, which enable multiband observation in sequence, is expected to be in the range 1, 20. While ET will detect all these events, LIGO/Virgo is expected to detect 50% of them. Finally, we estimate that the gravitational-wave background from unresolved sources in the LISA band may in principle be detected with a signal-to-noise ratio between 2.5 and 80.
Massive binary black holes (10
5
M
⊙
–10
9
M
⊙
) form at the centre of galaxies that experience a merger episode. They are expected to coalesce into a larger black hole, following the emission of ...gravitational waves. Coalescing massive binary black holes are among the loudest sources of gravitational waves in the Universe, and the detection of these events is at the frontier of contemporary astrophysics. Understanding the black hole binary formation path and dynamics in galaxy’s mergers is therefore mandatory. A key question poses: during a merger, will the black holes descend over time on closer orbits, form a Keplerian binary and coalesce shortly after? Here we review progress discussing the fate of black holes in different environments: from major mergers of collisionless galaxies to major and minor mergers of gas-rich disc galaxies, from smooth and clumpy circum-nuclear discs to circum-binary discs present on the smallest scales inside galactic nuclei.
ABSTRACT
Laser Interferometer Space Antenna (LISA) will extend the search for gravitational waves (GWs) at $0.1\, {-}\, 100$ mHz where loud signals from coalescing binary black holes of $10^4 \, ...{-}\, 10^7\, \, \rm {M}_{\odot }$ are expected. Depending on their mass and luminosity distance, the uncertainty in the LISA sky-localization decreases from hundreds of deg2 during the inspiral phase to fractions of a deg2 after the merger. By using the semi-analytical model L-Galaxies applied to the Millennium-I merger trees, we generate a simulated universe to identify the hosts of $z\, {\le }\, 3$ coalescing binaries with total mass of $3\, {\times }\, 10^{5}$, $3\, {\times }\, 10^6$, and $3\, {\times }\, 10^7\, \rm {M}_{\odot }$, and varying mass ratio. We find that, even at the time of merger, the number of galaxies around the LISA sources is too large (${\gtrsim }\, 10^2$) to allow direct host identification. However, if an X-ray counterpart is associated to the GW sources at $z\, {< }\, 1$, all LISA fields at merger are populated by ${\lesssim }\, 10$ active galactic nuclei (AGNs) emitting above ${\sim }\, 10^{-17} \, \rm erg\, cm^{-2}\, s^{-1}$. For sources at higher redshifts, the poorer sky-localization causes this number to increase up to ${\sim }\, 10^3$. Archival data from eRosita will allow discarding ${\sim }\, 10{{\ \rm per\ cent}}$ of these AGNs, being too shallow to detect the dim X-ray luminosity of the GW sources. Inspiralling binaries in an active phase with masses ${\lesssim }\, 10^6\, \rm {M}_{\odot }$ at $z\, {\le }\, 0.3$ can be detected, as early as 10 h before the merger, by future X-ray observatories in less than a few minutes. For these systems, ${\lesssim }\, 10$ AGNs are within the LISA sky-localization area. Finally, the LISA-Taiji network would guarantee the identification of an X-ray counterpart 10 h before merger for all binaries at $z\, {\lesssim }\, 1$.
Flares from tidal disruption events are unique tracers of quiescent black holes at the centre of galaxies. The appearance of these flares is very sensitive to whether the star is totally or partially ...disrupted, and in this paper we seek to identify the critical distance of the star from the black hole (rd) that enables us to distinguish between these two outcomes. We perform here mesh-free finite mass, traditional, and modern smoothed particle hydrodynamical simulations of star-black hole close encounters, with the aim of checking if the value of rd depends on the simulation technique. We find that the critical distance (or the so-called critical disruption parameter βd) depends only weakly on the adopted simulation method, being βd = 0.92 ± 0.02 for a γ = 5/3 polytrope and βd = 2.01 ± 0.01 for a γ = 4/3 polytrope.
Abstract
In the local Universe, black holes of $10^{5-6}\, {\rm M_\odot }$ are hosted in galaxies displaying a variety of stellar profiles and morphologies. These black holes are the anticipated ...targets of LISA, the Laser Interferometer Space Antenna that will detect the low-frequency gravitational-wave signal emitted by binary black holes in this mass interval. In this paper, we infer upper limits on the lifetime of binary black holes of $10^{5-6}\, {\rm M_\odot }$ and up to $10^8\, {\rm M_\odot }$, forming in galaxy mergers, exploring two underlying stellar density profiles, by Dehnen and by Prugniel & Simien, and by exploiting local scaling relations between the mass of the black holes and several quantities of their hosts. We focus on the phase of the dynamical evolution when the binary is transitioning from the hardening phase ruled by the interaction with single stars to the phase driven by the emission of gravitational waves. We find that different stellar profiles predict very distinct trends with binary mass, with lifetimes ranging between fractions of a Gyr to more than 10 Gyr, and with a spread of about one order of magnitude, given by the uncertainties in the observed correlations, which are larger in the low-mass tail of the observed black hole population.
The quest for binary and dual supermassive black holes (SMBHs) at the dawn of the multi-messenger era is compelling. Detecting dual active galactic nuclei (AGN) – active SMBHs at projected ...separations larger than several parsecs – and binary AGN – probing the scale where SMBHs are bound in a Keplerian binary – is an observational challenge. The study of AGN pairs (either dual or binary) also represents an overarching theoretical problem in cosmology and astrophysics. The AGN triggering calls for detailed knowledge of the hydrodynamical conditions of gas in the imminent surroundings of the SMBHs and, at the same time, their duality calls for detailed knowledge on how galaxies assemble through major and minor mergers and grow fed by matter along the filaments of the cosmic web. This review describes the techniques used across the electromagnetic spectrum to detect dual and binary AGN candidates and proposes new avenues for their search. The current observational status is compared with the state-of-the-art numerical simulations and models for formation of dual and binary AGN. Binary SMBHs are among the loudest sources of gravitational waves (GWs) in the Universe. The search for a background of GWs at nHz frequencies from inspiralling SMBHs at low redshifts, and the direct detection of signals from their coalescence by the Laser Interferometer Space Antenna in the next decade, make this a theme of major interest for multi-messenger astrophysics. This review discusses the future facilities and observational strategies that are likely to significantly advance this fascinating field.
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