Abstract
The origin of the black hole mergers detected by LIGO–Virgo remains an open question. While the unusual mass and spin of a few events constrain their possible astrophysical formation ...mechanisms, it is difficult to classify the bulk of the observed mergers. Here we consider the distribution of masses and spins in LIGO–Virgo’s first two observing catalogs. We show that, for black holes in the mass gap, our fiducial active galactic nucleus (AGN) model is preferred over a parametric mass–spin model fit to the full GWTC-2 merger sample (Bayes factor
>
10
). This preference, nevertheless, depends on uncertain AGN model parameters. We further show that a 20% fractional contribution of the detected events of an AGN-disk origin reproduces well the observed black hole mass distribution in the pair-instability mass gap, while only marginally contributing to the lower-mass detected population. The overall AGN contribution corresponds to a black hole merger rate of about 2.5 Gpc
−3
yr
−1
, comparable to theoretical expectations.
The heaviest neutron stars and lightest black holes expected to be produced by stellar evolution leave the mass range 2.2 M largely unpopulated. Objects found in this so-called lower mass gap likely ...originate from a distinct astrophysical process. Such an object, with mass 2.6 M was recently detected in the binary merger GW190814 through gravitational waves by LIGO/Virgo. Here we show that black holes in the mass gap are naturally assembled through mergers and accretion in active galactic nucleus (AGN) disks, and can subsequently participate in additional mergers. We compute the properties of AGN-assisted mergers involving neutron stars and black holes, accounting for accretion. We find that mergers in which one of the objects is in the lower mass gap represent up to 4% of AGN-assisted mergers detectable by LIGO/Virgo. The lighter object of GW190814, with mass 2.6 M , could have grown in an AGN disk through accretion. We find that the unexpectedly high total mass of 3.4 M observed in the neutron star merger GW190425 may also be due to accretion in an AGN disk.
Binary black hole mergers encode information about their environment and the astrophysical processes that led to their formation. Measuring the redshift dependence of their merger rate will help ...probe the formation and evolution of galaxies and the evolution of the star formation rate. Here we compute the cosmic evolution of the merger rate for stellar-mass binaries in the disks of active galactic nuclei (AGNs). We focus on recent evolution out to redshift z = 2, covering the accessible range of current Earth-based gravitational-wave observatories. On this scale, the AGN population density is the main contributor to redshift dependence. We find that the AGN-assisted merger rate varies by less than a factor of two in the range 0 < z ≤ 2, comparable to the expected level of evolution for globular clusters, but much smaller than the order-of-magnitude evolution for field binaries.
There is some weak evidence that the black hole merger named GW190521 had a non-zero eccentricity
. In addition, the masses of the component black holes exceeded the limit predicted by stellar ...evolution
. The large masses can be explained by successive mergers
, which may be efficient in gas disks surrounding active galactic nuclei, but it is difficult to maintain an eccentric orbit all the way to the merger, as basic physics would argue for circularization
. Here we show that active galactic nuclei disk environments can lead to an excess of eccentric mergers, if the interactions between single and binary black holes are frequent
and occur with mutual inclinations of less than a few degrees. We further illustrate that this eccentric population has a different distribution of the inclination between the spin vectors of the black holes and their orbital angular momentum at merger
, referred to as the spin-orbit tilt, compared with the remaining circular mergers.
Abstract
Active galactic nuclei (AGNs) can funnel stars and stellar remnants from the vicinity of the galactic center into the inner plane of the AGN disk. Stars reaching this inner region can be ...tidally disrupted by the stellar-mass black holes in the disk. Such micro tidal disruption events (micro-TDEs) could be a useful probe of stellar interaction with the AGN disk. We find that micro-TDEs in AGNs occur at a rate of ∼170 Gpc
−3
yr
−1
. Their cleanest observational probe may be the electromagnetic detection of tidal disruption in AGNs by heavy supermassive black holes (
M
•
≳ 10
8
M
⊙
) that cannot tidally disrupt solar-type stars. The reconstructed rate of such events from observations, nonetheless, appears to be much lower than our estimated micro-TDE rate. We discuss two such micro-TDE candidates observed to date (ASASSN-15lh and ZTF19aailpwl).
We present post-Newtonian N-body simulations on mergers of accreting stellar-mass black holes (BHs), where such general relativistic effects as the pericentre shift and gravitational wave (GW) ...emission are taken into consideration. The attention is concentrated on the effects of the dynamical friction and the Hoyle–Lyttleton mass accretion by ambient gas. We consider a system composed of 10 BHs with initial mass of 30 M⊙. As a result, we show that mergers of accreting stellar-mass BHs are classified into four types: a gas drag-driven, an interplay-driven, a three-body-driven, or an accretion-driven merger. We find that BH mergers proceed before significant mass accretion, even if the accretion rate is ∼10 Eddington accretion rate, and then all BHs can merge into one heavy BH. Using the simulation results for a wide range of parameters, we derive a critical accretion rate (
$\dot{m}_{\rm c}$
), below which the BH growth is promoted faster by mergers. Also, it is found that the effect of the recoil by the GW emission can reduce
$\dot{m}_{\rm c}$
especially in gas number density higher than 108 cm−3, and enhance the escape probability of merged BHs. Very recently, a gravitational wave event, GW150914, as a result of the merger of a ∼30 M⊙ BH binary has been detected. Based on the present simulations, the BH merger in GW150914 is likely to be driven by three-body encounters accompanied by a few M⊙ of gas accretion, in high-density environments like dense interstellar clouds or galactic nuclei.
Abstract
The astrophysical origin of over 90 compact binary mergers discovered by the LIGO and Virgo gravitational wave observatories is an open question. While the unusual mass and spin of some of ...the discovered objects constrain progenitor scenarios, the observed mergers are consistent with multiple interpretations. A promising approach to solve this question is to consider the observed distributions of binary properties and compare them to expectations from different origin scenarios. Here we describe a new hierarchical population analysis framework to assess the relative contribution of different formation channels simultaneously. For this study we considered binary formation in active galactic nucleus (AGN) disks along with phenomenological models, but the same framework can be extended to other models. We find that high-mass and high-mass-ratio binaries appear more likely to have an AGN origin compared to having the same origin as lower-mass events. Future observations of high-mass black hole mergers could further disentangle the AGN component from other channels.
MicroRNA (miRNA; miR) is a class of small regulatory RNA molecules, the aberrant expression of which can lead to the development of cancer. We recently reported that overexpression of miR-21 and/or ...miR-155 leads to activation of the phosphoinositide 3-kinase (PI3K)-AKT pathway in malignant lymphomas expressing CD3(-)CD56(+) natural killer (NK) cell antigen. Through expression analysis, we show in this study that in both NK/T-cell lymphoma lines and samples of primary lymphoma, levels of miR-150 expression are significantly lower than in normal NK cells. To examine its role in lymphomagenesis, we transduced miR-150 into NK/T-cell lymphoma cells, which increased the incidence of apoptosis and reduced cell proliferation. Moreover, the miR-150 transductants appeared senescent and showed lower telomerase activity, resulting in shortened telomeric DNA. We also found that miR-150 directly downregulated expression of DKC1 and AKT2, reduced levels of phosphorylated AKT(ser473/4) and increased levels of tumor suppressors such as Bim and p53. Collectively, these results suggest that miR-150 functions as a tumor suppressor, and that its aberrant downregulation induces continuous activation of the PI3K-AKT pathway, leading to telomerase activation and immortalization of cancer cells. These findings provide new insight into the pathogenesis of malignant lymphoma.
The proto-oncogene BMI1 and its product, Bmi1, is overexpressed in various types of tumors, particularly in aggressive tumors and tumors resistant to conventional chemotherapy. BMI1/Bmi1 is also ...crucially involved in cancer-initiating cell maintenance, and is recurrently upregulated in mantle cell lymphoma (MCL), especially aggressive variants. Recently, side population (SP) cells were shown to exhibit tumor-initiating characteristics in various types of tumors. In this study, we show that recurrent MCL cases significantly exhibit upregulation of BMI1/Bmi1. We further demonstrate that clonogenic MCL SP shows such tumor-initiating characteristics as high tumorigenicity and self-renewal capability, and that BMI1 was upregulated in the SP from recurrent MCL cases and MCL cell lines. On screening for upstream regulators of BMI1, we found that expression of microRNA-16 (miR-16) was downregulated in MCL SP cells by regulating Bmi1 in the SPs, leading to reductions in tumor size following lymphoma xenografts. Moreover, to investigate downstream targets of BMI1 in MCL, we performed cross-linking/chromatin immunoprecipitation assay against MCL cell lines and demonstrated that Bmi1 directly regulated pro-apoptotic genes such as BCL2L11/Bim and PMAIP1/Noxa, leading to enhance anti-apoptotic potential of MCL. Finally, we found that a proteasome inhibitor bortezomib, which has been recently used for relapsed MCL, effectively induced apoptosis among MCL cells while reducing expression of Bmi1 and increasing miR-16 in MCL SP. These results suggest that upregulation of BMI1 and downregulation of miR-16 in MCL SP has a key role in the disease's progression by reducing MCL cell apoptosis. Our results provide important new insight into the pathogenesis of MCL and strongly suggest that targeting BMI1/Bmi1 might be an effective approach to treating MCL, particularly refractory and recurrent cases.
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