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1. Explaining the LIGO black hole mass function with field binaries: Revisiting Stellar Evolution at low Metallicity or Invoking Growth via gas accretion?

   Safarzadeh, Mohammadtaher; Ramirez-Ruiz, Enrico (May 2021, ArXivorg)

   null (Ed.)

   Our understanding of the formation and evolution of binary black holes (BBHs) is significantly impacted by the recent discoveries made by the LIGO/Virgo collaboration. Of utmost importance is the detection of the most massive BBH system, GW190521. Here we investigate what it takes for field massive stellar binaries to account for the formation of such massive BBHs. Whether the high mass end of the BH mass function is populated by remnants of massive stars that either formed at extremely low metallicities and avoid the pair-instability mass gap or increase their birth mass beyond the pair-instability mass gap through the accretion of gas from the surrounding medium. We show that assuming that massive stars at very low metallicities can form massive BHs by avoiding pair-instability supernova, coupled with a correspondingly high formation efficiency for BBHs, can explain the observed BH mass function. To this end, one requires a relation between the initial and final mass of the progenitor stars at low metallicities that is shallower than what is expected from wind mass loss alone. On the other hand, assuming pair-instability operates at all metallicities, one can account for the observed BH mass function if at least about 10% of the BHs born at very low metallicities double their mass before they merge because of accretion of ambient gas. Such BBHs will have to spend about a Gyr within a parsec length-scale of their parent atomic cooling halos or a shorter timescale if they reside in the inner sub-parsecs of their host dark matter halos. Future stellar evolution calculations of massive stars at very low metallicity and hydrodynamical simulations of gas accretion onto BBHs born in atomic cooling halos can shed light on this debate. 

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2. A New Diagnostic Method for Distinguishing Binary from Single Supermassive Black Holes Using Broad Emission Lines

   Ji, Ziming (July 2024, Rochester Institute of Technology)

   During cosmic timescales, supermassive binary black holes (SMBBHs) form by galaxy merg- ers, where each galaxy hosts a supermassive black hole (SMBH) at its center. By studying SMBBHs, we can gain insights into galaxy evolution and black hole growth. However, the typical separation between black holes in SMBBHs is usually below 1 pc, making them dif- ficult to resolve using direct imaging or photometry. To be able to distinguish binary black holes (BBHs) from typical AGN powered by single black holes (SBHs), we conducted this research project to develop a new diagnostic method to identify a unique feature of SMBBHs, which can be used to distinguish AGN powered by BBHs from those powered by SBHs. The basic idea of this method is that BBHs have different configurations compared with those of SBHs, such as the circumbinary disk enveloping the whole binary system, the mini- disks around each black hole, and the streams between the circumbinary disk and minidisks. It is these different configurations of accretion disks of black holes that lead to the difference in the spectral energy distributions (SEDs), which in turn will differently photoionize the broad line region (BLR) and produce different strengths of broad emission lines. Thus, it is these differences in strengths of broad emission lines that we expect to see between two different configurations of black holes, either binary or single black holes. By identifying these differences in line strengths, we can distinguish between binary and single black holes. In order to achieve this goal of distinguishing BBHs from SBHs, we organized our project in the steps below: (1) obtained BBH SEDs from Gutiérrez et al., 2022, (2) generated SBH SEDs using XSPEC modeling code OPTXAGNF, (3) produced single-cloud models, which represent the BLR, input BBH and SBH SEDs into the models, and simulated the single0cloud response with a photoionization code Cloudy, (4) built cloud-ensemble models, which represent a more realistic BLR, input BBH and SBH SEDs into the models, and simulated the emission-line response within those clouds using a broad emission line mapping code BELMAC. In step (3), we have explored the differences in line ratios between BBH and SBH for a representative single-cloud photoionization model. The emission lines we used here are: Si IV λ1400Å, C III] λ1909Å, C IV λ1549Å, Mg II λ2798Å, and Lyα λ1216Å. It turned i out that differences do exist, but they are too small to be identified in observational data. Furthermore, we have investigated the line equivalent widths predicted by SBH and BBH models respectively. By doing so, we found some apparent differences between BBHs and SBHs in some specific emission lines: Lyα, CIV, and Hα. However, these differences vanish at the highest mass of black holes (109M⊙). In step (4), we continued the investigation of the equivalent width between SBH and BBH BLR cloud-ensemble models and found that some emission lines show the difference between BBHs and SBHs, such as CIV in the case of BH mass 107M⊙, U ∝ r−2, and log n/1 cm−3 = 10.5, 11.0. For the highest mass in the case of U ∝ r−2, the results are similar to the one in single-cloud models: no difference is shown between BBHs and SBHs across all emission lines. Most importantly, in this step, we found that for CIV λ1549 Å in the case of a black hole mass of 107M⊙, s = −2, and log n/1 cm−3 = 10.5, only the SBH EW falls inside the range of the observed range in SDSS DR7 Quasar Catalog while the BBH EW falls outside the range and becomes an outlier. This is what we want to find to distinguish BBHs from SBHs in the observational data. 

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3. Effects of an Immortal Stellar Population in AGN Disks

   https://doi.org/10.3847/1538-4357/ac5d40  

   Jermyn, Adam S.; Dittmann, Alexander J.; McKernan, B.; Ford, K. E. S.; Cantiello, Matteo (April 2022, The Astrophysical Journal)

   Abstract Stars are likely embedded in the gas disks of active galactic nuclei (AGN). Theoretical models predict that in the inner regions of the disk, these stars accrete rapidly, with fresh gas replenishing hydrogen in their cores faster than it is burned into helium, effectively stalling their evolution at hydrogen burning. We produce order-of-magnitude estimates of the number of such stars in a fiducial AGN disk. We find numbers of order 10^2–4, confined to the innerr_cap∼ 3000r_s∼ 0.03 pc. These stars can profoundly alter the chemistry of AGN disks, enriching them in helium and depleting them in hydrogen, both by order-unity amounts. We further consider mergers between these stars and other disk objects, suggesting that star–star mergers result in rapid mass loss from the remnant to restore an equilibrium mass, while star–compact object mergers may result in exotic outcomes and even host binary black hole mergers within themselves. Finally, we examine how these stars react as the disk dissipates toward the end of its life, and find that they may return mass to the disk fast enough to extend its lifetime by a factor of several and/or may drive powerful outflows from the disk. Post-AGN, these stars rapidly lose mass and form a population of stellar mass black holes around 10M_⊙. Due to the complex and uncertain interactions between embedded stars and the disk, their plausible ubiquity, and their order-unity impact on disk structure and evolution, they must be included in realistic disk models. 

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4. Measuring the Properties of Active Galactic Nuclei Disks with Gravitational Waves

   https://doi.org/10.3847/1538-4357/ac6180  

   Vajpeyi, Avi; Thrane, Eric; Smith, Rory; McKernan, Barry; Saavik Ford, K. E. (May 2022, The Astrophysical Journal)

   Abstract Active galactic nuclei (AGNs) are promising environments for the assembly of merging binary black hole (BBH) systems. Interest in AGNs as nurseries for merging BBHs is rising, following the detection of gravitational waves from a BBH system from the purported pair-instability mass gap, most notably GW190521. AGNs have also been invoked to explain the formation of the high-mass-ratio system GW190814. We draw on simulations of BBH systems in AGNs to propose a phenomenological model for the distribution of black hole spins of merging binaries in AGN disks. The model incorporates distinct features that make the AGN channel potentially distinguishable from other channels, such as assembly in the field and in globular clusters. The model parameters can be mapped heuristically to the age and density of the AGN disks. We estimate the extent to which different populations of mergers in AGNs can be distinguished. If the majority of merging black holes are assembled in AGNs, future gravitational-wave observations may provide insights into the dynamics of AGN disks. 

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5. Binary black hole merger rates in AGN discs versus nuclear star clusters: loud beats quiet

   https://doi.org/10.1093/mnras/stac2861  

   Ford, K. E. Saavik; McKernan, Barry (October 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Galactic nuclei are promising sites for stellar origin black hole (BH) mergers, as part of merger hierarchies in deep potential wells. We show that binary black hole (BBH) merger rates in active galactic nuclei (AGNs) should always exceed merger rates in quiescent galactic nuclei (nuclear star clusters, NSCs) around supermassive black holes (SMBHs) without accretion discs. This is primarily due to average binary lifetimes in AGNs that are significantly shorter than those in NSCs. The lifetime difference comes from rapid hardening of BBHs in AGNs, such that their semimajor axes are smaller than the hard–soft boundary of their parent NSC; this contrasts with the large average lifetime to merger for BBHs in NSCs around SMBHs, due to binary ionization mechanisms. Secondarily, merger rates in AGNs are enhanced by gas-driven binary formation mechanisms. Formation of new BHs in AGN discs is a minor contributor to the rate differences. With the gravitational wave detection of several BBHs with at least one progenitor in the upper mass gap, and signatures of dynamical formation channels in the χeff distribution, we argue that AGNs could contribute $$\sim 25{\!-\!}80{{\ \rm per\ cent}}$$ of the LIGO–Virgo measured rate of $$\sim 24\, \rm {Gpc}^{-3} \rm {yr}^{-1}$$. 

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