@article{17552,
  abstract     = {We examine the light curves of two quasars, motivated by recent suggestions that a supermassive black hole binary (SMBHB) can exhibit sharp lensing spikes. We model the variability of each light curve as due to a combination of two relativistic effects: the orbital relativistic Doppler boost and gravitational binary self-lensing. In order to model each system we extend previous Doppler plus self-lensing models to include eccentricity. The first quasar is identified in optical data as a binary candidate with a 20-yr period (Ark 120), and shows a prominent spike. For this source, we rule out the lensing hypothesis and disfavor the Doppler-boost hypothesis due to discrepancies in the measured vs. recovered values of the binary mass and optical spectral slope. The second source, which we nickname Spikey, is the rare case of an active galactic nucleus (AGN) identified in Kepler's high-quality, high-cadence photometric data. For this source, we find a model, consisting of a combination of a Doppler modulation and a narrow symmetric lensing spike, consistent with an eccentric SMBHB with a total mass of approximately 30 million solar masses, rest-frame orbital period T=418 days, eccentricity e=0.5, and seen at an inclination of 8 degrees from edge-on. This interpretation can be tested by monitoring Spikey for periodic behavior and recurring flares in the next few years. In preparation for such monitoring we present the first X-ray observations of this object taken by the Neil Gehrels Swift observatory.},
  author       = {Hu, Betty X and D’Orazio, Daniel J and Haiman, Zoltán and Smith, Krista Lynne and Snios, Bradford and Charisi, Maria and Di Stefano, Rosanne},
  issn         = {0035-8711},
  journal      = {Monthly Notices of the Royal Astronomical Society},
  number       = {4},
  pages        = {4061--4070},
  publisher    = {Oxford University Press},
  title        = {{Spikey: self-lensing flares from eccentric SMBH binaries}},
  doi          = {10.1093/mnras/staa1312},
  volume       = {495},
  year         = {2020},
}

@article{17554,
  abstract     = {The bright quasar PG1302-102 has been identified as a candidate supermassive black hole binary from its near-sinusoidal optical variability. While the significance of its optical periodicity has been debated due to the stochastic variability of quasars, its multiwavelength variability in the ultraviolet (UV) and optical bands is consistent with relativistic Doppler boost caused by the orbital motion in a binary. However, this conclusion was based previously on sparse UV data that were not taken simultaneously with the optical data. Here, we report simultaneous follow-up observations of PG1302-102 with the Ultraviolet Optical Telescope on the Neil Gehrels Swift Observatory in six optical + UV bands. The additional nine Swift observations produce light curves roughly consistent with the trend under the Doppler boost hypothesis, which predicts that UV variability should track the optical, but with a ∼2.2 times higher amplitude. We perform a statistical analysis to quantitatively test this hypothesis. We find that the data are consistent with the Doppler boost hypothesis when we compare the the amplitudes in optical B-band and UV light curves. However, the ratio of UV to V-band variability is larger than expected and is consistent with the Doppler model, only if either the UV/optical spectral slopes vary, the stochastic variability makes a large contribution in the UV, or the sparse new optical data underestimate the true optical variability. We have evidence for the latter from comparison with the optical light curve from All-Sky Automated Survey for Supernovae. Additionally, the simultaneous analysis of all four bands strongly disfavours the Doppler boost model whenever Swift V band is involved. Additional, simultaneous optical + UV observations tracing out another cycle of the 5.2-yr proposed periodicity should lead to a definitive conclusion.},
  author       = {Xin, Chengcheng and Charisi, Maria and Haiman, Zoltán and Schiminovich, David and Graham, Matthew J and Stern, Daniel and D’Orazio, Daniel J},
  issn         = {0035-8711},
  journal      = {Monthly Notices of the Royal Astronomical Society},
  number       = {2},
  pages        = {1683--1696},
  publisher    = {Oxford University Press},
  title        = {{Testing the relativistic Doppler boost hypothesis for the binary candidate quasar PG1302-102 with multiband Swift data}},
  doi          = {10.1093/mnras/staa1643},
  volume       = {496},
  year         = {2020},
}

@article{17564,
  abstract     = {The existence of ≈10^9 Msun supermassive black holes (SMBHs) within the first billion year of the universe has stimulated numerous ideas for the prompt formation and rapid growth of BHs in the early universe. Here we review ways in which the seeds of massive BHs may have first assembled, how they may have subsequently grown as massive as ≈10^9 Msun, and how multi-messenger observations could distinguish between different SMBH assembly scenarios. We conclude the following: (1) The ultra-rare ≈10^9 Msun SMBHs represent only the tip of the iceberg. Early BHs likely fill a continuum from stellar-mass (approx. 10 Msun) to the super-massive (≈10^9 Msun) regime, reflecting a range of initial masses and growth histories. (2) Stellar-mass BHs were likely left behind by the first generation of stars at redshifts as high as z=30, but their initial growth was typically stunted due to the shallow potential wells of their host galaxies. (3) Conditions in some larger, metal-poor galaxies soon became conducive to the rapid formation and growth of massive `seed' holes, via gas accretion and by mergers in dense stellar clusters. (4) BH masses depend on the environment (such as the number and properties of nearby radiation sources and the local baryonic streaming velocity), and on the metal enrichment and assembly history of the host galaxy. (5) Distinguishing between assembly mechanisms will be difficult, but a combination of observations by LISA (probing massive BH growth via mergers) and by deep multi-wavelength electromagnetic observations (probing growth via gas accretion) is particularly promising.},
  author       = {Inayoshi, Kohei and Visbal, Eli and Haiman, Zoltán},
  issn         = {0066-4146},
  journal      = {Annual Review of Astronomy and Astrophysics},
  number       = {1},
  pages        = {27--97},
  publisher    = {Annual Reviews},
  title        = {{The Assembly of the First Massive Black Holes}},
  doi          = {10.1146/annurev-astro-120419-014455},
  volume       = {58},
  year         = {2020},
}

@article{17566,
  abstract     = {The formation of supermassive stars has generally been studied under the assumption of rapid accretion of pristine metal-free gas. Recently it was found, however, that gas enriched to metallicities up to Z∼10−3 Z⊙ can also facilitate supermassive star formation, as long as the total mass infall rate onto the protostar remains sufficiently high. We extend the analysis further by examining how the abundance of supermassive star candidate haloes would be affected if all haloes with super-critical infall rates, regardless of metallicity were included. We investigate this scenario by identifying all atomic cooling haloes in the Renaissance simulations with central mass infall rates exceeding a fixed threshold. We find that among these haloes with central mass infall rates above 0.1 M⊙ yr−1 approximately two-thirds of these haloes have metallicities of Z>10−3 Z⊙. If metal mixing within these haloes is inefficient early in their assembly and pockets of metal-poor gas can remain then the number of haloes hosting supermassive stars can be increased by at least a factor of four. Additionally the centres of these high infall-rate haloes provide ideal environments in which to grow pre-existing black holes. Further research into the (supermassive) star formation dynamics of rapidly collapsing haloes, with inhomogeneous metal distributions, is required to gain more insight into both supermassive star formation in early galaxies as well as early black hole growth.},
  author       = {Regan, John A. and Haiman, Zoltán and Wise, John H. and O'Shea, Brian W. and Norman, Michael L.},
  issn         = {2565-6120},
  journal      = {The Open Journal of Astrophysics},
  number       = {1},
  publisher    = {Maynooth University},
  title        = {{Massive star formation in metal-enriched haloes at high redshift}},
  doi          = {10.21105/astro.2006.14625},
  volume       = {3},
  year         = {2020},
}

@article{17579,
  abstract     = {Approximately 200 supermassive black holes (SMBHs) have been discovered within the first ∼gigayear after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here, we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation; gas accretion; dynamical friction from stars and gas; stellar collisions; and gas ejection. We find that when the collapsing gas fragments at a density of ≲3 × 1010 cm−3, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a ≲104 M⊙ object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with Z ≲ 10−5 Z⊙ with suppressed H2 abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as ∼105–106 M⊙. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.},
  author       = {Tagawa, Hiromichi and Haiman, Zoltán and Kocsis, Bence},
  issn         = {0004-637X},
  journal      = {The Astrophysical Journal},
  number       = {1},
  publisher    = {American Astronomical Society},
  title        = {{Making a supermassive star by stellar bombardment}},
  doi          = {10.3847/1538-4357/ab7922},
  volume       = {892},
  year         = {2020},
}

@article{17581,
  abstract     = {We present analysis of Chandra X-ray observations of seven quasars that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on the apparent periodicity in their optical light curves. Simulations predict that close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs, including harder or softer X-ray spectra, ripple-like profiles in the Fe K-α line, and distinct peaks in the spectrum due to the separation of the accretion disk into a circumbinary disk and mini disks around each SMBH. We obtained Chandra observations to test these models and assess whether these quasars could contain binary SMBHs. We instead find that the quasar spectra are all well fit by simple absorbed power-law models, with the rest-frame 2–10 keV photon indices, Γ, and the X-ray-to-optical power slopes, αOX, indistinguishable from those of the larger quasar population. This may indicate that these seven quasars are not truly subparsec binary SMBH systems, or it may simply reflect that our sample size was too small to robustly detect any differences. Alternatively, the X-ray spectral changes might only be evident at energies higher than probed by Chandra. Given the available models and current data, no firm conclusions are drawn. These observations will help motivate and direct further work on theoretical models of binary SMBH systems, such as modeling systems with thinner accretion disks and larger binary separations.},
  author       = {Saade, M. Lynne and Stern, Daniel and Brightman, Murray and Haiman, Zoltán and Djorgovski, S. G. and D’Orazio, Daniel and Ford, K. E. S. and Graham, Matthew J. and Jun, Hyunsung D. and Kraft, Ralph P. and McKernan, Barry and Vikhlinin, Alexei and Walton, Dominic J.},
  issn         = {0004-637X},
  journal      = {The Astrophysical Journal},
  number       = {2},
  publisher    = {American Astronomical Society},
  title        = {{Chandra observations of candidate subparsec binary supermassive black holes}},
  doi          = {10.3847/1538-4357/abad31},
  volume       = {900},
  year         = {2020},
}

@article{17587,
  abstract     = {The astrophysical origin of gravitational wave (GW) events discovered by LIGO/VIRGO remains an outstanding puzzle. In active galactic nuclei (AGNs), compact-object binaries form, evolve, and interact with a dense star cluster and a gas disk. An important question is whether and how binaries merge in these environments. To address this question, we have performed one-dimensional N-body simulations combined with a semianalytical model that includes the formation, disruption, and evolution of binaries self-consistently. We point out that binaries can form in single–single interactions through the dissipation of kinetic energy in a gaseous medium. This "gas-capture" binary formation channel contributes up to 97% of gas-driven mergers and leads to a high merger rate in AGN disks even without preexisting binaries. We find the merger rate to be in the range of ∼0.02–60 Gpc−3 yr−1. The results are insensitive to the assumptions on the gaseous hardening processes: we find that once they are formed, binaries merge efficiently via binary–single interactions even if these gaseous processes are ignored. We find that the average number of mergers per black hole (BH) is 0.4, and the probability for repeated mergers in 30 Myr is ∼0.21–0.45. High BH masses due to repeated mergers, high eccentricities, and a significant Doppler drift of GWs are promising signatures that distinguish this merger channel from others. Furthermore, we find that gas-capture binaries reproduce the distribution of low-mass X-ray binaries in the Galactic center, including an outer cutoff at ∼1 pc due to the competition between migration and hardening by gas torques.},
  author       = {Tagawa, Hiromichi and Haiman, Zoltán and Kocsis, Bence},
  issn         = {0004-637X},
  journal      = {The Astrophysical Journal},
  number       = {1},
  publisher    = {American Astronomical Society},
  title        = {{Formation and evolution of compact-object binaries in AGN disks}},
  doi          = {10.3847/1538-4357/ab9b8c},
  volume       = {898},
  year         = {2020},
}

@article{17591,
  abstract     = {The rotational kinematic Sunyaev-Zeldovich (rkSZ) signal, imprinted on the cosmic microwave background (CMB) by the gaseous halos (spinning “atmospheres”) of foreground galaxies, would be a novel probe of galaxy formation. Although the signal is too weak to detect in individual galaxies, we analyze the feasibility of its statistical detection via stacking CMB data on many galaxies for which the spin orientation can be estimated spectroscopically. We use an “optimistic” model, in which fully ionized atmospheres contain the cosmic baryon fraction and spin at the halo’s circular velocity 𝑣circ, and a more realistic model, based on hydrodynamical simulations, with multiphase atmospheres spinning at a fraction of 𝑣circ. We incorporate realistic noise estimates into our analysis. Using low-redshift galaxy properties from the MaNGA spectroscopic survey (with median halo mass of 6.6×1011  𝑀⊙), and CMB data quality from Planck, we find that a 3⁢𝜎 detection would require a few×104 galaxies, even in the optimistic model. This is too high for current surveys, but upcoming higher-angular resolution CMB experiments will significantly reduce the requirements: stacking CMB data on galaxy spins in a ∼10 deg2 can rule out the optimistic models, and ≈350  deg2 will suffice for a 3⁢𝜎 detection with ACT. As a proof-of-concept, we stacked Planck data on the position of ≈2,000 MaNGA galaxies, aligned with the galaxies’ projected spin, and scaled to their halos’ angular size. We rule out average temperature dipoles larger than ≈1.9  𝜇⁢K around field spiral galaxies.},
  author       = {Matilla, José Manuel Zorrilla and Haiman, Zoltán},
  issn         = {2470-0010},
  journal      = {Physical Review D},
  number       = {8},
  publisher    = {American Physical Society},
  title        = {{Probing gaseous galactic halos through the rotational kinematic Sunyaev-Zeldovich effect}},
  doi          = {10.1103/physrevd.101.083016},
  volume       = {101},
  year         = {2020},
}

@article{17595,
  abstract     = {We study the thermal evolution of UV-irradiated atomic cooling haloes using high-resolution three-dimensional hydrodynamic simulations. We consider the effect of H− photodetachment by Lyα cooling radiation trapped in the optically-thick cores of three such haloes, a process that has not been included in previous simulations. Because H− is a precursor of molecular hydrogen, its destruction can diminish the H2 abundance and cooling. We find that the critical UV flux for suppressing H2-cooling is decreased by ∼15–50 per cent in our fiducial models. Previous one-zone modelling found a larger effect, with Jcrit reduced by a factor of a few; we show that adopting a constant halo mass to determine the trapped Lyα energy density, as is done in the one-zone models, yields a larger reduction in Jcrit, consistent with their findings. Our results nevertheless suggest that Lyα radiation may have an important effect on the thermal evolution of UV-irradiated haloes, and therefore on the potential for massive black hole formation.},
  author       = {Wolcott-Green, Jemma and Haiman, Zoltán and Bryan, Greg L},
  issn         = {0035-8711},
  journal      = {Monthly Notices of the Royal Astronomical Society},
  number       = {1},
  pages        = {138--144},
  publisher    = {Oxford University Press},
  title        = {{Suppression of H2 cooling in protogalaxies aided by trapped Lyα cooling radiation}},
  doi          = {10.1093/mnras/staa3057},
  volume       = {500},
  year         = {2020},
}

@article{17596,
  abstract     = {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.},
  author       = {Yang, Y. and Bartos, I. and Haiman, Zoltán and Kocsis, B. and Márka, S. and Tagawa, H.},
  issn         = {0004-637X},
  journal      = {The Astrophysical Journal},
  number       = {2},
  publisher    = {American Astronomical Society},
  title        = {{Cosmic evolution of stellar-mass black hole merger rate in active galactic nuclei}},
  doi          = {10.3847/1538-4357/ab91b4},
  volume       = {896},
  year         = {2020},
}

@article{17597,
  abstract     = {Deep Neural Networks (DNNs) are powerful algorithms that have been proven capable of extracting non-Gaussian information from weak lensing (WL) data sets. Understanding which features in the data determine the output of these nested, non-linear algorithms is an important but challenging task. We analyze a DNN that has been found in previous work to accurately recover cosmological parameters in simulated maps of the WL convergence (κ). We derive constraints on the cosmological parameter pair (Ωm,σ8) from a combination of three commonly used WL statistics (power spectrum, lensing peaks, and Minkowski functionals), using ray-traced simulated κ maps. We show that the network can improve the inferred parameter constraints relative to this combination by 20% even in the presence of realistic levels of shape noise. We apply a series of well established saliency methods to interpret the DNN and find that the most relevant pixels are those with extreme κ values. For noiseless maps, regions with negative κ account for 86−69% of the attribution of the DNN output, defined as the square of the saliency in input space. In the presence of shape nose, the attribution concentrates in high convergence regions, with 36−68% of the attribution in regions with κ>3σκ.},
  author       = {Matilla, José Manuel Zorrilla and Sharma, Manasi and Hsu, Daniel and Haiman, Zoltán},
  issn         = {2470-0010},
  journal      = {Physical Review D},
  number       = {12},
  publisher    = {American Physical Society},
  title        = {{Interpreting deep learning models for weak lensing}},
  doi          = {10.1103/physrevd.102.123506},
  volume       = {102},
  year         = {2020},
}

@article{17600,
  abstract     = {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.},
  author       = {De Rosa, Alessandra and Vignali, Cristian and Bogdanović, Tamara and Capelo, Pedro R. and Charisi, Maria and Dotti, Massimo and Husemann, Bernd and Lusso, Elisabeta and Mayer, Lucio and Paragi, Zsolt and Runnoe, Jessie and Sesana, Alberto and Steinborn, Lisa and Bianchi, Stefano and Colpi, Monica and del Valle, Luciano and Frey, Sándor and Gabányi, Krisztina É. and Giustini, Margherita and Guainazzi, Matteo and Haiman, Zoltán and Herrera Ruiz, Noelia and Herrero-Illana, Rubén and Iwasawa, Kazushi and Komossa, S. and Lena, Davide and Loiseau, Nora and Perez-Torres, Miguel and Piconcelli, Enrico and Volonteri, Marta},
  issn         = {1387-6473},
  journal      = {New Astronomy Reviews},
  publisher    = {Elsevier BV},
  title        = {{The quest for dual and binary supermassive black holes: A multi-messenger view}},
  doi          = {10.1016/j.newar.2020.101525},
  volume       = {86},
  year         = {2020},
}

@article{17601,
  abstract     = {Using numerical hydrodynamics calculations and a novel method for densely sampling parameter space, we measure the accretion and torque on a binary system from a circumbinary disk. In agreement with some earlier studies, we find that the net torque on the binary is positive for mass ratios close to unity, and that accretion always drives the binary toward equal mass. Accretion variability depends sensitively on the numerical sink prescription, but the torque and relative accretion onto each component do not depend on the sink timescale. Positive torque and highly variable accretion occurs only for mass ratios greater than around 0.05. This means that for mass ratios below 0.05, the binary would migrate inward until the secondary accreted sufficient mass, after which it would execute a U-turn and migrate outward. We explore a range of viscosities, from α = 0.03 to α = 0.15, and find that this outward torque is proportional to the viscous torque, so that torque per unit accreted mass is independent of α. Dependence of accretion and torque on mass ratio is explored in detail, densely sampling mass ratios between 0.01 and unity. For mass ratio q > 0.2, accretion variability is found to exhibit a distinct sawtooth pattern, typically with a five-orbit cycle that provides a smoking gun prediction for variable quasars observed over long periods, as a potential means to confirm the presence of a binary.},
  author       = {Duffell, Paul C. and D’Orazio, Daniel and Derdzinski, Andrea and Haiman, Zoltán and MacFadyen, Andrew and Rosen, Anna L. and Zrake, Jonathan},
  issn         = {0004-637X},
  journal      = {The Astrophysical Journal},
  number       = {1},
  publisher    = {American Astronomical Society},
  title        = {{Circumbinary disks: Accretion and torque as a function of mass ratio and disk viscosity}},
  doi          = {10.3847/1538-4357/abab95},
  volume       = {901},
  year         = {2020},
}

@article{17604,
  abstract     = {Numerical studies of gas accretion onto supermassive black hole binaries (SMBHBs) have generally been limited to conditions where the circumbinary disk (CBD) is 10-100 times thicker than expected for disks in active galactic nuclei (AGN). This discrepancy arises from technical limitations, and also from publication bias toward replicating fiducial numerical models. Here we present the first systematic study of how the binary's orbital evolution varies with disk scale height. We report three key results: (1) Binary orbital evolution switches from outspiralling for warm disks (aspect ratio ~0.1), to inspiralling for more realistic cooler, thinner disks at a critical aspect ratio ~0.04, corresponding to orbital Mach number ~25. (2) The net torque on the binary arises from a competition between positive torque from gas orbiting close to the black holes, and negative torque from the inner edge of the CBD, which is denser for thinner disks. This leads to increasingly negative net torques on the binary for increasingly thin disks. (3) The accretion rate is modestly suppressed with increasing Mach number. We discuss how our results may influence modeling of the nano-Hz gravitational wave background, as well as estimates of the LISA merger event rate.},
  author       = {Tiede, Christopher and Zrake, Jonathan and MacFadyen, Andrew and Haiman, Zoltán},
  issn         = {0004-637X},
  journal      = {The Astrophysical Journal},
  number       = {1},
  publisher    = {American Astronomical Society},
  title        = {{Gas-driven inspiral of binaries in thin accretion disks}},
  doi          = {10.3847/1538-4357/aba432},
  volume       = {900},
  year         = {2020},
}

@article{17605,
  abstract     = {Despite the rapidly growing number of stellar-mass binary black hole mergers discovered through gravitational waves, the origin of these binaries is still not known. In galactic centers, black holes can be brought to each others' proximity by dynamical processes, resulting in mergers. It is also possible that black holes formed in previous mergers encounter new black holes, resulting in so-called hierarchical mergers. Hierarchical events carry signatures such as higher-than-usual black hole mass and spin. Here we show that the recently reported gravitational-wave candidate, GW170817A, could be the result of such a hierarchical merger. In particular, its chirp mass ∼40 M⊙ and effective spin of χeff ∼ 0.5 are the typically expected values from hierarchical mergers within the disks of active galactic nuclei. We find that the reconstructed parameters of GW170817A strongly favor a hierarchical merger origin over having been produced by an isolated binary origin (with an odds ratio of > 10^3).},
  author       = {Gayathri, V. and Bartos, I. and Haiman, Zoltán and Klimenko, S. and Kocsis, B. and Márka, S. and Yang, Y.},
  issn         = {2041-8205},
  journal      = {The Astrophysical Journal Letters},
  number       = {2},
  publisher    = {American Astronomical Society},
  title        = {{GW170817A as a hierarchical black hole merger}},
  doi          = {10.3847/2041-8213/ab745d},
  volume       = {890},
  year         = {2020},
}

@article{17607,
  abstract     = {The heaviest neutron stars and lightest black holes expected to be produced by stellar evolution leave the mass-range 2.2 M⊙≲m≲5 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 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.},
  author       = {Yang, Y. and Gayathri, V. and Bartos, I. and Haiman, Zoltán and Safarzadeh, M. and Tagawa, H.},
  issn         = {2041-8205},
  journal      = {The Astrophysical Journal Letters},
  number       = {2},
  publisher    = {American Astronomical Society},
  title        = {{Black hole formation in the lower mass gap through mergers and accretion in AGN disks}},
  doi          = {10.3847/2041-8213/abb940},
  volume       = {901},
  year         = {2020},
}

@article{17902,
  abstract     = {Probing structural changes of a molecule induced by charge transfer is important for understanding the physicochemical properties of molecules and developing new electronic devices. Here, we interrogate the structural changes of a single diketopyrrolopyrrole (DPP) molecule induced by charge transport at a high bias using scanning tunneling microscope break junction (STM-BJ) techniques. Specifically, we demonstrate that application of a high bias increases the average nonresonant conductance of single Au–DPP–Au junctions. We infer from the increased conductance that resonant charge transport induces planarization of the molecular backbone. We further show that this conformational planarization is assisted by thermally activated junction reorganization. The planarization only occurs under specific electronic conditions, which we rationalize by ab initio calculations. These results emphasize the need for a comprehensive view of single-molecule junctions which includes both the electronic properties and structure of the molecules and the electrodes when designing electrically driven single-molecule motors.},
  author       = {Zang, Yaping and Fung, E-Dean and Fu, Tianren and Ray, Suman and Garner, Marc H. and Borges, Anders and Steigerwald, Michael L. and Patil, Satish and Solomon, Gemma and Venkataraman, Latha},
  issn         = {1530-6992},
  journal      = {Nano Letters},
  number       = {1},
  pages        = {673--679},
  publisher    = {American Chemical Society},
  title        = {{Voltage-induced single-molecule junction planarization}},
  doi          = {10.1021/acs.nanolett.0c04260},
  volume       = {21},
  year         = {2020},
}

@article{17903,
  abstract     = {Light emission from tunnel junctions are a potential photon source for nanophotonic applications. Surprisingly, the photons emitted can have energies exceeding the energy supplied to the electrons by the bias. Three mechanisms for generating these so-called overbias photons have been proposed, but the relationship between these mechanisms has not been clarified. In this work, we argue that multielectron processes provide the best framework for understanding overbias light emission in tunnel junctions. Experimentally, we demonstrate for the first time that the superlinear dependence of emission on conductance predicted by this theory is robust to the temperature of the tunnel junction, indicating that tunnel junctions are a promising candidate for electrically driven broadband photon sources.},
  author       = {Fung, E-Dean and Venkataraman, Latha},
  issn         = {1530-6992},
  journal      = {Nano Letters},
  number       = {12},
  pages        = {8912--8918},
  publisher    = {American Chemical Society},
  title        = {{Too cool for blackbody radiation: Overbias photon emission in ambient STM due to multielectron processes}},
  doi          = {10.1021/acs.nanolett.0c03994},
  volume       = {20},
  year         = {2020},
}

@article{17904,
  abstract     = {The creation of stable molecular monolayers on metallic surfaces is a fundamental challenge of surface chemistry. N-Heterocyclic carbenes (NHCs) were recently shown to form self-assembled monolayers that are significantly more stable than the traditional thiols on Au system. Here we theoretically and experimentally demonstrate that the smallest cyclic carbene, cyclopropenylidene, binds even more strongly than NHCs to Au surfaces without altering the surface structure. We deposit bis(diisopropylamino)cyclopropenylidene (BAC) on Au(111) using the molecular adduct BAC–CO2 as a precursor and determine the structure, geometry, and behavior of the surface-bound molecules through high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Our experiments are supported by density functional theory calculations of the molecular binding energy of BAC on Au(111) and its electronic structure. Our work is the first demonstration of surface modification with a stable carbene other than NHC; more broadly, it drives further exploration of various carbenes on metal surfaces.},
  author       = {Doud, Evan A. and Starr, Rachel L. and Kladnik, Gregor and Voevodin, Anastasia and Montes, Enrique and Arasu, Narendra P. and Zang, Yaping and Zahl, Percy and Morgante, Alberto and Venkataraman, Latha and Vázquez, Héctor and Cvetko, Dean and Roy, Xavier},
  issn         = {1520-5126},
  journal      = {Journal of the American Chemical Society},
  number       = {47},
  pages        = {19902--19906},
  publisher    = {American Chemical Society},
  title        = {{Cyclopropenylidenes as strong carbene anchoring groups on Au surfaces}},
  doi          = {10.1021/jacs.0c10743},
  volume       = {142},
  year         = {2020},
}

@article{17905,
  abstract     = {A series of four oligomers of cyclopentadithiophene-vinylenes end capped with pyridine groups was prepared and their optical and electronic properties studied. Treatment with trifluoroacetic acid (TFA) leads to the bisprotonation of the nitrogens of the pyridine, which has an important impact on the optical properties. Excess treatment with TFA provokes the oxidation of the conjugated core, generating radical cations and dications. The ease of the TFA treatment in solution was extended to protonation in the solid-state where further characterization of the neutral and TFA-treated samples was carried out in electrically active substrates in organic field-effect transistors. Finally, the new molecules were found to be excellent conductors in single-molecule junctions thanks to strong electron delocalization and resonance orbital mediated transport. These studies show the opening of a spectrum of possibilities by suitable terminal substitution of π-cores.},
  author       = {Guijarro, Fernando G. and Medina Rivero, Samara and Gunasekaran, Suman and Arretxea, Iratxe and Ponce Ortiz, Rocío and Caballero, Rubén and Cruz, Pilar de la and Langa, Fernando and Venkataraman, Latha and Casado, Juan},
  issn         = {2046-2069},
  journal      = {RSC Advances},
  number       = {68},
  pages        = {41264--41271},
  publisher    = {Royal Society of Chemistry},
  title        = {{Synthesis and electronic properties of pyridine end-capped cyclopentadithiophene-vinylene oligomers}},
  doi          = {10.1039/d0ra08220a},
  volume       = {10},
  year         = {2020},
}