The origin of the most massive black holes at high-z: BlueTides and the next quasar frontier

The growth of the most massive black holes in the early Universe, consistent with the detection of highly luminous quasars at z > 6 implies sustained, critical accretion of material to grow and power them. Given a black hole (BH) seed scenario, it is still uncertain which conditions in the early Universe allow the fastest BH growth. Large-scale hydrodynamical cosmological simulations of structure formation allow us to explore the conditions conducive to the growth of the earliest supermassive BHs. We use the cosmological hydrodynamic simulation BLUETIDES, which incorporates a variety of baryon physics in a (400 h−1Mpc)3 volume with 0.7 trillion particles to follow the earliest phases of BH critical growth. At z = 8 the most massive BHs (a handful) approach masses of 108 M⊙ with the most massive (with MBH = 4 × 108 M⊙) being found in an extremely compact (compared to present day) spheroid-dominated host galaxy. Examining the large-scale environment of hosts, we find that the initial tidal field is more important than overdensity in setting the conditions for early BH growth. In regions of low tidal fields gas accretes ‘cold’ on to the BH and falls along thin, radial filaments straight into the centre forming the most compact galaxies and most massive BHs at the earliest times. Regions of high tidal fields instead induce larger coherent angular momenta and influence the formation of the first population of massive compact discs. The extreme early growth depends on the early interplay of high gas densities and the tidal field that shapes the mode of accretion. Mergers may play a minor role in the formation of the first generation, rare massive BHs

Forecasts for the WFIRST High Latitude Survey using the BlueTides Simulation

We use the BlueTides simulation to predict the properties of the high-z galaxy and active galactic nucleus (AGN) populations for the planned 2200 deg2 Wide-Field Infrared Survey Telescope's (WFIRST) High Latitude Survey (HLS). BlueTides is a cosmological hydrodynamic simulation, which incorporates a variety of baryon physics in a (400 h−1 Mpc)3 volume evolved to z = 8 with 0.7 trillion particles. The galaxy luminosity functions in the simulation show good agreement with all the current observational constraints (up to z = 11) and predict an enhanced number of UV bright galaxies. At the proposed depth of the HLS (mUV < 26.75), BlueTides predicts 106 galaxies at z = 8 with a few up to z ∼ 15 due to the enhanced bright end of the galaxy luminosity function. At z = 8, galaxies in the mock HLS have specific star formation rates of ∼10 Gyr−1 and ages of ∼80 Myr (both evolving linearly with redshift) and a non-evolving mass–metallicity relation. BlueTides also predicts ∼104 AGN in WFIRST HLS from z = 8 out to z ∼ 14. These AGN host black holes of M ∼ 106–108 M⊙ accreting close to their Eddington luminosity. Galaxies and AGN have host halo masses of Mhalo ∼ 1011–12 M⊙ and a linear bias b ≈ 13–20. Given the expected galaxy space densities, their high bias and large volume probed, we speculate that it may be feasible for WFIRST HLS to detect the baryon acoustic oscillation peak in the galaxy power spectrum out to z = 8–9

Quantum spin Hall edge states and interlayer coupling in twisted-bilayer WTe2_2

The quantum spin Hall (QSH) effect, characterized by topologically protected spin-polarized edge states, was recently demonstrated in monolayers of the transition metal dichalcogenide (TMD) WTe$_2$. However, the robustness of this topological protection remains largely unexplored in van der Waals heterostructures containing one or more layers of a QSH insulator. In this work, we use scanning tunneling microscopy and spectroscopy (STM/STS) to explore the topological nature of twisted bilayer (tBL) WTe$_2$ which is produce from folded monolayers, as well as, tear-and-stack fabrication. At the tBL bilayer edge, we observe the characteristic spectroscopic signature of the QSH edge state that is absent in topologically trivial as-grown bilayer. For small twist angles, a rectangular moir\'e pattern develops, which results in local modifications of the band structure. Using first principles calculations, we quantify the interactions in tBL WTe$_2$ and its topological edge states as function of interlayer distance and conclude that it is possible to tune the topology of WTe$_2$ bilayers via the twist angle as well as interlayer interactions

Monsters in the dark: predictions for luminous galaxies in the early Universe from the BlueTides simulation

Using deep Hubble and Spitzer observations Oesch et al. have identified a bright (MUV ≈ −22) star-forming galaxy candidate at z ≈ 11. The presence of GN-z11 implies a number density ∼10−6 Mpc−3, roughly an order of magnitude higher than the expected value based on extrapolations from lower redshift. Using the unprecedented volume and high resolution of the BLUETIDES cosmological hydrodynamical simulation, we study the population of luminous rare objects at z > 10. The luminosity function in BLUETIDES implies an enhanced number of massive galaxies, consistent with the observation of GN-z11. We find about 30 galaxies at MUV ≈ −22 at z = 11 in the BLUETIDES volume, including a few objects about 1.5 mag brighter. The probability of observing GN-z11 in the volume probed by Oesch et al. is ∼13 per cent. The predicted properties of the rare bright galaxies at z = 11 in BLUETIDES closely match those inferred from the observations of GN-z11. BLUETIDES predicts a negligible contribution from faint AGN in the observed SED. The enormous increase in volume surveyed by WFIRST will provide observations of ∼1000 galaxies with MUV < −22 beyond z = 11 out to z = 13.5

Mixed-dimensional moir\'e systems of graphitic thin films with a twisted interface

Moir\'e patterns formed by stacking atomically-thin van der Waals crystals with a relative twist angle can give rise to dramatic new physical properties. The study of moir\'e materials has so far been limited to structures comprising no more than a few vdW sheets, since a moir\'e pattern localized to a single two-dimensional interface is generally assumed to be incapable of appreciably modifying the properties of a bulk three-dimensional crystal. Layered semimetals such as graphite offer a unique platform to challenge this paradigm, owing to distinctive properties arising from their nearly-compensated electron and hole bulk doping. Here, we perform transport measurements of dual-gated devices constructed by slightly rotating a monolayer graphene sheet atop a thin bulk graphite crystal. We find that the moir\'e potential transforms the electronic properties of the entire bulk graphitic thin film. At zero and small magnetic fields, transport is mediated by a combination of gate-tunable moir\'e and graphite surface states, as well as coexisting semimetallic bulk states that do not respond to gating. At high field, the moir\'e potential hybridizes with the graphitic bulk states owing to the unique properties of the two lowest Landau bands of graphite. These Landau bands facilitate the formation of a single quasi-two-dimensional hybrid structure in which the moir\'e and bulk graphite states are inextricably mixed. Our results establish twisted graphene-graphite as the first in a new class of mixed-dimensional moir\'e materials.Comment: 18 pages, 14 figures, 5 supplementary videos in ancillary file

Photo-physics and electronic structure of lateral graphene/MoS2 and metal/MoS2 junctions

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS2) interface. A 10x larger photocurrent is extracted at the EG/MoS2 interface when compared to metal (Ti/Au) /MoS2 interface. This is supported by semi-local density-functional theory (DFT), which predicts the Schottky barrier at the EG/MoS2 interface to be ~2x lower than Ti/MoS2. We provide a direct visualization of a 2D material Schottky barrier through combination of angle resolved photoemission spectroscopy with spatial resolution selected to be ~300 nm (nano-ARPES) and DFT calculations. A bending of ~500 meV over a length scale of ~2-3 micrometer in the valence band maximum of MoS2 is observed via nano-ARPES. We explicate a correlation between experimental demonstration and theoretical predictions of barriers at graphene/TMD interfaces. Spatially resolved photocurrent mapping allows for directly visualizing the uniformity of built-in electric fields at heterostructure interfaces, providing a guide for microscopic engineering of charge transport across heterointerfaces. This simple probe-based technique also speaks directly to the 2D synthesis community to elucidate electronic uniformity at domain boundaries alongside morphological uniformity over large areas