The University of Florida Theoretical Astrophysics Seminar is held on Wednesdays at 1:45pm in Room 2165 of the New Physics Building, unless otherwise noted.*

* Please note that in Jan. & Feb. 2020, some seminars will also be held on Mondays at 1:45pm.

Spring 2020 Schedule

Wed., January 29: Maria Charisi (Caltech)

Title: Searching for supermassive black hole binaries in the era of multi-messenger astronomy


Supermassive black hole binaries (SMBHBs) are a natural consequence of galaxy mergers. They should be fairly common in galactic nuclei, but they remain undetected at small separations. These sources are extremely important both for extragalactic astronomy and for fundamental physics, since they are the strongest sources of low-frequency gravitational waves (GWs). They can be identified as quasars with periodic variability in electromagnetic waves or from their strong gravitational radiation. I will discuss the discovery of SMBHB candidates in time-domain surveys, along with ongoing efforts to confirm their binary nature with multi-wavelength observations. I will also describe efforts to detect GWs from merging SMBHBs with Pulsar Timing Arrays, as well as constraints on tentative binaries in the local universe derived from the most recent NANOGrav dataset. With the first detection on the horizon, I will explore synergies between electromagnetic and GW observations and the tremendous promise for multi-messenger discoveries of SMBHBs, especially in the upcoming era of the Vera Rubin Observatory (formerly known as LSST).

Mon., February 3: Sownak Bose (CfA Harvard/Smithsonian)

Title: Constraining Cosmology with Artificial Universes


Understanding the nature of the dark matter — the elusive substance that dominates the matter density of the universe — and dark energy — a mysterious force thought to drive the accelerated expansion of the cosmos — are perhaps two of the most intriguing, open questions that keep cosmologists awake at night. Over the past four decades, an extensive programme of numerical simulations has established a “standard model”, in which the dark matter is a cold, collisionless particle, interacting predominantly via gravity. This simple starting point has achieved impressive success in making accurate predictions for the formation and evolution of structures through cosmic time. On the other hand, experimental efforts at detecting the “cold” dark matter (CDM) have failed to detect such a particle, and have only narrowed down the parameter space in which this particle could feasibly exist. In this talk, I will present results from state-of-the-art simulations of structure formation in models of dark matter that extend beyond the canonical CDM paradigm. In particular, I will showcase two generic cases — “warm” and “interacting” dark matter — to demonstrate how more “exotic” dark matter phenomenology can manifest in the formation of galaxies, and will show how targeted observational campaigns can be used to constrain the identity of the dark matter. Finally, I will discuss the exciting avenues by which we will be able to get closer than ever in our quest to constraining our cosmological model, by exploiting the unprecedented confluence of new experiments that are set to come online in the new decade.

Wed., February 5: Sarah Wellons (Northwestern Univ.)

Title: Simulating Galaxy Formation in the Early Universe


In the last decade, new observational techniques have allowed us to peer deep into the Universe’s history and glimpse galaxies which formed in the first few billion years after the Big Bang. Evidence is mounting that galaxies in the early Universe appear and behave very differently from those nearby - for example, the most massive galaxies are extremely compact, and star-forming disks appear to have strange clumpy morphologies. In this talk, I will discuss galaxy formation at these early epochs from a theoretical perspective, reviewing the physical processes that are important for producing realistic galaxies and presenting results drawn from both large-volume cosmological simulations (which allow us to compare and predict the statistical properties of galaxy populations) and high-resolution zoom-in simulations (which allow us to drill down on the physics governing individual systems). I will focus in particular on massive compact galaxies, whose varied formation and evolution present a case study on how galaxy populations evolve over time, and on massive disks, whose rapid rotation can tell us about the distribution of normal and dark matter in the earliest galaxies. I will conclude by discussing some of the remaining open questions in galaxy formation and the prospects for future discoveries with upcoming facilities like JWST and WFIRST.

Wed., February 12: Ke Fang (Stanford Univ.)

Title: Multi-messenger Astrophysics: Probing Compact Objects with Cosmic Particles


The study of compact stellar remnants such as black holes and neutron stars is an important component of modern astrophysics. Recent observations of the first neutron star merger event and an active galactic nucleus as potential high-energy neutrino source open a new way to study compact objects using multi-messengers. The key to coordinated detection and interpretation of multiple messenger signals, namely, electromagnetic radiation, cosmic rays, neutrinos, and gravitational waves, is to understand the link between the messengers. Motivated by this question, we study the theory of high-energy particle propagation in the vicinity of magnetar-powered transients and black hole jets using numerical simulation. We also investigate data analysis frameworks aiming to exploit data across multiple wavelengths and messengers. We close the talk by overlooking the future of Multi-messenger Astrophysics, in light of upcoming facilities for astroparticles and new questions brought by recent observations.

Mon., February 17: Daniel D'Orazio (CfA Harvard/Smithsonian)

Title: Multi-Messenger and Multi-Band Interrogation of Compact-Object Binaries


Binary systems consisting of two compact objects span at least ten orders of magnitude in mass, from the neutron stars and stellar-mass black holes paired via binary stellar evolution or dynamical encounters, to the supermassive black holes that meet at the centers of galactic nuclei. Accordingly, these systems arise from an enormously diverse range of astrophysical environments. What they share is their potential role in generating luminous, high-energy electromagnetic radiation and their ability to generate detectable gravitational radiation upon merger. I will discuss work aimed at electromagnetically identifying a yet undetected population of sub-parsec separation supermassive black hole binaries, which are targets of ongoing monitoring by the pulsar timing arrays as well as the future LISA gravitational-wave observatory. I will also discuss work that leverages detection of gravitational waves in multiple frequency bands to elucidate the astrophysical origin of the LIGO gravitational-wave events. In the coming years, present and upcoming time domain surveys (e.g., the Vera C. Rubin Observatory) and gravitational-wave observatories (e.g., LISA, LIGO and its evolutions) will drive forward investigations of compact-object binaries across the mass scale, and drastically expand our knowledge of compact-object binary populations and the environments that shape them.

Mon., February 24: Tuguldur Sukhbold (Ohio State Univ.)



Wed., April 8: Gongjie Li (Georgia Tech)



Wed., April 15: Kayhan Gultekin (Univ. of Michigan)



Fall 2019 Schedule

August 28: Evan Schneider (Princeton)

Title: The Origin of Multiphase Galaxy Outflows


Star-forming galaxies are often observed to host galactic winds - gas that is flowing out of the galaxy in phases ranging from cold molecular clouds to hot X-ray emitting plasma. While these multiphase outflows are routinely observed, theoretically constraining their origin and evolution has proven difficult. Explaining the prevalence and velocities of the cool ionized phase (T~10^4 K) in particular poses a challenge. In this talk, I will discuss a potential dual origin for this cool gas. Through a series of extremely high-resolution simulations run with the GPU-based Cholla code, I will show that in high star formation surface density systems, dense disk gas can be pushed out by the collective effect of clustered supernovae, explaining the low-velocity material. Subsequent shredding and mixing of these clouds creates gas with intermediate densities and temperatures that is prone to radiative cooling, allowing momentum to transfer between phases and producing high velocity cool gas. In addition to explaining the nature of outflows themselves, these multiphase winds could potentially be a source of the cool photo-ionized gas that is found in abundance in galaxy halos.

September 18: Aklant Bhowmick (UF)

Title: Clustering of quasars and high redshift galaxies: New frontiers for structure formation


Galaxy clustering from large scales (>20 Mpc) to small scales (<1 Mpc) serves as a key probe for the physics of non-linear structure and galaxy formation. At large scales, it enables us to probe the galaxy linear bias, which establishes the connection between galaxy properties with the properties of underlying haloes that correlate with the halo bias. At small scales, we can probe the halo substructure and how galaxies are distributed within it, and also look for possible signatures of hierarchical structure growth. In this talk, I will be presenting the recent results on the clustering statistics of two rare classes of objects in the observable universe, namely, high redshift (z > 7) galaxies and quasars (0 < z < 4) using two state of the art cosmological hydrodynamic simulations, BlueTides and MassiveBlackII. I will discuss how the simulation results compare with existing observational constraints, and prospects of detecting these objects in upcoming surveys.

October 9: Zachary Slepian (UF)

Title: Triangulating the Universe’s Contents, Laws, and Origin with Galaxy Correlation


The Universe is now in an epoch of accelerated expansion of space-time itself, presumed due to dark energy. 3D maps of the distribution of galaxies such as the Sloan Digital Sky Survey (SDSS) have yielded percent-level constraints on dark energy using the imprint of Baryon Acoustic Oscillations (BAO) as a standard ruler to map the Universe's expansion history. Dark Energy Spectroscopic Instrument (DESI; 2019-2024) will map 30 million galaxies (30X SDSS) to improve dark energy constraints by an order of magnitude.

The standard analysis approach for these surveys uses the clustering of galaxy pairs. However, fully exploiting the powerful DESI dataset will demand new ideas. The clustering of galaxy triplets—triangles—measured through the 3-Point Correlation Function (3PCF), offers a powerful new window on cosmic acceleration and dark energy. First, the BAO method can be used in the 3PCF exactly as it is already used with pairs, to sharpen our history of the Universe’s expansion. Second, the 3PCF offers information on the fidelity with which galaxies trace the underlying matter (galaxy biasing), enhancing their utility for cosmology. Third, the direction-dependent 3PCF enables probing modified gravity, another possible explanation for accelerated cosmic expansion. I will present a transformatively fast 3PCF algorithm, and demonstrate its use to make the first application of the BAO method to the 3PCF and place high-precision constraints on galaxy biasing. I will then show how the direction-dependent 3PCF can provide a complementary probe of modified gravity. I will close with further exciting 3PCF cosmology opportunities, such as constraining inflation and the neutrino mass, as well as a novel application to understand the physics of the turbulent interstellar medium as relevant for star formation.

October 23: Jeremiah Murphy (FSU)

Title: How Do Massive Stars Die?


The title of this talk remains an important fundamental question in theoretical astrophysics. The explosive deaths of massive stars, core-collapse supernovae, are some of the most energetic events in the Universe; they herald the birth of neutron stars and black holes, are a major site for nucleosynthesis, influence galactic hydrodynamics, trigger further star formation, and are prodigious emitters of neutrinos and gravitational waves. Though these explosions play an important and multifaceted role in many cosmic phenomena, the details of the explosion mechanism have remained elusive for many decades. The fundamental challenge of core-collapse theory is to understand what makes the difference between a fizzled result (black hole formation) and successful explosions. Ultimately, answering this question will require both theory and observational constraints. In this talk, I will present recent progress in theory and observations that constrain the theory.

November 6: Dan D'Orazio (CfA)

Title: Hunting for Black Hole Binaries with Gravitational Lensing


The merger of two galaxies, each with a supermassive black hole (SBH) at its heart, results in the formation of a supermassive black hole binary (SBHB) at the center of the new galaxy. The eventual inspiral and merger of SBHBs generates gravitational waves that will be detectable by the space interferometer gravitational wave observatory LISA, and the Pulsar Timing Arrays. However, the steps between galactic merger and black hole merger are poorly understood. It is not even clear if most SBHBs will merge. To address this outstanding issue, commonly referred to as the “final parsec problem,” we must find SBHBs during the compact, sub-parsec-separation stage of their lives. While identification of such a population is notoriously difficult, it would open the field of SBHB demography, elucidating the processes at play in galactic nuclei that facilitate (or hinder) SBHB mergers. Here I discuss techniques for identifying sub-parsec separation SBHBs, and highlight a novel signature that arises via periodic gravitational lensing of an accretion disk around one SBH in the binary by the other SBH. I will present a recently discovered self-lensing binary candidate and discuss further applications of this technique to the stellar-mass black hole binaries that merge in LIGO/VIRGO.

November 27: [Thanksgiving Week: No Seminar]

Dec 4: Chiara Mingarelli (Flatiron Institute)

Title: Probing supermassive black hole mergers with pulsar timing


Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes should in-spiral together and eventually merge, generating a background of gravitational radiation in the nanohertz regime. An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in this band. I describe the current efforts to develop and extend the pulsar timing array concept, together with recent limits which have emerged from international efforts to constrain astrophysical phenomena at the heart of supermassive black hole mergers.

Dec 11: Pedro Capelo (University of Zurich)

Title: Dual active galactic nuclei and the pairing of supermassive black holes in the LISA era


Pairs of accreting supermassive black holes (SMBHs), known as dual active galactic nuclei (AGN), and the generation of gravitational waves (GWs) from the inspiral and coalescence of SMBHs are two faces of the same coin. The SMBHs that, by coalescing, generate GWs detectable by the Laser Interferometer Space Antenna (LISA) are the same systems which, millions or billions of years earlier, were potentially both accreting gas at the centre of a galactic merger remnant and producing a dual AGN, akin to the few systems we currently observe via electromagnetic (EM) probes. Connecting these two physical events is paramount to understanding (i) how the large scale structure of the Universe evolved, (ii) how SMBHs and their host galaxies co-evolve, and (iii) how SMBH accretion and AGN feedback work. To do so, we need to perform high-resolution numerical simulations that model a huge variety of astrophysical systems and processes, compare our results to current EM observations, and be ready to interpret the signals that will come from evolved EM probes (e.g. ATHENA) and, soon, from LISA. In this talk, I will show that both SMBH pairing and dual AGN activity can depend significantly on a wide assortment of astrophysical parameters, including the mass ratio between two merging galaxies, the occurrence of gas instabilities, and the dark matter content of the interacting systems, to name a few, making the field of multi-messenger astrophysics both challenging and exciting.

Other Events

Students may receive credit for attending the Astrophysics Seminar by registering for PHY 6391.