List of Past Astronomy Colloquia : 01-Jan-2019 to 01-Jun-2019

Date:   Wednesday 06-Feb-2019
Speaker:   Dr. Tess Jaffe (GSFC/UMD)
Title:  IMAGINE’ing Galactic Magnetic Fields

The Galactic magnetic field (GMF) plays an important role in a variety of astrophysical processes but is not well understood. Magnetic fields in the interstellar medium are difficult to study directly but affect phenomena as diverse as: the propagation of cosmic rays; the formation of stars; the morphology of supernova remnants; the deflections in the arrival directions of extragalactic ultra-high energy cosmic rays (UHECRs); the cosmic microwave background (CMB). I will review how data across the electromagnetic spectrum are giving us new and different views of the fields (for example, polarized dust emission from Planck, or anisotropies in the arrival directions of UHECRs seen with Auger or IceCube) and how these data are difficult to interpret because of the complexity of the different contexts and the degeneracies in the parameter space. The Interstellar MAGnetic field INference Engine (IMAGINE) is our new framework for combining all available observables as well as theoretical knowledge into a statistically rigorous Bayesian analysis. This will allow us to incorporate *all* available information to break some of these degeneracies as well as to explicitly quantify how well different models reproduce the same observables with the Bayesian evidence. I will summarize the project, the infrastructure we have made publicly available, and our plans to build more realistic GMF models based on magneto-hydrodynamical dynamo equations and to attempt both parametric and non-parametric reconstructions of the GMF.

Date:   Wednesday 13-Feb-2019
Speaker:   Dr. Josiah Schwab (UCSC)
Title:  The Progenitors and Descendants of Peculiar Thermonuclear Supernovae

As transient surveys push deeper, wider and faster, new classes of events are revealed. I will discuss recent progress made in understanding the Type Iax supernovae, a class of thermonuclear events similar to Type Ia supernovae, but with distinct and intriguing differences. Multiple lines of evidence, both theoretical and observational, are now converging to give us a coherent picture of these systems. While many questions remain, these events are exemplars of how the extragalactic discovery and follow up of transient events can be combined with galactic studies of stellar populations to understand their progenitor systems and the remnants they leave behind.

Date:   Thursday 21-Feb-2019
Speaker:   Dr. Evan Schneider (Princeton University)
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. Simultaneously, shredding and mixing of these clouds increases the density of the hot phase of the wind, leading to large-scale radiative losses that produce 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.

Date:   Wednesday 27-Feb-2019
Speaker:   Dr. Massimo Gaspari (Princeton University)
Title:  Raining on Galaxies and Black Holes: Unifying the Micro and Macro Properties of AGN Feeding and Feedback

Feeding and feedback tied to supermassive black holes (SMBHs) play central role in the cosmic evolution of galaxies, groups, and clusters of galaxies. The self-regulated active galactic nucleus (AGN) cycle is matter of intense debate. I review key results of our numerical campaign to unveil how SMBHs are tightly coupled to the multiphase gaseous halos, linking the inner gravitational radius to the large Mpc scale and vice versa. Massively parallel magnetohydrodynamic simulations show the turbulent plasma halo radiatively cools via a top-down multiphase condensation rain of warm filaments and molecular clouds. The multiphase precipitation inherits the hot halo kinematics and thermodynamics, ultimately establishing a 'cosmic weather'. In the nuclear region, the recurrent collisions between the clouds and filaments promote angular momentum cancellation and boost the SMBH accretion rate through a mechanism known as Chaotic Cold Accretion (CCA). The CCA rapid variability triggers powerful AGN outflows, which quench the macro cooling flow and star formation, while preserving the atmospheres of galaxies, groups, and clusters in global thermal equilibrium throughout cosmic time. I highlight the key imprints of AGN feedback and feeding, such as bubbles, shocks, turbulence, and condensed structures, with a critical eye toward observational concordance, including the X-ray plasma, optical filaments, and radio molecular clouds.

Date:   Wednesday 06-Mar-2019
Speaker:   Dr. Chang-Goo Kim (Princeton University)
Title:  Introducing TIGRESS: where gravity and feedback meet the real ISM

The interstellar medium (ISM) is the heart of the cosmic evolution, where the cosmic baryonic cycle ends with star formation and begins again by stellar feedback. Since gravity and feedback are tightly coupled with the ISM state and evolution, numerical simulations are paramount to develop a detailed, quantitative understanding of star formation and feedback as well as the ISM itself. In this talk, I will describe recent achievements in modeling the star-forming ISM using a novel numerical framework, called TIGRESS (Three-phase ISM in Galaxies Resolving Evolution with Star formation and Supernova feedback). In the TIGRESS framework, a portion of galactic disks is self-consistently simulated by solving MHD equations with self-gravity and supernova/FUV radiation feedback from massive young stars. With the help of realistic simulations, I will present how do we build (1) effective theories for star formation/ISM/feedback and (2) synthetic observations to interpret detailed and diverse observations. I will primarily focus on theories for self-regulation of star formation rates and multiphase galactic outflows to provide a physical understanding of observed correlations between star formation rates, outflow properties, and galactic conditions. I will introduce our ongoing projects on multiwavelength synthetic observations with a detailed case study for dust polarization maps.

Date:   Wednesday 13-Mar-2019
Speaker:   Dr. Coral Wheeler (CalTech)
Title:  Be it therefore resolved: cosmological simulations of dwarf galaxies with extreme resolution

The currently favored cosmological paradigm, Lambda Cold Dark Matter Theory (LCDM), has been widely successful in predicting the counts, clustering, colors, morphologies, and evolution of galaxies on large scales, as well as a variety of cosmological observables. Despite these successes, several challenges have arisen to this model in recent years, most of them occurring at the smallest scales — those of dwarf galaxies (Mstar < 10^9 Msun). To investigate these challenges, I will introduce a suite of extremely high-resolution cosmological (GIZMO/FIRE2) simulations of dwarf galaxies (Mhalo ~10^10 Msun), run to z = 0 with 30 Msun and sub-pc resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. Every halo with Mhalo > 10^8.6 Msun is populated by a resolved stellar galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. This new generation of simulations allows us to probe smaller physical scales than previously possible in cosmological simulations, and to make more detailed predictions for the counts, star formation histories, morphologies and chemical composition of the lowest mass galaxies ever observed. My simulations confirm many results at lower resolution, suggesting they are numerically robust (for a given physical model), but I also discover several intriguing discrepancies with observations. I will also discuss what these simulations can tell us about the emerging low surface-brightness sky.

Date:   Wednesday 27-Mar-2019
Speaker:   Dr. Benedikt Diemer (Harvard University)
Title:  The edge of darkness, and other halo surprises

Structure in the universe arises from the non-linear collapse of primordial perturbations, forming dark matter halos that host galaxies at their centers. Despite the complexity of this process, the resulting dark halos are thought to obey a number of universal laws. In this talk, I combine simple, analytical modeling with supercomputer simulations to show that the structure of dark halos is intimately connected to their history and dynamics. I demonstrate that, contrary to conventional wisdom, halos have a well-defined physical boundary called the splashback radius. I investigate this boundary by tracking the orbits of billions of simulated particles in unprecedented detail. Finally, I discuss how the recent discovery of the splashback radius in the real universe marks the beginning of a new era for observations of the halo outskirts.

Date:   Wednesday 03-Apr-2019
Speaker:   Dr. Padi Boyd (GSFC)
Title:  The Transiting Exoplanet Survey Satellite (TESS) Early Science Results

TESS’s primary science goal is to discover planets smaller than Neptune orbiting nearby bright stars. It does so by monitoring millions of stars with high time resolution, high accuracy, long time-baseline time series data (light curves) in a wide bandpass optimized for nearby small, cool M-stars. Launched in April 2018, TESS is now more than halfway through the first year of its 2-year near-all-sky survey.

The unprecedented time domain astronomical dataset that will result from the TESS survey allows identification and study of numerous additional types of astronomical phenomena, from comets and asteroids in our own solar system, starspots, flares and oscillations on stars, to supernovae in nearby galaxies and monitoring the brighter active galaxies.

In this talk, I will give an overview of the TESS mission status, discuss some early science results including confirmed small planets, flaring behavior in small stars, and supernovae on the rise, and discuss the many ways in which the scientific and general community are jumping in and making quick progress, with the focus on maximizing the science impact of this innovative survey mission.

TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. George Ricker of MIT’s Kavli Institute for Astrophysics and Space Research serves as principal investigator for the mission. Additional partners include Orbital ATK, NASA’s Ames Research Center, the Harvard-Smithsonian Center for Astrophysics, and the Space Telescope Science Institute. The TESS science instruments were jointly developed by MIT’s Kavli Institute for Astrophysics and Space Research and MIT’s Lincoln Laboratory. More than a dozen universities, research institutes, and observatories worldwide are participants in the mission.

Date:   Wednesday 10-Apr-2019
Speaker:   Dr. Lorenzo Sironi (Columbia University)
Title:  Fast and furious: magnetic reconnection in relativistic jets and black hole coronae

Relativistic jets of blazars and magnetized coronae of low-luminosity accretion flows, like Sgr A* at our Galactic Center, routinely display fast and bright flares of high-energy emission. Yet, the “engine” responsible for accelerating the emitting particles to ultra-relativistic energies is still unknown. With fully-kinetic particle-in-cell (PIC) simulations, we argue that magnetic reconnection — a process by which magnetic field lines of opposite polarity annihilate, releasing their energy to the particles — can satisfy all the basic conditions for the emission. In blazar jets, we show that reconnection can naturally explain the puzzling ultra-fast bright flares observed at GeV and TeV energies, whose duration can be even shorter than the light-travel time across the black hole that powers the jet. In low-luminosity accretion flows like Sgr A*, we show that reconnection can power both thermal and non-thermal emission, and we produce physically-grounded synthetic images and spectra to be compared with infrared and X-ray observations and with the upcoming results of the Event Horizon Telescope.

Date:   Wednesday 17-Apr-2019
Speaker:   Dr. Chris Fragile (College of Charleston)
Title:  Mysteries of Black Hole Accretion

Black hole accretion has fascinated the general public and been the subject of intense astrophysical study for many decades, yet there are a number of basic questions that still have not been answered. In fact, we seem to be getting to the point where some of the mysteries have been around for so long that many researchers have forgotten about them or at least stopped investing time in trying to solve them. In this talk, I will (re-)introduce the audience to some of these mysteries and mention some of the computational tools and simulations that are being developed to bring fresh insights into the field. Over the course of the talk, I will discuss the unusual behavior of tilted orbits in relativity, strange timing signals (quasi-periodic oscillations) in the light curves of accreting black holes, different states of black hole accretion, and the lingering problems with our “standard” accretion disk model.

Date:   Wednesday 24-Apr-2019
Speaker:   Dr. Martine Rothblatt (United Therapeutics)
Title:  Moonshots to Earthshots

Dr. Rothblatt will discuss her career. Starting with an early interest in space, she came to the DC area to use her MBA/JD degree to work on space communications and spent several years as an astronomy graduate student at the University of Maryland. She then left UMD to work with NASA on tracking and data relay satellites and to protect radio astronomy quiet bands, after which she started several satellite communications companies including Sirius Satellite Radio. She then entered the life sciences field, founding United Therapeutics and obtaining a Ph.D. in medical ethics. Dr. Rothblatt is also a leading advocate for transgender rights.

Date:   Wednesday 01-May-2019
Speaker:   Dr. M. Coleman Miller (UMD)
Title:  Peering Inside Neutron Stars

Neutron stars have long beckoned to physicists from many fields as realms of extreme physics beyond what we can test in terrestrial laboratories. In the realm of nuclear physics, the lure is that the cores of neutron stars are several times denser than atomic nuclei and yet are technically cold; this state is unique in the universe and holds clues to the nature of very dense matter. After many years of sparse hints, astronomical observations are opening this realm to our gaze. These observations include recent observations of an especially massive neutron star, the first information about neutron stars from gravitational waves, and mass-radius information anticipated from the NICER mission. I will describe recent work on these subjects with collaborators at UMd and elsewhere, and will discuss future prospects.

Date:   Wednesday 08-May-2019
Speaker:   Dr. Kevin France (University of Colorado)
Title:  “The Potential Habitability of Rocky Planets around Red Dwarfs: Stellar Drivers for Atmospheric Chemistry and Stability”

High-energy photons and particles from stars regulate the atmospheric temperature structure and photochemistry on orbiting planets, influencing the long-term stability of planetary atmospheres and the production of potential “biomarker” gases. Rocky planets orbiting low-mass stars (M dwarfs) will likely be the first exoplanets directly probed for signs of life, however, relatively few observational and theoretical constraints exist on the high-energy irradiance from typical (i.e., weakly active) M dwarf exoplanet host stars. In this talk, I will describe results from an ongoing panchromatic survey (Chandra/XMM/Hubble/ground) of M and K dwarf exoplanet hosts. The MUSCLES* Treasury Survey combines UV, X-ray, and optical observations, reconstructed Lyman-alpha and EUV (10-90 nm) radiation, and next-generation stellar atmosphere models to provide realistic inputs for modeling the stability and climate on potentially habitable planets around red dwarfs.

I will present an overview of the project and focus on three main results – 1) the evolution of the high-energy spectral energy distribution as a star’s habitable zone moves inward from 1 to 0.1 AU, including implications for the possible abiotic production of the suggested biomarkers O2 and O3, 2) new estimates of the high-energy particle fluxes (from CMEs/SEPs) from these stars based on solar UV flare/particle flux measurements, and 3) characterization of the high-energy variability on active and inactive M dwarfs, with an emphasis on the potential for these impulsive events to drive large-scale atmospheric mass loss. I will conclude by presenting a short summary of our prospects for spectroscopically characterizing the atmospheres of potentially inhabited planets around M dwarfs in the next ~20 years.

* Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems

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