Astrophysics Science Division
Astrophysics Science Division Colloquium Series Schedule: Fall 2020

ASD Colloquium Series - Fall 2020

ASD Colloquium Series - Fall 2020

The Astrophysics Science Division colloquia occur virtuallly on Tuesdays at 3:45 pm. Schedules from past colloquium seasons are available.

Contact: Knicole Colon


Aug 25 Virtual Colloquium
A high-energy neutrino coincident with a Tidal Disruption Event
Robert Stein (DESY Zeuthen)


Sep 1 No Colloquium
Sep 8 Virtual Colloquium
Fundamental Stellar Astrophysics in the Gaia Era: A New Understanding of Local Stellar Populations
Keivan G. Stassun and Marina Kounkel (Vanderbilt University, Western Washington University)
Sep 15 Virtual Colloquium
Gaseous halos of galaxies as a key testing ground for galaxy formation models
Andrey Kravtsov (University of Chicago)
Sep 22 No Colloquium
Sep 29 No Colloquium


Oct 6 Virtual Colloquium
Galactic Astronomy with LISA
Valeriya Korol (University of Birmingham)
Oct 13 Virtual Colloquium
The Rise and Fall of Galaxies in the Early Universe
Steven Finkelstein (University of Texas at Austin)
Oct 20 Virtual Colloquium
A Universe of Black Holes
Tiziana DiMatteo (Carnegie Mellon University)
Oct 27 Virtual Colloquium
eROSITA - the next generation All Sky Survey
Joern Wilms (Remeis-Observatory Bamberg, FAU Erlangen-Nuernberg)


Nov 03 No Colloquium
Nov 10 Virtual Colloquium
The 511 keV Positron Puzzle
Thomas Siegert (University of California, San Diego)
Nov 17 Virtual Colloquium
A Solution to the Peas-in-a-Pod Problem for Extrasolar Planetary Systems
Fred Adams (University of Michigan)
Nov 24 No Colloquium - Thanksgiving Week


Dec 01 Virtual Colloquium
Soft X-rays and the Earth's Magnetosphere
David Sibeck & Scott Porter (GSFC)
Dec 08 Virtual Colloquium
The NANOGrav search for nanohertz gravitational waves
Xavier Siemens (University of Wisconsin-Milwaukee)
Dec 15 Virtual Colloquium
Direct Imaging of Newborn Worlds - How we do it and why you should care.
Kate Follette (Amherst College)
A high-energy neutrino coincident with a Tidal Disruption Event
Robert Stein
DESY Zeuthen
Tuesday, Aug 25, 2020


IceCube discovered a diffuse flux of high-energy neutrinos in 2013, and recently identified the flaring blazar TXS 0506+056 as a likely neutrino source. However, a combined analysis of many similar blazars revealed no significant population excess, leaving the vast majority of the neutrino flux unexplained. I will discuss the identification of a second likely neutrino source, the Tidal Disruption Event (TDE) AT2019dsg, found as part of a systematic search for optical counterparts to high-energy neutrinos using the Zwicky Transient Facility. The probability of finding such a TDE with our follow-up program by chance is just 0.2%. Our multi-wavelength observations reveal the presence of a central engine powering particle acceleration in AT2019dsg, and confirm that this object can satisfy all necessary conditions for PeV neutrino production.

Fundamental Stellar Astrophysics in the Gaia Era: A New Understanding of Local Stellar Populations
Keivan G. Stassun and Marina Kounkel
Vanderbilt University, Western Washington University
Tuesday, Sep 08, 2020


We begin with an overview of the precision stellar astrophysics enabled by the confluence of Gaia parallaxes with large-scale photometric and spectroscopic surveys. We then give examples of the new understanding of local Galactic structure that this has enabled, including in particular the detailed history of canonical star-forming regions, and the discovery of "stellar strings" reflecting the local conditions of recent star formation. We conclude with the example of a newly discovered, young, triple star system including an eclipsing binary, that provides prima facie evidence supporting the new "stellar strings" paradigm.

Gaseous halos of galaxies as a key testing ground for galaxy formation models
Andrey Kravtsov
University of Chicago
Tuesday, Sep 15, 2020


Presence of diffuse extended gaseous halos - or circumgalactic medium (CGM) - around galaxies is indicated by observations and is generally predicted by galaxy formation models. Properties of these halos should bear imprint of the key processes driving galaxy formation, such as accretion history of gas and outflows driven by feedback. I will briefly review what we currently know about CGM properties and the evidence that CGM has complex multiphase structure quite different from gaseous halos of group- and cluster-sized systems. I will present results of recent studies of gaseous halos in cosmological galaxy formation simulations that show that multiphase structure arises due to thermal instability seeded by the nonlinear density perturbations arising during gas accretion from IGM and winds from both the central and satellite galaxies.

Galactic Astronomy with LISA
Valeriya Korol
University of Birmingham
Tuesday, Oct 06, 2020


White dwarf stars are a well-established tool for studying Galactic stellar populations. Two white dwarfs in a tight orbit forming a double white dwarf (DWD) binary offer us an additional messenger - gravitational waves - for exploring the Milky Way and its immediate surroundings. Gravitational waves produced by DWDs can be detected by the future Laser Interferometer Space Antenna (LISA). I will discuss what we will learn about our Galaxy from the LISA sample of DWDs. In particular, I will demonstrate how well the density distribution of DWDs constrains scale parameters of the Milky Way's bulge, disc and central bar. Finally, I will show that massive Galactic satellites can be seen on gravitational wave sky and I will present which of their properties we will be able to investigate with LISA.

The Rise and Fall of Galaxies in the Early Universe
Steven Finkelstein
University of Texas at Austin
Tuesday, Oct 13, 2020


Reionization was the last major phase transition of the universe, and both the time evolution and spatial variation of this process encode key information about the onset of luminous objects in the universe. While we think that massive stars within star-forming galaxies provide the needed ionizing photons, the observed escape fractions of these photons may be too low to complete reionization when combined with typical assumptions. We have devised a new semi-empirical model which resolves this tension by using simulation-motivated escape fractions, where the smallest halos have the highest escape fractions, leading to a successful completion of reionization by z=5.5 with low (<5%) average ionizing photon escape fractions. Our model makes a number of testable predictions, including: 1) AGNs contribute non-negligibly to the end of reionization, 2) the neutral fraction at z~7 is only 20%, and 3) significant star-formation must be occurring at z~9-10. I will show observational results from my group at UT Austin testing all of these assumptions, including results from ultra-deep Keck spectroscopy for Lyman-alpha emission at z~7-10, and the discovery of several remarkably bright galaxy candidates at z > 9. The abundance of bright galaxies at early times implies that the first quenched galaxies may exist earlier than predicted. I will finish by discussing a new moderate-depth (K~23) imaging survey over the ~20 square degree SHELA (the Spitzer/HETDEX Exploratory Large Area) survey, which we have used to discover ~5600 massive (log M/Msol > 11) galaxies at 3 < z < 5. Our analysis of the stellar populations in these objects shows that ~10% are quiescent (sSFR < 10^-11 yr^-1), nearly 20X the largest previous sample of massive quiescent galaxies in this epoch. Simulations have struggled to replicate this, finding that increasing AGN feedback to hasten quenching slows down galaxy growth enough such that massive galaxies form too late. I will discuss ongoing followup efforts with DECam, ALMA and Keck to confirm the nature of a significant fraction our sample, and accurately quantify the contamination rate.

A Universe of Black Holes
Tiziana DiMatteo
Carnegie Mellon University
Tuesday, Oct 20, 2020


Massive black holes are fundamental constituents of our cosmos. Understanding their formation at cosmic dawn, their growth, and the emergence of the first, rare quasars in the early Universe remains one of our greatest theoretical and observational challenges. Hydrodynamic cosmological simulations self-consistently combine the processes of structure formation at cosmological scales with the physics of smaller galaxy scales. They capture our most realistic understanding of massive black holes and their connection to galaxy formation. I will focus on the predictions for the first quasars and their host galaxies in the BlueTides simulation. Next generation faciities and the advent of multi-messenger astrophysics brings new exciting prospects for tracing the origin, growth and merger history of massive black holes across cosmic ages.

eROSITA - the next generation All Sky Survey
Joern Wilms
Remeis-Observatory Bamberg, FAU Erlangen-Nuernberg
Tuesday, Oct 27, 2020


Launched from Baikonur on 13 July 2019, over the next years the eROSITA instrument on the Spectrum-X-Gamma satellite will perform the deepest all sky survey in the 0.5-10keV band, far surpassing its predecessor, the ROSAT All Sky Survey. In this presentation I will discuss the scientific motivation behind eROSITA, present the survey strategy and instrument design, I will discuss select initial results from the first year of eROSITA operations, in which already more than one million of X-ray sources have been identified.

The 511 keV Positron Puzzle
Thomas Siegert
University of California, San Diego
Tuesday, Nov 10, 2020


Despite being the strongest persistent diffuse gamma-ray line signal known, the 511 keV emission line from the annihilation of electrons with positrons from the centre of the Galaxy poses an unsolved problem for half a century. The general questions are “where do all the positrons come from?”, and “why is the emission concentrated in the bulge of the Milky Way?”. The first question can be answered by a budgeting approach from different positron-producing sources. These included, for example, beta-plus-decays of radioactive isotopes like 22Na from novae, 26Al from massive stars, 44Ti from supernovae, or 56Co for type Ia explosions. But also higher-energy processes in the strong electro-magnetic fields of neutron stars and black holes can produce vast amounts of electron-positron pairs. Alternative explanations such as annihilating dark matter particles or a complete misinterpretation of the signal are also not ruled out entirely. However the second question, the distributions of these sources, predominantly in the Galactic disk compared to the annihilation regions appear mostly uncorrelated. This ‘positron puzzle’ thus requires more ingredients, either related to cosmic-ray propagation, or in-situ annihilation in additional source types, such as billions of flaring stars. The positron annihilation sky has been mapped with the coded-mask spectrometer SPI, but its imaging capabilities result in ambiguities for clear interpretations. A more advanced imaging technique is coming from compact Compton telescopes like the Compton Spectrometer and Imager, COSI. The prototype instrument had a successful long-duration balloon flight in 2016 in which also the 511 keV signal could be measured. I will present an overview of astronomical positron annihilation physics, possible fallacies when interpreting MeV data, and how improved imaging techniques can contribute in solving this puzzle.

A Solution to the Peas-in-a-Pod Problem for Extrasolar Planetary Systems
Fred Adams
University of Michigan
Tuesday, Nov 17, 2020


Motivated by the trends found in the observed sample of extrasolar planets, this talk discusses tidal equilibrium states for forming planetary systems --- subject to conservation of angular momentum, constant total mass, and fixed orbital spacing. In the low-mass limit, valid for superearth-class planets, we show that energy optimization leads to nearly equal mass planets, with circular orbits confined to a plane. We then generalize the treatment to include the self-gravity of the planetary bodies. For systems with sufficiently large total mass in planets, the optimized energy state switches over from the case of nearly equal mass planets to a configuration where one planet contains most of the material. This transition occurs for a critical mass threshold of approximately 40 earth masses (where the value depends on the semimajor axes of the planetary orbits, the stellar mass, and other system properties). These considerations of energy optimization apply over a wide range of mass scales, from binary stars to planetary systems to the collection of moons orbiting the giant planets in our solar system.

Soft X-rays and the Earth’s Magnetosphere
David Sibeck & Scott Porter
Tuesday, Dec 01, 2020


To predict space weather, magnetospheric physicists seek to understand the processes that govern the flow of solar wind energy through the Earth’s magnetosphere. Years of empirical studies and in situ measurements like those of the spectacularly successful THEMIS and MMS missions demonstrate that magnetic reconnection is the crucial process but don’t tell what mode of reconnection predominates: steady or bursty?, triggered or intrinsic?, patchy or extended?, at locations where shocked solar wind and magnetospheric magnetic field lines lie precisely antiparallel to one another, or simply sheared? Global magnetohydrodynamic simulations offer some clues, but don’t contain the fundamental microscale physics. Global observations are needed to understand the global solar wind interaction. In the absence of an extensive fleet of spacecraft, global imagers can supply the requisite observations. Fortuitously, the region of space just outside (but not inside) Earth’s magnetopause shimmers in the soft X-rays emitted when high charge state solar wind ions exchange electrons with exospheric neutrals. This enables researchers to identify and track the location of the Earth’s magnetopause, whose motion provides crucial information concerning the nature of magnetic reconnection. This talk describes the joint ESA and CAS soft X-ray mission SMILE and tells how researchers employing the unique GSFC heritage in wide field-of-view soft X-ray imaging are preparing the Heliophysics MIDEX STORM proposal.

The NANOGrav search for nanohertz gravitational waves
Xavier Siemens
University of Wisconsin-Milwaukee
Tuesday, Dec 08, 2020


Supermassive black hole binaries (SMBHBs), and possibly other sources, generate gravitational waves in the nanohertz part of the spectrum. For over a decade and a half the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has been using the Green Bank Telescope, the Arecibo Observatory, and, more recently, the Very Large Array to observe millisecond pulsars. Our goal is to directly detect nanohertz gravitational waves, which cause small correlated perturbations to the times of arrival of radio pulses from millisecond pulsars. We currently monitor almost 80 millisecond pulsars with sub-microsecond precision and weekly to monthly cadences. I will present an overview of NANOGrav Physics Frontiers Center activities and summarize the results of our most recent search for a stochastic background of gravitational-waves on the 12.5-yr dataset.

Direct Imaging of Newborn Worlds - How we do it and why you should care.
Kate Follette
Amherst College
Tuesday, Dec 15, 2020


Of the thousands of known extrasolar planets, why are the dozen or so directly imaged exoplanets among the most important, despite their apparently anomalous properties within the general exoplanet population (>10AU, >2MJ)? What are the prospects for (and recent successes in) detecting younger, lower-mass and/or closer-in planets via direct imaging? I will discuss the current state of the art in the field of high contrast imaging of extrasolar planets and circumstellar disks, with a particular emphasis on a subset of objects that host both disks and (likely) planets - the so-called “transitional disks”. These young circumstellar disks are almost certainly actively undergoing planet formation, yet the presence of disk material complicates our ability to isolate light from planets and/or protoplanets embedded within them. Recent successes in this area have generated an explosion of interest, a healthy amount of debate, and a plethora of new questions about how to best isolate and interpret accretion signatures for embedded protoplanets. I will describe the recent results, challenges, and prospects of this burgeoning field.

Past Colloquia Schedules