List of Past Astronomy Colloquia : 01-Sep-2017 to 31-Dec-2017

Date:   Wednesday 06-Sep-2017
Speaker:   Dr. Megan Johnson (U.S. Naval Observatory)
Title:  The HI Neighborhoods Around STARBIRDS

Starbursts are finite periods of intense star formation (SF) that can dramatically impact the evolutionary state of a galaxy. Recently, published results have redefined both the duration and spatial extent of starbursts in dwarf galaxies to be longer and more distributed than previously thought, with star formation efficiencies (SFEs) comparable to spiral galaxies. The contrast between these SFEs and those typical of non-bursting dwarfs highlights the inconsistencies between two SF modes that can operate in low-mass galaxies. These inconsistencies might be explainable if the starburst mode of SF is externally triggered by gravitational interactions with other nearby systems. We present new, sensitive neutral hydrogen observations of 18 starburst dwarf galaxies, which are part of the STARburst IRregular Dwarf Survey (STARBIRDS) and each were mapped with the Green Bank Telescope (GBT) and/or Parkes Telescope in order to study the low surface brightness gas distributions, a common tracer for tidal interactions.

Date:   Wednesday 13-Sep-2017
Speaker:   Dr. Ke Fang (JSI Fellow) & Dr. Geoffrey Ryan (JSI Fellow) (UMD)
Title:  See Abstracts

High-energy Neutrinos from Magnetars Formed from the Merger of Binary Neutron Stars-Ke Fang

The merger of a neutron star (NS) binary may result in the formation of a long-lived, or indefinitely stable, millisecond magnetar remnant surrounded by a low-mass ejecta shell. A portion of the magnetar’s prodigious rotational energy is deposited behind the ejecta in a pulsar wind nebula, powering luminous optical/X-ray emission for hours to days following the merger. Ions in the pulsar wind may also be accelerated to ultra-high energies, providing a coincident source of high energy cosmic rays and neutrinos. At early times, the cosmic rays experience strong synchrotron losses; however, after a day or so, pion production through photomeson interaction with thermal photons in the nebula comes to dominate, leading to efficient production of high-energy neutrinos. After roughly a week, the density of background photons decreases sufficiently for cosmic rays to escape the source without secondary production. We will discuss the astroparticle production resulted from these competing effects. Specifically, we will show that a neutrino flux that peaks on a few day timescale near an energy of ~ 1018 eV may be detectable for individual mergers out to ~ 10 (100) Mpc by current (next-generation) neutrino telescopes, providing clear evidence for a long-lived NS remnant and an highest-energy counterpart signal for the gravitational wave event.

Lighting Up Accretion Disks With Binary Black Holes - Geoffrey Ryan

Accretion can provide an electromagnetic counterpart, or even precursor, to binary black hole mergers seen by LIGO and eventually, LISA. Gas flows on to binary systems are highly dynamical and display a rich phenomenology. I will present a series of numerical simulations of accretion on to binary black holes, with a focus on "minidisks", the accretion disks which form around each member of the binary, and the response of a circumbinary disk to the recoiling post-merger black hole. We find minidisks display prominent spiral shock waves, excited by the tidal forces of the binary companion. These shocks efficiently heat the disk and drive accretion by purely hydrodynamic means, resulting in a thin accretion disk bright in soft x-rays. Post-merger, the recoiling remnant black hole drives a one-armed spiral shock wave through any surrounding circumbinary disk, providing a prompt x-ray counterpart to the merger. I will also briefly review the tool used to produce these simulations, the general relativistic moving-mesh hydrodynamics code Disco.

Date:   Wednesday 20-Sep-2017
Speaker:   Dr. James Rhoads (GSFC)
Title:  "Lyman Alpha Galaxies in the Epoch of Reionization"

Lyman alpha emission from galaxies affords powerful tests of cosmological reionization, because Lyman alpha photons are resonantly scattered by neutral hydrogen. This leads to several possible ways of using Lyman alpha line ongoing surveys sensitive to Lyman alpha in the epoch of reionization, including LAGER (Lyman Alpha Galaxies in the Epoch of Reionization), DAWN (the Deep And Wide Narrowband survey), and FIGS (the Faint Infrared Grism Survey). I will close by discussing prospects for studying reionization history using JWST and WFIRST.

Date:   Wednesday 27-Sep-2017
Speaker:   Dr. Suvi Gezari (UMD)
Title:  Exploring Supermassive Black Hole Demographics with Time Domain Observations

We are entering an era of increasingly powerful wide-field optical synoptic surveys that are transforming the study of the variable night sky. I will highlight the capability of time domain observations to study supermassive black holes (SMBHs), and in particular, to study their demographics (mass, spin, binarity). I will present our past results from the Pan-STARRS1 Medium Deep Survey and new results from the Intermediate Palomar Transient Factory that probe SMBHs in the nuclei of galaxies, including dormant SMBHs via the tidal disruption and accretion of a star, binary SMBHs via periodic quasar variability, and active SMBHs caught in the act of turning on. I will conclude with the exciting potential of the upcoming Zwicky Transient Facility and the Large Synoptic Survey Telescope to map the demographics of SMBHs over cosmic time.

Date:   Wednesday 04-Oct-2017
Speaker:   Dr. Leo Singer (GSFC)
Title:  GW170814: Accurate Localization and New Physics Enabled by A Triple Coincident GW Detection

Just two weeks after Advanced Virgo became operational, it detected its first binary black hole merger GW170814 in coincidence with the Advanced LIGO detectors at Hanford and Livingston. For the first time, the worldwide gravitational-wave detector network is able to measure GW polarization and thereby place novel constraints on alternative theories of gravity. Moreover, the addition of just one more detector and one more independent timing baseline resulted in an order of magnitude improvement in position accuracy on the sky, from over a thousand square degrees to sixty square degrees. Although no electromagnetic emission is expected in the case of this binary black hole merger event, the sub-100 square degrees localization will be typical of future triply coincident detections, whether they are binary black holes or binary neutron stars which are expected to produce copious emission across the electromagnetic spectrum. GW170814 is the prelude to a "bright" future of multimessenger observations with Advanced LIGO and Virgo.

Date:   Wednesday 11-Oct-2017
Speaker:   Dr. Catherine Espaillat (Boston University)
Title:  Tracking Planet Footprints in Dusty Disks

We know that most stars were once surrounded by protoplanetary disks. How these young disks evolve into planetary systems is a fundamental question in astronomy. Observations of T Tauri stars (TTS) may provide insights, particularly a subset of TTS with “transitional disks” that contain holes or gaps in their dust disk. Many researchers have posited that these holes and gaps are the “footprints” of planets given that theoretical simulations predict that a young, forming planet will clear the material around itself, leaving behind a cavity in the disk. In this talk, I will review the key observational constraints on the dust and gas properties of transitional disks and examine these in the context of theoretical planet-induced disk clearing models. I will also discuss possibilities for future work in this field in the era of ALMA and JWST observations.

Date:   Wednesday 18-Oct-2017
Speaker:   Dr. Tod Strohmayer (GSFC)
Title:  A NICER View of Compact Stars

X-ray observations provide a window into nature's extreme physics laboratories, neutron stars and black holes, the collapsed remnants of exploded massive stars. Nowhere else in the universe is the gravity stronger or the matter denser. Among the unanswered fundamental physics questions that can be addressed by their study are the equation of state (EOS) and basic constituents of ultra-dense matter, and the nature of extreme gravity. I will provide an X-ray tour of these objects, highlighting how X-ray observations enable addressing such questions, and with a focus on the new capabilities provided by NASA's recently launched Neutron Star Interior Composition Explorer (NICER) mission.

Date:   Wednesday 25-Oct-2017
Speaker:   Dr. Ryan Foley (UC Santa Cruz)
Title:  Discovery of the First Electromagnetic Counterpart to a Gravitational Wave Source

Two months ago, on August 17, LIGO and Virgo made the first detection of a binary neutron star merger, GW170817. Two seconds later, an associated gamma-ray burst was detected by Fermi and INTEGRAL, GRB 170817A. Soon after sunset in Chile, my team, One-Meter, Two-Hemisphere (1M2H), used the 1-m Swope telescope at Las Campanas Observatory to discover the optical counterpart, Swope Supernova Survey 2017a (SSS17a). In this talk, I will tell the tale of this discovery. I will also describe our observations taken over the next several weeks. With our electromagnetic data alone, we are able to independently and definitively say that GW170817 came from the merger of a binary neutron star system. These data show that a significant amount of r-process material was generated and ejected in the merger -- a so-called ‘kilonova’ -- providing enough material to solve the question of where the majority of r-process elements are created. Examining the host galaxy, we find that the progenitor system was likely several Gyr old. This data-rich event is just the beginning. GW170817 and SSS17a herald the beginning of a new era of astronomy.

Date:   Wednesday 01-Nov-2017
Speaker:   Dr. Jackie Hewitt (MIT)
Title:  Probing the Cosmic Dawn and the Epoch of Reionization with the 21cm Hydrogen Line

Measurements of the cosmic microwave background at redshift 1100 give us information about the initial density fluctuations that seeded structure formation. Observations of galaxies at redshift 7 give us information about the outcome of this structure formation. Between those redshifts lies a modern frontier of cosmology -- the cosmic dawn -- the formation of the first stars and the reionization of the intergalactic medium. Several recently completed low frequency radio arrays have provided us with an early glimpse into the Epoch of Rionization through the redshifted neutral hydrogen 21cm radio line. A new instrument, the Hydrogen Epoch of Reionization Array (HERA) is under construction. HERA will be significantly more capable, and presents interesting opportunities and challenges.

Date:   Wednesday 08-Nov-2017
Speaker:   Dr. Nick Law (UNC)
Title:  The Evryscopes: high-cadence gigapixel observations of the entire accessible sky

The Evryscope is a new type of array telescope which monitors the entire accessible sky in each exposure. One Evryscope has covered the Southern hemisphere from Chile since 2015, and we will soon deploy another Evryscope to cover the North from Mount Laguna Observatory in California. Each telescope, with 700 MPix covering an 8000-square-degree field of view, builds many-year-length, high-cadence light curves for every accessible object brighter than ~16th magnitude. Every night, we add more than a billion object detections to our databases, enabling the detection of exoplanet transits, microlensing events, nearby extragalactic transients, gravitational wave electromagnetic counterparts, and a wide range of other short timescale events. I will discuss our science plans, the big-data analysis required to explore the petabyte-scale dataset we are collecting over the next few years, and the first results from the Southern Evryscope.

Date:   Wednesday 15-Nov-2017
Speaker:   Dr. Elena Gallo ( University of Michigan)
Title:  The local massive black hole occupation fraction

Whereas mergers and intermittently efficient accretion in larger black holes spur growth and remove observational signatures of their birth,smaller black holes have more subdued histories and undergo mostly secular evolution. Consequently, both the mass distribution and the very rate of occupancy of massive black holes in lower-mass galaxies contain archaeological information on the initial “seeding” mechanism. I will present recent results from a number of observational campaigns aimed at characterizing the massive black hole population in nearby low-mass galaxies. Most notably, I will discuss how a measurement of the local active fraction down to very low X-ray Eddington ratios can be employed to yield quantitative constraints on the massive black hole occupation fraction. As this measurement relies on Chandra X-ray Telescope data for hundreds of nearby galaxies, it enables us to concurrently quantify the incidence and intensity of low-level black hole accretion as a function of host stellar mass as well as environment (such as living in isolation vs. belonging to a cluster or group), and characterize the relation between massive black holes and nuclear star clusters, also as a function of environment and galactic morphology.

Date:   Wednesday 29-Nov-2017
Speaker:   Dr. Marshall Perrin (STScI)
Title:  Direct Imaging of Planetary Systems: Progress and Prospects

High-contrast imaging of nearby planetary systems allows us to directly observe both their largest and smallest components: their giant Jovian planets and the tiny planetesimals that make up dusty circumstellar disks. For planets, measurements of atmospheric spectra and orbital motions are advancing our understanding of planetary demographics and atmospheric physics. For disks, we can not only place constraints on the physical properties and composition of the dust particles, we are increasingly able to investigate the dynamical interactions between disks and planets. These observations are enabled by sophisticated instrumentation that combines wavefront control, coronagraphy, integral field spectroscopy, and advanced data processing methods. I will highlight recent results from the Gemini Planet Imager and our team’s ongoing GPI Exoplanet Survey. In the next few years JWST will be a powerful facility for further characterizing such planets at longer infrared wavelengths, for a handful of worlds at relatively wide separations. Looking further ahead the WFIRST Coronagraphic Imager and future proposed missions like HabEx or LUVOIR will employ many of the same high contrast technologies as GPI but operating in space to reach unprecedented contrasts and begin the study of nearby worlds in reflected light.

Date:   Wednesday 06-Dec-2017
Speaker:   Dr. Laura Lopez (Ohio State University)
Title:  Observational Assessment of Stellar Feedback in Nearby Galaxies

Massive stars have a profound astrophysical influence throughout their tumultuous lives and deaths. Stellar feedback – the injection of energy and momentum by stars to the interstellar medium (ISM) – occurs through a variety of mechanisms: radiation, photoionization heating, winds, jets/outflows, supernovae, and cosmic-ray acceleration. Despite its importance, stellar feedback is cited as one of the biggest uncertainties in astrophysics today, stemming from a dearth of observational constraints and the challenges of considering many feedback modes simultaneously. In this talk, I will discuss how a systematic approach to multiwavelength observations can be used to overcome these issues. I will summarize results from application of these methods to massive-star regions in the Milky Way and nearby galaxies, where feedback processes are best resolved. Finally, I will highlight exciting prospects of using current and upcoming facilities to explore feedback in diverse conditions.

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