The Quenching of the Ultra-faint Dwarf Galaxies in the Reionization Era

Tom Brown

STScI

Tuesday, Aug 30, 2016

Abstract

I will present results from a Hubble Space Telescope survey of the ultra-faint dwarf galaxies. These Milky Way satellites, discovered in the Sloan Digital Sky Survey, appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in the efforts to understand the missing satellite problem. Because they are the least luminous, most dark matter dominated, and least chemically evolved galaxies known, the ultra-faint dwarfs are the best candidate fossils from the early universe. The primary goal of the survey is to measure the star-formation histories of these galaxies and discern any synchronization due to the reionization of the universe.

Insights into Binary Stars, Stellar Winds, and Astrophysical Plasmas from X-ray Observations of Planetary Nebulae

Rodolfo Montez

SAO

Tuesday, Sep 13, 2016

Abstract

Planetary nebulae (PNe) provide textbook examples of astrophysical plasma and shock processes and provide essential constraints for theories of stellar evolution and the chemical enrichment of the universe. The varied shapes of PNe reveal the actions of interacting stellar winds from the late stages in the life of intermediate-mass stars, and growing evidence suggests that many PNe are the products of interacting binary star systems. As a result, studies of PNe can yield insight into other astrophysical objects governed by binary processes, such as, low mass X-ray binaries and Type Ia supernovae. Best known for their ten thousand degree optical line emission, the Chandra X-ray Observatory has established that a fraction of PNe display extended X-ray emission from shock-heated plasmas of a few million degrees and that the central stars harbor hotter than expected point-like emission from plasmas that reach tens of millions of degrees. I describe the discoveries, insights, and questions raised by Chandra observations of PNe with emphasis on those results gleaned from the Chandra Planetary Nebulae Survey (ChanPlaNS), which is the first systematic X-ray survey of PNe in the solar neighborhood.

Freeform Optics at NASA

Joseph Howard

GSFC

Monday, Sep 19, 2016

Abstract

Freeform optics are non-rotationally symmetric optics with large departures from a base sphere, typically from many microns to mm. They provide additional degrees of freedom to optical designs, which enable instruments with better image quality, larger fields of view, and smaller package size. The past, present, and future usage of Freeform Optics at NASA are discussed in this presentation.

The role of cosmic rays in stellar and supermassive black hole feedback

Mateusz Ruszkowski

Michigan

Tuesday, Sep 27, 2016

Abstract

I will discuss the role of cosmic rays in stellar and supermassive black hole feedback. I will argue that cosmic rays are likely to play a very important role across a large range of distance scales -- from the scales of individual galaxies to the scales comparable to those of cool cores of galaxy clusters.

Regarding the galactic scale feedback, I will focus on supernova and cosmic ray driven winds. Galactic outflows play an important role in galaxy evolution and, despite their importance, detailed understanding of the physical mechanisms responsible for the driving of these winds is lacking. Although cosmic rays comprise only a tiny fraction of interstellar particles by number, they carry energy comparable to that in the thermal gas. I will describe a suite of global 3D MHD numerical simulations that focus on the dynamical role of cosmic rays injected by supernovae, and specifically on the impact of cosmic ray streaming along the magnetic fields. Our results show that this microphysical effect can have a significant impact on the wind launching depending on the details of the plasma physics.

Regarding the feedback on galaxy cluster scales, I will discuss results from our simulations of black hole jets in cool cores of galaxy clusters including the effects of cosmic rays. I will argue that cosmic ray heating of the intracluster medium (ICM) may be a very important heating mechanism both in the tenuous and cold phases of the ICM while not violating observational constraints.

While largely an unexplored territory in the context of galactic winds and AGN feedback, cosmic ray feedback is an important process facilitating launching and efficient driving of galactic-scale winds in starburst galaxies and heating of the ICM and remains the subject of intense research.

What is the contribution of blazars to the IceCube neutrino flux?

Maria Petropoulou

Purdue

Tuesday, Oct 4, 2016

Abstract

The discovery of 0.1 – 2 PeV neutrinos of astrophysical origin by IceCube has opened a new window in the high-energy Universe. There are various astrophysical candidate sources, including active galactic nuclei (AGN) and starburst galaxies. Although a firm association of the detected neutrinos with one (or more) of them is still lacking, IceCube has started constraining various theoretical models. In this talk, I will present our model predictions for (i) the blazar contribution to the diffuse neutrino flux and (ii) for the possibility of detecting individual blazars as neutrino point sources. I will discuss the implications of the latest IceCube searches on the theoretical models for blazar emission.

Diagnostics on Acceleration and Turbulence Near Shocks in Blazar Jets

Matthew Baring

Rice

Tuesday, Oct 11, 2016

Abstract

Acceleration at relativistic shocks is likely to be important in various astrophysical jet sources, including blazars and other radio-loud active galaxies. An important recent development for blazar science is the ability of Fermi-LAT data to pin down the power-law index of the high energy portion of emission in these sources, and therefore also the index of the underlying non-thermal particle population. This talk highlights how multiwavelength spectra including X-ray band and Fermi data can be used to probe diffusive acceleration in relativistic, oblique, MHD shocks in blazar jets. The spectral index of the non-thermal particle distributions resulting from Monte Carlo simulations of shock acceleration, and the fraction of thermal particles accelerated to non-thermal energies, depend sensitively on the particles' mean free path scale. Models of the radiative synchrotron/Compton signatures of the resulting thermal and non-thermal particle distributions are presented. Important constraints on the frequency of particle scattering and the level of field turbulence are identified for the blazars BL Lac, AO 0235+164 and Mrk 501. The possible interpretation that turbulence levels decline with remoteness from jet shocks, and a significant role for non-gyroresonant diffusion, are addressed, and analogies to heliospheric conditions are drawn.

Lessons Learned Designing, Building and Testing the Chandra Observatory

Jonathan Arenberg

Northrop-Grumman

Tuesday, Oct 18, 2016

Abstract

Launched in 1999, the Chandra X-ray Observatory, which began its existence as the Advanced X-ray Astrophysics Facility (AXAF) has been a highly successful, productive mission. The discussion offers some of the major lessons learned by members of the Chandra Telescope team. The lessons discussed are gleaned from our experiences developing, designing, building and testing the telescope and its subsystems, with the benefit of 16+ years of hindsight and mission success. Among the topics to be discussed are the early developmental tests, known as VETA-I and VETA-II, requirements derivation, the impact of late requirements and reflection on the conservatism in the design process. This discussion will conclude with some ideas on building highly motivated and largely badgeless teams like Chandra's for future missions of all sizes.

Planetary System Evolution: Giant Planet Eccentricity & Nonconservative Dynamics

David Tsang

University of Maryland

Tuesday, Oct 25, 2016

Abstract

I will discuss two topics relevant to the evolution of planetary systems, eccentricity evolution of giant planets within their natal disks, and a new nonconservative action formalism that can be used for N-body dynamics. In the first part of the talk I will show how entropy gradients within the disk can modify the non-coorbital corotation torques which dominate the eccentricity evolution for giant planets, and that when insolation heats the gap in the disk due to the giant planet, eccentricity can be pumped. This may have a signature in the existing exoplanet radial velocity data, which may bear the imprint of the disk structure as a function of metallicity. In the second part, I will briefly outline a new nonconservative variational principle that we have recently developed, which allows for nonconservative processes to be generically modeled with an action. I will demonstrate how this allows for the formulation of nonconservative (slimplectic) variational integrators: numerical integrators with the Noether charge (Energy, angular momentum etc) accuracy properties of symplectic integrators but for systems that have dissipation or other nonconservative physics (e.g. tides, radiation reaction, drag).

The Intricate Role of Cold Gas and Dust in Galaxy Evolution at Early Cosmic Epochs

Dominik Riechers

Cornell

Tuesday, Nov 1, 2016

Abstract

Dusty starburst galaxies at very high redshift represent an important phase in the early evolution of massive galaxies. They typically represent large-scale, gas-rich major mergers that trigger intense, short-lived bursts of star formation, which consume most of the available gas and drive the morphological transition to spheroids. At early cosmic epochs, these hyper-luminous galaxies commonly trace regions of high galaxy overdensity, and may be directly related to the formation of galaxy clusters and their giant central ellipticals. Molecular and atomic gas plays a central role in our understanding of the nature of these often heavily obscured distant systems. It represents the material that stars form out of, and its mass, distribution, excitation, and dynamics provide crucial insight into the physical processes that support the ongoing star formation and stellar mass buildup. I will discuss the most recent progress in studies of the cold gas content of dusty starburst galaxies at high redshift, back to the first billion years of cosmic time using CARMA, the Jansky Very Large Array, the IRAM NOEMA interferometer, and the Atacama Large (sub)Millimeter Array (ALMA). I will also highlight our recent successful first detections of the interstellar medium in "normal" (~L*) galaxies at z>5 with ALMA, and discuss the impact of our findings on future studies back to even earlier epochs.

The new era of gravitational-wave astrophysics

Alessandra Buonanno

Albert Einstein Institute/UMD

Friday, Nov 4, 2016

Abstract

The detection by LIGO of gravitational waves emitted by coalescing binary black holes heralded a new era in astrophysics. I will review the theoretical work that has paved the way to observe such gravitational-wave signals, highlight the main implications of the discovery focusing on its astrophysical and fundamental physics aspects, and discuss the unique science that lies ahead of us with upcoming gravitational-wave observations.

Neutron Star Binary Mergers in the Era of Gravitational Wave Astronomy

Brian Metzger

Columbia

Tuesday, Nov 8, 2016

Abstract

The discovery of merging binary black holes by Advanced LIGO heralds the birth of a new field of research: gravitational wave (GW) astronomy. Coalescing binary systems containing neutron stars (NS) are among the new GW sources expected over the next few years. Maximizing the scientific return from such discoveries will require identifying a coincident electromagnetic counterpart. One promising counterpart is an optical/IR transient powered by the radioactive decay of neutron-rich elements synthesized in the merger ejecta (a `kilonova'). Beyond providing a beacon to the GW chirp, kilonovae directly probe the most promising astrophysical site for rapid neutron capture (r-process) nucleosynthesis. I will describe how the lifetime of the hyper-massive NS which is created during a NS-NS merger impacts the kilonova light curves and color. Free neutrons in the outermost layers of the ejecta could power a bright 'precursor' to the main kilonova emission, enhancing the prospects for its detection from the ground. A small fraction of short gamma-ray bursts are accompanied by long-lived X-ray emission. This may suggest that some mergers result in the formation of long-lived - or even indefinitely stable - NS remnants, thus placing stringent constraints on the high density equation of state.

ALMA observations of the Sunyaev-Zel'dovich effect in galaxy clusters

Tetsu Kitayama

Toho Univ.

Tuesday, Nov 15, 2016

Abstract

The Sunyaev-Zel'dovich effect (SZE), inverse Compton scattering of the cosmic microwave background photons off weakly relativistic electrons, provides a unique probe of cosmic plasma up to high redshifts. I will present new measurements of the thermal SZE by the Atacama Large Millimeter/submillimeter Array (ALMA). We have obtained an SZE image toward a distant galaxy cluster RXJ1347.5-1145 with 5" resolution (corresponding to the physical size of 20 kpc/h), the highest angular and physical spatial resolutions achieved to date for imaging the SZE, while retaining extended signals out to 40". Being proportional to the integrated electron pressure, the SZE image shows distinct morphology from the X-ray image and offers a new probe of small-scale structures of the intracluster medium. I will also discuss implications of these results on the evolution of galaxy clusters as well as prospects for further SZE observations with ALMA.

Bringing the Hard X-ray Sky into Focus: NuSTAR’s view of supernova remnants (and the Sun)

Brian Grefenstette

Caltech

Tuesday, Nov 29, 2016

Abstract

Understanding the origin of supernova explosions remains one of the outstanding problems in astrophysics. X-ray and gamma-rays emitted promptly during these explosions as well as those produced in the decay of radioactive elements produced in the supernova explosion can be used to study the energetics and the dynamics of the explosion. I will provide an overview of recent observations of supernovae and supernova remnants and demonstrate how these data represent a new diagnostic tool for studying the supernova explosion and explosive nucleosynthesis. Closer to home and roughly 10 orders of magnitude brighter in measured flux, I will provide a brief overview of early NuSTAR observations of the Sun and show how focusing optics in the hard X-ray bandpass are opening a new window for studying high temperature or non-thermal emission from the quiet Sun.

WFIRST: A Powerful Tool for a Diverse Astrophysics Program

David Spergel

Princeton

Tuesday, Dec 6, 2016

Abstract

The WFIRST mission will utilize a 2.4 meter telescope with a wide field of view to address many of the most exciting problems in astrophysics. The mission can enable transformation science in a range of areas: (1) its microlensing program should discover ~2000 exoplanets and complete Kepler’s demographic survey; (2) its coronagraph will characterize the atmospheres of nearby planets; (3) its dark energy program will use multiple tools (supernova, gravitational lensing, large-scale structure) to study the origin of cosmic acceleration; and (4) its hundred-fold increase in field-of-view over Hubble will enable a wide-ranging general observer program. As one example, I will discuss a potential astrometric program that WFIRST could conduct to detect exoplanets and to study dark matter in the galactic halo.

Exoplanet Demographics from the Kepler and K2 Missions

Tom Barclays

NASA Ames

Tuesday, Dec 13, 2016

Abstract

The Kepler spacecraft launched in 2009 and precipitated a revolution in our understanding of our place in the Universe. We have learnt that giant planets are rare and Earth-sized planets are common. Perhaps most astonishingly of all, the majority of planets found by Kepler are super-Earths that have no peer in our own planetary system. I will present the latest estimates of planet occurrence rates, focusing on the all-important Habitable Zone. Despite Kepler's unprecedented photometric quality, it’s the most challenging regions of parameter space that intrigue us the most. I will talk about some data analysis technique that I’ve applied in recent years to overcome boundaries that limited the detection and characterization of planets in the Kepler data. I will then discuss how we are using these techniques to evaluate planet candidates using data from Kepler’s current voyage as the K2 Mission.

Superluminous Supernovae and Their Utility as Cosmic Probes

Ryan Chornock

Ohio U.

Tuesday, Dec 20, 2016

Abstract

Large wide-field optical surveys in the last decade have uncovered a rare class of supernovae with peak luminosities up to 100 times larger than those of normal supernovae. First, I will discuss the ongoing search to understand the sources of this extra power. While some objects appear to be caused by extreme circumstellar interaction, the hydrogen-poor superluminous supernovae (SLSNe) have remained mysterious. The most popular current models invoke a central engine, possibly the spindown of a rapidly-rotating magnetar. This invites some broad comparisons of SLSNe to the relationship of gamma-ray bursts to normal core-collapse SNe. I will then look forward to the upcoming decade of major space missions and large ground-based telescopes. The enormous ultraviolet output of SLSNe is shifted into the observed-frame optical and near-infrared at high redshifts, making them promising probes of the environments of star formation in the early universe.