Astrophysics Science Division
Astrophysics Science Division - Seminars & Meetings

Through the courtesy of the speakers since 2004, some presentations are available on line.

Future schedules:

Recent schedules:

  • 2008, Third Quarter
  • 2008, Second Quarter
  • 2008, First Quarter

    ASD Colloquia are Tuesdays at 3:45 pm (Meet the Speaker at 3:30 pm)
    in Bldg 21, Room 183 unless otherwise noted.


    Sept 2 Leslie Sage (Nature) - How to Publish in Nature
    Sept 9
    Christopher Stubbs (Harvard Univ.) - Exploring Fundamental Physics with Astrophysical Techniques
    Sept 16
    Alan Kogut (GSFC) - Inflation, Gravity Waves, and the Cosmic Microwave Background
    Sept 23
    Margaret Meixner (STScI) - Space Telescope Science Institute -- Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE)
    Sept 30
    Andrew Smith (UMD) - A wide-field view of the TeV sky: The results of the Milagro Experiment


    Oct 7
    Asaf Pe'er (STScI) - Thermal emission from Gamma-ray bursts
    Oct 14
    Szabolcs Marka (Columbia University) - Gravitational Waves in Multimessenger Astrophysics
    Building 2, Room 8
    Oct 21
    Martin Harwit (Cornell University) - Conceiving and Marketing NASA's Great Observatories
    Oct 28
    Joe Lazio (NRL) - The Square Kilometer Array: The Next-Generation Radio Telescope


    Nov 4
    Francis Halzen (U. Wisconsin) - High-Energy Neutrino Astronomy: Towards a Kilometer-Scale Neutrino Observatory
    Nov 11
    Nov 18
    Jim Condon (NRAO) - Dark Energy and the Hubble Constant
    Nov 25


    Dec 2
    Alberto Sesana (Penn State) - Massive black hole binaries, LISA and Pulsar Timing: upcoming new windows in astrophysics and cosmology
    Dec 9
    David Charbonneau (CfA) - The Dynamics-Based Approach to Characterizing Habitable Exoplanets
    Dec 16
    Piero Madau (Univ. of California) - The Fabulous Lives of Massive Black Holes
    Dec 23

    How to Publish in Nature

    Leslie Sage

    Nature Magazine

    Tuesday, September 2, 2008



    Exploring Fundamental Physics with Astrophysical Techniques

    Christopher Stubbs

    Harvard University

    Tuesday, September 9, 2008


    The best evidence we have for physics beyond the standard model comes from astrophysical observation: Dark Energy, Dark Matter, and even the abundance of matter over antimatter are each indications of shortcomings in our understanding of basic physics. I will describe our efforts to understand the nature of the Dark Energy, and why this particular problem merits our collective attention. I'll also present the status of PanSTARRS and LSST, two ambitious ground-based optical systems that will provide new capabilities for studying diverse topics in astronomy and astrophysics.

    Inflation, Gravity Waves, and the Cosmic Microwave Background

    Alan Kogut


    Tuesday, September 16, 2008


    The polarization of the cosmic microwave background contains a contribution from gravity waves excited during the epoch of inflation, shortly after the Big Bang. A positive detection of this signal would have extraordinary consequences for both cosmology and physics: not only would it establish inflation as a physical reality, but it would also provide a model-independent determination of the relevant energy scale. I will present the scientific motivation behind measurements of the CMB polarization and discuss how recent experimental progress could lead to a detection of the inflationary signal in the not-very-distant future.

    Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE)

    Margaret Meixner

    Space Telescope Science Institute

    Tuesday, September 23, 2008


    The recycling of matter between the interstellar medium (ISM) and stars are key evolutionary drivers of a galaxy's visible matter. The SAGE team is performing a Spitzer Legacy imaging survey of the Large Magellanic Cloud (LMC), using the IRAC (3.6, 4.5, 5.8 and 8 microns) and MIPS (24, 70, and 160 microns) instruments on board Spitzer. The Spitzer wavelengths provide a sensitive probe of circumstellar and interstellar dust and hence, allows us to study the physical processes of the ISM, the formation of new stars and the injection of mass by evolved stars and their relationships on the galaxy-wide scale of the LMC. Due to its proximity, favorable viewing angle, multi-wavelength information, and measured tidal interactions with the Milky Way (MW) and Small Magellanic Cloud (SMC), the LMC is uniquely suited for surveying the agents of a galaxy's evolution (SAGE), the ISM and stars. Our uniform and unbiased survey of the LMC (7x7 degrees) will have much better wavelength coverage, up to ~1000 times better point source sensitivity and ~11 times better angular resolution than previous IR surveys. SAGE will reveal over 6 million sources including ~150,000 evolved stars, ~50,000 young stellar objects and the diffuse ISM with column densities >1.2x10^{21} H/cm^2. The diffuse IR emission in the LMC can be associated with individual gas/dust clouds, thereby permitting unique studies of dust processes in the ISM. SAGE's complete census of newly formed stars with masses >1-3 Msun will reveal whether tidally-triggered star formation events in the LMC are sustained or short-lived. SAGE's complete census of evolved stars with mass loss rates >1x10^{-8} Msun/yr will quantitatively measure the rate at which evolved stars inject mass into the ISM. In this talk, I will present an overview of the SAGE survey including preliminary results on ISM, star formation and evolved stars.

    A wide-field view of the TeV sky: The results of the Milagro Experiment

    Andrew Smith

    University of Maryland

    Tuesday, September 30, 2008


    The recently launched Fermi gamma-ray space telescope will map the gamma-ray sky above 30 MeV. However, as energies increase the particle flux from high energy gamma-ray sources wanes, until the paucity of events limits the sensitivity. Above 1 TeV, for example, a bright gamma-ray source only radiates ~10 photons/m2/year. At these energies, large ground-based instruments are necessary. The Milagro experiment, a novel water Cherenkov air-shower detector that operated for 8 years from 2000-2008, recently completed a 20 TeV survey of the northern sky. I will present the results of the Milagro sky survey and show energy spectra for all high-significance sources. I will also introduce the HAWC experiment, a planned extension of the Milagro technique to a higher elevation site. When completed, HAWC will be 15x more sensitive than Milagro.

    Thermal emission from Gamma-ray bursts

    Asaf Pe'er

    Space Telescope Science Institute

    Tuesday, October 7, 2008


    I will show evidence for a thermal emission component that accompanies the overall non-thermal spectra of the prompt emission phase in GRBs. Both the temperature and flux of the thermal emission shows a well-defined temporal behavior, a broken power law in time. The temperature is nearly constant during the first few seconds, afterwards it decays with power law index alpha~0.7. The thermal flux also decays at late times as a power law with index beta~2.0. This behavior is very ubiquitous, and was found in a sample currently containing 32 BATSE bursts. I will show that this result is naturally explained by considering emission from the photosphere.

    The photosphere of a relativistically expanding plasma wind strongly depends on the angle to the line of sight, theta. As a result, thermal emission can be seen after tens of seconds. I will introduce the probability density function P(r,theta) of a thermal photon to escape the plasma at radius r and angle theta, and show how it is used in calculating the flux. I will discuss the propagation of photons below the photosphere, and show that as a result of the slight misalignment of the scattering electrons velocity vectors the photons energy decreases with radius, epsilon'(r)~r^{-2/3}, resulting in a decay of the observed temperature. Finally, I will show that understanding the thermal emission is essential in understanding the high energy, non-thermal spectra. Moreover, I will show how thermal emission can be used to directly measure the Lorentz factor of the flow and the initial radius of the jet.

    Gravitational Waves in Multimessenger Astrophysics

    Szabolcs Marka

    Columbia University

    Tuesday, October 14, 2008
    Building 2, Room 8


    Gamma-ray, X-ray, optical and neutrino observations of cataclysmic cosmic events with plausible gravitational wave emission can be used in combination with searches for gravitational waves. Information on the progenitor, such as trigger time, direction and expected frequency range, shall enhance our ability to identify gravitational wave signatures with amplitudes close to the noise floor of the detector. Even in the absence of detection, the association of the astrophysical trigger with a particular source distance allows to interpret upper limits on the observed flux of gravitational waves in terms of the energy emitted in the form of gravitational waves. After a summary of past multimessenger based gravitational wave searches, I will discuss the implications of these results. I will close by giving an outlook on the future.

    Conceiving and Marketing NASA's Great Observatories

    Martin Harwit

    Cornell University

    Tuesday, Octobber 21, 2008


    In late 1984, Dr. Charles P. (Charlie) Pellerin Jr., director of the Astrophysics Division of NASA's Office of Space Science and Applications (OSSA) faced a dilemma. Congress and the White House had given approval to work that would lead to the launch of the Gamma Ray Observatory and the Hubble Space Telescope, but competing segments of the astronomical community were clamoring for two additional missions, the Space Infrared Telescope Facility (SIRTF) and the Advanced X-ray Astrophysics Facility (AXAF). Pellerin knew that Congress would not countenance a request for another costly astronomical space observatory so soon after approving GRO and HST. He also foresaw that if he arbitrarily assigned priority to either AXAF or SIRTF he would split the astronomical community. The losing faction would be up on Capitol Hill, lobbying Congress to reverse the decision; and Congress would do what it always does with split communities --- nothing. Pellerin called a meeting of leading astrophysicists to see how a persuasive argument could be made for both these new observatories and to market them as vital to a first comprehensive inventory of the universe conducted across all wavelength ranges. The group provided Pellerin a rotating membership of astrophysicists, who could debate and resolve issues so that decisions he reached would have solid community support. It also helped him to market his ideas in Congress. Ultimately, the concept of the Great Observatories came to be accepted; but its implementation faced myriad difficulties. False starts, political alliances that never worked out, and dramatic changes of direction necessitated by the Challenger disaster of early 1986 continually kept progress off balance. My paper follows these twists and turns from late 1984 to the announcement, on February 1, 1988, that President Reagan's FY89 budget proposal to Congress had designated AXAF for a new start.

    The Square Kilometer Array: The Next-Generation Radio Telescope

    Joe Lazio

    Naval Research Laboratory

    Tuesday, October 28, 2008


    The Square Kilometre Array (SKA) is the next-generation centimeter- and meter-wavelength telescope. Its Key Science Projects include

      (a) The end of the Dark Ages, involving searches for a neutral hydrogen signature from the intergalactic medium;

      (b) Galaxy assembly and evolution, primarily via a "billion galaxy survey,"

      (c) Testing theories of gravitation, using an array of pulsars to search for gravitational waves and relativistic binaries to probe the strong-field regime; and

      (d) Astrobiology, including planetary formation within protoplanetary disks.

    I will review the key science, with a particular eye toward the SKA's role in the future multi-wavelength and multi-messenger astronomy, then describe the on-going technology development.

    High-Energy Neutrino Astronomy: Towards a Kilometer-Scale Neutrino Observatory

    Francis Halzen

    University of Wisconsin

    Tuesday, November 4, 2008


    Kilometer-scale neutrino detectors such as IceCube are discovery instruments covering nuclear and particle physics, cosmology and astronomy. Examples of their multidisciplinary missions include the search for the particle nature of dark matter and for additional small dimensions of space. In the end, their conceptual design is very much anchored to the observational fact that Nature produces photons and protons with energies in excess of one hundred and one hundred million Terraelectronvolts, respectively. The cosmic ray connection sets the scale of cosmic neutrino fluxes. The problem has been to develop a robust and affordable technology to build the kilometer-scale neutrino detectors required to detect candidate sources such as supernova remnants and active galxies. The AMANDA telescope transforming ultra-clear deep Antarctic ice into a Cherenkov detector of muons and showers initiated by neutrinos of all three flavors, has met this challenge. Having collected more than 6000 well-reconstructed muon neutrinos of 50 GeV ~ 500 TeV energy, AMANDA represents a proof of concept for the ultimate kilometer-scale neutrino observatory, IceCube, now half complete and already producing results exceeding seven years of AMANDA data in sensitivity.

    Dark Energy and the Hubble Constant

    Jim Condon


    Tuesday, November 18, 2008


    Dark energy (DE) dominates the energy density and future expansion of the universe, and its nature is a major unsolved problem for theoretical physics. The equation-of-state and energy density of DE can be constrained observationally by comparing an accurate (< 3% rms) value of the Hubble constant with the known distance to the last-scattering surface of the cosmic microwave background. The Megamaser Cosmology Project (MCP) is now measuring geometric distances to water masers in the nuclei of distant Seyfert galaxies in order to obtain an accurate value of H0 free from the systematic errors associated with "standard candles." I will review the effect of DE on the Hubble constant and present the latest MCP results.

    Massive black hole binaries, LISA and Pulsar Timing: upcoming new windows in astrophysics and cosmology

    Alberto Sesana

    Pennsylvania State University

    Tuesday, December 2, 2008
    ***Building 2, Room 8***


    In the next decade the detection of gravitational waves (GW) will (hopefully) be a reality, opening a completely new window on the Universe. The primary actors on this upcoming stage are expected to be massive black hole (MBH) binaries (MBHBs). After a short introduction about hierarchical galaxy and MBH formation, and GW detection, I will discuss the possibility of constraining different models of black hole formation and cosmic evolution using the planned LISA and pulsar timing arrays (PTAs), assessing their capability of providing unique high(and low)-redshift information difficult to obtain by other means.

    The Dynamics-Based Approach to Characterizing Habitable Exoplanets

    David Charbonneau

    Harvard-Smithsonian Center for Astrophysics

    Tuesday, December 9, 2008


    When exoplanets are observed to transit their parent stars, we are granted direct estimates of their masses and radii, and we can undertake studies of their atmospheres. Such systems have profoundly impacted our understanding of giant exoplanets akin to Jupiter or Neptune, yet no transiting examples of rocky exoplanets have yet been found. By targeting nearby M-dwarf stars, a transit search using modest equipment is capable of discovering planets as small as 2 Earth radii in their stellar habitable zones. The discovery of such planets would provide fundamental constraints on the physical structure of planets that are primarily rock and ice in composition. Moreover, by differencing spectra gathered when the planet is in view from those when it is occulted by the star, we can study the atmospheric chemistry of potentially habitable worlds.

    The Fabulous Lives of Massive Black Holes

    Piero Madau

    University of California

    Tuesday, December 16, 2008


    The study of the formation, merging history, and environmental impact of the massive black holes that are ubiquitous in the nuclei of luminous galaxies today provides invaluable insight into the quasar phenomenon, the evolution of active galactic nuclei, the relation between black holes and their host halos, the hierarchical growth of cosmic structure, and the astrophysics of some of the earliest formed sources of light. I'll discuss the conditions under which MBHs become incorporated through a series of mergers into larger and larger halos, sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. The spin distribution of MBHs is determined by gas accretion, and is predicted to be heavily skewed towards fast-rotating holes and already in place at early epochs. Decaying MBH binaries will eject hypervelocity stars, shape the innermost central parsec of galaxies, and should be detected in significant numbers by LISA.

    Randall Smith