MIT Astrophysics Colloquia - Fall 2008

Tuesdays at 4:00 PM in the Marlar Lounge, Room 37-252
EXCEPT *Wednesday December 10*
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
Refreshments are served at 3:45 PM.

Sponsored by
the Astrophysics Division of the MIT Department of Physics
and
the MIT Kavli Institute for Astrophysics and Space Research.

September 09:
HYPERVELOCITY STARS AND MASSIVE BLACK HOLES
Dr. Warren R. Brown
Harvard-Smithsonian Center for Astrophysics
Host: Scott Hughes

Abstract: Hypervelocity stars ejected from the Galaxy at ~1000 km/s are a natural consequence of the massive black hole in the Galactic center. We discovered the first hypervelocity star in 2005, and we designed a survey that has found 12 more of these unbound stars. Hypervelocity stars are interesting because their observed spatial and velocity distributions are linked to the black hole ejection mechanism.

September 16:
THE LICK OBSERVATORY SUPERNOVA SEARCH, AND FOLLOW-UP STUDIES OF SUPERNOVAE
Prof. Alexei V. Filippenko
UC Berkeley
Host: Ed Bertschinger

Abstract: The Lick Observatory Supernova Search (LOSS) conducted with the 0.76-m Katzman Automatic Imaging Telescope (KAIT) has been by far the world's most successful search for nearby supernovae, having discovered about 700 of them over the past decade. The search and its results will be described. LOSS supernova rates as a function of host-galaxy Hubble type will be presented. As an aside, KAIT's utility in rapidly obtaining follow-up observations of the optical afterglows of gamma-ray bursts will also be illustrated. Finally, some follow-up studies of supernovae will be described, concentrating on several objects having massive progenitors. The extremely powerful SN 2006gy is perhaps the first pair-instability SN ever observed and could be a link to the earliest stars. SN 2006jc is a peculiar Type Ib supernova that had a luminous outburst 2 years before explosion and whose progenitor was probably a Wolf-Rayet star.

September 23:
"EXCITING" DARK MATTER, POSITRONS, AND PAMELA
Prof. Douglas Finkbeiner
Harvard-Smithsonian Center for Astrophysics
Host: Angelica de Oliveira-Costa

Abstract: Since late July, the cosmic-ray community has scrambled to explain the positron/electron ratio spectrum from the PAMELA satellite at 1 < E < 50 GeV. Combining this with previous data on the electron cosmic-ray spectrum, we now have evidence for a "bump" in the spectrum, half electrons and half positrons. I will explore the possibilities for making this with WIMP annihilation, and show that generic candidates from supersymmetry cannot make such a signal without violating other constraints on gamma-rays. I will then introduce a toy model (developed with Neal Weiner) involving a new light particle in the dark sector which couples strongly to the WIMP, but only very weakly to the standard model. In this scenario, the WIMP annihilates through this new particle, evading constraints on gammas and anti-protons. Such a model can have the correct mass, annihilation cross section, and scattering cross section to explain the PAMELA data and the WMAP haze excess, while naturally having the right thermal relic cross section. The model can also explain the INTEGRAL 511 keV line in the inner few degrees of the galaxy via inelastic scattering.

September 30:
Galaxies in Nearby Clusters: Cored Giants, Nucleated Dwarfs and Everything in Between
Prof. Andres Jordan
Pontificia Universidad Catolica de Chile
Host: Paul Schechter

Abstract: Nearly a century after the nature of galaxies as "island universes" was established, their formation and evolution remain important problems in modern astrophysics. Elliptical galaxies, whether giants or dwarfs, share a continuum of properties across the entire range of galaxy mass. On nuclear scales, these galaxies are found to exhibit a gradual progression from a light "deficit" (cores) to a light "excess" (stellar nuclei), while on larger scales their brightness profiles are well described by Sersic models. The central regions of elliptical galaxies hold a constant fraction of their mass in a central massive object: a super-massive black hole, a nuclear star cluster, or a combination thereof. Despite the tumultuous times galaxies experience as they form and evolve, their end products show clear trends that all galaxy formation and evolution models must be able to explain.

October 07:
GALEX at Five: New Discoveries and Fresh Perspectives on Galaxy Evolution
Prof. David Schiminovich
Columbia University
Host: Paul Schechter `

Abstract: The Galaxy Evolution Explorer (GALEX) is a NASA Small Explorer mission that was launched in April 2003 and is continuing to conduct nested surveys and targeted observations of the space ultraviolet sky. A principal goal of the GALEX mission is to study the history of star formation in the Universe and determine the physical drivers for this evolution. In this talk I will present key science results from GALEX after five years in orbit, with an emphasis on novel approaches that have led to an improved, though still very incomplete, understanding of how galaxies evolve. Along the way, I will describe several new galaxy scaling relations and explore their physical implications. Finally, I will also highlight a number of recent discoveries of energetic transient and dynamic phenomena detected in the ultraviolet by GALEX.

October 14:
Gravitational radiation from pulsar glitches: nuclear physics with LIGO
Dr. Andrew Melatos
Senior Lecturer, University of Melbourne
Host: Scott Hughes

Abstract: Pulsars are fantastically stable clocks, with spin periods measured to 15 significant figures in some objects. They spin down steadily over millions of years under the action of electromagnetic torques. However, roughly 5% of known pulsars also experience "glitches", which are tiny, randomly timed, discontinuous spin-up events. The physical origin of glitches remains a mystery after 40 years. Recent radio pulsar timing data, drawn primarily from the Parkes Multibeam Survey, has quadrupled the glitch database and effected a sea change in our ideas about the glitch phenomenon. In this talk, I present the latest data and discuss their implications for the long-standing superfluid vortex paradigm as well as for current and future experiments with the Laser Interferometer Gravitational Wave Observatory (LIGO). It is shown that the gravitational-wave signal following a glitch, once detected, can be inverted to infer the compressbility and viscosity of bulk nuclear matter. The results of such a detection will be comparable to experimental data from terrestrial relativistic heavy-ion colliders.

October 21:
Testing Gravity in the Solar System
Dr. James Battat
Massachusetts Institute of Technology
Host: Gabriella Sciolla (joint LNS-Astro colloquium)

Abstract: Ten years ago, shocking news came from the world of astrophysics: we live in an accelerating Universe. General Relativity (GR) with a cosmological constant can accommodate these observations, but no adequate explanation for the observed dark energy density yet exists. A plausible alternative to dark energy is that GR breaks down on cosmological scales. Indeed, speculative "post-Einstein" gravity theories abound. Many of these theories predict observable signatures in the solar system, and can therefore be constrained with Lunar Laser Ranging (LLR) and planetary ranging observations. LLR, in fact, provides the tightest constraints on many facets of gravitational physics including the strong equivalence principle, the inverse square law of gravity, gravitomagnetism, and Lorentz symmetry in the gravitational sector. I will describe the ongoing Apache Point Observatory Lunar Laser-ranging Operation (APOLLO), a project which currently delivers millimeter-precision measurements of the Earth-Moon distance. In addition, I will present new constraints on Lorentz invariance and theories that produce cosmic acceleration without dark energy.

October 28:
TESTS OF GRAVITY USING LENSING AND DYNAMICS
Prof. Bhuvnesh Jain
University of Pennsylvania
Host: Angelica de Oliveira-Costa

Abstract:

November 04:
Approaching the event horizon: Submm-VLBI of Sgr-A*
Dr. Sheperd Doeleman
MIT Haystack Observatory
Host: Scott Hughes

Abstract: There is now very strong evidence that SgrA*, the compact source of radio, IR, and x-ray emission at the Galactic Center, marks the position of a 4 million solar mass black hole. Only 8 kilo-parsecs away, Very Long Baseline Interferometry (VLBI) has the potential to model, and eventually image, emission on scales of a few Schwarzschild radii, at the innermost accretion region of this black hole. This requires pushing the VLBI technique to short wavelengths where scattering by the ionized ISM is reduced and the intrinsic structure of SgrA* can be observed. VLBI observations in April 2007 at a wavelength of 1.3mm have now confirmed structure in SgrA* on scales of just a few Schwarzschild radii. More sensitive observations, using additional VLBI stations, are planned over the next few years, and will be sensitive to time variable structures predicted by models of flaring activity in SgrA*. I will describe the instrumentation efforts that enable these observations, and discuss what current and future VLBI observations of SgrA* tell us about this closest super-massive black hole.

November 18:
The epochs of hydrogen and helium reionization
Prof. Matias Zaldarriaga
Harvard-Smithsonian Center for Astrophysics
Host: Angelica de Oliveira-Costa

Abstract: I will summarize our current state of understanding of the epochs of hydrogen and helium reionization from both a theoretical and observational perspective. I will discuss the implications of our theoretical models for future observations including measurements of the redshifted 21cm line by arrays such as the MWA.

November 25:
Empirical Evidence for AGN Feedback
Dr. Christy Tremonti
Max Planck Institute for Astronomy
Host: Scott Hughes

Abstract: Feedback from massive stars and accreting black holes has proven to be a key ingredient in successful models of galaxy evolution. Yet much about the feedback process is still poorly understood due to a lack of direct observational constraints. To help remedy this, we are studying a population of massive post-starburst galaxies at z~0.6. These objects are the likely remnants of major mergers, observed a few hundred million years after the peak of their star formation and AGN activity. In a 70% of our sample we detect Mg II absorption lines which are blueshifted by 500 - 2000 km/s with respect to the stars. We hypothesize that the absorbing material represents a fossil galactic wind launched near the peak of the galaxy's activity. We estimate the mass and energy of the outflow and conclude that feedback from AGNs played a role in expelling the cool gas and quenching star formation. Finally, we consider the various physical mechanisms that may be at work in driving AGN-powered outflows.

December 02:
Asteroids, Comets and Planetary Systems
Prof. Chas Beichman
Caltech/JPL
Host: Sara Seager

Abstract: The Spitzer Space Telescope has greatly advanced our knowledge of disks of material orbiting nearby stars, starting with protostellar disks around young stars containing primordial material and ending with debris disks containing dust replenished by? collisions from larger bodies. I will concentrate on recent Spitzer results on debris disks associated with analogs of the asteroid belt and Kuiper (comet) belt in our solar system. Kuiper Belt analogs are found at 70 um around approximately 17% of mature F,G and K stars. Spitzer IRS spectroscopy identifies the continuation of this emission to 25-30 um in almost all of these objects. Hot dust, or the potential analogs of "asteroid belt" emission, is quite rare, being found in only 1% of stars examined. When present, this material may be due to the destruction of large objects thrown in from larger orbital distances analogous to Late Heavy Bombardment (LHB) period in our own solar system. I will discuss the possible relationship between spatial distribution of disks and presence of planets.

Wednesday, December 10: NOTE UNUSUAL DATE!!
Gamma-Ray Bursts: A New Probe of the High-Redshift Universe
Dr. Edo Berger
Harvard-Smithsonian Center for Astrophysics
Host: Scott Hughes

Abstract: Gamma-ray bursts (GRBs) are the extremely powerful explosions of the most massive stars in the universe. Their optical and infrared luminosities are bright enough to be seen at least to z~7 and they thus provide a powerful probe of high redshift galaxies and the intergalactic medium. In this talk I will show that the location of GRBs within the star forming regions of galaxies affords an in-situ view of metal enrichment across a wide redshift range. In addition, follow-up observations of the host galaxies with Spitzer and HST allow us to investigate the galaxy mass-metallicity relation beyond z~2, and to delineate for the first time the properties of damped Lyman-alpha absorbers. I will also highlight some exciting recent results from GRB observations, including a naked-eye burst at z~1 and a burst at z~6.6.

This page is maintained by Angelica de Oliveira-Costa (angelica@space.mit.edu)