MIT Astrophysics Colloquia - Spring 2013

Tuesdays at 4:00 PM in the Marlar Lounge, Room 37-252
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
(unless location otherwise noted)
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.


Tuesday Mar 5:
Our Screwy Universe: Probing Fundamental Physics from the World's Extremes with CMB Polarization
Brian Keating
UC San Diego
Host: Andy Friedman

Abstract: Abstract:The Background Imager of Cosmic Extragalactic Polarization (BICEP) experiment was the first cosmic microwave background (CMB) polarimeter designed to measure the unique "B-mode" polarization pattern hypothesized to originate during the Inflationary cosmological epoch. BICEP observed 3% of the sky from our observatory at the Amundsen-Scott South Pole Research Station in Antarctica and produced the first meaningful limits on the energy scale of Inflation to come from CMB polarization. Soon after BICEP's initial results were released, a first-detection of parity-violating "cosmic birefringence" effects was claimed using publicly available BICEP data (Xia, Li & Zhang, 2009). I will discuss the challenges of polarimetry at the few parts per billion level and explain why systematic effects are particularly pernicious for probes of cosmic parity violation. I will conclude by discussing how BICEP, its successor BICEP2, and the POLARBEAR/Simons Array experiments will constrain Inflationary cosmology, Dark Matter and Dark Energy and cosmic parity violation.
Tuesday Mar 12:
The Dark Side of Dark Matter
Annika Peter
UC Irvine and The Ohio State University
Host: Meng Su

Abstract: Abstract: The nature of dark matter is one of the major "known unknowns" of physics of the universe. From astronomical observations, we know that dark matter exists, makes up 23% of the mass budget of the universe, clusters strongly to form the load-bearing frame of structure for galaxy formation, and hardly interacts with ordinary matter Although most of what we know about dark matter comes from astronomy, most of the effort now to characterize dark matter is focused on non-gravitational interactions between dark matter and standard-model particles. However, dark matter may live in a new, dark sector of physics with only tenuous connections to the standard model. The challenge is to learn about the physics of this new sector without relying on measurable non-gravitational interactions with the standard model. In this talk, I argue that astronomical searches can elucidate some of the dark physics of dark matter. I will show how the physics within a new dark sector may imprint itself on the formation and growth of cosmological structure in the universe, which may then be probed using the gravitational interactions between dark matter and standard-model particles. I will outline which currently existing data sets can provide interesting constraints already, and what kinds of theoretical and observational work should be done in the future for even better constraints on the physics of dark matter.
Tuesday Mar 19:
Origin of Retrograde Hot Jupiters
Smadar Naoz
CfA
Host: Ed Bertschinger

Abstract: The search for extra-solar planets has led to the surprising discovery of many Jupiter-like planets in very close proximity to their host star, the so-called “hot Jupiters” (HJs). Even more surprising, many of these HJs have orbits that are eccentric or highly inclined with respect to the equator of the star, and some (about 25%) even orbiting counter to the spin direction of the star. This poses a unique challenge to all planet formation models. We show that secular interactions between Jupiter-like planet and another perturber in the system can easily produce retrograde HJ orbits. We show that in the frame of work of secular hierarchical triple system (the so-called Kozai mechanism) the inner orbit's angular momentum component parallel to the total angular momentum (i.e., the z-component of the inner orbit angular momentum) need not be constant. In fact, it can even change sign, leading to a retrograde orbit. A brief excursion to very high eccentricity during the chaotic evolution of the inner orbit allows planet-star tidal interactions to rapidly circularize that orbit, decoupling the planets and forming a retrograde hot Jupiter. We estimate the relative frequencies of retrograde orbits and counter to the stellar spin orbits using Monte Carlo simulations, and find that they are consistent with the observations. The high observed incidence of planets orbiting counter to the stellar spin direction may suggest that three body secular interactions are an important part of their dynamical history.
Tuesday Mar 26:
Spring Break
no colloquium


Tuesday Apr 2:
Galaxy Evolution in the First Billion Years: How HST has Revolutionized Our View of the Universe
Steven Finkelstein
U Texas
Host: Anna Frebel

Abstract: The field of high-redshift galaxy evolution has experience an explosion over the past three years, thanks to the installation of the infrared-sensitive Wide Field Camera 3 on the Hubble Space Telescope. In the first few months after the servicing mission, numerous papers were published detailing the discovery of the first robust galaxy samples at z = 7 and 8, with the most distant galaxies dating only 500 million years from the Big Bang. Since that time, new deep and wide area surveys, such as the HST Multi-cycle Treasury Program CANDELS, has allowed us to take a detailed look into the properties of these galaxies. These data allow us to probe into the epoch when we expect to see galaxies vigorously building up their stellar, chemical, and dust reservoirs. I will discuss my ongoing work examining these enigmatic galaxies, first on understanding the colors of these galaxies, from which we have learned about the build-up of dust in the universe, as well as the possible presence of primordial stars. I will also show how we use these galaxies to probe the reionization of the intergalactic medium, which was likely ending around this epoch. Finally, I will detail key tests for the future, including those which can only be done with the James Webb Space Telescope.
Tuesday Apr 9:
Stellar Explosions and Element Synthesis
Gail McLaughlin
North Carolina State Univ.
Host: Anna Frebel

Abstract: The astrophysical origin of many of the heaviest elements remains a mystery. About half of the elements with mass number greater than 100 are thought to be made by a rapid neutron capture process (r-process) that requires many neutrons. Finding a site for this process which is consistent with observation presents a challenge. I will review the leading suggestions and discuss new avenues for making progress on this subject.
Tuesday Apr 16:
Patriots Day
No colloquium


Tuesday Apr 23
Order out of chaos: towards understanding galaxy formation in the cosmological context
Andrey Kravtsov
U Chicago
Host: Paul Schechter

Abstract: Galaxy formation is a complex, hierarchical, highly non-linear process, which involves gravitational collapse of dark matter and baryons, supersonic, highly compressible and turbulent flows of gas, star formation, stellar feedback, as well as heating, cooling, and chemical processes that affect the gas and, indirectly, the stellar and dark matter distributions. Nevertheless, despite the apparent complexity of processes accompanying galaxy formation, galaxies exhibit a number of striking regularities, such as tight correlations between galaxy sizes, masses, luminosities, and internal velocities and surprisingly tight correlations between properties of stars and gas in galaxies and the mass and extent of their parent halos dominated by dark matter. Existence of such correlations indicates that powerful processes operate to bring order out of chaos. Understanding what these processes are and how they operate is not only fascinating scientifically, but is critical for interpreting the avalanche of current and future observations of galaxies across cosmic time. I will describe recent progress in our understanding of how such regularities can arise in a seemingly chaotic and nonlinear process of galaxy formation.
Tuesday Apr 30:
Damped Lyman alpha Systems (DLAs): The most well-understood, enigmatic galaxies
Regina Jorgenson
U Hawaii
Host: Anna Frebel/Rob Simcoe

Abstract: Damped Lyman alpha Systems (DLAs) are observed as absorption features in the lines of sight toward background, high redshift quasars and defined as clouds of neutral gas having N(HI) >= 2x10^20 cm^-2. DLAs dominate the neutral gas content of the Universe between z=[0,5] and are believed to be the progenitors of modern spiral galaxies. However, despite 25 years of study and over 1000 DLAs discovered, thanks to the SDSS survey, the true nature of these important reservoirs of neutral gas remains elusive. I will discuss recent efforts to determine the physical conditions in DLAs via absorption line physics as well as complimentary attempts to detect the DLA host galaxies in emission by taking advantage of the Keck/OSIRIS IFU and LGSAO. In addition, I will discuss recent work using the most metal-poor DLAs as probes of early nucleosynthesis.
Tuesday May 7:
The Imprint of the First Stars on Dwarf Galaxies
John Wise
Georgia Tech
Host: Anna Frebel

Abstract: I present results from a suite of cosmological radiation hydrodynamics simulations that focus on the transition from Population III star formation to the first galaxies. Each simulation captures the radiative and chemical feedback from ~300 Population III stars, leading to the formation of a 10^9 solar mass galaxy at redshift 7. We have investigated the variations in galaxy properties when changing the Pop III characteristic mass, critical metallicity, UV backgrounds, metal cooling, and feedback from radiation pressure. One constant result from all the simulations is a metallicity floor between [Z/H] = -3 to -4. We show that momentum transfer from ionizing radiation plays an important role in providing turbulent support and mixing metals, preventing the overproduction of stars and metals. This results in a stellar population with a tight metallicity distribution function centered at [Z/H] = -2, agreeing with the observed luminosity-metallicity relation in dwarf galaxies.
Tuesday May 14:
The Bones of the Milky Way
Alyssa Goodman
CfA
Host: Anna Frebel

Abstract: The Milky Way is typically thought of as a spiral galaxy, but our understanding of its detailed structure remains fuzzy thanks to our observational vantage point within its disk. Most of what we do know about the Milky Way's three-dimensional geometry comes from velocity-resolved observations of gas and stars. But, recently, it has become possible to combine exquisitely sensitive observations of dust with more traditional kinematically-resolved observations of gas to reveal totally new structures within the Milky Way. In this talk, I will explain why we now believe that some extraordinarily long and thin so-called "infrared dark clouds" are in fact "bones" of the Galaxy, centered within spiral arms, and, in at least one case, marking the geometric mid-plane the disk to within less than a few parsecs. The talk will highlight how both large surveys and new visualization tools have been critical in this investigation.
NOTE UNUSUAL DAY: Monday May 20:
Cosmological Galaxy Simulations Compared with Observations
Joel Primack
UCSC
Host: Rob Simcoe

Abstract: I will give an overview of the latest galaxy simulations by the group led by Avishai Dekel and me compared with observations from the large HST CANDELS survey, and the new Assembling Galaxies of Resolved Anatomy (AGORA) high-resolution galaxy simulation comparison project (http://AGORAsimulations.org) that Piero Madau and I initiated last year. I will also briefly summarize new work using the Bolshoi cosmological simulations, including new Semi-Analytic Models of the evolving galaxy population.
Tuesday May 21:
Time-Domain Informatics and Smart Data Analytics in the Era of Large Astronomical Surveys
Hakeem Oluseyi
Florida Institute of Technology
Host: Ed Bertschinger

Abstract: Current and upcoming large surveys for studying dark energy and the Lambda-CDM paradigm, such as the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST), will require reliable identification, classification and analyses of large numbers of transient and variable sources. One subset of these efforts are the near-field cosmology studies that map the distributions of Galactic stars in spatial-kinematic-metallicity-age space to test models of Galactic assembly and evolution. This talk will describe our work on developing new astroinfomatics techniques for taking advantage of the chromo-temporal data that will be available from the LSST and our current work with data from Kepler and LINEAR surveys.

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