MIT Astrophysics Colloquia - Fall 2012

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 Sept 11:
Hot on the Trail of Warm Planets Orbiting Cool M Dwarfs
John A. Johnson
CalTech
Host: Sara Seager

Abstract: Just three years ago the prospect of finding temperate, rocky worlds around other stars was still the subject of science fiction: none had been found and reasonable estimates put us years or decades away from such a momentous discovery. All of that has changed very recently on the heels of the extraordinarily successful NASA Kepler mission. By searching for the tiny diminutions of starlight indicative of an eclipsing planet, Kepler has produced thousands of new planet candidates orbiting distant stars. Careful statistical analyses have shown that the majority of these candidates are bona fide planets, and the number of planets increases sharply toward Earth-sized bodies. Even more remarkably, many of these planets are orbiting right "next door," around tiny red dwarf stars. I will describe our multi-telescope campaign to validate and characterize these tiny planetary systems, and present some early, exciting results that point the way to the first detection of the first Earth-sized planet in the habitable zone of a star.
Tuesday Sept 18:
Cosmology on a Moving Mesh
Lars Hernquist
CfA
Host: Anna Frebel

Abstract: Understanding the formation and evolution of galaxies in a qcosmological context using numerical simulations remains an elusive goal. In this talk, I describe a new approach to modeling the hydrodynamics of galaxy formation in which the equations of motion are solved on a moving mesh. The use of a moving mesh makes the scheme fully Lagrangian, unlike popular particle-based codes which are pseudo-Lagrangian in nature, and mitigates against advection errors when a spatially fixed grid is used. I present results from an initial study comparing results for a moving mesh with those obtained using a smoothed particle hydrodynamics solver. This preliminary work suggests that the new approach offers promise for resolving the long-standing problems which have plagued this field for nearly two decades.
Tuesday Sept 25:
Probing Black Hole Spin and AGN Structure with X-ray Spectra
Laura Brenneman
CfA
Host: Ron Remillard

Abstract: The angular momentum of a supermassive black hole (SMBH) is a vitally important quantity in astrophysics. Measuring the spins of SMBHs in active galactic nuclei (AGN) can inform us about the relative role of gas accretion vs. mergers in recent epochs of the life of the host and its AGN. Black hole spin is also thought to play a pivotal role in triggering relativistic jets, enabling the SMBH to influence its surroundings out to scales much larger than its gravitational sphere of influence allows. Advances in theoretical modeling as well as observational sensitivity in the Chandra/XMM-Newton/Suzaku era are finally producing robust constraints on the spins of a handful of SMBHs, as well as the physical properties of the accretion disk and the structure of the AGN as a whole. This science is still very much in its infancy, however. I will discuss our current knowledge of the distribution of SMBH spins in the local universe, with an emphasis on addressing the complexities involved in using X-ray spectroscopy to obtain these measurements. I will also address prospects for improving the accuracy, precision and quantity of these spin constraints in the next decade and beyond with instruments such as NuSTAR, Astro-H and ATHENA. Additionally, I will demonstrate the importance of considering spectral variability on a wide variety of timescales when modeling the emission and absorption properties of the AGN system. Proper modeling of this variability can yield critical insights into the physical structure of the nucleus.
Tuesday Oct 2:
Microarcsecond Astronomy: Reaching Toward the Schwarzschild Limit, and Other Matters Arising
Bernard Burke
MIT
host: TBD

Abstract: Two fundamental questions have to be addressed if astronomical objects are to be studied with microarcsecond resolution. First, will there be anything to see; second, how are you going to do it. An aperture of 10^11-10^12 wavelengths is not easy to come by -- 100 kilometers at optical wavelengths, or the earth-moon distance at centimeter wavelengths. The first question has an easy answer -- studies of radio source scintillations caused by the interstellar medium give direct evidence that there are microarcsecond structures. The second question has an easy answer: the VLBI studies by the group led by Haystack's Shep Doeleman using millimeter wavelengths at an earth diameter have already demonstrated structures at the galactic center, close to the Schwarzschild limit. Most of my talk will focus on a new instrument that has lately come on line, the RADIOASTRON VLBI satellite. It was launched last summer, and underwent checkout during the past winter. The satellite is performing well, and a review in May 2012 by the Radioastron International Science Committee (RISC) led to the conclusion that an AO should be issued as soon as possible. Interesting resuls have already been obtained by observing pulsar scintillations at the maximum baseline of 200,000 km, and the various possible scientific programs that can be undertaken will be examined.
Tuesday Oct 9: SPECIAL TIME: 4:15 to 5:15
SPECIAL COLLOQUIUM: The Pioneering Decade of X-ray Astronomy
Walter Lewin
MIT


Tuesday Oct 16:
The Bridges and Tails of Interacting Dwarf Galaxies: efficient baryon removal and the formation of dwarf Spheroidals
Gurtina Besla
Columbia
Host: Paul Schechter

Abstract: Interactions between isolated dwarf galaxies are a largely understudied mode of galaxy evolution that could potentially represent a major channel for the removal of baryons from low mass systems and thereby facilitate the morphological transformation from dwarf Irregular type galaxies to gas-poor dwarf Spheroidals. The ongoing tidal interactions between our nearest pair of dwarf galaxies, the Magellanic Clouds, illustrate that such interactions may be an efficient mode of gas loss. In addition, the properties of the Magellanic Bridge connecting them indicate that hydrodynamic forces during a direct collision may enhance the gas removal process, facilitating a morphological transformation and inducing star formation in the Bridge. With the discovery of the gas-deficient tidally disrupted dwarf about the starbursting Magellanic Irregular dwarf galaxy, NGC 4449, it is becoming increasingly apparent that such interactions may be generic, in lines with theoretical expectations of hierarchical structure formation. Such interactions likely represent a generic mode of dwarf galaxy evolution and baryon loss that is independent of proximity to a massive host.
Tuesday Oct 23:
What is the Circumgalactic Medium?
Jason Tumlinson
StSci
Host: Anna Frebel/Rob Simcoe

Abstract: Gas flows in accretion, star formation, and feedback in galaxies are at the critical frontier in galaxy evolution studies. Many of the flows that govern how galaxies form and evolve pass through the "Circumgalactic Medium", the region of diffuse gas within the dark matter halo but outside the disk. Thanks to major advances in instrumentation and dedicated surveys we are now making major strides in characterizing the CGM and working out its relationship to galaxies. I will report results from large programs using Hubble's Cosmic Origins Spectrograph to examine the CGM. We find that the CGM is an important reservoir of cosmic baryons and metals, and that it shows clear evidence of both galaxy accretion and feedback. I will also connect these results to other recent characterizations of the CGM at high redshift, and discuss their implications for galaxy formation at large.
Tuesday Oct 30
The Bones of the Milky Way
Alyssa Goodman
CfA
Host: Anna Frebel/Rob Simcoe

Abstract: The Milky Way is typically thought of as a spiral galaxy, but our understanding of its detailed structure remains vague 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 so-called "infrared dark clouds" are in fact the "bones" of the Galaxy, marking out the true mid-plane of its disk to within less than a few parsecs. We call the long features "bones" thanks to recent numerical simulations of spiral galaxies that show a network of over-dense filaments within and between the arms that resemble an endoskeleton for a galaxy. The talk will highlight how both large surveys and new visualization tools have been critical in this investigation. In the end, my goal will be to convince the audience that the "Nessie" Infrared Dark Cloud is a nearly-continuous, many-hundreds-of-pc-long, few-pc-thick, structure, lying within a few pc of the mid-plane of the MIlky Way.
Tuesday Nov 6:
Star Formation in Extreme Environments near Supermassive Black Holes and in Massive Star Clusters
Jessica Lu
U Hawaii
Host: Deepto Chakrabarty

Abstract: The formation of stars is a fundamental astrophysical process; and yet we still debate whether it varies with environment. Milky Way young star clusters range in mass over four orders of magnitude; but, the best-studied star forming regions, such as Taurus and Orion, represent only a small range of initial conditions at the low mass end. Young star clusters with masses greater than 10,000 solar masses are promising targets for determining whether the initial mass function (IMF) that results from the star formation process is universal or depends on environment. Such clusters are challenging observational targets as they require high spatial resolution at infrared wavelengths and are heavily contaminated by field stars. I present results from a Keck adaptive optics and HST study of several massive young star clusters in the Milky Way, including around the supermassive black hole at the Galactic Center. IMFs are constructed by using high-precision astrometry and spectroscopy to distinguish individual cluster stars. I will discuss whether the measured IMFs differ for massive clusters at a range of Galactoentric radii and how they compare to the "universal" IMF established locally.
Tuesday Nov 13:
Molecular Probes of Protoplanetary Disks
Karin Oberg
U Virginia
Host: Anna Frebel

Abstract: Planets form in disks around young stars and the structures and compositions of these protoplanetary disks should shape the planet formation process. Small molecules provide exquisite probes of key disk characteristics that are inaccessible through other means, including kinematics, ionization levels, and the location of snowlines. Some of these molecules are also important in their own right, as tracers of the prebiotic evolution present during planet formation. I will discuss our latest results on how spatially resolved observations of molecular emission together with an increasing understanding of disk chemistry are shaping our understanding of protoplanetary disks. In the age of ALMA, molecular probes are predicted to play a transformational role and I will conclude with some of the exciting phenomena, in disks and elsewhere, that we will be able to address using ALMA's high spatial resolution, high sensitivity molecular line imaging capabilities.
Tuesday Nov 20:
G-CLEF - a versatile optical high-resolution echelle spectrograph for the Giant Magellan Telescope
Gabor Furesz
CfA
Host: Anna Frebel

Abstract: The next generation of optical telescopes are expected to see first light at the dawn of the next decade. Out of the three ongoing ELT projects only the GMT has selected an optical high-resolution spectrograph as a first light instrument. The G-CLEF echelle spectrograph, being built by a consortium led by the Harvard-Smithsonian Center for Astrophysics, has been designed to serve a broad array of science goals and projects, such as detection and characterization of Earth-like planets and measuring chemical abundances in stars in the Milky Way and dwarf satellite galaxies and globular clusters. Multiplexing capabilities are also being considered. After a short overview of these research programs I will describe the essential design aspects of the G-CLEF, focussing on the challenge of building an instrument that simultaneously fulfills the need of precision radial velocity measurements, good blue efficiency for stellar abundance work and also being capable of multi-object spectroscopy of very faint stars. Accordingly, we have developed a design that includes high internal efficiency, several resolution modes and a spectral coverage from 3500 to 9000 A and a multi-object capability of up to ~30 targets. Furthermore, I'll describe our efforts towards enabling radial velocities precision measurements at the 10 cm/s level. Even at a resolution of 100k, 10cm/s corresponds only to a physical shift in the recorded spectra as small as 1/10,000th of a pixel. This is comparable to the lattice constant of the silicon crystal in the detector, and within a 25 metric ton, mini-van sized instrument the slightest environmental change can easily produce a much larger signal.
Tuesday Nov 27:
Rare Isotopes in Cosmic Explosions and in the Laboratory
Hendrik Schatz
Michigan State
Host: Anna Frebel

Abstract: Rare isotopes play a critical role in a variety of astrophysical sites. Despite their fleeting existence of often less then a second, they imprint their properties onto the chemical composition of the universe, and the properties of stellar explosions. Progress has been made in astronomical observations of the signatures of rare isotopes and the chemical evolution of the Galaxy. Similar progress is imminent in understanding the relevant properties of rare isotopes through experiments at rare isotope beam accelerators and nuclear theory. I will review the important role that rare isotopes play in understanding stellar explosions, show some examples of laboratory measurements that can already be done today, and give an outlook for the prospects of this field with the Facility for Rare Isotope Beams currently being established at Michigan State University by the Department of Energy’s Office of Science.
Tuesday Dec 4:
The First Stars and Galaxies
Volker Bromm
U Texas
Host: Anna Frebel

Abstract: How and when did the cosmic dark ages end? I present simulations of the formation of the first stars and galaxies, discuss their feedback on the intergalactic medium, and describe ways to probe their signature with missions such as WMAP and the James Webb Space Telescope. The properties of the first stars are determined by the interplay between cold dark matter and the atomic and molecular physics of hydrogen. I will identify the key processes and outline the major remaining uncertainties.
Tuesday Dec 11:
Connecting the Light and Dark Side of Galaxy Formation
Piero Madau
UCSC
Host: Anna Frebel

Abstract: While the LCDM paradigm of cosmic structure formation has been tremendously successful at explaining the large scale distribution of matter in our universe, the mapping from dark matter halos to their baryonic components, to the properties of the galaxies embedded within halos, is far from straightforward and currently poorly understood. I will describe recent ultra-high resolution cosmological simulations in LCDM that lead to the formation of realistic late-type galaxies and appear to shed light on some key puzzles in the field. Such "zoom-in" simulations follow the dark, stellar, and gaseous matter components and include a number of critical ingredients like a blastwave scheme for supernova feedback, a star formation recipe based on a high gas density threshold, and metal-dependent radiative cooling.

This page is maintained by Anna Frebel