MIT Astrophysics Colloquia - Spring 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.



Regular Spring 2012 Astrophysics Colloquia will begin March 6
Tuesday March 6:
Gleaming through the Dark: Passive Galaxies as Probes of Dark Matter & Dust
Genevieve Graves
UC Berkeley
Host: Nevin Weinberg

Abstract: Galaxies---and particularly massive, passive, early type galaxies---obey a large number of tight scaling relations. These scaling relations not only provide important clues about galaxy evolution, they also make passive galaxies useful tools for studying other astrophysical phenomena. In this talk, I present two separate studies that make use of a detailed understanding of galaxy scaling relations in order to probe "dark" components of our universe: cold dust and dark matter. In the first project, we compare predictions for the intrinsic colors of massive galaxies to their observed colors in order to map out variations in Milky Way dust absorption. We detect coherent, statistically-significant deviations from the standard Schegel-Finkbeiner-Davis dust maps. These deviations correlate with the local dust temperature, suggesting that the dust model used to convert IR flux into dust absorption maps needs revision. In the second project, we use a photometry-only version of the Fundamental Plane of early type galaxies to measure magnification due to weak gravitational lensing. By using a "photoFP" relation to predict intrinsic galaxy radii, we are able to make magnification measurements that have substantially higher S/N per source than previous magnification techniques. Combining this new magnification method with existing weak lensing techniques will maximize the science return from upcoming major lensing surveys, including the Dark Energy Survey, the Hyper-Suprime Cam Survey, LSST, and Euclid.
Tuesday March 13:
Keck Spectroscopy of Faint Lyman Break Galaxies with 3 < z < 8: Implications for Cosmic Reionization
Richard Ellis
Caltech
Host: Nevin Weinberg

Abstract: Did star-forming galaxies reionize the Universe? Deep imaging with Hubble Space Telescope has provided the first broad census of the ultraviolet luminosity distribution of galaxies in the first billion years of cosmic history. But determining the redshifts and intrinsic properties of these sources represents a formidable challenge. I will summarize the status of an ambitious spectroscopic campaign being undertaken at the Keck observatory to address the question of whether early galaxies were largely responsible for cosmic reionization.
Tuesday March 20:
A monadic portrait of dynamical systems: From moons and exo-planets to star clusters and massive black holes
Hagai Perets
CfA and Technion
Host: Nevin Weinberg

Abstract: Many-body gravitating systems dominated by a massive central object are ubiquitous at all scales. Examples range from the satellites of Kuiper Belt objects, to the moons and planets in the Solar system, exoplanet systems, multiple stellar systems and up to stars near massive black holes (MBH). I will touch upon some of the physical processes underlying the dynamical evolution of such systems, and show how similar processes manifest themselves in different systems. Among these are dynamical relaxation through scattering, long term secular and coherent relaxation processes (Kozai-Lidov evolution, resonant relaxation), tidal disruption, mass loss, tidal friction, gravitational wave emission, as well as moon/planet/star formation and migration, and dynamical ejection and capture of objects. These give rise to extraordinary similarities in the structure and evolution of such systems. Such a “unified” picture includes a cold disk like component around the central object (e.g. planetary rings and the regular moons, the Solar system and planetesimal disks, the stellar disks in the Galactic center), a heated population of objects on inclined and eccentric objects (e.g. irregular moons, many exoplanet systems, the cusp and B-stars in the Galactic center), a population of peculiar objects on orbits very close to the central mass (e.g. Triton orbiting Neptune, Hot Jupiters in Extrasolar systems, close inner binaries in triple systems, the S-stars near the MBH in the Galactic center), and a population of ejected bodies (e.g. runaway moons, free floating planets, hypervelocity stars). As I'll show, in many cases these similarities can originate from similar processes, although the differences can give rise to different manifestations; scaling up/down of the same processes could therefore help us learn about the dynamics of the systems and offer clues about the evolution of objects as small as comets to bodies as massive as nuclear black holes.
Tuesday March 27:

No Colloquium: Spring Break


Abstract:
Tuesday April 3:
Galaxy Evolution: Progress Towards Understanding the Role of Baryonic Gas
Chris Churchill
New Mexico State University
Host: Nevin Weinberg

Abstract: In the Lambda-CDM paradigm, as dark matter halos develop, baryonic gas accretes onto the halos from the intergalactic cosmic web of baryonic gas. Galaxy evolution itself, including the resulting observed range of galaxy properties, and the global star formation rate of the universe, are governed in large part by the dissipative response of baryonic gas during its journey to becoming bound within a dark matter potential well. In general, this response is governed by the trade off between the cooling and dynamical timescales. Three distinct modes of accretion onto halos are theorized: the "hot" mode", the "cold mode", and the "hot+cold" mode. The mode is most strongly dependent upon the dark matter halo mass, with some dependence on galaxy environment (local overdensity). It is postulated that the universe of galaxies can be understood in the context of the physics governing these modes and their secondary processes, such as subsequent feedback (winds) from the formed galaxy.

After providing a brief review of the theoretical scenario of how baryons respond to dark matter halos and under what conditions gas accretes onto the galaxy itself versus onto the halo only, we show new, highly detailed observational results that both support and contradict the theoretical predictions. An expanded campaign is described, requiring HST, in which we aim to observationally test the theories with a large sample of detailed, case-by-case examples.
Tuesday April 10:
Tidal Disruption of Stars by Massive Black Holes
Enrico Ramirez-Ruiz
UC Santa Cruz
Host: Nevin Weinberg

Abstract:
A star interacting with a massive black hole cannot be treated as a point mass if its gets so close to the black hole that it becomes vulnerable to tidal distortions and even disruption. When a rapidly changing tidal force starts to compete with a star's self-gravity, the material of the star responds in a complicated way, being stretched along the orbital direction, squeezed at right angles to the orbit and strongly shocked. This phenomenon poses an as yet unmet challenge to computer simulations. The art of modeling tidal disruption of stars forms the main theme of my talk. Detailed simulations should tell us what happens when stars of different types get tidally disrupted, and what radiation a distant observer might detect as the observational signature of such events.
Tuesday April 17:

No Colloquium: Patriots Day


Abstract:
Tuesday April 24:
The Physics of Galaxy Cluster Plasmas
Eliot Quataert
UC Berkeley
Host: Nevin Weinberg

Abstract: Galaxy clusters are among the largest gravitationally bound objects in the universe. The majority of the baryonic mass in clusters resides in a hot, low density plasma that pervades the intracluster medium (rather than in stars). The heating and cooling processes in this plasma must be understood in order to make progress on a number of key problems in galaxy formation, including the formation of the most massive galaxies and black holes in the universe. An understanding of galaxy cluster thermodynamics is also important for the use of clusters as cosmological probes into the nature of dark matter and dark energy. In this talk, I will describe new insights into the physics of galaxy cluster plasmas, focusing on (1) the physics of novel forms of convection that arise in dilute plasmas, and (2) the subtle imbalance between heating and cooling processes that leads to the development of cool gas embedded in the ambient hot intracluster medium. I will also describe the importance of this physics for galaxy formation and for the use of clusters as cosmological probes.
Tuesday May 1:
The Theory of Core-Collapse Supernovae: Where we stand and where we are heading.
Christian Ott
Caltech
Host: Nevin Weinberg

Abstract: Core-collapse supernovae from massive stars are among the most energetic events in the universe. They liberate a mass-energy equivalent of ~15% of a solar mass in the collapse of their progenitor star's core. The majority (~99%) of this energy is carried away by neutrinos, while (~1%) is transferred to the kinetic energy of the explosive outflow. A smaller, yet still tremendous amount of energy is emitted in electromagnetic and gravitational waves. In 1934, Baade and Zwicky proposed that a "[...] super-nova represents the transition of an ordinary star into a neutron star". 78 years and one core-collapse supernova (SN 1987A) in the Large Magellanic Cloud later, our understanding of core-collapse supernovae is still very incomplete. In particular, the details of the physical process(es) responsible for converting the gravitational energy liberated in collapse into energy of the explosion are still uncertain. I review the current state of core-collapse supernova theory and discuss recent progress made and insights gained from new multi-dimensional computational models. These models are yielding new predictions for the signature of core-collapse supernovae in neutrinos and gravitational waves and I outline how their observation from the next nearby core collapse event can shed light on the supernova mechanism. While a key goal is to understand the mechanism driving regular supernova explosions, a core-collapse supernova is not always the outcome of collapse. I delineate the ensemble of alternative outcomes and present new results on stellar-mass black hole formation and on the core-collapse supernova -- long GRB connection.
Tuesday May 8:
Massive star forming galaxies at the peak of the galaxy formation epoch
Reinhard Genzel
MPE Garching and UC Berkeley
Host: Nevin Weinberg

Abstract: I will discuss the results of three major programs of studying star formation, cold gas, feedback and dynamics of massive 'normal' star forming galaxies near the peak of the epoch of galaxy formation (z~1-3). Our observations, carried out with the IRAM Plateau de Bure interferometer and with two large instruments developed at MPE (the VLT near-IR integral field spectrometer SINFONI and the far-IR spectrometer/photometer PACS on Herschel) show that massive galaxies near the star formation-stellar mass 'main-sequence' were gas rich, highly turbulent and clumpy, disky systems with various degrees of rotational support. Star formation in these galaxies was plausibly driven by continuous, rapid accretion of gas and minor mergers from the cosmic web. The evolution of their disks and central bulges was probably strongly influenced by disk fragmentation and instabilities, as well as by powerful galactic outflows driven from the large star forming clumps. I will discuss the impact of these new observations on our understanding of galaxy evolution in the early Universe.
Tuesday May 15:
PAPER: the Precision Array for Probing the Epoch of Reionization
James Aguirre
University of Pennsylvania
Host: Nevin Weinberg

Abstract: Between the last-scattering surface of the CMB at 400 kyr after the Big Bang to the appearance of the first quasars at around 900 Myr after the Bang, we have precious little direct observational evidence for the formation process of stars and galaxies. During this period, however, the first stars and galaxies are known to produce enough ionizing radiation to completely ionize the initially neutral intergalactic medium. I will discuss current efforts to detect the reionization epoch using the Precision Array for Probing the Epoch of Reionization (PAPER). PAPER is a focused experiment comprising an array of 128 dual-polarization antennae operating from 100 – 200 MHz to detect the redshifted signal from the hyperfine 21 cm transition of neutral hydrogen. PAPER employs a unique strategy for optimizing its sensitivity to the power spectrum of fluctuations. I will discuss the instrument and observing strategy and summarize the current state.

This page is maintained by Nevin Weinberg