MIT Astrophysics Brown Bag Lunch Series - Summer/Fall 2013
Mondays at 12:05 PM in the Marlar Lounge
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
The Brown Bag lunch is a forum for visiting astronomers/astrophysicists to speak about their research. Talks begin at 12:05 and speakers should plan 40 minutes of material, to leave room for questions during and after the presentation. If you are visiting MIT and would like to give a lunch talk, you may either contact the organizers directly or have your local colleagues arrange a time. The present organizers of the series are Rob Simcoe, Nevin Weinberg, and Laura Lopez.
Monday September 23
The link between the cosmic near-IR and X-ray backgrounds: Signatures of the first black holes?
Host: Mike McDonald
The first galaxies and miniquasars will remain largely inaccessible to future telescopes, but may be detectable via their contribution to the Cosmic near-Infrared Background (CIB) anisotropies. A significant extragalactic clustering signal from an unresolved population is now well established by CIB fluctuation measurements. I will review the current status of the field, focusing on a recent discovery of a cross-correlation signal between the CIB fluctuations and the unresolved Cosmic X-ray Background. I present a population study of all galaxies and AGN and predict their contributions to the CIB and CXB including the mutual cross-correlation signal. I discuss the signal's potential for probing the early Universe in several upcoming CIB measurements, including LIBRAE, an ESA selected science program dedicated to CIB fluctuation exploration with the Euclid telescope.
Monday October 7
Modeling multiphase gas and sub-mm line emission in galaxies
Host: Rob Simcoe
Abstract: The star-formation activity of the Universe has gone through strong
changes from redshifts of z~3 until present day. Information about the
gas content of typical star-forming galaxies responsible for these
changes has so far largely been missing. In this talk I present several
theoretical approaches to make predictions for and support upcoming
observations of the gas content of high redshift galaxies with ALMA and
SKA. First, I present a novel method to indirectly estimate the HI and
H2 content of a sample of ~20000 galaxies covering a wide range of
redshifts and highlight the HI and H2 evolution in this sample of
galaxies. Second, I present a new model in which I explicitly track the
formation of molecular hydrogen and implement a physically-motivated
H2-based star formation recipe within a semi-analytic cosmological
galaxy formation model. I discuss our predictions for the atomic and
molecular content of galaxies, how they evolve and how they constrain
our understanding of galaxy formation and evolution. Third, I present a
combination of this model of galaxy formation with a three-dimensional
radiative transfer code. I discuss the CO, HCN, C, [CII], and [OI]
emission from galaxies and use this tool to study how the detailed gas
properties of galaxies during cosmic noon compares to galaxies in the
local Universe and make direct predictions for ALMA observations.
Monday October 14
No talk scheduled
SPECIAL DATE: Thursday October 17 at 2PM
The Regulation of Black Hole Winds and Jets Across the Mass Scale
University of Michigan
Host: Mike Nowak
Mechanical power in black holes of all masses is generated by both winds and jets. With Chandra grating spectra, we characterize outflows and find that the kinetic power of the winds scales with increasing bolometric luminosity, suggesting that supermassive black holes may be more efficient than stellar-mass black holes in launching winds. We also examine jet production, estimating the power required to inflate local bubbles. Consistency between wind and jet scaling relations suggests that a common launching mechanism may drive both flows. High-velocity outflows resemble jets more than the winds, suggesting that there is a regime in which winds become jets, with jet power dominant at low Eddington fractions, and winds dominant at high fractions.
Monday October 21
Simulating Damped Lyman-alpha Systems on a Moving Mesh
Host: Rob Simcoe
I will discuss recent work modeling the distribution of cold gas and metals around DLAs, using the moving mesh code Arepo.
DLAs are high column density HI absorption systems and probe directly the gas distribution in halos. Using synthetic metal and HI spectra extracted from recent Arepo simulations incorporating metal enrichment and feedback models, I will compare the properties of DLAs to high-redshift observations, focusing on the column density distribution function, the bias of DLAs to the linear matter power spectrum, and the metallicity distribution, showing that these are (mostly) in good agreement with reality.
Monday October 28
Asteroseismology of Intermediate Mass Stars
Host: Simon Albrecht
Helioseismology is a mature field that has led to remarkably accurate
knowledge of the solar interior. Recently, with the advent of the CoRoT
and Kepler space missions, asteroseismology of solar-like oscillators
has begun to make similar contributions to our knowledge of the
interiors of stars cooler than approximately 6500 K. This has been
achieved through the study of acoustic oscillations stochastically
excited in the outer convection zones of these stars.
Main sequence stars hotter than 6500 K are also known to pulsate, but
their oscillations are triggered by different mechanisms, which are more
challenging to understand. Their complexity is caused by the drastic
structural change in the stellar envelopes: the transition from deep and
effective to shallow convective envelopes takes place within the group
of A and F type stars. This has not only a deep impact on pulsational
stability but also on several other astrophysical processes, such as
rotation, activity, magnetic fields, mixing, diffusion and even the
alignment of planetary systems. In this talk I will show why
intermediate mass stars are important astrophysical laboratories and how
the Kepler and CoRoT satellites changed our understanding of these stars.
Monday November 4
Thermal SZ Results from ACT and Planck: New Constraints on Intracluster Gas and the Amplitude of Density Fluctuations
Host: Max Tegmark
The thermal Sunyaev-Zel'dovich (tSZ) effect arises from the inverse Compton scattering of CMB photons off hot electrons in the universe, which are predominantly found in the intracluster medium (ICM) of massive galaxy clusters. Because these objects are exponentially rare, the tSZ signal is extremely sensitive to the amplitude of density fluctuations, sigma_8. However, interpretation of the tSZ signal is complicated by uncertainties regarding how the ICM gas follows the matter distribution. I will present new results involving data from the Atacama Cosmology Telescope (ACT) and the Planck satellite that aim to shed light on these questions. First, I will describe the first detection of the cross-correlation of the tSZ field with the large-scale matter distribution traced by the Planck CMB lensing potential. This measurement provides initial clues about the amount of hot gas found in moderately massive groups and clusters (M ~ 10^13-10^14.5 M_sun). Second, I will describe the first measurement of the PDF of the tSZ field using ACT 148 GHz maps. This measurement contains information from all (zero-lag) moments of the tSZ field, beyond simply the 2- or 3-point functions. It is a very sensitive probe of sigma_8 and may also provide a method with which to break the degeneracy between sigma_8 and uncertainties in the physics of the ICM gas.
Monday November 11
No talk scheduled
Monday November 18
Advances in galaxy-formation simulations: calculating mock observables & using a more-accurate numerical technique
Host: Anna Frebel
Galaxy formation has been studied using idealized numerical
simulations of isolated disk galaxies and galaxy mergers for decades,
but most simulations performed to date have suffered from two
potentially significant limitations: First, when comparing simulations
with observations, physical quantities - rather than observables -
from the simulations are used. Second, the most-commonly used
techniques, smoothed-particle hydrodynamics (SPH) and adaptive mesh
refinement, suffer from numerical inaccuracies that can potentially
jeopardize the results of simulations performed with those techniques.
I will discuss methods for solving both of these limitations. I
address the first limitation by performing 3-D dust radiative transfer
on hydrodynamical simulations to calculate spatially resolved UV-mm
spectral energy distributions of simulated galaxies. I will present an
application to submillimeter galaxies, for which a realistic
comparison with observables yields results that are qualitatively
different from those of more naive comparisons. I address the second
limitation by using the more-accurate moving-mesh hydrodynamics code
Arepo. I will discuss how merger simulations performed with the
moving-mesh technique differ from otherwise identical simulations
performed using SPH. Finally, based on this comparison and other work,
I will outline the types of galaxy-formation simulations for which the
traditional formulation of SPH is sufficiently accurate and describe
when and why this is not the case.
Monday November 25
Host: Rob Simcoe
Monday December 2
Cosmology with the Secondary Anisotropies in the Cosmic Microwave Background
Host: Bryce Croll
Measurements of the primary anisotropies in the CMB have been the
backbone of modern precision cosmology. Recently, high resolution CMB
measurements from experiments, such as the Atacama Cosmology
Telescope, the South Pole Telescope, and the Planck satellite are
probing scales where the secondary anisotropies dominate over the
primary. I will focus on the secondary anisotropies caused by the
thermal and kinetic Sunyaev Zel'dovich effects. Our ability to obtain
cosmological information from these secondaries is limited by our
theoretical understanding of the baryons in the large-scale structure
between us and the primary CMB. I will present numerical simulations
that model these baryons and attempt to constrain various cosmological
parameters. Additionally, I will discuss the wealth of astrophysical
large-scale structure information (in particular galaxy cluster
astrophysics) that is interconnected with these secondaries.
Monday January 27
Atmosphere of Exo-Neptune HAT-P-11b
University of Maryland
Host: Mike McDonald
We present new observations of the transiting exo-Neptune HAT-P-11b from a joint HST and warm Spitzer program to measure the transmission spectrum of its thick atmosphere. Our data cover a wide span of wavelength space, including warm Spitzer IRAC 3.6 & 4.5 micron photometry and Hubble WFC3 1.1-1.7 micron spectroscopy from our observations, as well as Kepler's optical photometry centered at 632nm. Our WFC3 spectroscopic observations are among the first using HST's new spatial scanning mode for optimised signal-to-noise spectroscopy. In addition, HAT-P-11 is one of the most active planet-hosting stars; observations of HAT-P-11b's atmosphere therefore allow us to shed light on the role that stellar activity may play in shaping the atmospheric chemistry of Neptune-sized planets. We use the Kepler photometry to model and remove the effects of the stellar activity during and surrounding our warm Spitzer transit observations. Our combined observations provide constraints on the atmospheric chemistry at the day-night terminator and conclude in favor of a cloud-free atmosphere.
Thursday February 20 [NOTE SPECIAL TIME]
Thirty Meter Telescope: The Next Generation of Ground Based Optical/Infra Red Observatory
Thirty Meter Telescope Project
Host: Sara Seager
I will discuss some of the scientific capabilities that the Thirty Meter Telescope will provide and some of the areas of study that will benefit from the TMT's capabilities. I'll describe how the telescope design was developed to support a broad range of observing capabilities and how the observatory is being engineered. Finally I'll describe the avenues through which individuals can actively participate in the project and in planning for a potential TMT/NSF partnership.
Monday March 3
Known Unknowns of Dark Matter Annihilation over Cosmic Time
University of Melbourne
Host: Jackie Hewitt
Dark matter annihilation has the potential to affect early structure formation
by injecting energy into surrounding gas. I will discuss major unsolved
problems relevant to the abundance and structure of dark matter halos, and how
these uncertainties affect predictions of dark matter annihilation power over
cosmic time. [Paper: arXiv:1309.7783]
Monday March 10
Testing Quantum Mechanics and Bell's Inequality with Cosmological Observations
Host: Max Tegmark
We discuss a thought experiment which would leverage cosmology to test quantum mechanics using astronomical observations. Specifically, we aim to close the "settings-independence" loophole in experimental tests of Bell's inequality by choosing the detector settings (e.g. polarizer orientations) using real-time observations of causally disconnected cosmic sources. This would help close one of the most important remaining Bell test loopholes whereby a local hidden-variable theory could mimic the quantum predictions if the experimental settings choices shared even a small correlation due to unknown local causal influences prior to the experiment. The talk will focus on the theoretical cosmology constraints needed to choose optimal sources for such an experiment, describing general conditions for pairs of cosmic events with arbitrary redshifts and angular separations to have shared causal pasts in Friedman-Lemaitre-Robertson-Walker universes with arbitrary curvature, including flat, dark energy dominated, accelerating universes like our own. While causally disjoint patches of the cosmic microwave background radiation at redshift z ~ 1090 could be used to set the detectors, z > 3.65 quasars observed at optical wavelengths are arguably the optimal candidate source pairs using present technology that meet the condition of having no shared causal past since the end of any period of inflation, 13.82 Gyr ago. Results are illustrated for our universe with causal structure animations to help visualize the intersections of past light cones for arbitrary event pairs.
Monday March 17
Microlensing: Beyond Planet Detection
Microlensing uses the gravitational bending of light to detect exoplanets. Because the typical timescale of the events is ~20 days and the individual events are not repeated, microlensing is generally thought of as giving only an instantaneous snapshot of the planetary system. In the context of recent microlensing discoveries, I will show how additional data and the inclusion of higher order effects in the analysis enables us to more fully characterize these planetary systems. These techniques expand the scope of microlensing beyond planet detection and toward a better understanding of brown dwarfs, stellar remnants, and the mass function of the inner galaxy.
Monday March 31
Learning about interstellar dust from X-ray scattering
Host: Fred Baganoff
Interstellar (IS) dust grains are an integral part of IS evolution in galaxies, but our knowledge of their properties primarily derives from observations of the UV/optical extinction, infrared emission, and polarization from grains. We can also learn about grains by studying the X-ray halos seen around absorbed X-ray sources that are created by small-angle X-ray scattering in dust grains along the line of sight. X-ray halos are strongly affected by the size distribution of the grains, and to a lesser extent their position, composition, and shape. I will describe how X-ray observations with Chandra and XMM-Newton have been used to survey Galactic X-ray halos to answer the many outstanding questions about IS dust grains, such as the total grain mass, density, and composition. Combining this X-ray halo data with the other UV, optical, and IR observations have allowed us to place tight constraints on all dust models. By analyzing data from many lines of sight, we have characterized both the mean X-ray halo and the variation. This will include sightlines through dense clouds where UV/optical extinction cannot be measured, and in-depth studies towards lightly-absorbed sources where absorption lines can be used to characterize the sightline. With the upcoming launch of Astro-H, we expect an explosion in usable lines of sight for such work which should lead to a revolution in our understanding of IS dust grains.
Monday April 7
The metallicity of the z~2.5 circumgalactic medium
Host: Rob Simcoe
Observations of the gaseous halos of galaxies (the circumgalactic medium or CGM) are able to constrain important but poorly-understood mechanisms that govern galaxy formation, such as supernovae-driven outflows and gas inflowing along cold-accretion streams. I will present the first results from our group's survey of cool gas in the CGM of Lyman-break selected galaxies at z~2.5, near the peak of the cosmic star formation rate. By comparing the gas absorption features seen towards a background QSO sightline nearby a foreground galaxy to photo-ionization models we measure the metallicity of the gas, which is crucial to determine its origin. Using a new method that includes variations in the shape of the UV radiation field illuminating the gas, we derive robust metallicity measurements. We show that metal-enriched gas is found in the CGM up to ~100 kpc from foreground galaxies.
Monday April 14
Morphology of the Galactic Center with NuSTAR
Host: Fred Baganoff
The inner arcminutes of the Galaxy contains the highest concentration of high-energy sources in the Milky Way. The supermassive black hole, pulsar wind nebulae (PWN), supernova remnants, X-ray binaries, and hot interstellar gas are copious emitters of X-rays and gamma-rays. NuSTAR provides a view of the hard X-ray (3-79 keV) band, a critical bridge between the soft X-ray and gamma-ray emission, with unprecedented angular resolution. I will present the first sub-arcminute images of the Galactic Center above 20 keV, obtained with NuSTAR. The hard X-ray emission from the Galactic Center is dominated by a diffuse component extending along the Galactic plane and a strong point-like source, spatially consistent with the ultra-high energy gamma-ray emission detected by HESS. The dominance of these sources provides new insight into possible hard X-ray emitting populations near Sgr A*, including black hole LMXBs, CVs, neutron stars, PWN and non-thermal filaments, dark matter, and the hard X-ray source previously reported by INTEGRAL.
Monday May 5
Science with the Space Surveillance Telescope
MIT Lincoln Labs
Host: Rob Simcoe
This page is maintained by Rob Simcoe