Colloquia Archives

Stella Offner (Yale)
Thursday, February 7, 2013
3:45 p.m.
LGRT 1033
Title:
Symbiotic Star Formation: Modeling the Complex Ecology of Molecu
Abstract:
The details of star formation are intimately related to the natal molecular cloud environment. This environment is in turn shaped by radiative and kinematic feedback from embedded forming stars. In this talk, I will present gravito-radiation-hydrodynamic simulations of clustered star formation. I will explore how protostellar radiation and mass outflows impact the stellar initial mass function, multiplicity, and molecular cloud evolution. I will discuss the importance of producing "synthetic observations", for example by modeling dust and CO line emission, in order to connect numerical results directly to observables.
Smadar Naoz (CfA)
Tuesday, February 5, 2013
3:45 p.m.
LGRT 1033
Title:
The Origin of Retrograde Hot Jupiters
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 the 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.
Catherine Espaillat (CfA)
Thursday, January 31, 2013
3:45 p.m.
LGRT 1033
Title:
Characterizing Planet-Forming Disks Around Young Stars
Abstract:
Theoretical simulations predict that a young planet will interact with the accretion disk surrounding its host star, clearing the material around itself and leaving behind an observational signature in the form of a clearing in the disk. Circumstellar disks with large, inner holes have been detected and are known as "transitional disks." A few years ago, Spitzer identified a new class of "pre-transitional disks" which have gaps rather than holes - they have an inner disk, a gap, and an outer disk. These gapped disks are the strongest evidence to date for disk clearing by planets and serve as signposts to guide young planet searches in the ALMA era.
Kristian Finlator (University of California Santa Barbara)
Tuesday, January 29, 2013
3:45 p.m.
LGRT 1033
Title:
Galaxy Formation and Feedback During the Cosmic Dark Ages
Abstract:
The study of structure formation during the first billion years is advancing rapidly, driven by pathfinding discoveries with the Hubble Space Telescope and motivated by anticipated studies that will wield next-generation facilities including ALMA, JWST, and LOFAR. I will show how detailed comparisons between numerical simulations and observations such as these inform our understanding of galaxy growth well into the cosmic dark ages. I will discuss observational and theoretical arguments that constrain how young galaxies ionized and heated the intergalactic medium. I will describe how the intergalactic medium's structure regulated the progress of cosmological reionization. Finally, I will show how a new class of cosmological radiation hydrodynamic simulations can be used to interpret existing observations of low-ionization metal absorbers that trace the earliest stages of structure formation.
Viviana Acquaviva (New York City College of Technology)
Thursday, January 24, 2013
3:45 p.m.
LGRT 1033
Title:
From galaxies to cosmic acceleration
Abstract:
The accelerated expansion of the Universe could be due either to Dark Energy filling space, or to General Relativity failing on large scales. By studying the evolution and clustering of galaxies, we can probe both the expansion history and the linear growth of structure, thereby testing these two scenarios across cosmic time. I will present my efforts at improving our understanding of galaxy properties through Spectral Energy Distribution (SED) fitting, the process of comparing theoretical templates to observations in order to find which models best resemble the data. I will introduce GalMC, our publicly available Markov Chain Monte Carlo algorithm for SED fitting, and show how it can be used to recover the age, mass, dust content, metallicity and star formation history of galaxies, with reduced systematics. I will also present SpeedyMC, the fast kin of GalMC optimized for very large surveys, and describe our ongoing work as a joint analysis of photometric redshifts and SED fitting parameters. Finally, I will describe the science goals of the Hobby Eberly Telescope Dark Energy eXperiment (HETDEX), which will discover about a million Lyman Alpha Emitting galaxies at 2 < z < 3.5 and use them to shed light on the behavior of dark energy and gravity in this largely unexplored redshift range.
Aaron Evans (NRAO)
Thursday, April 26, 2012
3:45 p.m.
LGRT 1033
Title:
GOALS: The Great Observatories All-sky LIRG Survey
Abstract:
Luminous Infrared Galaxies (LIRGs) are observed primarily to be interacting and merging galaxies. They are the sites of rampant star formation and active galactic nuclei (AGN), which are fed by abundant supplies of molecular gas. However, the very property that led to their initial discovery as a significant population - their high infrared luminosity - also makes them difficult to study; the majority of the UV and optical light from young, massive stars and AGN is absorbed by obscuring dust and re-emitted in the infrared. The Great Observatories All-sky LIRGs Survey thus makes use of the diversity in wavelength coverage of the present space-based telescopes to probe the activity in a large (~ 100 - 200), flux-limited sample of LIRGs from the Revised Bright Galaxy Sample (RBGS). The majority of the talk will be devoted to discussing the survey as a whole. The latter part of the talk will be focussed specifically on a GOALS analysis of NGC 2623.
Stella Offner (CfA)
Thursday, April 12, 2012
3:45 p.m.
LGRT 1033
Title:
The Role of Episodic Accretion in Star Formation
Abstract:
A number of young stars, like FU Orionis, have been observed to experience short abrupt changes in their luminosity of an order of magnitude or more. These changes are commonly attributed to episodic fluctuations in the accretion rate onto the star. However, the characteristics, frequency and importance of these bursts in the star formation process are poorly constrained. Recently, episodic accretion has been proposed as the origin of the stellar age spread in low-mass stars inferred in young clusters. Episodic accretion has also been suggested as a solution for the protostellar "luminosity problem," wherein protostars are observed to be dimmer than predicted by star formation models. In this talk, I will give an overview of each of these issues and evaluate whether episodic accretion could provide a solution. I will also discuss some current and future observational programs that may help to constrain protostellar accretion histories.
Bruce Draine (Princeton)
Thursday, April 5, 2012
3:45 p.m.
LGRT 1033
Title:
Infrared, Submm, and Microwave Emission from Interstellar Dust
Abstract:
Interstellar dust radiates most powerfully in the far-infrared, typically peaking in the 100-200um region. However, the emission at longer wavelengths is often unexpectedly strong. This is particularly true at microwave frequencies (~cm wavelengths), where sensitive studies of the CMB revealed emission that was far stronger than expected. This "anomalous microwave emission" is almost certainly produced in part by dust grains (PAHs) spinning at tens of GHz, although other processes may also contribute. The emission at mm- and submm wavelengths has also been problematic. Some galaxies (e.g., the SMC) show much stronger emission near ~1 mm than expected from "normal" dust models. This is sometimes attributed to large masses of very cold dust, but more likely it is telling us about new emission processes. I will argue that much of the submm excess in the SMC may be magnetic dipole emission from iron nanoparticles. If true for the SMC, this may presumably apply to other low-metallicity galaxies as well.
Daniel Eisenstein (CfA)
Thursday, March 29, 2012
3:45 p.m.
LGRT 1033
Title:
Dark Energy and Cosmic Sound
Abstract:
I will discuss how the acoustic oscillations that propagate in the photon-baryon fluid during the first million years of the Universe provide a robust method for measuring the cosmological distance scale. The distance that the sound can travel can be computed to high precision and creates a signature in the late-time clustering of matter that serves as a standard ruler. Galaxy clustering results from the Sloan Digital Sky Survey reveal this feature, giving a geometric distance to a redshift of 0.35 and an accurate measurement of Omega_matter. I will review our recent work on the theory and practice of the acoustic oscillation method and our latest cosmology results from SDSS-II. I will then present SDSS-III, which will use the acoustic method to produce 1% distance measurements in order to map the curvature and expansion history of the Universe and measure the evolution of dark energy.
Alexey Vikhlinin (CfA)
Thursday, March 15, 2012
3:45 p.m.
LGRT 1033
Title:
On the long-term future of X-ray astronomy
Abstract:
Chandra has revolutionized X-ray astronomy by being a versatile observatory for studying objects ranging in scale from the inner structure of the Crab pulsar nebula to high-redshift AGNs, clusters of galaxies, and cosmology. Chandra is operating well and is expected to last into 2020's. In the next few years, we expect a launch of an array of smaller missions, which will open new areas of X-ray astronomy such as polarimetry, imaging at E>10 keV, sensitive surveys, high-resolution X-ray spectroscopy, ensuring a continues short-term vibrancy of the field. However, in the post-Chandra era, X-ray astronomy cannot be sustained by small-scale experiments, and there are no concrete plans for a powerful, observatory-class mission. What an X-ray observatory for the 2020's can look like? We are developing a concept, SMART-X, for a next-generation X-ray observatory with large-area, 0.5" resolution grazing incidence adjustable X-ray mirrors, high-throuput critical transmission gratings, and X-ray microcalorimeter and CMOS-based imager in the focal plane. High angular resolution is enabled by new technology based on controlling the shape of mirror segments using thin film piezo actuators deposited on the back surface. Science application include observations of growth of supermassive black holes sinse redshifts of ~10, ultra-deep surveys overs 10's of square degrees, galaxy assembly at z=2-3, as well as new opportunities in the high-resolution X-ray spectroscopy and time domain.
Caleb Scharf (Columbia)
Thursday, March 8, 2012
3:45 p.m.
LGRT 1033
Title:
Outstanding questions for exoplanetary science
Abstract:
Exoplanetary science has gone from non-existence to a burgeoning field in less than 20 years. I'll present an overview of where it stands now, from the extraordinary abundance of exoplanets to the remarkable diversity of system architectures and planetary characteristics. Many huge questions remain, and I'll discuss some work on planet formation and the nature of terrestrial-like worlds, including the application of new computer technology to planet discovery and system simulation.
David Hogg (NYU)
Thursday, March 1, 2012
3:45 p.m.
LGRT 1033
Title:
Finding the dark matter
Abstract:
One of the principal goals of Gaia and surveys like it is to infer the density map and formation history of the Milky Way. I present a few toy problems in which we use phase-space information for a snapshot of tracer particles to infer the matter density within the system. All known methods for performing this inference make multiple assumptions that are known to be false for the Galaxy, and few have made proper use of the observational noise model. I will express some optimism for methods that make use of cold phase-space structures and other kinds of rare but informative tracers.
Michael Kuhlen (UC Berkeley)
Tuesday, February 28, 2012
3:45 p.m.
LGRT 1033
Title:
Computational Cosmology and Galaxy Formation
Abstract:
Fueled by continuing advances in numerical methods and computational capabilities, the future of galaxy formation theory is going to be driven by numerical simulations. Yet computational galaxy formation is extremely challenging, owing to the multitude of important physical processes and the wide range of scales over which they operate. Much of the galaxy formation simulation work to date has relied on simple, and often ad-hoc, subgrid models for star formation and feedback. In this talk I will describe my recent efforts to improve this situation by including more realistic and physics-driven treatments of some of the relevant processes. As one example, I will discuss cosmological adaptive mesh refinement simulations in which star formation is regulated by the local abundance of molecular hydrogen. These simulations reproduce much of the observational phenomenology of star formation rates as a function of atomic and molecular gas content and metallicity. At the same time this new piece of physics leads to a suppression of the stellar content of low mass dark matter halos, thereby helping to explain the vexing dwarf galaxy problem.
Mark Krumholz (UC Santa Cruz)
Thursday, February 23, 2012
3:45 p.m.
LGRT 1033
Title:
The Origin of the IMF
Abstract:
Star-forming environments vary by orders of magnitude in density, pressure, metallicity, and other properties, yet the initial mass function (IMF) of the stars they produce remains stubbornly unchanged. Explaining the origin and universality of the IMF is one of the oldest problems in theoretical astrophysics, but in the last few years theoretical advances in understanding how gas fragments, together with algorithmic advances that have allowed simulations to include improved physics such as radiative transfer, have produced significant progress. I describe these advances, and lay out the beginnings of a theoretical model capable of explaining the IMF. This model suggests that the IMF is nearly but not perfectly universal, and the subtle variations that it admits provide avenues for future observational tests.
Daniel Wang (UMass, Amherst)
Tuesday, February 21, 2012
3:45 p.m.
LGRT 1033
Title:
Stellar Feedback and Galaxy Evolution
Abstract:
TBA
Alyson Brooks (U. Wisconsin - Madison)
Thursday, February 16, 2012
3:45 p.m.
LGRT 1033
Title:
Toward the Formation of Realistic Galaxies
Abstract:
Much progress has been made in recent years in forming realistic disk galaxies in fully cosmological simulations. Computational advances have allowed for unprecedented resolution, which in turn allows for a more realistic treatment of star formation and energy feedback. These improvements have led to a new examination of gas accretion, consumption, and loss in the formation of galaxy disks. I will show that a more realistic treatment of gas in simulated disk galaxies leads to a better match with observational results as a function of redshift. I will demonstrate that because star formation (i.e., gas consumption) varies with galaxy mass, the structure of dark matter within galaxies varies with mass. This leads to observable scaling relations as a function of galaxy mass, and resolves a number of long standing challenges within the CDM model. Realistic simulated galaxies are the necessary starting point for interpreting observations in light of galaxy formation theory.
Romeel Dave (U. Arizona)
Thursday, February 9, 2012
3:45 p.m.
LGRT 1033
Title:
Baryon Cycling: A Modern View of Galaxy Evolution
Abstract:
Traditionally, galaxy formation theory has been based on connecting observable baryons to dark matter halos and their merger history. But in the past decade, hydrodynamic simulations of galaxy formation have elucidated a somewhat different view, in which galaxy growth is governed by an evolving balance between continual inflow from the intergalactic medium and strong, ubiquitous outflows. In this talk I will discuss the formalism of baryon cycling and the simulations that led to it, and highlight some of the new insights and interpretations that it yields for the stellar, gas, and metal content of galaxies. Of central importance in baryon cycling is the role of circum-galactic gas in regulating galaxy growth, and I will present results from simulations detailing this connection and its observational implications. I will argue that in the coming years, the holistic study of galaxies and their surrounding gas will be the key to understanding galaxy evolution across cosmic time.
Andrew Benson (Caltech)
Thursday, February 2, 2012
3:45 p.m.
LGRT 1033
Title:
Galaxy Formation Theory: The Next Decade
Abstract:
I will present my view of how the field of galaxy formation theory should, and must, progress over the next decade to keep pace with expected observational advances. I will describe my current work aimed at establishing robust modelling techniques that can generate detailed realizations of astronomical datasets, and will discuss the challenges that must be met to reach the primary goal of my research program: inferring the underlying physical mechanisms of galaxy formation from observational measures.
Lars Hernquist (CfA)
Thursday, January 26, 2012
3:45 p.m.
LGRT 1033
Title:
Cosmology on a Moving Mesh
Abstract:
Understanding the formation and evolution of galaxies in a cosmological 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 quasi-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.
Els Peeters (University of Western Ontario)
Thursday, December 8, 2011
3:45 p.m.
LGRT 1033
Title:
PAH emission features as astrophysical probes
Jiangtao Li (UMass)
Thursday, December 1, 2011
3:45 p.m.
LGRT 1033
Title:
CHANDRA SURVEY OF GALACTIC CORONAE AROUND NEARBY EDGE-ON DISK GA
Abstract:
CHANDRA SURVEY OF GALACTIC CORONAE AROUND NEARBY EDGE-ON DISK GALAXIES
Pete Schloerb (UMass)
Thursday, November 17, 2011
3:45 p.m.
LGRT 1033
Title:
LMT
Caleb Fassett (Mt. Holyoke)
Thursday, November 10, 2011
3:45 p.m.
LGRT 1033
Title:
TBA
Sugata Kaviraj (Imperial College London)
Thursday, November 3, 2011
3:45 p.m.
LGRT 1033
Title:
Early-type galaxies: the last 8 billion years
Jen Andrews (UMass)
Thursday, October 27, 2011
3:45 p.m.
LGRT 1033
Title:
The Formation of Dust in Core Collapse Supernovae
Lynn Mathews (MIT Haystack Observatory)
Thursday, October 20, 2011
3:45 p.m.
LGRT 1033
Title:
Tails of Stellar Mass Loss: The HI 21-cm Line as a Probe of the
Abstract:
Tails of Stellar Mass Loss: The HI 21-cm Line as a Probe of the Late Stages of Stellar Evolution
Desika Narayanan (University of Arizona)
Thursday, October 13, 2011
3:45 p.m.
LGRT 1033
Title:
The Efficiency of Star Formation in High Redshift Galaxies
Kristen Coppin (McGill University)
Thursday, October 6, 2011
3:45 p.m.
LGRT 1033
Title:
Probing the Evolutionary Sequence of the Formation of Massive Galaxies
Abstract:
Probing the Evolutionary Sequence of the Formation of Massive Galaxies
Jim Geach (McGill University)
Thursday, September 29, 2011
3:45 p.m.
LGRT 1033
Title:
The Evolution of the Molecular Gas Fraction of Star-Forming Galaxies
Abstract:
The Evolution of the Molecular Gas Fraction of Star-Forming Galaxies
Ben Oppenheimer (Leiden Observatory)
Thursday, September 22, 2011
3:45 p.m.
LGRT 1033
Title:
A Full Confrontation of the Low-Redshift Galaxy-Intergalactic Medium Relationship
Abstract:
A Full Confrontation of the Low-Redshift Galaxy-Intergalactic Medium Relationship

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