KICP Seminars & Colloquia, Winter 2008

Seminar schedule for Winter 2008
January 4, 2008
Friday noon seminar
Stefan Hilbert
Max Planck Institute for Astrophysics
Ray-tracing through the Millennium Simulation   [Abstract]
January 9, 2008
Wednesday colloquium
Christopher McKee
UC Berkeley
Star Formation in Galaxies   [Abstract]
January 11, 2008
Friday noon seminar
Tony Wong
University of Illinois at Urbana-Champaign
Atomic and Molecular Gas in Galaxies: The Case of the LMC   [Abstract]
January 16, 2008
Astronomy Colloquium
Vasiliki Pavlidou
KICP, University of Chicago
Deciphering the Gamma-ray Sky: GeV Astronomy in the Era of GLAST   [Abstract]
January 18, 2008
Friday noon seminar
Martin K. Pohl
Iowa State University
Energetic particles and turbulence in the Universe   [Abstract]
January 23, 2008
Wednesday colloquium
Dmitri A. Uzdensky
Princeton University
Magnetized Coronae of Stars and Accretion Disks   [Abstract]
January 25, 2008
Friday noon seminar
Joe Silk
January 30, 2008
Astronomy Colloquium
Yoram Lithwick
Dynamics of Protoplanetary Disks: Vortices and Turbulence   [Abstract]
February 1, 2008
Friday noon seminar
Mark C Wyman
Perimeter Institute for Theoretical Physics
CMB B-mode polarization from Cosmic Strings   [Abstract]
February 6, 2008
Astronomy Colloquium
Philip Chang
University of California, Berkeley
Star Formation and Gasdynamics in the Nuclear Regions of Galaxies   [Abstract]
February 8, 2008
Friday noon seminar
Christian Reichardt
UC Berkeley
Recent Results from ACBAR   [Abstract]
February 13, 2008
Astronomy Colloquium
Avery Broderick
The University of Toronto
At the horizon: Imaging black holes and testing General Relativity   [Abstract]
February 15, 2008
Friday noon seminar
Justin Khoury
Perimeter Institute for Theoretical Physics
Gravity on the Largest Scales and Cosmic Acceleration   [Abstract]
February 20, 2008
Wednesday colloquium
Ann Zabludoff
Unversity of Arizona/Steward Observatory
The Importance of Lens Environments   [Abstract]
February 22, 2008
Friday noon seminar
Greg Bryan
Columbia University
Blowing Hot and Cold on Galaxy Clusters   [Abstract]
February 27, 2008
Wednesday colloquium
Natalia Ivanova
The University of Toronto, CITA
X-ray binaries: puzzle bonanza   [Abstract]
February 29, 2008
Friday noon seminar
Steven Furlanetto
Observing the First Galaxies and the Reionization Epoch   [Abstract]
March 5, 2008
Wednesday colloquium
Xiaohui Fan
University of Arizona
The Highest Redshift Quasars and the End of Reionization   [Abstract]
March 7, 2008
Friday noon seminar
Yu Qingjuan
University of California at Santa Cruz
Hypervelocity Stars: What They Can Tell Us About the Galactic Center and Halo   [Abstract]
March 12, 2008
Astronomy Colloquium
Martin Houde
The University of Western Ontario
Probing the Turbulence Dissipation Range and Magnetic Field Strengths in Molecular Clouds   [Abstract]
March 14, 2008
Friday noon seminar
Mark R Krumholz
Princeton University
From Massive Cores to Massive Stars   [Abstract]
March 19, 2008
Astronomy Colloquium
Noel Swerdlow
The University of Chicago
Kepler's Cosmology and Proof of the Heliocentric Theory   [Abstract]

  • January 9, 2008 | 3:30 PM | RI 480
    Star Formation in Galaxies
    Christopher McKee, UC Berkeley

    Star formation is at the nexus of astrophysics: stars are believed to be responsible for the re-ionization of the universe, they created all the heavy elements, they control the formation and evolution of galaxies, and the formation of stars naturally leads to the formation of planets. It is therefore critical to understand the rate at which stars form. Observations of the star formation rate are encapsulated in the Kennicutt-Schmidt relations between the star formation rate and the amount of gas in a galaxy. Observations have also shown that star formation is inefficient. This inefficiency, and the Kennicutt-Schmidt relations, are naturally explained in the theory of turbulence-regulated star formation (Krumholz & McKee 2005). Star formation in turn is a major driver of turbulence in giant molecular clouds, the sites of star formation. The observed star formation rate is generally inferred from observations of the ionized gas produced by massive stars. However, massive stars can form only in regions of high surface density (~ 1 g/cm^2 or greater). This threshold for massive star formation leads to a truncation of the initial mass function at high masses in the outer parts of galaxies, and can account for the difference in the star formation rates inferred from observations of UV continuum emission by GALEX and of Halpha emission from ionized gas.
  • January 23, 2008 | 3:30 PM | RI 480
    Magnetized Coronae of Stars and Accretion Disks
    Dmitri A. Uzdensky, Princeton University

    In this talk I will paint a general physical picture of magnetically-active astrophysical coronae, relevant to both stars and accretion disks. I first present a statistical theory of a turbulent accretion disk's magnetically-dominated corona, represented by an ensemble of loops. Each loop constantly evolves under the Keplerian shear, radnom footpoint motions on the disk surface, and reconnection with other loops. I construct and then solve numerically a kinetic equation for the loop distribution function. I use the resulting steady state solution to calculate the vertical distribution and the total amount of the magnetic energy in the corona, the overall coronal torque, etc. I demonstrate that these quantities grow strongly with increased strength of Keplerian shear relative to reconnection. This underscores the importance of reconnection in accretion disk coronae. In the second part of my talk I argue that recent advances in magnetic reconnection enable significant progress in understanding of magnetized coronae as self-regulating, marginally collisioness systems. I propose a novel view on the coronal heating problem, based on an interplay of two effects: (1) transition between the slow collisional and the fast collisionless reconnection regimes and (2) plasma evaporation from the star's or disk's surface due to coronal energy release. I show how these ideas work both for the solar corona and for the inner part of a black hole accretion disk corona.
  • February 20, 2008 | 3:30 PM | RI 480
    The Importance of Lens Environments
    Ann Zabludoff, Unversity of Arizona/Steward Observatory

    Analyses of strong lensing galaxies have provided constraints on quantities as important and varied as the Hubble constant and substructure in the dark matter halos of galaxies. However, the environments in which most lens galaxies lie are not well-known and may contribute significantly to the lensing potential. Not including the environment in lens models can introduce uncertainties and biases of 5-20%, preventing lensing constraints from competing with those derived from CMB or distance ladder measurements. I will describe results from our theoretical modeling of environment effects and from our five-year observational survey of the fields of 70 lenses. We find that the majority of lenses lie in overdense environments -- poor groups or rich clusters of galaxies -- that perturb lens models. We are using these data not only to identify "golden lenses" whose properties provide useful cosmological constraints, but also to measure the evolution of groups and clusters over the large redshift range probed by our sample. I will discuss some of our recent results measuring the baryon content of nearby groups and clusters, which suggest that our lens survey will ultimately allow us to explore baryon evolution in such common environments.
  • February 27, 2008 | 3:30 PM | RI 480
    X-ray binaries: puzzle bonanza
    Natalia Ivanova, The University of Toronto, CITA

    The abundance of X-ray binaries in our and other galaxies and their brightness at many wavelengths make them key to our understanding of different physical processes (accretion and gravitational waves radiation) and of advanced questions in stellar evolution (the origin and evolution of binary systems, the final stages of massive stars, the formation of compact objects). Within the next decade, observations at existing and planned facilities in almost all wavelengths will put a heavy demand on further studies of compact binaries. In my talk, I will discuss what is our current knowledge of X-ray binaries formation and evolution, where we had a progress and where we have failed, what are new breakthrough ideas. Specific attention will be given to globular clusters, where dynamical interactions between objects lead to the enhanced formation of X-ray binaries. Their study surprisingly helped to understand the field population of X-ray binaries and millisecond pulsars better. Other topics to be discussed are whether we can put constraints on the stellar evolution by comparing the observed populations and theoretical results, which uncertainties in the theory seem to play the most important role and what questions must be answered in the future.
  • March 5, 2008 | 3:30 PM | RI 480
    The Highest Redshift Quasars and the End of Reionization
    Xiaohui Fan, University of Arizona

    Luminous quasars at high redshift provide direct probes of the evolution of supermassive black holes (BHs) and the intergalactic medium (IGM) at early cosmic time. Over the last six years, more than 20 quasars have been discovered at z>6 from various wide-field surveys. Two main results emerge from the studies of these quasars. Detections of such objects indicate the existence of billion M_sun BHs merely a few hundred Myrs after the first star formation in the Universe. Theyare surrounded by metal-enriched gas and young galaxies with intense star formation, providing the strongest constraints on early growth of supermassive BHs and co-evolution with their host galaxies. Meanwhile, Absorption spectra of the highest redshift quasars reveal complete Gunn-Peterson absorption, indicating a rapid increase in the IGM neutral fraction, marking the end of the reionization epoch at z~6, suggesting a peak of reionization activity and emergence of the earliest galaxies and AGNs at 67 will likely produce the first dection of quasars in this crucial epoch in the next few years.


  • January 4, 2008 | 12:00 PM | LASR Conference Room
    Ray-tracing through the Millennium Simulation
    Stefan Hilbert, Max Planck Institute for Astrophysics

    Gravitational lensing is playing an increasingly important role in astrophysics and cosmology. In collaboration with people from MPA and Bonn, I investigate gravitational lensing by carrying out ray-tracing through the Millennium Simulation, one of the largest simulations of cosmic structure formation.

    In this talk, I will present results for the statistics of strong lensing. Furthermore, I will talk about our efforts towards realistic simulations of weak galaxy-galaxy lensing and cosmic shear surveys. Finally, I will discuss the potential capabilities of future radio telescopes for imaging the cosmic matter distribution.
  • January 11, 2008 | 12:00 PM | LASR Conference Room
    Atomic and Molecular Gas in Galaxies: The Case of the LMC
    Tony Wong, University of Illinois at Urbana-Champaign

    I discuss the role of the molecular gas (or perhaps more accurately, the dense gas) fraction in the evolution of galaxies, with emphasis on recent data from the Large Magellanic Cloud (LMC).
  • January 18, 2008 | 12:00 PM | LASR Conference Room
    Energetic particles and turbulence in the Universe
    Martin K. Pohl, Iowa State University

    The Universe is filled with energetic particles that are collectively referred to as cosmic rays. It appears that efficient acceleration of cosmic rays proceeds in systems with outflow phenomena, in which a fraction of the flow kinetic energy can be transferred to cosmic rays. One class of those systems are shell-type supernova remnants (SNR). The question of cosmic-ray acceleration in SNRs includes aspects of the generation, interaction, and damping of magnetic turbulence in non-equilibrium plasmas. I will report on recent results of kinetic simulations, that may shed light on the strange symbiosis of cosmic rays and magnetic turbulence.
  • January 25, 2008 | 12:00 PM | LASR Conference Room
    Joe Silk, Oxford

  • February 1, 2008 | 12:00 PM | LASR Conference Room
    CMB B-mode polarization from Cosmic Strings
    Mark C Wyman, Perimeter Institute for Theoretical Physics

    Detecting the parity-odd, or B-mode, polarization pattern in the cosmic microwave background radiation due to primordial gravity waves is considered to be the final observational key to confirming the inflationary paradigm. The search for viable models of inflation from particle physics and string theory has (re) discovered another source for B-modes: cosmic strings. Strings naturally generate as much vector mode perturbation as they do scalar, producing B-mode polarization with a spectrum distinct from that expected from inflation itself. In a large set of models, B-modes arising from cosmic strings are more prominent than those expected from primordial gravity waves. In light of this, we study the physical underpinnings of string- sourced B-modes and the model dependence of the amplitude and shape of the $C_l^{BB}$ power spectrum. Observational detection of a string-sourced B-mode spectrum would be a direct probe of post-inflationary physics near the GUT scale. Conversely, non-detection would put an upper limit on a possible cosmic string tension of Gmu < 10^{-7} within the next three years.
  • February 8, 2008 | 12:00 PM | LASR Conference Room
    Recent Results from ACBAR
    Christian Reichardt, UC Berkeley

    The Arcminute Cosmology Bolometer Array Receiver (ACBAR) is a multi-frequency 16-element bolometer array which observed from the 2m Viper telescope at the South Pole during the 2001, 2002, 2004 and 2005 Austral winters. ACBAR's small (5') beams allow it to probe the damping tail of the cosmic microwave background (CMB) power spectrum, making it highly complementary to experiments at larger angular scales such as WMAP and Boomerang. I will present recent results from the complete set of ACBAR's CMB temperature anisotropy observations. We include new data from the final 2005 observing season, expanding the number of detector-hours by 210% and the sky coverage by 490% over the data set used in the previous ACBAR release. As a result, the band-power uncertainties have been reduced by more than a factor of two on angular scales encompassing the third to fifth acoustic peaks as well as the damping tail of the CMB power spectrum. The calibration uncertainty has been reduced from 6% to 2.2% in temperature through a direct comparison of the CMB anisotropy measured by ACBAR with that of the dipole-calibrated WMAP3 experiment. The measured power spectrum is consistent with a spatially flat, LambdaCDM cosmological model. We see evidence for weak gravitational lensing of the CMB by comparing the likelihood for the best-fit lensed/unlensed models to the ACBAR+WMAP3 data. On fine angular scales, there is weak evidence (1.7 sigma) for excess power above the level expected from primary anisotropies. The source of this power cannot be constrained by the ACBAR 150 GHz observations alone; however, if it is the same signal seen at 30 GHz by the CBI and BIMA experiments, then it has a spectrum consistent with the Sunyaev-Zel'dovich effect.
  • February 15, 2008 | 12:00 PM | LASR Conference Room
    Gravity on the Largest Scales and Cosmic Acceleration
    Justin Khoury, Perimeter Institute for Theoretical Physics

    We present a class of modified theories of gravity in which vacuum energy is screened or degravitated. We argue that any such theory must, at the linear level, reduce to a theory of massive or resonance graviton. The immediate implication is that there are new degrees of freedom, associated with the extra polarization states of the graviton, the most interesting of which is the helicity-0 or longitudinal mode. Phenomenologically, this mode leads to a fifth force which is suppressed in regions of high density but kicks in on large (>Mpc) scales. We discuss the implications for cosmological evolution and structure formation.
  • February 22, 2008 | 12:00 PM | LASR Conference Room
    Blowing Hot and Cold on Galaxy Clusters
    Greg Bryan, Columbia University

    The new X-ray observatories have unleashed an explosion of data about the hot gas in galaxy clusters, in the process overturning cherished theories and posing new puzzles, particularly about the impact of AGN on the thermal state of the cluster gas. I review what can be understood from observations, simple theory and high-resolution numerical simulations, showing that in some areas the latest observational results are in surprisingly good agreement with theory. On the other hand, the impact of cooling and heating on cluster cores and -- to a lesser extent -- on global scaling relations, is still not well understood. This uncertainty is important when using clusters for constraining cosmological parameters and I will talk about some ways to improve both our understanding of AGN feedback, and our cosmological constraints.
  • February 29, 2008 | 12:00 PM | LASR Conference Room
    Observing the First Galaxies and the Reionization Epoch
    Steven Furlanetto, UCLA

    Finding and understanding the earliest generations of galaxies is one of the frontiers of modern cosmology. Although enormous strides have been made in the past decade, the current observational evidence is ambiguous at best. I will describe two routes toward improving the situation. First, searches for high-redshift galaxies through their Lyman-alpha emission lines can teach us not only about the galaxies themselves but also about the intergalactic medium (IGM). While current measurements constrain their abundance, the clustering of these objects promises to reveal even more information. Second, three-dimensional ''tomography'' 21 cm emission (or absorption) by the neutral IGM has the potential to unlock the detailed distribution of baryons between recombination and reionization. I will describe how this cosmic background can teach us about the eras of the first stars, first black holes, and reionization itself. I will also describe some of the challenges facing these measurements.
  • March 7, 2008 | 12:00 PM | LASR Conference Room
    Hypervelocity Stars: What They Can Tell Us About the Galactic Center and Halo
    Yu Qingjuan, University of California at Santa Cruz

    Hypervelocity stars, with velocities up to 10^3 km/s, were predicted to exist in the Galaxy as a consequence of dynamical interaction of stars with the central supermassive black hole. This prediction has been confirmed with the discovery of the first hypervelocity star in the Galactic halo in 2005. In this talk, I will review the dynamical mechanisms of ejecting hypervelocity stars and recent observations. I will talk about how hypervelocity stars can reveal the properties of the Galactic center and the binarity of the central supermassive black hole. I will also talk about the kinematics of hypervelocity stars and how they can be used as a diagnostic tool to constrain the triaxial shape of the Galactic halo potential. Future observational tests have potential implications in the cosmological contexts, as triaxial shapes of galactic halos and the existence of binary black holes are the generic predictions of the hierarchical structure formation model in the Lambda-CDM cosmogony.
  • March 14, 2008 | 12:00 PM | LASR Conference Room
    From Massive Cores to Massive Stars
    Mark R Krumholz, Princeton University

    The similarity between the mass and spatial distributions of pre-stellar gas cores in star-forming clouds and young stars in clusters provides strong circumstantial evidence that these gas cores are the direct progenitors of individual stars. I describe a physical model for the evolution of massive cores into stars, starting with the intial phases of collapse and fragmentation, through disk formation and fragmentation, the later phases of stellar feedback, and finally interaction of the newly formed stars with their environments. This model shows that a direct mapping from cores to stars is the natural physical outcome of massive core evolution, and thereby allows us to explain many of the properties of young star clusters as direct imprints of their gas-phase progenitors.


  • January 16, 2008 | 3:30 PM | RI 480
    Deciphering the Gamma-ray Sky: GeV Astronomy in the Era of GLAST
    Vasiliki Pavlidou, KICP, University of Chicago

    The GeV sky was first studied in detail by the Energetic Gamma-Ray Experiment Telescope (EGRET) in the 1990s. Together with an impressive array of data on high-energy systems, the legacy of the EGRET mission includes persisting puzzles, such as the nature of the many yet-unidentified gamma-ray sources, the details of the mechanism producing the Galactic diffuse emission, and the origin of the diffuse, isotropic, persumably extragalactic gamma-ray background. The launch of the Gamma-ray Large Area Space Telescope in the summer of 2008 will once again open the GeV-window of energies to observations, with unprecendented sensitivity and angular resolution. I will discuss recent progress, using model-independent approaches, on the interconnected questions of unidentified sources and the gamma-ray diffuse backgrounds, as well as novel tests that GLAST will enable us to perform to constrain models of nonthermal high-energy processes in Galactic and extragalactic scales.
  • January 30, 2008 | 3:30 PM | RI 480
    Dynamics of Protoplanetary Disks: Vortices and Turbulence
    Yoram Lithwick,

    Protoplanetary disks are gaseous accretion disks around young stars. I will show how, in hydrodynamical accretion disks such as these, vortices form out of a nonlinear instability. I will also show that a vortex's ultimate fate in 3D depends on whether it is weak or strong. Weak vortices live essentially forever. Strong vortices decay into turbulence via a 3D instability. One interesting implication is that in protoplanetary disks dust can be trapped by weak vortices, triggering planet formation. A second implication is that strong vortices can be responsible for the turbulence that makes these accretion disks accrete.
  • February 6, 2008 | 3:30 PM | RI 480
    Star Formation and Gasdynamics in the Nuclear Regions of Galaxies
    Philip Chang, University of California, Berkeley

    How are active galactic nuclei (AGNs) and supermassive black holes (SMBHs) fed? To feed them in the absence of major mergers suggests that gas must be transported from kpc-scales the 100s of parsec scales to parsec and sub-parsec scales. I will discuss two interesting cases that might shed light on some of the relevant physics: the formation of the young stellar disk in the nuclei of M31 and the formation of star clusters in the nuclear rings of galaxies. In the case of M31, I will show how a preexisting eccentric stellar disk can radially transport gas on parsec scales, which is reflected in the morphology of the young stellar disk. In the case of star clusters in nuclear rings, tidal interactions between the clusters and the ring result in their separation. The physics of these systems may help elucidate the mechanism by which AGNs and SMBHs are fed.
  • February 13, 2008 | 3:30 PM | RI 480
    At the horizon: Imaging black holes and testing General Relativity
    Avery Broderick, The University of Toronto

    We are now at the advent of a golden age for observational tests of strong-field General Relativity. Gravitational wave experiments such as LIGO and LISA have promised to provide extraordinary insights into the structure and evolution of compact objects. However, I will discuss a more modest, complementary approach: directly imaging the horizons of nearby super massive black holes. Such observations are already technically feasible and demonstration experiments are presently underway. I will describe how such experiments can be used to address outstanding problems in black hole astrophysics, provide evidence for the existence of horizons and directly verify that the Kerr metric provides an appropriate description of the spacetime around black holes.
  • March 12, 2008 | 3:30 PM | RI 480
    Probing the Turbulence Dissipation Range and Magnetic Field Strengths in Molecular Clouds
    Martin Houde, The University of Western Ontario

    I will present a study of the turbulent velocity dispersion spectra as a function of length scale for the coexistent HCN and HCO+ molecular species in the M17 star-forming molecular cloud. I will show that the observed downward shift of the ion's spectrum relative to that of the neutral is readily explained by the existence of an ambipolar diffusion range within which ion and neutral turbulent energies dissipate differently. I will also demonstrate how these observations can be used to evaluate this decoupling scale and estimate the strength of the plane-of-the-sky component of the embedded magnetic field in a completely novel way.
  • March 19, 2008 | 3:30 PM | RI 480
    Kepler's Cosmology and Proof of the Heliocentric Theory
    Noel Swerdlow, The University of Chicago