KICP Seminars & Colloquia, Spring 2010

Seminar schedule for Spring 2010
March 31, 2010
Astronomy Colloquium
Andrea Lommen
Franklin and Marshall College
The North American Nanohertz Observatory of Gravitational Waves (NANOGrav)   [Abstract]
April 2, 2010
Friday noon seminar
John R Peterson
Purdue University
Exploring the Dark Sector with Clusters of Galaxies   [Abstract]
April 7, 2010
Wednesday colloquium
Kollmeier Juna
Carnegie Institution of Washington
Oldies but Goodies: Old Hypervelocity Stars and RR Lyrae as Probes of the Inner and Outer Milky Way   [Abstract]
April 9, 2010
Friday noon seminar
Eric Dahl
The University of Chicago
News from COUPP: New dark matter limits, acoustic alpha discrimination, and progress towards a deep site detector (or two)   [Abstract]
April 14, 2010
Astronomy Colloquium
Jeter Hall
Fermi National Laboratory
Searching for the Dark Matter   [Abstract]
April 16, 2010
Friday noon seminar
Kumiko Kotera
The University of Chicago
Ultrahigh energy cosmic rays, neutrinos and gamma rays: a multi-messenger approach to Astroparticle Physics.   [Abstract]
April 21, 2010
Wednesday colloquium
John Carlstrom
The University of Chicago
Early Results from the South Pole Telescope
April 23, 2010
Friday noon seminar
Orly Gnat
Non-Equilibrium Ionization Processes in Metal-Ion Absorbers   [Abstract]
April 28, 2010
Astronomy Colloquium
Zeljko Ivezic
University of Washington
Reaching for the sky: from SDSS to LSST   [Abstract]
April 30, 2010
Friday noon seminar
Pedro Facal
The University of Chicago
Microwave detection of ultra-high energy cosmic rays: the MIDAS experiment at the University of Chicago   [Abstract]
May 5, 2010
Wednesday colloquium
Paolo Gondolo
University of Utah
Dark stars, or how dark matter can make a star shine: introduction and update   [Abstract]
May 7, 2010
Friday noon seminar
Andrew R Zentner
The University of Pittsburgh
Is the Near-Power-Law Galaxy Correlation Function a Cosmic Coincidence?   [Abstract]
May 12, 2010
Astronomy Colloquium
James Kasting
Pennsylvania State University
How to Find a Habitable Planet   [Abstract]
May 14, 2010
Friday noon seminar
Guilhem Lavaux
University of Illinois at Urbana-Champaign
Peculiar velocities of galaxies: a versatile probe of cosmology   [Abstract]
May 19, 2010
Wednesday colloquium
Adam A Lidz
University of Pennsylvania
Hydrogen and Helium Reionization   [Abstract]
May 21, 2010
Friday noon seminar
Eli Rykoff
University of California, Santa Barbara
Cosmology with galaxy clusters: a multi-wavelength approach   [Abstract]
May 26, 2010
Astronomy Colloquium
Scott A. Hughes
The gravitational two-body problem in general relativity   [Abstract]
May 28, 2010
Friday noon seminar
Juan Collar
The University of Chicago
Light WIMPs, the Plot Thickens?
June 2, 2010
Astronomy Colloquium
Brian Fields
When Stars Attack! Live Radioactivities as Signatures of Near-Earth Supernovae   [Abstract]
June 4, 2010
Friday noon seminar
Janice A Hester
California Institute of Technology
The Environment of Major Mergers between Dark Matter Halos   [Abstract]
June 10, 2010
Astronomy Colloquium
Robert P. Kirshner
Harvard-Smithsonian Center for Astrophysics
The Past, Present, and Future of Supernova Cosmology   [Abstract]

  • April 7, 2010 | 3:30 PM | RI 480
    Oldies but Goodies: Old Hypervelocity Stars and RR Lyrae as Probes of the Inner and Outer Milky Way
    Kollmeier Juna, Carnegie Institution of Washington

    The Milky Way provides an opportunity for a close-up investigation of the complex processes of galaxy and star formation. I will discuss recent efforts to do this by using rare, but important, probes of these phenomena. In the first portion of the talk I will present results on hypervelocity stars primarily from the Sloan Digital Sky Survey. The distribution of these stars, in physical properties and in space, allows us to place interesting limits on star formation and dynamics at the Galactic Center as well as the possibility to constrain the shape of the Milky Way's dark matter halo. I will discuss progress we have made toward these goals. In the second portion of the talk, I will discuss how one can use RR Lyrae stars to probe the outer halo of the Milky Way and find new and distant substructures which are difficult to probe by other means. Our recent confirmation of a distant structure in RR Lyrae stars highlights the power of this approach to unraveling the outer halo and showcases exciting possibilities for future all-sky time-domain surveys.
  • April 21, 2010 | 3:30 PM | RI 480
    Early Results from the South Pole Telescope
    John Carlstrom, The University of Chicago
  • May 5, 2010 | 3:30 PM | RI 480
    Dark stars, or how dark matter can make a star shine: introduction and update
    Paolo Gondolo, University of Utah

    The first stars in the universe may have shined due to dark matter annihilation instead of nuclear fusion. They were dark matter-powered stars, or for short Dark Stars. In this talk, I will report on the story of Dark Stars including recent developments: how they formed, evolved and might have died, and how they might be detected.
  • May 19, 2010 | 3:30 PM | RI 480
    Hydrogen and Helium Reionization
    Adam A Lidz, University of Pennsylvania

    A key period in our story of structure formation is the Epoch of Reionization (EoR), when early populations of galaxies and/or quasars formed, emitted ultraviolet light and ionized 'bubbles' of gas around them, eventually filling the entire volume of the intergalactic medium (IGM) with ionized gas. Reionization studies aim to determine the filling factor and size distribution of ionized bubbles during the EoR, which in turn constrain the properties of the first luminous sources. Current observations suggest that hydrogen is reionized sometime before z>~6 by star-forming galaxies. These sources should simultaneously singly ionize helium, but are unlikely to also doubly ionize it. Helium may be doubly-ionized only later on, perhaps near z~3, by bright quasars. I will describe efforts to theoretically model the Epochs of Hydrogen and Helium Reionization, and focus on some of their observational implications. First, I will forecast the prospects for learning about hydrogen reionization from upcoming 21 cm observations. I will then discuss an analysis of existing HI Ly-a forest data aimed at identifying signatures of helium reionization near z~3.


  • April 2, 2010 | 12:00 PM | LASR Conference Room
    Exploring the Dark Sector with Clusters of Galaxies
    John R Peterson, Purdue University

    Cosmological data indicates that the Universe is dominated by two unseen components, dark energy and dark matter. A number of astrophysical techniques are capable of making improved measurements that may help us to study their properties in greater detail. One way of doing this is by using cluster of galaxies, the largest structures in the Universe. They contain large quantities of dark matter and map out the growth of structure which is believed to be dominated by dark energy. We first present a new large X-ray selected seredipitous cluster survey based on a novel joint analysis of archival Chandra and XMM-Newton data. The survey provides enough depth to reach clusters near a redshift of 1 and simultaneously a large enough sample to find evidence for the strong evolution of clusters expected from structure formation theory. We detected a total of 723 clusters of which 462 are newly discovered clusters. The survey exploits a tech nique which combines the exquisite Chandra imaging quality with the high throughput of the XMM-Newton telescopes using overlapping survey regions. We measure the log N-log S distribution and find that the number density falls exponentially, indicative of rapid cluster evolution, as expected for cosmic structure formation. This data set can then be used for precision cosmological measurements of dark matter and dark energy. We also discuss the physics of clusters of galaxies, and mention attempts to understand the perplexing physics in cores of clusters. Finally, we highlight some recent detailed optical simulations of the future telescope, LSST (Large Synoptic Survey Telescope). This telescope and others will find enormous numbers of clusters of galaxies, and will perform the next generation of dark sector studies.
  • April 9, 2010 | 12:00 PM | LASR Conference Room
    News from COUPP: New dark matter limits, acoustic alpha discrimination, and progress towards a deep site detector (or two)
    Eric Dahl, The University of Chicago

    New results from COUPP have it poised to become the world's leading experiment for direct detection of WIMP dark matter. The bubble chamber technique is immune to the gamma and beta backgrounds that other direct detection experiments must face, but previous COUPP bubble chambers have suffered a substantial alpha-decay background. We can now distinguish bubbles arising from alpha-decays from those nucleated by nuclear recoils (from neutrons or WIMPs) based on their acoustic signature. A recent run in the NuMI tunnel at Fermilab with a 4kg chamber demonstrated >80% alpha rejection and produced the world's best limit on the spin-dependent WIMP-proton cross section, reaching the cosmic-muon induced neutron background and saturating the physics reach of the NuMI site. The 4kg chamber will move to SNOLAB this summer, followed shortly by a 60kg chamber. With alpha discrimination expected to be >99%, these chambers will probe interesting parameter space for spin-dependent interactions and will be competitive with world-leading experiments in the spin-independent sector.
  • April 16, 2010 | 12:00 PM | LASR Conference Room
    Ultrahigh energy cosmic rays, neutrinos and gamma rays: a multi-messenger approach to Astroparticle Physics.
    Kumiko Kotera, The University of Chicago

    Ultrahigh energy cosmic rays remain a puzzle to the Astrophysics community, as their sources have not been discovered yet, in spite of decades of experimental and theoretical research. The trajectories of these charged particles are indeed deviated by the magnetic fields of the Universe, making it difficult to identify their origin. The quest for sources of ultrahigh energy cosmic rays can be associated with the search of their secondary neutrino and gamma-ray signatures produced during their propagation in the intergalactic medium. Both neutrinos and gamma rays travel in a straight manner and bear valuable information on the birthplace of their progenitors. I will present estimates of these secondary fluxes for point sources embedded in filaments or clusters of galaxies, and for given distributions of sources in the Universe. Each of these studies explores the influence of a wide variety of parameters, including cosmic ray chemical composition, source luminosity, distance, intergalactic magnetic field configuration, etc. I will introduce the complete propagation and interaction code that we developed for this purpose, as well as our flexible modeling of intergalactic magnetic fields. In light of these works, I will discuss the detectability of neutrino and gamma-ray signals with current and upcoming instruments and the impact such detection could have on the understanding of ultrahigh energy cosmic rays.
  • April 23, 2010 | 12:00 PM | LASR Conference Room
    Non-Equilibrium Ionization Processes in Metal-Ion Absorbers
    Orly Gnat, Caltech

    In this talk I will discuss several processes that give rise to non-equilibrium ionization in metal-ion absorbers. These include radiative cooling, fast shock waves, and conductive evaporation of warm clouds. In each case, I will demonstrate the impact that departures from equilibrium ionization have on the absorption line signatures. I will first describe computations of the equilibrium and non-equilibrium ionization states and cooling efficiencies in radiatively cooling gas. I will then discuss models of the non-equilibrium cooling column densities associated with fast radiative shocks, including the effect of the shock self-radiation on the "downstream" ionization states. In these models I self-consistently follow the time-dependent dynamics, ionization, cooling and radiative transfer equations in one-dimensional stable shocks. I will describe how the observational signatures depend on the controlling parameters, including the shock velocity, gas metallicity, magnetic field, and shock age. Finally, I will present recent computations of thermally conductive interface layers, that may surround evaporating clouds embedded in a hot medium. These models include photoionization by an external radiation field. I will describe how departures from equilibrium affect the conditions for which self-consistent evaporating solutions exist, and the metal-ion columns produced in the evaporating layers.
  • April 30, 2010 | 12:00 PM | LASR Conference Room
    Microwave detection of ultra-high energy cosmic rays: the MIDAS experiment at the University of Chicago
    Pedro Facal, The University of Chicago

    There are two main techniques for detecting high energy cosmic rays: fluorescence detectors provide a nearly calorimetric energy measurement and an accurate determination of the primary mass but are limited to run in clear, moonless nights; surface detectors run continuously but are not as accurate as fluorescence detectors and must be deployed over large areas. Molecular bremsstrahlung, a microwave emission due to the interaction of the free-electrons (that form a low density plasma produced by the cascade ionization) with the neutrals in the atmosphere, has been measured in the laboratory and is isotropic, unpolarized and scales quadratically with the primary energy. As such, is an excellent candidate for a better detector for ultra high energy cosmic rays: an imaging wide aperture telescope that uses the atmosphere to obtain a calorimetric measurement of the energy in the air shower and of the primary mass (via the depth of the shower maximum) with an 100% duty cycle. MIDAS (MIcrowave Detection of Air Showers)is a prototype of such a detector, a 4.5 diameter parabolic dish operating at 4 GHz, instrumented with 53 imaging pixels that has been installed in February 2010 at the University of Chicago campus. I'll will discuss the design, performance and first results of MIDAS as well as its future.
  • May 7, 2010 | 12:00 PM | LASR Conference Room
    Is the Near-Power-Law Galaxy Correlation Function a Cosmic Coincidence?
    Andrew R Zentner, The University of Pittsburgh

    I started as a postdoc at the Center for Cosmological Physics (now the KICP) nearly 7 years ago. I came, in part, to study (with other KICP members) possible simple, theoretical interpretations of observations of nearly power-law galaxy correlation functions. I'm happy to report we've made some progress. I will discuss galaxy correlation functions from an abstract theoretical perspective, using observational data only for gross guidance. I will then assume that galaxies occupy dark matter halos and discuss the popular "halo model" for computing galaxy correlation statistics. I will assume that galaxies within group- and cluster-sized halos occupy self-bound subhalos and use a model for subhalo evolution to identify the physical processes that lead to the present-day clustering patterns of galaxies. I will show that the power-law nature of the galaxy correlation function relies on a particular balance between the accretion of new subhalos onto group-sized halos and the destruction of those subhalos by dynamical processes within these groups. In the context of the same model, I will discuss the dependence of halo clustering on galaxy luminosity and color as well as redshift. One interesting outcome of this general exploration is that the galaxy correlation functions should deviate from power laws at high-redshifts (z >~ 1, as observed, at least approximately) and in the future. Based on this discussion, I will conclude by suggesting that the simple observation of a near power-law galaxy correlation function at low-redshifts is largely coincidental.
  • May 14, 2010 | 12:00 PM | LASR Conference Room
    Peculiar velocities of galaxies: a versatile probe of cosmology
    Guilhem Lavaux, University of Illinois at Urbana-Champaign

    A valuable source of information on the distribution of dark matter and the growth of structures lies in the peculiar velocity field of the galaxies. I present here a short review of peculiar velocity field reconstruction methods and in particular the more recently Monge-Ampere-Kantorovitch reconstruction of galaxy trajectories. I then illustrate the use of peculiar velocities in three contexts. First, I apply this method to a real galaxy sample: the 2MASS Redshift survey. I compare the predicted peculiar velocities from the distribution of galaxies to the observed peculiar velocities in our neighborhood of 30 Mpc/h. I study the origin of the motion of the Local Group relative to the Cosmic Microwave Background dipole. I discuss how these two studies may help at putting additional constraints on the Lambda CDM models. Second, I study the peculiar velocities as a tracer of the evolution of cosmic voids and how they can be used to constrain the physical properties of Dark Energy at different redshifts. Finally, I use the reconstructed peculiar velocities to produce a precise re-simulation of the dark matter distribution in the Local Universe. I discuss some of the applications of this simulation.
  • May 21, 2010 | 12:00 PM | LASR Conference Room
    Cosmology with galaxy clusters: a multi-wavelength approach
    Eli Rykoff, University of California, Santa Barbara

    A multi-wavelength approach to cluster cosmology is necessary to overcome systematic uncertainties in the relation between cluster mass and observable mass tracers. I highlight recent results from the SDSS maxBCG cluster catalog, a highly pure and complete optically selected cluster sample, containing ~14,000 rich clusters in the redshift range 0.1 < z < 0.3 from SDSS DR5 imaging data. Through a cross-correlation of the maxBCG catalog with X-ray photon maps from the ROSAT All-Sky Survey (RASS), we trace how the average X-ray properties scale with optical richness. Combined with stacked weak lensing observations, we obtain the first constraints on the covariance among cluster richness, X-ray luminosity, and halo mass. We have also used RASS data to optimize a new matched filter richness estimator, and demonstrate its utility as a reduced scatter mass proxy, as compared to other published richness estimators. Finally, I introduce preliminary results from maRCS (maxBCG Rich Cluster Sample), a volume and richness limited cluster sample in the redshift range 0.2
  • May 28, 2010 | 12:00 PM | LASR Conference Room
    Light WIMPs, the Plot Thickens?
    Juan Collar, The University of Chicago
  • June 4, 2010 | 12:00 PM | LASR Conference Room
    The Environment of Major Mergers between Dark Matter Halos
    Janice A Hester, California Institute of Technology

    Major mergers are a popular theoretical mechanism for creating early type galaxies. Under the right conditions, simulated major mergers of gaseous spiral galaxies pass through a star burst phase, an active AGN phase, and finally form a red and dead elliptical remnant after feedback from the AGN heats and expels the gas from the galaxy. The remnants formed in these simulations are supported by random stellar motions, are centrally concentrated, have old red stellar populations, are surrounded by hot gaseous halos, and host dormant massive black holes. Anecdotal evidence supports this proposed mechanism; several classes of observed galaxy appear to correspond to the theoretical stages of the transformation of a major merger of two spiral galaxies into a red elliptical galaxy. Before we can claim to have revealed the source of the Hubble diagram, however, rigorous observational tests of the major merger driven evolutionary scenario must be developed and applied. Statistical tests, which can add the rigor of large numbers, cannot rely on detailed observations or analyses of individual galaxies. A novel probe is required in order to determine whether major mergers constitute a significant fraction of the populations of interest, such as E+A galaxies and luminous AGN. We studied major mergers between dark matter halos in the Millennium Simulation in order to determine whether environment can provide this probe. We found an excess of less luminous companion galaxies located at small physical distances from merger remnants which is a strong, potentially observable, signature of major merger populations. This diagnostic is robust against general assumptions about the role gas physics plays in determining whether a major merger follows the prescribed evolutionary path. We are working close to the limit of the simulation's ability to accurately simulate subhalos within the dense dark matter background of their hosts' halos. It is therefore imperative that we develop a physical understanding of the dynamics that determine the major merger rate and use this understanding to distinguish whether the measured environmental signatures are true diagnostics of major mergers or artifacts of the simulation. I will present a break down of subhalo dynamics and demonstrate their effects in the simulation. A subhalo's opportunities to transfer its orbital energy and angular momentum with respect to its host halo into the internal orbits of itself, its host, or its companions, play a key role in determining if and when the subhalo will merge with its host. Inelastic collisions between subhalos of the same host therefore greatly enhance the merger rate. This is the root cause of the measured excess of companions around merger remnants.


    • March 31, 2010 | 3:30 PM | RI 480
      The North American Nanohertz Observatory of Gravitational Waves (NANOGrav)
      Andrea Lommen, Franklin and Marshall College

      NANOGrav is a consortium of radio astronomers and gravitational wave physicists whose goal is to detect gravitational waves using an array of millisecond pulsars as clocks. Whereas interferometric gravitational wave experiments use lasers to create the long arms of the detector, NANOGrav uses earth-pulsar pairs. The limits that pulsar timing places on the energy density of gravitational waves in the universe are on the brink of limiting models of galaxy formation and have already placed limits on the tension of cosmic strings. Pulsar timing has traditionally focused on stochastic sources, but most recently I have been investigating the idea of detecting individual gravitational wave bursts wherein there are some interesting advantages. I will also demonstrate how the array can be used to reconstruct the waveform and obtain its direction.
    • April 14, 2010 | 3:30 PM | RI 480
      Searching for the Dark Matter
      Jeter Hall, Fermi National Laboratory

      There is a large body of evidence that ~85% of the matter in the Universe is in the form of cold, non-baryonic dark matter. I describe how terrestrial particle detectors are searching for clues on the nature of the dark matter. I focus on two leading experiments, current results, and challenges in searching for an unknown form of matter.
    • April 28, 2010 | 3:30 PM | RI 480
      Reaching for the sky: from SDSS to LSST
      Zeljko Ivezic, University of Washington

      Despite a several thousand years long history, sky surveying is experiencing a bonanza as detectors, telescopes and computers become ever more powerful. I will discuss how the unprecedentedly accurate and diverse data from the optical Sloan Digital Sky Survey have recently enabled numerous exciting discoveries. I will use three specific examples (asteroids, quasar variability, and mapping of the Milky Way stellar distribution) to give a preview of what to expect from the upcoming next-generation surveys, such as the Dark Energy Survey and the Large Synoptic Survey Telescope.
    • May 12, 2010 | 3:30 PM | RI 480
      How to Find a Habitable Planet
      James Kasting, Pennsylvania State University

      Over 400 planets have been found around nearby stars, but none of them is thought to be at all like Earth. The goal now is to identify rocky planets within the habitable zones of their stars and to search their atmospheres spectroscopically for signs of life. To do this, we need new space-based telescopes such as NASA’s proposed Terrestrial Planet Finders or ESA’s Darwin mission (all of which are indefinitely postponed at the moment). If spectra of extrasolar planet atmospheres can be obtained, the presence of O2, which is produced from photosynthesis, or O3, which is produced photochemically from O2, would under most circumstances provide strong evidence for life beyond Earth. But “false positives” for life may also exist, and these need to be clearly delineated in advance of such missions, if at all possible. I will also contrast my optimism about the search for complex life with the more pessimistic view expressed by Ward and Brownlee in their book, Rare Earth.
    • May 26, 2010 | 3:30 PM | RI 480
      The gravitational two-body problem in general relativity
      Scott A. Hughes, MIT

      A simple problem in Newtonian gravity, the motion of two bodies about one another is far more challenging in general relativity (GR). Motivated largely by the anticipated importance of compact binaries as gravitational-wave sources, many years of effort have produced a suite of tools for modeling binaries with GR. In this talk, I will present an overview of how we model these sources in GR and what we have learned from the relativistic two-body problem. I will focus in particular on how unique aspects of relativistic gravity flavor the gravitational waves which binaries generate, and how these flavorings can be exploited to learn about compact bodies, especially black holes. I will emphasize analogs between the GR analysis and electromagnetic theory, hopefully demonstrating that the rich features of these models are in fact surprisingly intuitive.
    • June 2, 2010 | 3:30 PM | RI 480
      When Stars Attack! Live Radioactivities as Signatures of Near-Earth Supernovae
      Brian Fields,

      The lifespans of the most massive stars are a symphony of the fundamental forces, culminating in a spectacular and violent supernova explosion. While these events are awesome to observe, they can take a more sinister shade when they occur closer to home, because an explosion inside a certain "minimum safe distance" would pose a grave threat to the biosphere on Earth or elsewhere. We will discuss these cosmic insults to life, and ways to determine whether a supernova occurred nearby over the course of the Earth's existence. We will then present recent evidence that a star exploded near the Earth about 3 million years ago. Radioactive iron-60 atoms have been found in ancient samples of deep-ocean material, and are likely to be debris from this explosion. Recent data confirm this radioactive signal, and for the first time allow sea sediments to be used as a telescope, probing the nuclear reactions that power exploding stars. Furthermore, an explosion so close to Earth was probably a "near-miss," which emitted intense and possibly harmful radiation. The resulting environmental damage may even have led to extinction of species which were the most vulnerable to this radiation.
    • June 10, 2010 | 4:15 PM | KPTC 106
      The Past, Present, and Future of Supernova Cosmology
      Robert P. Kirshner, Harvard-Smithsonian Center for Astrophysics

      Supernova have been developed into a powerful tool for cosmological distance measurement. In the (recent) past, supernovae showed that we live in an accelerating universe. In the present supernovae are a key element in constraining the properties of dark energy. While the present data are consistent with a cosmological constant, today's constraints are not very rigorous. As a community, we are beginning to learn where the systematic problems arise in tightening the noose and improving our knowledge. I'll review some of the problems we have encountered with dust absorption and supernova environments and I will show some promising developments using thermonuclear supernovae in the near-infrared that may mitigate these difficulties. The future will not be as easy as the past, but the conclusion of programs like ESSENCE, Supernova Legacy Survey and the Sloan Supernova Survey plus the Palomar Transient Factory, Pan-STARRS, and the Dark Energy Survey all promise real progress in the years just ahead.