Talks & Events
KICP Colloquia: 2008
Star Formation in Galaxies
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.
Magnetized Coronae of Stars and Accretion Disks
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.
The Importance of Lens Environments
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.
X-ray binaries: puzzle bonanza
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.
The Highest Redshift Quasars and the End of Reionization
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 6
The echo of Einstein's greatest blunder
The coupling of baryons and photons by Thomson scattering in the early universe leads to a rich structure in the power spectra of the cosmic microwave background photons and the matter. The study of the former has revolutionized cosmology and allowed precise measurement of a host of important cosmological parameters. The
study of the latter is still in its infancy, but holds the potential to constrain the nature of the dark energy believed to be causing the accelerated expansion of the universe. I will discuss how we can measure this cosmic sound, and the theoretical developments that need to be made before we can realize the promise of future missions.
Measuring CMB Polarization
The Inflationary paradigm has been remarkably successful in passing observational tests, largely from Cosmic Microwave Background temperature anisotropy measurements. Gravitational waves produced during the epoch of Inflation may imprint a detectable polarization signal in the CMB. This polarization signal, which has a distinctive 'B-mode' pseudo-vector field that cannot be duplicated by matter over/under-densities, depends on the physics of Inflation and thus can be used to distinguish among Inflationary models.
We have developed the BICEP (Background Imaging of Cosmic Extragalactic Polarization) experiment to measure degree-scale CMB polarization in a search for evidence of an inflationary gravitational wave background. I discuss the unique design of the receiver and its performance after two seasons of observations from the South Pole. Recent advances in antenna-coupled TES bolometers will enable significant improvement in system sensitivity. We are developing two new instruments based on these new detectors, SPIDER to measure large-scale polarization from a long-duration balloon, and BICEP2 to deeply integrate in a small region of the sky with low Galactic foregrounds from the ground. These measurements will ultimately culminate in a space-borne measurement with large detector arrays, NASA's Einstein Inflation Probe.
Star Formation in Turbulent Molecular Clouds
Star formation, despite its importance, is not well understood up to now. This situation is however changing quickly. High-resolution observations now provide detailed insight into the complex structure of the turbulent interstellar medium and its various gas phases. The star formation history of molecular cloud regions like Taurus and Orion has now been investigated in great details. At the same time, numerical simulations have achieved enough resolution and complexity in order to explore the origin of turbulent molecular clouds and their fragmentation into stars and stellar clusters in great details.
At the moment the physics of star formation is still in an early phase of exploration. A major step forward has recently been made by identifying several key questions which need to be solved to make progress. I will discuss some of the most puzzling and challenging questions. I then will present recent ideas and and new numerical simulations that have the potential to solve some of these puzzles and by this provide crucial steps towards a consistent theory of star formation.
Searching for Dark-Matter Axions
The axion is a hypothetical elementary particle whose existence would explain the baffling absence of CP violation in the strong interactions. It's properties make it a good dark-matter candidate. Even though dark-matter axions would make up the overwhelming majority of mass in the universe, they are extraordinarily difficult to detect. We have developed a detector of dark-matter axions that is at heart an exquisitely sensitive detector of microwave radiation. This colloquium will briefly review the role of axions in particle and astrophysics, and will describe the progress we've made in the experimental search.
New CMB Polarization Results from QUaD
Measurements of the intensity fluctuations of the Cosmic Microwave Background have already taught us an enormous amount about the nature of the Universe in which we live. Polarization measurements have the potential to tell us even more. After briefly reviewing the motivation for polarization measurements I will move on to the QUaD experiment at South Pole, which is currently the world's most sensitive CMB polarimeter in the multipole range 200 to 2000. I will describe the instrument, observation strategy, analysis and newly released results. Finally I will mention a new project called SPUD and the push to detect the gravitational wave B modes.
CN-cycle Solar Neutrinos and the Solar System Metalicity
The CN-cycle plays a modest role in solar energy generation, but produces a significant flux of neutrinos that will be measured in SNO+ and other future detectors. Because past measurements have precisely calibrated other conditions in the solar core -- and because of progress made in constraining neutrino flavor physics -- I argue this new measurement will determine the primordial solar core abundances of C and N to 10%. This will eliminate a key assumption in the standard solar model, and address the current conflict between helioseismology and photospheric abundance determinations. I discuss the possibility that this conflict is real and associated with late-stage metal differentiation in the solar-system disk associated with planetary formation -- and point to observational implications.
Indeterminacy of Holographic Quantum Geometry
Theoretical motivations will be reviewed for the idea that spacetime is holographic: there is a minimum time or length in nature, and the third dimension of space emerges in quantum theory from time evolution, in the normal direction to a light sheet. A new phenomenon will be described that arises in such holographic geometry: "holographic noise", caused by an indeterminacy of transverse position. It will be shown that currently operating interferometric gravitational-wave detectors have the capability to detect holographic noise. These experiments provide a zero-parameter test of the holographic hypothesis. In a holographic world, they provide a direct, precise measurement of the fundamental minimum interval of time.
The Chirality of Life: From Phase Transitions to Astrobiology
Life is chiral. Amino acids that make up biomolecules are left-handed, while all sugars are right-handed. And yet, when synthesized in the laboratory, the solutions come out 50-50. Is life's chirality simply an accident, or is it the result of dynamical processes that occurred in early Earth, during prebiotic times? If life exists elsewhere in the Universe, will it choose the same chirality? In this talk, I will address these and other questions of interest in astrobiology, using techniques from nonequilibrium statistical mechanics and field theory. In particular, I will argue that life's chirality might be due to a symmetry-breaking phase transition. Exploring this possibility, I will obtain bounds on possible processes that may have selected life's chirality here and possibly in other life-bearing planetary platforms. Coupling the polymerization reactions to a simple model of the early-Earth environment, we conclude that Earth's chirality was selected randomly. A large sample of planetary platforms with stereochemistry will have a 50-50 chiral distribution.
Cosmological Inhomogeneities and Dark Energy
The issue of whether the backreaction of inhomogeneities can mimic the effects of dark energy has been a subject of much debate. I review the evidence on both sides of this issue and discuss what needs to be done to resolve it. I also discuss the extent to which inhomogeneities perturb high precision measurements of dark energy.
Early results from the Fermi gamma-ray space telescope
The Fermi Gamma-ray Space Telescope, recently launched in June 2008, is a satellite based observatory to study the high energy gamma-ray sky.
There are two instruments on Fermi: the Large Area Telescope (LAT) which provides coverage from 20 MeV to over 300 GeV, and the Gamma-ray Burst Monitor (GBM) which provides observations of transients from 8 keV to 30 MeV. Since its launch, Fermi has detected over one hundred gamma-ray bursts (three above 100 MeV), dozens of flaring AGN, a couple of Galactic transients, radio-loud and radio-quiet pulsars, and is enabling a detailed study of the many other persistent sources in the high energy gamma-ray sky. In this talk, I will review the early results from Fermi, and discuss the current status and future plans for the observatory.
Clustering, Quenching, and Feedback: Galaxies and AGN at z=1
Roughly half of the red elliptical galaxies observed today have formed since z=1. I will present galaxy clustering results from the DEEP2 Redshift Survey that strongly constrain the mechanism responsible for the quenching or cessation of star formation in these galaxies. I will show where this quenching is occurring on large scales and how it can not be due primarily to cluster-specific physics. I will also present results on the clustering of optically-bright quasars and X-ray selected AGN and show how AGN accretion correlates with the star formation activity in galaxies at z=1. I will also show new results on the prevalence of outflowing galactic winds at z=1 and discuss their role in quenching star formation. Finally, I will present a new wide-area prism survey that will allow further studies of galaxy evolution to z=1 with the largest faint galaxy survey to date.