Talks & Events
Some Recent Developments in the Theory of MHD Turbulence
There are many situations of astrophysical interest, like the solar wind or the interstellar medium, in which the turbulent motions of an electrically conducting fluid are affected by a background magnetic field. The resulting coupled system, which goes under the name of magneto-hydrodynamic (MHD) turbulence,has been the subject of study for over forty years. There are two competing theories of MHD turbulence, following Kolmogorov, both describe an energy cascade process from large to small scales, but their phenomenological underpinning and their predictions are drastically different. The first, developed independently by Iroshnikov and Kreichnan in the sixties, is based on the assumption that the energy cascade is isotropic at all scales. It predicts that the nonlinear interactions weaken at small scales and that the slope of the energy spectrum in the inertial range is -3/2. By contrast, the second theory, developed jointly by Goldreich and Shreidar in the nineties, assumes that the energy cascade is anisotropic, and that the degree of anisotropy increases at small scales. It assumes further that the nonlinear interactions remain strong at all scales and that the slope of the energy spectrum in the inertial range is -5/3 in the directions transverse to the local magnetic field. Interestingly, numerical simulations over the last decade, rather than clarifying the issues, have produced a picture in which the energy cascade is clearly anisotropic, in agreement with the Goldreich-Shridar phenomenology, but the slope of the energy spectrum in the inertial range is close to -3/2 as predicted by the Iroshnikov-Kreichnan theory. In my talk I will show how the numerical results and the Goldreich-Shridar theory can be reconciled by the introduction of the idea of dynamical alignment. According to this picture, the polarization vectors of the velocity and magnetic field fluctuations become increasingly more aligned at small scales. I will show that this tendency is associated with the conservation of cross-helicity in the ideal problem, and that one of the consequences is that the nonlinear interactions are depleted so that the slope of the energy spectrum agrees with the observed results.
Proof of dark matter existence and other results from a collision of galaxy clusters
Collisions of galaxy clusters let us study the properties of normal and dark matter that are inaccessible by other means. I will review recent results obtained using the merging cluster 1E0657-56. A combination of a long Chandra X-ray observation with accurate weak and strong gravitational lensing maps has provided the first direct, model-independent proof of the dark matter existence (as opposed to the modified gravity paradigm), and a direct constraint on self-interaction cross-section of the dark matter particles. This cluster also exhibits a rare example of a shock front in the intergalactic gas. Its X-ray observations can be used for interesting physical tests, such as determining the electron-ion equilibration timescale in magnetized astrophysical plasmas.
The First Supermassive Black Holes
The existence of a supermassive black hole in nearly every galactic nucleus is no longer in doubt, but the question of how these black holes formed is wide open. I will argue that they could have formed via the direct infall and collapse of gas in pregalactic haloes at redshifts ~10-20, without the intermediate stage of Pop III star formation. Global gravitational instabilities get rid of excess angular momentum, and the infalling gas forms a self-gravitating, optically thick structure - a "quasistar". As matter piles on, the core of the quasistar heats up until it undergoes runaway neutrino cooling and collapses to form a 10 solar mass black hole. The black hole then grows by accreting from the quasistar at an extremely super-Eddington rate, reaching thousands of solar masses in less than a million years. Concurrently, the quasistar expands to form a radiation pressure-dominated, convective envelope reminiscent of a red giant. I will discuss the structure and evolution of quasistars and their detectability with the James Webb Space Telescope.
The Fossil History of the Solar System: Links to Interstellar Chemistry
In a few key instances the chemical composition of meteorites and comets provides a fossil record of the physical conditions present at their creation. In this talk I will explore lines of evidence that link the chemistry of the solar nebula to that seen in the interstellar medium today. †In particular, I will present results that address two long-standing questions in solar system chemistry: (1) the overall nitrogen deficiency in comets (when compared to carbon and oxygen) and† (2) the origin of oxygen isotopic anomalies seen in primitive meteorites. I will show how an answer to these questions has implications for the origin of other anomalies seen in comets and meteorites, but also implies the birth of the Sun in a large star cluster.
The Structure of the Gaseous Cosmic Web
Over the past decade, observational and theoretical studies of the general intergalactic medium (IGM) have transformed our understanding of the material universe at high redshift (z>2). The IGM has been recognized as the dominant baryonic component of a large scale cosmic web. It is the matrix from which galaxies are forming, and at the same time provides galaxies with a sink of metal-enriched gas, energy and radiation. The astrophysical simplicity of the IGM and its strong evolution over time have made it an excellent laboratory for studying the baryon contents, initial density fluctuations, temperature, enrichment history, and the ionizing background of the universe.
I'll describe some new aspects of the IGM that have recently come to light, using QSO absorption lines seen in the spectra of lensed background QSOs. In particular, we will see how it is possible to study the kinematics of the cosmic web, as a function of density and spatial scale. I shall further discuss the possible impact of galactic winds on the properties of the IGM, and will report briefly on some recent results that may require a revision in our current understanding of the course of reionization.
The Galaxy-Dark Matter Connection
Understanding how galaxies are biased with respect to the dark matter is of crucial importance if we are to use observations of large scale structure in order to constrain cosmological parameters. In this talk I present a powerful, statistical method that links galaxies to their dark matter haloes, and which allows us to completely specify their bias. I present applications of this method to both the 2dFGRS and the SDSS, and discuss implications for both cosmology and galaxy formation. I will also present a new method to identify galaxy groups in redshift surveys. Applying this method to the 2dFGRS and SDSS yields the largest galaxy group catalogues constructed to date, which put tight constraints on the galaxy-dark matter connection.
An Explosion of Cosmic Explosions
Only about a hundred years ago astronomers came to recognize cosmic explosive events. What was once termed as Stella Nova are now divided into two major families, novae and supernovae (with real distinct classes in each). The speaker will first summarize new developments in the field of cosmic explosions (namely gamma-ray bursts) and proceed to speculate on (and announce) new classes of cosmic explosions. Such a discussion is timely given the imminent commissioning of wide-angle optical surveys for transient sources (e.g. SkyMapper, PS1, LSST).
How Collisionless Shocks Work (And How They Don't)
Collisionless shocks are ubiquitous in astrophysics. Besides their primary role of terminating supersonic flows, collisionless shocks are commonly inferred to accelerate nonthermal particles and cosmic rays,and to generate significant magnetic fields. How and if this actually happens remains largely a realm of speculations. I will present the results of a systematic study of relativistic collisionless shocks through ab-initio particle-in-cell simulations, focusing on the basic physics that mediates a shock. I will show how shock properties depend on magnetization and composition of the flow and describe which parameter regimes are conducive to particle acceleration. In particular, I will show the first evidence for self-consistent Fermi particle acceleration in simulations, and address the issue of the electron-ion temperature ratio in relativistic shocks. These simulations begin to place constraints on the composition and magnetization of relativistic outflows in astrophysics.
Dark Energy Constraints from Growth of Structure: Results from the 400 deg2 X-ray Cluster Survey
The growth of matter density perturbation is a dark energy probe complementary to the geometrical cosmological tests provided, e.g., by SN Ia or baryonic acoustic oscillations in the galaxy power spectrum. Evolution of the mass function of galaxy clusters is a sensitive measure of growth of structure and hence, dark energy. The cluster sample used in this work is selected in X-ray using ROSAT pointed observation, covering 400 deg^2 of the extragalactic sky. The high-quality X-ray data later obtained with Chandra provide accurate total mass estimates in individual cluster. This sample provides the most precise at present determination of the evolution in the cluster mass function. The dark energy equation of state parameter, w, is constrained to +-0.2 by clusters alone; to +-0.1 by combination of the cluster and CMB data; to +- 0.07 by combining CMB, SN Ia, BAO, and clusters.
From Protostars to Planets to Debris Around Young Suns
This talk will summarize our basic understanding of the evolutionary time scales associated with Circumstellar material. I will cover both the dissipation of primordial dust and gas, which is associated with the formation of planets, and the generation and evolution of secondary or debris dust, which is the most readily apparent astronomical signature of our own mature planetary system.
New Tests of Strong-Field General Relativity with Black Holes and Neutron Stars
In contrast to gravity in the weak-field regime, which has been subjected to numerous experimental tests, gravity in the strong-field regime is largely unconstrained by experiments. Indeed, a large class of gravity theories can be constructed that obey the Einstein equivalence principle and cannot be rejected by solar system tests, but that diverge from general relativity in the strong-field regime. I show that such theories predict black holes and neutron stars with significantly different properties than their general relativistic counterparts. I then discuss how recent observations with current telescopes have provided interesting new constraints on scalar-tensor and braneworld gravity models that are comparable to solar-system and table-top experiments.
Star Formation as probed by Spitzer
Recent science results from the Spitzer Infrared Nearby Galaxies Survey (SINGS) will be reviewed. In particular, the unprecedented angular resolution of Spitzer has enabled the investigation of the mid-IR emission as a tracer of star formation both of and within galaxies. One immediate application of the mid-IR tracers is to the spatially-resolved Schmidt Law, which I will discuss for the specific case of the nearby galaxy M51. Applicability of the mid-IR SFR tracers to samples of galaxies at cosmological distances will be also discussed.
Three-Year Results from WMAP (with Commentary)
The data from the first three years of operation of the Wilkinson Microwave Anisotropy Probe (WMAP) satellite provide detailed full-sky maps of the cosmic microwave background temperature anisotropy and new full-sky maps of the polarization. Together, the data provide a wealth of cosmological information, including the age of the universe, the epoch when the first stars formed, and the overall composition of baryonic matter, dark matter, and dark energy. The results also provide constraints on the period of inflationary expansion in the very first moments of time. These and other aspects of the mission results will be discussed and commented on.
WMAP, part of NASA's Explorers program, was launched on June 30, 2001. The WMAP satellite was produced in a partnership between the Goddard Space Flight Center and Princeton University. The WMAP team also includes researchers at the Johns Hopkins University; the Canadian Institute of Theoretical Astrophysics; University of Texas; University of Chicago; Brown University; University of British Columbia; and University of California, Los Angeles.
Recent results from the SDSS Lens + ACS survey
I'll present some of the latest results from our (SLACS) survey for galaxy-galaxy strong gravitational lenses discovered by following up the spectroscopic survey of SDSS with direct ACS imaging. As the largest lens survey, we have confirmed more than 87 multiply imaged systems with a myriad of image configurations. This unique and homogeneous data set constrains the intrinsic properties of the lensing galaxies and allows us to construct a more "fundamental" plane of early-type galaxies.
Injection of Small Bodies into the Interstellar Medium by Planetary Nebulae
Recent studies of the nearest planetary nebulae using the Hubble Space Telescope indicate much and perhaps most of the material ejected by intermediate mass stars during the planetary nebula phase is trapped into dense knots of planetary mass. I will report on a detailed study of these knots in the nearest bright planetary nebula, NGC 7293-the Helix Nebula. Using data spanning x-ray to radio wavelengths, we have been able to establish the basic characteristics and the physics governing the knots. We have been able to establish the source of the intense luminosity of these knots in the ubiquitous H2 molecule. The tantalizing question that remains is whether or not these condensations survive in the Interstellar Medium and play a role in the formation of stars and planets.
Echoes of Galaxy Assembly: Faint Light Around Nearby Galaxies
Galaxies are not simple superpositions of disks and spheroids. Instead, most disk galaxies host multiple faint stellar components. Studies within the Milky Way suggest that these additional components are old and chemically unevolved, and that they trace distinct epochs in the early history of the Galaxy. I will discuss recent work on the structure, kinematics, and stellar populations of thick disks and stellar halos, and the important constraints that they place on the assembly of disk galaxies.
Wide-Field Surveys of the Optical Sky: The Large Synoptic Survey Telescope (LSST)
Recent technological advances have now made it possible to carry out deep optical surveys of major fractions of the visible sky. Such surveys enable a diverse array of astronomical investigations, ranging from the search for small moving objects in the solar system to studies of the assembly history of the Milky Way. In terms of cosmology, wide-field surveys can yield tight constraints on models of dark energy using a variety of independent techniques.
The Large Synoptic Survey Telescope (LSST) is the most ambitious project of this kind that has yet been proposed. With an 8.4 m primary mirror, and its 3.2 Gigapixel, 10 square degree camera, LSST will provide a nearly an order of magnitude improvement in survey speed over all existing surveys, or those which are currently in development. Over its ten years of operation, LSST will survey 20,000 square degrees of sky in six optical colors down to 27th magnitude. At least a thousand distinct images will be acquired of every field, enabling a plethora of statistical investigations for intrinsic variability and for control of systematics in deep imaging studies.
I will describe some of the science that will be made possible by the construction of LSST and give a brief overview of the technical design.
Star Formation at High z: Evidence for two types of DLAs?
Damped Ly_ absorption systems (DLAs) are a population of objects that act as neutral-gas reservoirs for star formation at high redshift. If the star formation efficiency in DLAs is the same as in :current galaxies, a significant fraction of the sky would be covered by emission from low surface-brightness objects. I describe results of a recent survey for such emission using deep images from the Hubble Ultra Deep Field. The low rate of detection implies a low efficiency for in situ star formation throughout the neutral gas. But evidence that the gas emits cooling radiation suggests it is being heated. I discuss why in DLAs with cooling rates below a critical value the gas is heated by FUV background radiation, and in DLAs with cooling rates above the critical value local sources of FUV radiation are required. The local sources are likely to be compact Lyman Break galaxies (LBGs) embedded in the DLA gas. I discuss evidence that the critical cooling rate divides the DLA sample into two distinct populations with different physical properties.
Three Mysteries of Matter
Particle physicists are on the verge of a direct experimental look at the mysterious Higgs field, which allegedly gives mass to all the elementary particles. Beyond the Higgs, other mysteries of matter face us, such as the identity of dark matter and the origin of the baryon asymmetry of the universe, both of which require new laws of physics beyond the Standard Model. I will summarize our present theoretical understanding of how these mysteries may be related within the context of TeV-scale Supersymmetry, and discuss experimental probes of this scenario.
Probing the Extremes of Surface Brightness at the Faint End of the Galaxy Luminosity Function
Models of structure formation based on cold dark matter generate a mass spectrum of dark-matter halos that is much steeper than the mass function of observed galaxies. In environments comparable to the Local Group, this “missing satellite” problem is particularly acute for satellite masses below approximately a hundred million solar masses, where the evolution of satellite galaxies is complex, and where the observations are least certain. I will discuss how two ongoing surveys targeting galaxies at opposite extremes of surface brightness are probing the evolution of faint satellite galaxies in environments ranging from low-mass groups to the most massive clusters of galaxies.
The SDSS Supernova Survey
Some Uses of Gravitational Lensing
The first proposed use of gravitational lenses was to measure the Hubble constant. Recent measurements are consistent with those obtained from other determinations and the challenge is to reduce the systematic error on selected sources. Attempts to do this for the source B1608+656 will be discussed and prospects for application to a larger sample will be considered. A more recent proposal to use weak lensing of the clustering of the faintest "galaxies" -- to help understand their redshift distribution, physical properties and role in galaxy formation -- will be described.
GammeV - a gamma to milli-eV particle search
The GammeV experiment uses a high-power laser and an accelerator magnet for photons to potentially oscillate into new milli-eV mass astroparticles. I will describe how I spent my Summer vacation to set interesting limits for these new class of particles.
Dark Matter and Galaxies
I will discuss various aspects of the relation between galaxies and dark matter and more general issues in the theory of galaxy formation, including: disk galaxy scaling relations and the fraction of baryons in galaxies, the relation between satellite galaxies and dark matter substructures, the universality of the relation between central galaxies and dark matter halos,the relative accretion/merger rates of central and satellite galaxies, and the origin of bimodality in the galaxy color distribution and the shape of the galaxy luminosity function. While recent theoretical and observational developments have shed light on many of these issues, many puzzles remain.
Early Results from the Local Cluster Substructure Survey (LoCuSS)
Gravitational lensing has long been recognized as a powerful tool for mapping the distribution of dark matter in clusters, and yet until recently, it has only been applied routinely to individual clusters. I will show that recent results are changing this picture; both strong and weak lensing are developing rapidly into tools that can routinely be applied to samples rivaling, for example, those considered by X-ray-only scaling relation studies. I will introduce a new survey, the Local Cluster Substructure Survey (LoCuSS) that is assembling a sample of ~100 clusters with HST, Subaru, Chandra, XMM, SZE and Spitzer data. The overall goal is to study how the recent assembly history of the clusters (as revealed by lensing-based mass maps) influences the baryons trapped in the clusters. After outlining the results that motivated the survey (e.g.structural segregation in the mass-temperature plane), I will present new LoCuSS results on the mass-observable scaling relations and the relationship between cluster merger history and the propoerties and thus evolution of brightest cluster galaxies.
Weak Lensing by Large Scale Structure
Intervening structures in the universe give rise to small distortions in the shapes of distant galaxies. By measuring this tiny coherent signal, we can study the mass distribution in the universe directly, without relying on baryonic tracers. This makes weak lensing by large-scale structures a powerful probe of cosmology. I will review the topic of "cosmic shear" and discuss how the signal is extracted from the data. I will present results from recent surveys, most notably the CFHT Legacy Survey. Finally I will discuss what will be required to significantly improve constraints on the properties of dark energy.
Noble Travails: Noble Liquid Detectors Searching for Particle Dark Matter
Particle dark matter is thought to be the overwhelming majority of the matter in the Universe, dwarfing the contribution from conventional material that we, the earth and the stars, are composed of. However, we still have no direct evidence for the existence of particle dark matter. This may soon change...
I will report on the latest results from the XENON10 liquid xenon- based detector which began searching for particle dark matter at Gran Sasso in late 2006 (http://xenon.brown.edu). The experiment has demonstrated (http://arxiv.org/abs/0706.0039) a world class direct search sensitivity which is a factor 4 better than its current nearest rival (CDMS II). I will discuss some of the details of this experiment.
I will also discuss some of the other noble liquid target experiments that are also providing competitive sensitivities in the race for the direct detection of particle dark matter. Theoretical estimates, based on supersymmetric models predict dark matter interaction rates from the best sensitivity of existing direct detection experiments of
~1 evts/kg/month, down to rates of ~1 evts/100 kg/yr, and below this.
Current and future noble liquid experiments for dark matter searches, range in scale from 10's kg to tonnes, and are designed to rise to this challenge. The new liquid xenon detector, LUX, which has begun construction, will be 100 times more sensitive than current best search experiments.
Large Scale Structures and Galaxy Evolution in the COSMOS Survey
The COSMOS survey is the largest high redshift galaxy evolution survey ever done -- imaging 2 square degrees with all major space-based and ground based observatories. I will describe the key data in the survey and then present recent results on large-scale structures, the dark matter distributions and galaxy evolution.
Magnetic Fields in Star-Forming Clouds
The "strong field" picture of star formation involves a slow drift of neutrals through a magnetic field into the core of a self-gravitating cloud until magnetic support can no longer prevent dynamical collapse. A contrasting "weak field", picture involves support primarily by turbulence with clumps sometimes gaining sufficient mass to become self-gravitating. Attempts to compare observations with theoretical simulations have not been adequate to provide a convincing model.
I will discus the outlook for measuring relevant properties of the magnetic field such as the angular dispersion, the relative magnitudes of the systematic and turbulent components, and the size of the turbulent eddies.
Star Formation and Neutral Gas in Normal Galaxies
Great strides have been made in galaxy evolution in the last decade through a combination of improved observations, semi-analytic modeling of galaxy evolution, and simulations. Nevertheless, one of the biggest gaps in our knowledge has to do with how stars form on galactic scales. How does the number and distribution of new stars in galaxies get established? How does this change as a function of redshift? What is the role of "feedback," i.e. mass expulsion? How ubiquitous is the IMF? I will take the position that the distribution and state of the interstellar gas is paramount in answering these questions. I will describe several well-known observational facts about the distribution of atomic gas in spiral galaxies that have no agreed upon explanation. We will see that the neutral gas, the atomic plus the molecular, follows an approximate scaling relation. I will discuss the molecular gas depletion time problem in spiral galaxies, and some of the implications for galaxy evolution, and a possible solution. I will show that what determines how the star forming material, the dense molecular gas is formed in galaxies, is hydrostatic pressure, and how this relates to the star formation within them.
The Enriched Xenon Observatory for Double Beta Decay
The Enriched Xenon Observatory (EXO) is a project aiming at detecting neutrinoless double beta decays of Xe-136. A xenon-filled time projection chamber (TPC) supplemented with scintillation light readout detects ionising particle interactions within its volume. When candidate events are recorded, the Ba-136 ion daughters will be identified, event by event, by means of optical spectroscopy. This coincidence technique would allow for a measurement of double beta decays virtually immune to external radioactive contaminations. The EXO collaboration is planning on combining these experimental techniques in a ton-scale Xe detector using a phased approach.
A smaller detector, EXO-200, employing 200 kg of enriched xenon (80% Xe-136) in liquid form within a TPC with scintillation readout and with no Ba identification, is in advanced stage of assembly. Its cryogenic and xenon handling systems are being re-assembled at the Waste Isolation Pilot Plant (WIPP) underground site in New Mexico. The central detector is planned to be installed by early 2008. As a parallel effort to EXO-200, strategies for Ba tagging are being developed in the laboratory.
I will present the EXO experiment in the context of neutrinoless double beta decay searches and describe the EXO-200 detector in detail, discussing its physics goals, experimental challenges, and schedule. I will also illustrate some of the most promising approaches for tagging single Ba ions produced in a ton-scale Xe detector, show milestone results achieved in laboratory setups, and discuss the EXO timeline for the near future.
Hot on the trail of particle dark matter
Over seventy years, a body of evidence has steadily grown indicating that much of the Universe's mass is non-luminous. Still today, however, we have not identified what makes up this mysteriously dark substance. Many experimental programs that hope to change this are underway, including deep underground detectors, gamma-ray telescopes, neutrino and anti-matter detectors, as well as particle colliders. Each of these efforts are searching for clues of dark matter's identity. With the new technologies needed to observe these particles rapidly developing, the hunt to discover dark matter's identity is well underway.