Talks & Events
Astrophysics and Cosmology with HESS
The High Energy Stereoscopic System (HESS) of imaging atmospheric Cherenkov telescopes is a powerful multi-functional tool for spectrometric, morphological and temporal studies of nonthermal phenomena and objects representing different Galactic and Extragalactic source populations. I will overview the recent discoveries by HESS with an emphasis on the detection of TeV emission from relatively distant blazars, and discuss some cosmological implications of these results for the Extragalactic Background Light (EBL) at optical and near-infrared wavelengths.
The tension in Standard Big Bang Nucleosynthesis:the baryon density, D, He and Li.
Three methods of measuring the cosmological baryons density now agree within about 10%: the CMB, the D/H ratio using Standard Big Bang Nucleosynthesis, and the Lyman-alpha absorption from the IGM at z=2 to 3. We recently measured the mean amount of absorption in the IGM with an error of 1%, and we use these measurements, and a large set of hydrodynamic simulations to determine the cosmological and astrophysical parameters of the IGM, including the temperature and the baryon density. Using this baryon density, SBBN predicts a factor 3 more 7Li than is seen in halo stars, and systematically more 4He that most measurements. We review the systematic errors and new measurements, and we discuss modifications to BBN that might explain the tension between D, He and Li.
Well-posedness of the Cauchy problem of General Relativity
The most common approach to solving the Cauchy problem of General Relativity (GR) is by implementing free evolution schemes of the 3+1 decomposition of Einstein's equations. A long-standing problem in numerical relativity is achieving long-term and stable numerical integration of the GR equations. At present, this is possible for rather short times or for special symmetric cases. In this talk, we will present a general theory for the study of well-posedness of constrained evolution of 3+1 formulations of GR. This new approach incorporates the constraint equations directly and therefore is principally different from standard analyses of well-posedness of free evolution schemes. We will demonstrate that well-posedness of constrained evolution of 3+1 formulations of GR depends entirely on the properties of the gauge and comment on consequences of this result.
Carbon Enhancement in the Galaxy -- A New Probe of the First Stars
Recent large surveys of metal-poor stars in the Galaxy have revealed that a surprising fraction of them are enhanced in their carbon-to-iron ratios by factors of from 10-10,000 relative to the solar ratio. Although most of the stars in the metallicity interval -2.7 < [Fe/H] < -2.0 are likely to have arisen from Asymptotic Giant Branch processing (and subsequent dumping via mass transfer to a surviving companion), there exist many stars with [Fe/H[ < -3.0 (including the two lowest [Fe/H] stars known, with [Fe/H] < -5.0) that cannot be accounted for by this process. Rather, primordial (or nearly primordial) progenitors are implicated. I report on the existing information from present surveys, and describe the results that will come from the recently-funded extension of the SDSS, which includes the program SEGUE = Sloan Extension for Galactic Understanding and Exploration. SEGUE will identify some 20,000 stars with [Fe/H] < -2.0, several thousand of which are expected to be carbon enhanced.
Searching for Enlightenment
In a remote valley in western China stand 10,000 television antennas spread across ten square kilometers. Tied together by an array of hundreds of PC computers this system, the Primeval Structure Telescope (PaST), will soon be used to search for evidence of the early ionization of the Universe. Today most hydrogen gas in the cosmos is ionized. But, before the first stars formed, neutral hydrogen was ubiquitous. By imaging 21 cm hydrogen hyperfine emission from this gas at redshifts from 6 to 25 the ionization state of the early universe can be explored. As the first stars began to light up, the 21 cm emission was extinguished, leaving a patchy radio sky. This patchy structure will be imaged with the PaST array, allowing the era of the first stars to be dated and studied.
Reflections of AGN Outbursts In the Hot Gas in Galaxies and Clusters
Chandra X-ray images show the presense of shocks, jets, cavities and buoyant bubbles in the hot gas in galaxies, groups and clusters. These features all owe their origin to outbursts from the supermassive black hole (SMBH) at the nucleus of the system. In this talk I will review recent results on AGN outbursts in the rich clusters MS0735.6+7421, Perseus, Hercules A, Hydra A and Virgo as well as the effects of outbursts and the X-ray luminosities of low luminosity AGN in a sample of 160 early-type galaxies.
Probing fundamental physics with the Atacama Cosmology Telescope
The Atacama Cosmology Telescope (ACT) is a millimeter-wave telescope designed to map the CMB temperature at arcminute angular scales. ACT will image a few hundred square degrees of the CMB in the southern sky from a site in the Atacama desert. In addition to measuring the primary CMB power spectrum, ACT will survey for massive galaxy clusters through their Sunyaev-Zel'dovich (SZ) signal. Three frequency bands around the SZ null--145, 215, and 265 GHz--will help to separate cluster signals from primordial anisotropy and from point sources. The maps will offer data on a wide variety of questions in fundamental physics and cosmology, including the growth of structure, and the spectrum of primordial perturbations.
High-Energy Astrophysics with Gamma-Ray Telescopes
Gamma-rays are messengers of high-energy processes throughout the Universe: Nuclear reactions create radioactive isotopes, and particles which have been accelerated into the relativistic regime create non-thermal radiation. The penetrating nature of these gamma-rays implies that even sources which are invisible or optically thick in other wavelengths can be studied through their gamma-ray emission. The Compton Observatory had surveyed the gamma-ray sky, and had found a few surprises, beyond the images that have been obtained from cosmic-ray interactions with interstellar gas and from diffuse radioactivities originating from cosmic nucleosynthesis. ESA's INTEGRAL observatory recently made possible to probe the low-energy gamma-ray regime at better sensitivity, and, most importantly, add spectroscopy power to resolve characteristics gamma-ray lines. We will discuss the interpretations and findings from recent observations at high energies and the corresponding source models, specifically addressing sources of nucleosynthesis and of particle acceleration in our Galaxy.
Probing high-redshift luminous galaxies into the ALMA epoch
I will discuss the observational progress made in understanding the nature and evolution of luminous far-IR galaxies, and highlight the most promising future investigations that lead towards exquisite imaging quality of their optically thick emission from ALMA in about 2012.
Radiation transfer in neutron star magnetospheres: the binary pulsar and magnetars
This talk will be separated into two parts. First, I will discuss eclipses in the binary pulsar system PSR J0737-3039, where the faster pulsar A is eclipsed once per orbit. A simple model of eclipses based on synchrotron absorption on closed field lines of slower pulsar B reproduces the complicated observed light curve down to intricate details and provides a direct proof of the long-standing assumption of dipolar magnetic fields of neutron stars.
In a second part I will describe a model of non-thermal emission from magetars due to resonant cyclotron scattering of surface radiation in their magnetospheres. Applying the model to anomalous X-ray pulsar 1E 1048.1—5937 gives a fit just as good as less physically motivated “black body plus powerlaw” spectra and allows one to determine density and temperature of the magnetospheric plasma.
How Did Cassiopeia A Explode? A Chandra Very Large Project
In April/May 2004 the Chandra X-ray Observatory observed the Cassiopeia A supernova remnant for 1 million seconds as one of the first of the newly instituted 'Very Large Projects'. This is the deepest ever x-ray observation of a supernova remnant, and allows observers to exploit the angular resolution of the Chandra mirrors to the fullest extent, in that spectra of adequate signal/noise can be extracted from very small spatial regions in most locations within the remnant. I will describe progress to date, and future plans, for the analysis of this unique dataset, paying specific attention to quantifying the nature and degree of asymmetry in the explosion, and the abundances and locations of chemical elements synthesized therein.
The Formation of the Solar System
How do planets form and how long does it take? Why are their orbits circular and coplanar? What set the number of planets in our solar system? We address these fundamental questions providing a coherent story on the formation of our solar system.
Gravitational Lensing with Large Imaging Surveys
I will describe the ideas and challenges in using gravitational lensing for cosmology. Recent results in weak lensing and the observational challenges ahead will be discussed. With planned multi-color imaging surveys, weak lensing can probe dark energy and constrain alternate theories of gravity. I will compare the strengths and weaknesses of lensing with other observational methods.
Extrasolar Planets: From Hot Jupiters to Hot Earths and Beyond
We have entered a new era in planetary astrophysics with over 170 extrasolar giant planets now known. Physical properties of a subset of these planets---the hot transiting giant planets---have been measured, including mass, radius and thermal emission. Even more intriguing than the hot Jupiters are the seven known hot super-massive Earths (mass range 7 to 20 Earth masses), which are expected to consist of a large rocky component. There is a chance to observationally study this class of planets in the near future. Beyond the hot Earths, our own Earth has been studied as an exoplanet, both in its present state and in its paleoclimates. I will summarize which exoplanet physical characteristics can be inferred from spectra, the interpretation of the hot Jupiter spectral data, the possibilities for observation and interpretation of hot supermassive Earths, and the scientific highlights and prospects for the future detection and study of and planets like Earth.
Do Extragalactic Cosmic Rays Induce Cycles in Fossil Diversity
Recent work has revealed a highly statistically significant 62 +/- 3-million-year cycle in the number of marine genera. This implies a periodic process extending back 540 My. While astro- and geophysical phenomena may be periodic, no plausible mechanism has been found. The fact that the period of the diversity cycle is so close to the 64 My period of the vertical oscillation of the Solar system relative to the galactic disk is suggestive. We propose that the diversity cycle is caused by the anisotropy of cosmic ray (CR) production in the galactic halo/wind/termination shock and the shielding effect of the galactic magnetic fields. CRs affect the biosphere in a number of ways: they cause DNA damage, mutations and cancer due to increased radiation, affect the atmospheric ozone concentration and UV protection, initiate cloud formation, and can affect climate. The high statistical significance of (i) the phase agreement between maximum excursions of the Sun toward galactic north and minima of the diversity cycle and (ii) the correlation of the magnitude of diversity drops with cosmic ray peak values through all cycles provide solid support for our model.
Peeking into a Neutron Star: Neutrons, Condensates, or Quarks?
Neutron stars are the densest objects in the universe and may contain hyperon-dominated matter, condensed mesons, or even deconfined or strange quark matter. Because of their low temperatures and high chemical potentials, the physical conditions in their interiors differ greatly from the dense conditions of the early universe or those achieved at hadron colliders. This region of the QCD phase diagram can only be probed through astrophysical observations that measure the mass and radius of neutron stars. For decades, this effort has been hampered by a number of model uncertainties as well as by the lack of accurate measurements of different spectroscopic phenomena from a single source that would break the degeneracies between the neutron star parameters of interest. I discuss how we can now overcome these problems by combining recent developments in our understanding of neutron star atmospheres with observations of distinct phenomena from the same neutron star source. In particular, I report the first unique measurement of the mass and radius of the neutron star source EXO 0748-676. The high inferred mass and large radius of this neutron star rule out all the soft equations of state of neutron star matter. This result shows that condensates and unconfined quarks do not appear under the conditions found in the centers of the neutron stars.
Forming the Milky Way Halo: smooth vs chunky
I will summarize our current understanding of the formation of the Galaxy's halo, using a number of different samples of halo stars from surveys including SDSS-II's SEGUE. Evidence is mounting for a "chunky" origin for much of the halo, via infall of satellite galaxies and subsequent formation of star streams. I will also discuss constraints on the smooth halo from a new, high-quality sample of metal-poor stars in the solar neighborhood.
Report from the Dark Energy Task Force
Understanding the observed cosmic acceleration is widely ranked among the very most compelling of all outstanding problems in physical science. Many believe that nothing short of a revolution will be required in order to integrate the cosmic acceleration (often attributed to â€śdark energyâ€ť) with our understanding of fundamental physics. The DETF was formed at the request of DOE, NASA and NSF as a joint subcommittee of the Astronomy and Astrophysics Advisory Committee (AAAC) and the High Energy Physics Advisory Panel (HEPAP) to give advice on optimizing our program of dark energy studies. To this end we have assessed a wide variety of possible techniques and strategies, and developed a series of factual findings and recommendations. I will present our main conclusions and discuss their implications.
Gravitational Waves from Strings
Long discredited as the main source of cosmological perturbations, cosmic strings have re-emerged as natural structures in string theory and natural products of stringy inflation. Techniques will be discussed for detecting them, or setting limits on parameters such as the mass per unit length, using gravitational wave backgrounds. The most sensitive current probe is millisecond pulsar timing; in the future, it will be LISA.
The Giant Magellan Telescope
The Giant Magellan Telescope (GMT) is a 21-5-meter collecting-area optical/near-infrared telescope with a resolution of a 24.5-meter. The baseline site for the GMT is Las Campanas, Chile. The GMT primary mirror is comprised of seven borosilicate 8.4-meter segments, and the secondary contains seven fast-steering segments aligned to each of the primary mirrors. The first of the primary mirrors has been cast at the Steward Observatory Mirror Laboratory, and preparations are underway for its polishing and testing. Several instrument concepts have been developed covering the wavelength range from the UV/optical to the thermal infrared. The project has just undergone a Conceptual Design Review, with a recommendation to proceed to the Design Development Phase. The Science Working Group has identified several areas where the GMT will have an impact. These include: 1) the nature of dark matter and dark energy 2) the first stars and galaxies 3) star and planet formation 4) the evolution of galaxies and 5) the growth of black holes. Unique capabilities of the GMT include wide-field (~10-arcminute FOV) spectroscopy and the direct detection of exoplanets. The GMT is a consortium of research institutions consisting of the Australian National University, the Carnegie Institution of Washington, Harvard University, Massachusetts Institute of Technology, Texas A&M University, Smithsonian Astrophysical Observatory, University of Arizona, University of Michigan, and the University of Texas at Austin.
Type Ia Supernovae from the CFHT Legacy Survey
The Supernovae Legacy Survey aims at discovering and spectroscopically identifying 700 type-Ia SN during its five years of operations at CFHT in Hawaii. I will present the survey and the current constraints on matter and energy content of the Universe and on the dark energy equation of state parameter. I will focus on how we plan to increase the expected number of SN to about 1000 using an offline analysis and discuss perspectives of the survey.
Searching for protoplanetary fragments in the Sloan Digital Sky Survey
The main asteroid belt represents a significant repository of material left over from the formation of the terrestrial planets. However, the formation and dynamical history of the asteroid belt is complicated, with a significant fraction of asteroids having undergone varying degrees of mechanical and chemical processing. One very important such mechanism is the formation and subsequent disruption of large planetesimals thought to have occured in the early Solar System. Among the most highly processed asteroids are the Vtype. V-type asteroids exhibit absorption bands characteristic of a basaltic (i.e. volcanic) composition, indicating an origin on the crust of large differentiated parent bodies. Until recently all known V-type asteroids were dynamically and compositionally consistent with being fragments of 4 Vesta, the second largest of the asteroids. This talk describes a search for new V-type asteroids, dynamically independent of Vesta, using the Sloan Digital Sky Survey dataset and follow-up observations from Apache Point Observatory. The discovery of new independent V-type asteroids will give us important insights into the timing and processes involved in the formation of planets.
Massive Black Holes from Early Times to the Present
I'll discuss models for the hierarchical growth of supermassive black holes, feeding pregalactic black hole seeds. Mergers and dynamical interactions, as well as their implications, will be critically addressed. I'll also discuss the constraints on the early evolution of the black holes required by the observations of z=6 quasars.
Optically Selected Galaxy Clusters: Update 2006
The Red-Sequence Cluster Surveys (RCS-1, now complete, and RCS-2, ongoing) are a pair of large optical surveys comprising about 1100 square degrees of imaging, designed to find clusters to redshifts beyond one. I will present a number of recent results from these surveys, including the cosmological analysis of RCS-1, and new results on strong lensing in RCS-2. Additionally I will discuss the SDSS cluster catalog, and the future of cluster research within the SDSS.
The Origin of Spheroidal Galaxies
For decades, the standard paradigm for spheroidal galaxy formation was the monolithic collapse theory, in which spheroids collapsed gravitationally in bulk and formed all their stars at very high redshift. This talk will examine evidence that has accumulated over the past year that suggests an extended formation period for spheroidal systems, with many of them forming rather recently via the quenching of blue, star-forming galaxies AFTER z = 1. The mechanism for this quenching is not well understood, but it might be feedback from active black holes. If true, the symbiosis is complete in that the galaxy gives birth to the black hole, but the black hole eventually determines the star-formation history of the galaxy.
High Resolution Radio Cosmology Near and Far
Measurements of the angular power spectrum of temperature anisotropies in the Microwave Background strongly support a model in which "primordial" energy density fluctuations generated by inflation-- modified mainly by simple gravity-driven evolutionary processes-- grow to form the structures seen today. While the last scattering surface itself appears to be well-explained by a simple physical picture, important questions remain about the subsequent evolution of large scale structures. For example: what is the state of the present-day mass density field, and where are the baryons? I will discuss two projects with the Green Bank Telescope which address these questions: 30 GHz discrete source observations undertaken to improve the precision of arcminute-scale CMB measurements; and a search for the long millimeter-wave hyperfine transitions of metal ions. I will also briefly describe recent first-light observations with the GBT at 90 GHz and related science prospects in this waveband.
Neutrino Telescopes and Their Mission
The current generation of neutrino telescopes, such as AMANDA and Baikal, has proved that the neutrino measurement is feasible using natural gigantic radiators, such as the polar ice and lake deep water. The cubic kilometer scale detectors are becoming a reality with IceCube being built at the South Pole. Sea water is a critical environment but European collaborations are proving that detectors can operate with good angular resolution, but with larger optical background induced by bioluminescence and potassium 40. I will discuss what could be the most interesting signatures for neutrino telescopes, which could be the astrophysical impact of observations and which are the performances of existing and under construction experiments.
New Challenges in Astrophysical Particle Acceleration
It is nearly a century since Victor Hess understood the nature of GeV cosmic radiation and over sixty years since Pierre Auger demonstrated the presence of PeV particles. The last decade has seen detailed studies of the energy frontier just below a ZeV. In addition imaging of supernova remnants and extragalactic jets by Chandra X-ray Observatory and H.E.S.S. has allowed us to explore astrophysical acceleration sites in detail. High Mach number shock fronts are confirmed as powerful accelerators and it now appears that they also amplify magnetic field and create PeV particles. The manner by which this happens will be discussed. PeV electron acceleration in relativistic jets requires a different mechanism and some possibilities will be explained. Candidate "ZeVatrons" will be reviewed. Finally, prospects for GLAST, which is due for launch next
Fall, will be described.
Mergers of Massive Galaxies, Central Black Holes, and Dark Matter Halos
While mergers of gas-rich disk galaxies are thought to lead to the formation of elliptical galaxies, merging of elliptical galaxies is a potentially important process for building up the most massive galaxies and their central black holes in the low-redshift universe. I will discuss results of simulations of these mergers and the dynamical interplay among stars, dark matter, and black holes in forming the global as well as central properties of massive galaxies. New results from recent analyses of SDSS luminous galaxies find similar trends as seen in the simulations. I will also discuss recent work on understanding the mergers of dark matter halos and the halo mass function using the Smoluchowski coagulation equation.
The Swift Observatory and High Energy Emission from GRB Afterglows
Swift was launched 2004 November 20. Since that time, the Burst Alert Telescope has detected approximately 2 gamma ray bursts (GRBs) per week, and the pointed instruments, including the X-ray Telescope and the Ultraviolet Optical Telescope, have slewed to a large fraction of these bursts with unprecedented speed. The prompt observation of GRB positions has allowed the X-ray telescope to study GRB afterglows at times that are several orders of magnitude earlier than past observations. Many exciting results have emerged, including X-ray afterglow detections of multiple short-hard bursts, ubiquitous flares at late times (100-10000 s) which imply delayed sporadic internal engine activity, a new canonical afterglow light curve that includes the transition from the prompt emission and multiple breaks in the power law-decay slope, very high redshift afterglow measurements, as well as other new results. A summary of these recent observations and their implications will be discussed, with particular emphasis on the emergence of new phenomena in the early X-ray afterglows of long bursts.
Mapping Dark Matter and Dark Energy Using Gravitational Lensing
Results will be presented on the detailed distribution of dark matter and implications for the nature of dark matter using gravitational lensing of background sources by massive foreground clusters. Strong lensing arcs offer a unique probe of dark energy as well, and I will discuss the utility and feasibility of this technique.