Talks & Events
Cosmology with Large Datasets
Gravity at largest observable distances
On the Origin of the Red Sequence and Bimodality of Galaxy Properties
The color-magnitude relation and the related dichotomy in the morphological, structural, and spectro-photometric properties of galaxies are well-known and fundamental observed properties of galaxies. These relations seem to be largely in place as early as redshift one. However, standard models of galaxy formation set within the Cold Dark Matter paradigm fail to reproduce either the color-magnitude relation or the observed strong bimodality in galaxy properties. I will show that including the feedback from Active Galactic Nuclei in such models may hold the key to understanding these fundamental observations.
Pulsar Timing Array: Probing the Nanohertz Gravitational Wave Background
constraints on the redshift dependence of the dark energy potential
Cosmology has now a standard model. This model is described by a handful of cosmological parameters, now determined with unprecedented precision, many of them are measured in multiple independent ways. The standard cosmological model is simple, yet puzzling. The big challenge is to shed some light on the dark energy component, which reveals itself only through the acceleration of the universe. Under the assumtion that dark energy is a slowly rolling scalar field, We develop a formalism to characterize the redshift evolution of the dark energy potential. We show that in principle the shape of this potential can be recovered non-parametrically. Since presently available data do not allow a non-parametric reconstruction, we consider a generic parametric description and use observations of passively evolving galaxies derive constraints on the shape of the dark energy potential in the range 0.1
New Insights into Quasar Wind Structure
Seismology of Accreting White Dwarfs
Pulsation modes have recently been observed in a handful of white dwarf primaries of Cataclysmic Variables, allowing an interesting new probe into the structure of accreting white dwarfs. I will briefly discuss the seismology of these objects, how stellar properties may be inferred from the observed mode frequencies, and new work on mode driving mechanisms.
Early Structure Formation
Through a series of numerical simulations we have arrived at the conclusions that the first luminous objects in the universe were massive isolated stars. These predictions will be tested observationally in the near future. This talk will highlight some of our current attempts to extend such studies of early structure formation to the first metal enriched stars and small early galaxies.
A Tour in the Neutron Star Zoo
Neutron stars are the most common end state of stars, and hundreds of millions of them populate the Galaxy. Despite the fact that several of their properties, such as their mass and composition, vary over relatively narrow ranges, neutron stars give rise to a bewildering zoo of observational properties. In this talk, I will review the main characteristics of the various classes of neutron stars, highlighting differences and connections, and the extent to which these are due to intrinsic versus environmental properties. In particular, I will focus on the origin of the quiescent X-ray emission in Anomalous X-ray pulsars and Soft Gamma-Ray repeaters, the relation between jets in radio pulsars and their anomalous braking indices, and the ultimate fate of the large number of old, isolated neutron stars in the Galaxy. I will finally show how the statistical properties of radio pulsars associated with giant HI supershells can be used to constrain neutron stars birth parameters, and hence better understand the physics of supernova explosions.
Recent Results from the Spitzer Space Telescope: A New View of the Infrared Universe
The Spitzer Space Telescope, launched on 25 August 2003, is producing a significantly new view of the Universe at infrared wavelengths. Spitzer is the fourth and final element in NASA's Great Observatory series. It consists of an 85-cm telescope and three cryogenically-cooled instruments capable of imaging from 3 to 180 microns wavelength and spectroscopy from 5 to 40 microns wavelength. Combining the intrinsic sensitivity achievable with a cryogenic telescope in space with the high sensitivity of modern, large-area infrared detector arrays, Spitzer is providing the astronomical community with huge gains in capability for exploring the infrared Universe. Primary among Spitzer's scientific objectives are the study of the formation and evolution of galaxies in the early Universe, understanding energy sources in ultraluminous galaxies, the study of star formation and evolution, determining the structure and evolution of planetary disks around nearby stars, and exploring the nature and distribution of brown dwarfs. After a brief description of the Spitzer mission, results from Spitzer's extragalactic and galactic observational programs will be presented, showing many of Spitzerís very spectacular images.
Raptor Observations of GRBs and Data Mining
Tuning into Cosmic Rays and Neutrinos
Cosmic rays and neutrinos with energies sensibly expressed in joules per particle have become the subject of considerable interest. Cosmic ray fluxes at these energies are low, and detectors with areas of thousands of square kilometers are needed. The existence of these ultrahigh energy cosmic rays implies the presence of a related flux of neutrinos generated by interactions with the cosmic microwave background. These neutrinos have not yet been detected, because a volumetric exposure of about 100 km^3 sr yr is required. Radio frequency techniques offer an economical approach to obtaining the large collecting power required to make progress on these problems. I will review the recent progress made in radio detection of ultra-high energy cosmic rays and neutrinos, and the prospects for the first detection of the neutrino flux by a balloon payload viewing neutrino interactions in the Antarctic ice sheet.
Super Flare from SGR 1806-20
The Cosmic Evolution of Active Galactic Nuclei
Chandra detects X-rays emitted during accretion onto supermassive black holes, even when they are highly obscured. Using extensive follow-up observations of the X-ray sources detected in both deep and wide-area Chandra surveys, I will describe the cosmic evolution of the X-ray luminosity functions and the reconstruction of the accretion history of supermassive black holes.
Acceleration of Cosmic Rays in Supernova Remnants
Simulating Reionization: Yesterday, Today, Tomorrow
I will review the current status of modeling of the process of cosmic reionization with numerical simulations. With the rest of numerical cosmology, the study of reionization is going through a revolutionary transition from the first stage of primitive, semi-qualitative simulations to the realistic, quantitative models that can be sensisibly compared to observational data. This transition, when complete, will allow us to model reionization with spatial dynamic range in excess of 10 million. Armed with these new tools, we will be able to reach the next big prize in numerical cosmology - helium reionization.
A Cross-correlation Study of Galaxies and QSO Absorption-line Systems
The forest of absorption line systems observed in the spectra of background QSOs offers a sensitive probe of tenuous gas in extended halos around galaxies as well as in intergalactic medium. Understanding the origin of QSO absorption line systems (QALS) bears directly on our effort to uncover missing baryons and to apply known absorption line statistics to constrain statistical properties of faint galaxies in the distant universe. I will show based on comparisons of galaxies and QSO absorbers along common lines of sight that extended gaseous envelopes are a common and generic feature of galaxies of a wide range of luminosity and morphological type. In addition, I will present a cross-correlation analysis between galaxies and absorbers that yields further constraints on the large-scale distribution of tenuous gas with respect to galaxies. Finally, I will discuss the prospects of studying absorption lines systems identified along the sightlines toward the optical afterglow of gamma-ray bursts.
Probing the ISM of High z Galaxies with DLA and GRB Afterglows
I will describe new results related to studies of the HI content of damped Lya systems and high resolution spectroscopy of GRB afterglows. The talk will highlight cosmological evolution in neutral gas (measured from the SDSS Data Release 3) and physical properties of gas surrounding star forming regions in high z galaxies.
The Swift Gamma-Ray Burst Explorer: Early views into black hole formation
NASA's Swift Gamma-Ray Burst Explorer is designed to localize and study Gamma-Ray Bursts (GRBs) and their afterglows, beginning within a few minutes of the burst events. Swift has now discovered over 75 GRBs and has produced detailed X-ray light curves and spectroscopy on over 60 of these, exceeding in the past nine months the total sample of GRB afterglows found in the previous 8 years. Key findings to date include rapid decays at early times and giant X-ray flares, suggesting that the central engines of GRBs continue long past the end of the prompt gamma-ray emission. We have also localized 3 short GRBs, providing important supporting evidence for compact merger theories for short bursts. I will focus on the highlights of the mission so far, concentrating primarily on afterglow results from the Swift X-ray Telescope.
The Structure of Early-Type Galaxies
The inner few hundred parsecs of galaxies are of interests for a variety of reasons. Because gas, dust and dense stellar systems are naturally drawn to the bottom of the potential well, a galaxy history is best reflected in the central regions. Dynamical processes occur faster at the center, where dynamical timescales are shorter than elsewhere in the galaxy. Finally, the centers of most galaxies host supermassive black holes, the evolution of which is intimately related to that of the surrounding galaxy. The ACS Virgo Cluster Survey (ACSVCS) targeted 100 early-type members of the Virgo cluster, from giants to dwarfs, each imaged at 7pc resolution in the g- and z-bands using the Advanced Camera for Surveys on board HST. I will discuss some very recent results from the ACSVCS, focusing in particular on the core and nuclear strucure of this fascinating
High Resolution Observations of the Cosmic Microwave Background with ACBAR.
Observations of the Cosmic Microwave Background (CMB) radiation provide a window to the Universe as it existed 400,000 years after the Big Bang. This snapshot of the early Universe encodes a wealth of information about the constituents of the Universe and perhaps the mechanism of inflation. High resolution observations of the CMB are an important compliment to the results of experiments with coarser resolution such as WMAP. The Arcminute Cosmology Bolometer Array Receiver (ACBAR) is a 16 element 230mK bolometer array designed to observe the CMB with a 5' beam. ACBAR has just completed its 4th and final season of observation at the South Pole. During it's lifetime it has produced maps of the CMB with unprecedented resolution and sensitivity. I will discuss the results of the full 2-year data set (to be released soon) and extrapolate to what we can expect from the 4-year data set.
The top-down approach to inflation seeks to embed it in the fundamental theory. I will review recent ideas about inflation, generation of fluctuations and reheating in string theory. The bottom-up approach to inflation reconstructs from available onservables the acceleration history of the universe. I will discuss the methods of inflationary trajectories reconstruction.
Pulsars in X-rays
Chandra and XMM-Newton observations of about 60 isolated (nonaccreting) pulsars have provided valuable data on the X-ray properties of these objects. Thanks to the higher sensitivity and better spectral resolution of the Chandra ACIS and XMM EPIC detectors, we can separate the thermal and nonthermal components of their spectra and study the radiation from the neutron star surfaces and magnetospheres with much higher certainty than it was possible in the pre-Chandra era. Particularly important is the high spatial resolution of Chandra data, which allows one to separate the pulsar radiation from that of compact pulsar wind nebulae (PWNe) around young pulsars and study the PWN morphology and spectra. Monitoring observations of the brightest PWNe, around the Crab and Vela pulsars, revealed stunning pictures of their complicated dynamic behavior. The large sample of pulsars and PWNe observed with the Chandra and XMM-Newton observatories made it possible to examine the correlations between various properties of these objects and their evolution. I will overview the general X-ray properties of pulsars and PWNe and present most interesting results of observations of individual objects.
UV and X-ray Observations of the Warm-Hot Intergalactic Medium
Hydrodynamic cosmological simulations predict that most of the baryons at low redshift reside in warm-hot intergalactic medium, WHIM. One of the few prospects for detecting this shock heated, low-density gas is via the "X-ray forest" of absorption lines it should produce in quasar spectra. Such observations are now possible with Chandra X-ray Observatory. I will describe recent efforts to trace the WHIM and determine its physical properties, with Chandra as well as with HST and FUSE, and discuss the implications towards the missing baryon problem. I will also describe the attempts to detect the Local Group baryons with X-ray and UV observations.