Talks & Events
KICP Friday Noon Seminars: 2010
Observational Signatures of Hydrogen and Helium Reionization
The reionization of hydrogen and helium are intimately linked to the formation of the first galaxies and quasars, yet setting direct observational constraints on either process has proven to be highly challenging. Over the past few years, a variety of observations have started to shed lights on both. I will review progress made in the field, and discuss my group's recent work on metal enrichment by the first galaxies and the thermal history of the intergalactic medium. These studies are providing unique insights into the first three billion years of evolution in the IGM, and will complement upcoming observations in developing a consensus picture of hydrogen and helium reionization.
New Perspective on Galaxy Clustering as a Cosmological Probe: General Relativistic Effects
The hunt for dark matter continues
The first generations of galaxies and 21cm fluctuations
The formation of the first generation of galaxies in the Universe has been studied for many years. We studied this epoch taking into account important physical ingredients. We show that these ingredients play a major role in the evolution of over-densities both in the linear and non-linear regime and on the formation and properties of the first luminous objects in the Universe. We give a detailed set of predictions for these objects, and in particular show that the first observable star was most likely to form only 30 million years after the big bang (at redshift 65), with the first observable Gamma ray burst exploding only a few million years later. Observations of the 21cm radiation from these epochs will help unfold the cosmic evolution of the first generation of galaxies. These observations are strongly affected by the UV radiation from stars at this era which couples the properties of the 21cm signal and the distribution of the first galaxies. Using an accurate analysis of this coupling process, including the ionized gas bubble around each galaxy, we predict a clear signature of this process. We show that such observable signatures can be used to detect and study the population of galaxies that formed as early as 200 Myr after the big bang.
Deconvolving the GeV Sky: Deriving the Physics of Starforming Galaxies, Active Galaxies, and Dark Matter
The recently launched Fermi Gamma-ray Space Telescope promises a decade of excitement and discovery in the GeV waveband. Fermi represents a dramatic improvement in instrumental capabilities for point source observations in GeV gamma rays compared to its predecessors, and is expected to revolutionize our understanding of known gamma-ray sources such as star-forming galaxies and active galactic nuclei. However, Fermi also opens up for the very first time the window of observations between 20-200 GeV, and carries a significant potential for the discovery of a yet-unknown exotic signal, such as a signature of dark matter annihilation. I will discuss promising novel techniques for disentangling the different components of the gamma-ray sky, which will allow us to maximize the science return of the mission; to uncover and identify any potential dark matter signal that may be hiding among the diffuse GeV photons; and to use gamma-ray observations to directly address problems in particle physics and astrophysics that may be otherwise inaccessible.
Peering at the horizon: A close look at Sagittarius A* and other black holes
The compact radio/IR/X-ray source Sagittarius A* marks the location of the Milky Way's central supermassive black hole. Investigations at all available wavelengths have shown it to be extremely under-luminous for its mass, radiating just 10^-9 of its Eddington luminosity. Several theoretical models of Sgr A*, which differ in their structure and physics, are able to match the spectrum and luminosity, and the discrepancies have not been resolved through observations of the source spectrum and variability alone. Here I present a program to understand Sgr A* through observations at (mostly) submillimeter wavelengths, a special place in the spectrum where a combination of instrument capabilities and source physics allow us to study the emission just outside the event horizon. Using purpose-built instruments and diverse observational techniques, including polarimetric monitoring and micro-arcsecond resolution imaging, we are disentangling the layers of source structure and should ultimately measure the black hole spin. In the era of ALMA the techniques we are using to study Sgr A* can be applied to more distant objects, culminating in a survey of black hole spins.
Characterizing the Transient Sky
In the coming years, several astronomical surveys will night scan the sky with regular cadence, revealing an enormous number of supernovae and other optical transients, many never seen before. The study of these stellar disruptions is not only a vibrant topic in itself, but also impacts fundamental questions in cosmology, nucleosynthesis, compact objects, and the sources of gravitational waves. Insight from theory and modeling is required to physically interpret this data and to explain the new phenomena discovered. Here I discuss recent advances in large scale simulations which predict the signatures of a diverse range of explosive transients. I focus first on the thermonuclear, or Type Ia, supernovae. Using multi-dimensional light curve models, I show how variations in Type Ia brightness are driven by deviations from spherical symmetry, and illuminate the physical origin of the empirical correlations used to standardize these supernovae as measures of cosmic expansion. I then present predictions for two very different sorts of transients: the disruption of extremely massive stars via the electron-positron pair instability (believed to characterize the death of the first generation of stars) and the faint optical emission from the merger of neutron stars (considered a promising source for gravitational wave observatories).
Cosmology from the CMB at arcminute scales: first results from the Atacama Cosmology Telescope
The many measurements of the cosmic microwave background (CMB) over the last two decades have provided a new view of the universe and a standard model of cosmology. The data support the basic picture of inflation and the hot Big Bang, and they allow us to estimate multiple cosmological parameters with nearly percent-level precision. There remains much to learn from terrestrial observations of the arcminute-scale CMB anisotropies. They can tell us about the emergence of structure in the large-scale distribution of matter and the properties of the dark energy. Their power spectrum is sensitive to parameters of the quantum field governing the epoch of inflation. With the new Atacama Cosmology Telescope, we are starting to map the CMB at arcminute resolution to address these questions. These small-scale temperature anisotropies--along with the polarization of the CMB at a wide range of angular scales--offer fertile ground for many fruitful observations in the decade to come.
Statistics of SL2S Arcs from the CFHTLS
Formation of gravitational arcs in groups/clusters is sensitive to the parameters of the cosmological model. The observed statistics of arcs can hence be used to test the standard cosmological model. A meaningful comparison of the arc statistics has so far been hampered due to incompleteness and a variety of selection effects.
Strong Lensing Legacy Survey (SL2S) is a large survey of strong lens systems in the CFHTLS-Wide (170 sq. deg) and CFHTLS-Deep (4 sq. deg.). With the help of a systematic arc detection algorithm, Arcfinder, the SL2S arc sample is being compiled. We quantify the completeness of the SL2S arcs sample by simulating mock gravitational arcs. We present preliminary results of the statistical properties of the SL2S arcs sample and its completeness which could be compared with predictions of standard model.
The E and B Experiment
The E and B EXperiment (EBEX) is a long-duration balloon-borne mission to study the polarization of the Cosmic Microwave Background. In particular, the instrument is designed to measure or set a stringent upper limit on B-mode CMB polarization arising from inflationary gravitational waves. In this talk, I will review the science behind EBEX, describe the design of the experiment, and report on its current status following our recent North American test flight.
Exploring the Dark Sector with Clusters of Galaxies
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.
News from COUPP: New dark matter limits, acoustic alpha discrimination, and progress towards a deep site detector (or two)
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.
Ultrahigh energy cosmic rays, neutrinos and gamma rays: a multi-messenger approach to Astroparticle Physics.
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.
Non-Equilibrium Ionization Processes in Metal-Ion Absorbers
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"
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.
Microwave detection of ultra-high energy cosmic rays: the MIDAS experiment at 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.
Is the Near-Power-Law Galaxy Correlation Function a Cosmic Coincidence?
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.
Peculiar velocities of galaxies: a versatile probe of cosmology
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.
Cosmology with galaxy clusters: a multi-wavelength approach
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
Light WIMPs, the Plot Thickens?
The Environment of Major Mergers between Dark Matter Halos
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.
The Atacama Cosmology Telescope: Spectrum and parameters from the 2008
The Atacama Cosmology Telescope (ACT) has mapped the microwave sky to arcminute scales. We will present the spectrum at 148 and 218 GHz from the 2008 season of the Southern Survey of the Atacama Cosmology Telescope. Improved map-making and efficient spectrum estimation allow us to recover the second through seventh acoustic peaks, and provide independent confirmation of the LCDM paradigm. After discussing the telescope and tests of the spectra, we will describe constraints on both primary cosmological parameters and secondary parameters from the small-scale power spectrum, including diffuse SZ emission and foregrounds.
How uncertain is our interpretation of cosmic ray data at high energy?
After reviewing methods for deriving the primary energy and estimating the mass composition of cosmic rays from observables of extensive air showers, the current standard model of cosmic rays is discussed and the status of some speculations about exotic scenarios is presented. Possible interpretations and apparent discrepancies regarding the composition of cosmic rays at very high energy are addressed in some detail.
Precision Cosmology from Optical Galaxy Clusters
Dark energy constraints from future optical galaxy cluster surveys will depend on how various systematic errors are controlled. I will first focus on the observable-mass distribution, discussing self-calibration and follow-ups for constraining cluster mass. I will then talk about theoretical uncertainties, discussing how precise calibrations of the dark matter halo mass function, bias function, and assembly history using N-body simulations will impact precision cosmology.
Results from the ANITA Search for Ultra-High Energy Neutrinos
The ANITA (ANtarctic Impulsive Transient Antenna) experiment is a balloon-borne radio telescope, designed to detect coherent Cherenkov emission from cosmogenic ultra-high energy neutrinos with energy greater than 10^18 eV. The second flight of the ANITA experiment launched on December 21st, 2008, and collected data for 30 days. This new data set allows for the most sensitive search to date for GZK neutrinos, which would reveal information about the source of the highest energy cosmic rays. I will present the results from the second flight of ANITA and discuss calibration techniques, analysis methods, and background rejection.
Constraining the dawn of cosmic structure and the epoch of reionization with the 21 cm line
The first billion years of the Universe contains the formation of the first galaxies and reionization. This period lies beyond the current observational frontier presenting challenges to theory and observation. Low frequency observations of the redshifted 21 cm line of neutral hydrogen will be key in developing our understanding of this period. In this talk, I will describe two aspects of the 21 cm signal from the period of ''cosmic dawn'': the global 21 cm signal and 21 cm fluctuations. I will discuss what can be learnt about the first galaxies and reionization from this technique and explore some of the challenges and opportunities ahead for the first observations.
Low Threshold Dark Matter Search with CCDs
I will report on the status of DAMIC, a low threshold dark matter search being performed with CCDs. The technical advantages of using CCDs for this type of search will be presented and the recent results obtained in a shallow underground site at Fermilab will be discussed. Developed for improving this search in the future will also be discussed.
The IMF, the UV bump, and AGB stars: New Insights From Stellar Population Synthesis
Stellar population synthesis (SPS) combines stellar evolution and atmosphere calculations, an initial mass function, a dust model, and a star formation history in order to `predict' the emergent spectrum of a galaxy. SPS techniques have proliferated in the past decade, resulting in the routine estimation of stellar masses, star formation rates, and metallicities of large samples of galaxies. In this talk I will focus instead on the basic SPS ingredients, including the IMF, dust properties, and evolution of AGB stars, and demonstrate that these uncertain inputs can actually be constrained directly from the integrated light of galaxies. By confronting a flexible SPS model with a variety of data, I will show that the low-mass IMF is not universal, the 2175A dust feature seen in our Galaxy also exists in typical star-forming galaxies, and that AGB stars contribute substantially less light in the near-IR than state-of-the-art stellar evolution calculations predict. Implications of these results will also be highlighted.
Simulations of the Magellanic Stream in a First Infall Scenario
Recent high precision proper motions from the Hubble Space Telescope (HST) suggest that the Magellanic Clouds are either on their first passage or on an eccentric long period (>6 Gyr) orbit about the Milky Way (MW). This differs markedly from the canonical picture in which the Clouds travel on a quasi-periodic orbit about the MW (period of ~2 Gyr). Without a short period orbit about the MW, the origin of the Magellanic Stream, a young (1-2 Gyr old) coherent stream of HI gas that trails the Clouds ~150 degrees across the sky, can no longer be attributed to stripping by MW tides and/or ram pressure stripping by MW halo gas. We propose an alternative formation mechanism in which material is removed by LMC tides acting on the SMC before the system is accreted by the MW. Both Clouds are modeled as N-body systems using cosmologically motivated density profiles and infall masses. The orbit of the SMC about the LMC is chosen such that resonances maximize the efficiency of LMC tides. Contrary to previous models, the orbit of the LMC about the MW is not assumed to be a free parameter and is instead determined by the HST proper motions. The N-body simulations demonstrate that it is possible to explain the origin of the Stream under the assumption that the Clouds have not been long term satellites of the MW. More generally, they show that gas stripping is expected to occur between any gas-rich dwarf galaxy pair.
Overcoming astrophysical uncertainties to constrain fundamental physics with LSS measurements
Large scale structure (LSS) measurements complement CMB observations in constraining fundamental physics parameters, such as neutrino properties and non-Gaussianity. However, our uncertainties in how galaxies trace the underlying matter density field complicate our interpretation of LSS observations in terms of these fundamental parameters. I will discuss how these uncertainties were (partially) overcome in our analysis of the SDSS Luminous Red Galaxy sample. In the second part of the talk, I will show that a dependence of galaxy properties on its host halo's assembly history can introduce significant uncertainties in the interpretation of large-scale clustering in terms of the non-Gaussianity parameter f_NL.