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Colloqiua & Seminars
Colloquia & Seminars, 2016
Helical Magnetic Fields in the Cosmos
Andrew Long, The University of Chicago
It is often necessary to study cosmological phase transitions, such as inflation and thermal symmetry-breaking phenomena, through the relics that they leave behind. In this sense, the cosmological magnetic field may be a powerful probe of the early universe, which has yet to be tapped. In this talk, I will survey various topics related to primordial magnetic fields (PMFs) with helicity. We will ask: how might a PMF have been generated in the early universe, how would it have evolved in the cosmological medium, and how could it be detected today? In the first part, we discuss the interesting connection between baryogenesis and magnetogenesis, which may link the sign of the cosmological baryon asymmetry to the sign of the magnetic helicity. In the second part, we discuss the interplay between the PMF and other magnetically-active objects, such as monopoles and axions. Finally we turn to detection prospects, with an emphasis on the frontier field of magnetically-broadened TeV blazar halos.
Searching for Dark Matter With Bubble Chambers
Andrew Sonnenschein, Fermilab
PDF | Video
Development of bubble chamber detectors for WIMP dark matter was pioneered at KICP in the early 2000’s. In the intervening years, we scaled the technology from the initial test-tube sized detectors operated in the basement of the LASR building to a 60 kg chamber now installed 2-km underground at SNOLAB. I will review the history of these developments and the most recent results from the PICO-2L and PICO-60 experiments.
Modified gravity inside astrophysical bodies: breaking of the Vainshtein mechanism
Ryo Saito, AstroParticle and Cosmology (APC) laboratory
Any infrared modification of gravity, which explains the current cosmic acceleration, should not spoil the successes of general relativity in solar-system observations. In many theories of modified gravity, it is ensured by the Vainshtein mechanism that works near dense sources. Recently, it has been found that the Vainshtein mechanism can be broken inside a dense source, although not outside, in a general class of scalar-tensor theories. In this talk, after reviewing how the Vainshtein mechanism can be broken, I will discuss its impact on the density profile of a star, modeling simply it as a polytropic sphere. I also show the existence of a universal upper bound on the amplitude of this type of modification, independently of the details of the equation of state.
New tests of not-so-dark matter
Yacine Ali-Haimoud, Johns Hopkins University
The nature of dark matter remains one of the major unsolved problems in physics, and one must leave no stone unturned when exploring empirical probes. In this talk I will discuss two different test of dark matter properties. First, I will show that dark matter interactions with standard model particles at high redshift can be probed through spectral distortions of the cosmic microwave background. In particular, I will show that upper limits from FIRAS measurements allow to constrain dark matter interactions for masses below 0.1 MeV. In the second part, I will discuss an interesting possible consequence of clumpy dark matter: flares following tidal compression events by supermassive black holes.
Quantum Twists of Space: Exotic Rotational Correlations from Quantum Geometry, Their Effects on Interferometer Signals, and Their Connection with Cosmic Acceleration
Craig Hogan, University of Chicago
PDF | Video
The talk will review theoretical arguments that if space and time emerge from a quantum system at the Planck scale, there should be nonlocal exotic quantum correlations of positions of massive bodies, even on scales much larger than the Planck length. In relational theories with no fixed background space, these could take the form of rotational quantum fluctuations in the inertial frame. Basic quantum principles are used to derive their effect on correlations in the signals of interferometers. An experimental test is proposed, based on a reconfiguration of the Fermilab Holometer. It is conjectured that entanglement of these rotational correlations with the Standard Model vacuum could explain the value of the cosmological constant in terms of known scales of physics.
A 3D view of the Dark Universe: illuminating intergalactic gas at high redshift with fluorescent Lyman-alpha emission
Sebastiano Cantalupo, ETH Zurich
Gravitational collapse during the Universe's first billion years transformed a nearly homogeneous matter distribution into a network of filaments - the Cosmic Web - where galaxies form and evolve. Because most of this material is too diffuse to form stars, its study has been limited so far to absorption probes against background sources. In this talk, I will present the results of a new program to directly detect and study high-redshift cosmic gas in emission using bright quasars and galaxies as external "source of illumination’’. In particular, I will show results from ultra-deep narrow-band imaging and recent integral-field-spectroscopy as a part of the MUSE Guaranteed Time of Observation program that revealed numerous giant Lyman-alpha emitting filaments around quasars and bright galaxies. Finally, I will discuss how the unexpectedly high luminosities of the giant Lyman-alpha filaments, together with the constraints from Helium and metal extended emission, present a serious challenge for our current understanding of the Intergalactic and Circumgalactic media based on hydrodynamical cosmological simulations.
Putting the Cosmology in "21 cm Cosmology"
Jonathan Pober, Brown University
21 cm cosmology -- the concept of using radio telescopes to observe the highly redshifted 21 cm line of neutral hydrogen on cosmological scales -- is a field on the verge of a breakthrough. The first generation of 21 cm cosmology experiments (LOFAR, MWA, and PAPER, among others) have been operating for several years, and first results at the level of design sensitivity are potentially forthcoming. In this talk, I will focus on the work needed to establish the reliability of any putative detection of the cosmological signal and the paths forward for bringing 21 cm experiments into the "precision cosmology" fold, alongside the CMB and galaxy surveys. Recent results from the MWA and PAPER will be presented, including the first limits on the z = 8.4 IGM temperature from PAPER.
Improved Limits from the Large Underground Xenon Dark Matter Experiment
Nicole Larsen, KICP
PDF | Video
A wealth of astrophysical research supports the existence of dark matter in the universe, yet the exact identity and nature of this unknown particle remain elusive. The Large Underground Xenon (LUX) dark matter search is a 370-kg xenon-based time projection chamber (TPC) that operates by detecting light and ionization signals from particles incident upon a xenon target. With the 2013 report of the world’s first sub-zeptobarn spin-independent WIMP-nucleon cross section limit, the LUX (Large Underground Xenon) experiment emerged as a frontrunner in the field of dark matter direct detection. In December 2015, LUX released an updated analysis of its 2013 dataset with increased detector exposure, updates to the background model, upgraded event reconstruction algorithms, and novel calibrations leading to an overall 23% increase in sensitivity for high-mass WIMPs and even more significant improvements for low-mass WIMPs. This talk details the design of the LUX experiment and reviews the analysis and reanalysis of the 2013 dataset leading to the world’s most stringent constraints on spin-independent WIMP-nucleon scattering for WIMPs above mass 4 GeV.
Quantifying discordance in the 2015 Planck CMB spectrum
Friday noon seminar
Graeme Addison, Johns Hopkins University
In this talk I will discuss the internal consistency of the Planck 2015 cosmic microwave background (CMB) temperature anisotropy power spectrum and show that tension exists between the determination of some cosmological parameters from multipoles l<1000 (roughly the scales accessible to WMAP) and l>=1000. I will show that the l>=1000 constraints are also in tension with low-redshift data sets, including Planck’s own measurement of the CMB lensing power spectrum (2.4 sigma), and the most precise determinations of the baryon acoustic oscillation scale (2.5 sigma), and Hubble constant (3.0 sigma). Finally, I will discuss some possible explanations for these disagreements.
New Approaches to Dark Matter
Justin Khoury, University of Pennsylvania
PDF | Video
In this talk I will discuss a novel theory of superfluid dark matter. The scenario matches the predictions of the LambdaCDM model on cosmological scales while simultaneously reproducing the MOdified Newtonian Dynamics (MOND) empirical success on galactic scales. The dark matter and MOND components have a common origin, as different phases of a single underlying substance. This is achieved through the rich and well-studied physics of superfluidity. The framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not): due to the higher velocity dispersion in clusters, and correspondingly higher temperature, the DM in clusters is either in a mixture of superfluid and normal phases, or fully in the normal phase. The model makes various observational predictions that distinguishes it from both LambdaCDM and standard MOND. In the last part of the talk, I will discuss an on-going attempt at explaining cosmic acceleration as yet another manifestation of dark matter superfluidity.
Modeling the Outskirts of Galaxy Clusters
Camille Avestruz, The University of Chicago
The observational study of galaxy cluster outskirts is a new territory to probe the thermodynamic and chemical structure of the X-ray emitting intracluster medium (ICM). Cluster outskirts are particularly important for modeling the Sunyaev-Zel'dovich effect, which is sensitive to hot electrons at all radii and has been used to detect hundreds of galaxy clusters with recent microwave cluster surveys. In cluster-based cosmology, measurements of cluster outskirts are an important avenue for estimating the cluster mass, as the outskirts are less sensitive to baryonic processes that dominate the cluster core. However, recent observations of cluster outskirts deviate from theoretical expectations, indicating that cluster outskirts are more complicated than previously thought. Computational modeling of cluster outskirts is necessary to interpret these observations. I will present cosmological simulations of galaxy cluster formation that follow the thermodynamic and chemical structures in the virialization regions of the ICM and transition to the IGM. Specifically, I will discuss how observational signatures of galaxy clusters are affected by gas flows, inhomogeneities in the ICM, and non-equilibrium physics.
Gravity at the horizon: from the cosmic dawn to ultra-large scales
Miguel Zumalacarregui, Nordic Institute for Theoretical Physics
Recent advances in cosmology provide both the motivation and the data to probe gravity on the largest scales available to observation. I will revise the landscape of gravitational theories, focusing on modern scalar-tensor theories and their cosmological implications. Then I will present the ongoing effort to test gravity in novel regimes such as the early universe, non-linear effects and ultra-large scales. I will also introduce the hi_class code (www.hiclass-code.net), which is central to this program.
High-Scale Axions without Isocurvature from Inflationary Dynamics
John M Kearney, Fermi National Accelerator Laboratory
If the PQ-breaking scale f is larger than the inflationary Hubble scale HI, the PQ symmetry is broken during inflation. In the most straightforward models, this gives rise to a light axion field during inflation, which acquires isocurvature fluctuations. Such fluctuations are very stringently constrained by current CMB measurements---in fact, supposing the near-future observation of primordial tensor modes (i.e., a measurement of a non-zero scalar-to-tensor ratio r, indicating a high inflationary scale), these constraints would exclude simple models of QCD axion dark matter in which f is larger than HI. This is particularly problematic for the near-Planckian values of f favored by, for instance, string theory.
A variety of solutions have been proposed to ''resurrect'' high-scale axions. Many seek to leverage inflationary dynamics to modify the behavior or potential of the PQ field during inflation in order to suppress isocurvature. However, inflation and the axion potential are both very fragile, and readily disrupted by additional interactions or couplings. As such, it is important to carefully consider the viability of influencing the PQ field via inflationary dynamics; in other words, can this really be accomplished without messing up either inflation or the solution to the strong CP problem? In this talk, I'll discuss the variety of issues that can arise in these constructions, and highlight the steps one must take to build a viable model.
Probing the Cosmic Dawn and the Epoch of Reionization with the 21cm Hydrogen Line
Jacqueline Hewitt, MIT
Measurements of the cosmic microwave background at redshift z ~ 1100 give us information about the initial density fluctuations that seeded subsequent gravitational collapse and structure formation. Observations of galaxies and clusters at z <~ 7 give us information about the outcome of this structure formation. Between those redshifts lies a modern frontier of cosmology - the cosmic dawn that marked the formation of the first stars and galaxies and the deionization of the intergalactic medium. Direct observations of this phase of the universe's history are just beginning. A particularly promising technique is that of mapping hydrogen structures using the redshifted 21cm radio line. Several recently completed low frequency radio arrays are now operating and providing us with an early glimpse into the Epoch of Rionization. Building upon these results a next generation instrument, the Hydrogen Epoch of Reionization Array (HERA) is beginning construction. HERA will be significantly more capable, and presents interesting opportunities and challenges.
A New Measurement of the Hubble Constant
Dan Scolnic, The University of Chicago
I will present a new, local, measurement by the SHOES team of the current rate of expansion (H0) of the universe from HST observations of Cepheid variables in host galaxies of Type Ia Supernovae. This measurement is a significant improvement from past measurements, and reduces many systematic uncertainties in past analyses. I will discuss the level of consistency of local measurements with measurements of H0 from the CMB.
More Is Different: The Power of Multi-Probe CMB/LSS Cross-Correlations
Colin Hill, Columbia University
Overlapping multi-wavelength surveys allow qualitatively new cosmological constraints. In this talk, I will describe three recent such results. (1) I will present a measurement of the kinematic Sunyaev-Zel’dovich (SZ) effect with Planck, WMAP, and WISE data using a novel estimator that does not require redshift estimates for individual tracers. This measurement yields the tightest kinematic SZ-derived constraint on the low-redshift baryon fraction to date, and the result is consistent with the expectation from analyses of the primordial CMB and Big Bang nucleosynthesis. (2) I will describe an updated measurement of the thermal SZ - CMB lensing cross-correlation using the 2015 Planck full mission data. This signal constrains the mass dependence of the "hydrostatic mass bias" afflicting X-ray-based galaxy cluster mass estimates, a key systematic in cluster-based cosmological constraints. (3) I will discuss a constraint on the multiplicative shear bias in CFHTLenS data based on cross-correlations with Planck CMB lensing and CFHTLenS galaxy density maps, the first demonstration of this method on actual data. The result is consistent with a value of the shear bias that would alleviate the tension between cosmological constraints from CFHTLenS and the Planck CMB temperature power spectrum.
WIMPs taking selfies: the DAMIC experiment at SNOLAB
Paolo Privitera, KICP/The University of Chicago
PDF | Video
The DAMIC (Dark MAtter In CCDs) experiment employs the bulk silicon of ~mm-thick charge-coupled devices (CCDs) to detect coherent elastic scattering of Weakly-Interacting Massive Particles (WIMPs) - putative yet-to-be-discovered particles which may explain the dark matter in the universe. This novel technique features an unprecedentedly low energy threshold (few tens of eVee) for the detection of nuclear recoils, providing optimal sensitivity for low mass WIMPs (< 10 GeV). In addition, the spatial resolution of the CCDs, unique amongst dark matter detectors, provides powerful methods to identify and mitigate environmental and cosmogenic backgrounds. I will show recent results from DAMIC R&D data which demonstrate the potential of the CCD technology for WIMP detectors and first images from DAMIC100, a 100 g detector with 18 CCDs under installation at SNOLAB.
ETHOS – From Dark Particle Physics to the Matter Distribution of the Universe and Beyond
Francis-Yan Cyr-Racine, Harvard University
We formulate an effective theory of structure formation (ETHOS) that enables cosmological structure formation to be computed in a vast array of microphysical model of dark matter physics. This framework maps the detailed microphysical theories of particle dark matter interactions into the physical effective parameters that shape the linear matter power spectrum and the self-interaction transfer cross section of non-relativistic dark matter. These are the input to structure formation simulations, which follow the evolution of the cosmological and galactic dark matter distributions. These effective parameters in ETHOS allow the classification of dark matter theories according to their structure formation properties rather than their intrinsic particle properties, paving the way for future simulations to span the space of viable dark matter physics relevant for structure formation.
Results from the first year of the HAWC Gamma Ray Observatory
Jordan Goodman, Maryland
PDF | Video
The High Altitude Water Cherenkov (HAWC) Gamma-ray Observatory in the high mountains of Mexico was completed in March of 2015 and is now giving us a new view of the TeV sky. HAWC is 15 times more sensitive than the previous generation of widefield EAS gamma-ray instruments and is able to detect the Crab nebula at >6σ with each daily transit. In our first year of operation, HAWC has a 5σ detection sensitivity for a source of ~50mCrab. Unlike Imaging Atmospheric Cherenkov Telescopes (IACTs), HAWC operates 24hrs/day with over a 95% on-time and observes the entire overhead sky (~2sr). HAWC’s peak energy sensitivity is 2-10 TeV which is ~10x higher than IACTs such as VERITAS and HESS, which makes their observations quite complementary. This talk will present results from the first year of HAWC data including our study of the galactic plane including new sources not yet detected by IACTs as well as spectra and morphology of bright sources. In addition, results of our monitoring of transient AGN will be presented.
Resolving the Isotropic Gamma-Ray Background in the Search for Dark Matter
Mariangela Lisanti, Princeton University
The presence of all-sky, diffuse gamma-ray emission has been known for several decades, but its origins remain an open question. While astrophysical sources such as Active Galactic Nuclei and star-forming galaxies almost certainly contribute to this Isotropic Gamma-Ray Background (IGRB), dark-matter annihilation may also leave an imprint. Therefore, resolving the components of the IGRB is an important step in pushing the sensitivity to signals of dark matter annihilation, particularly in the well-motivated parameter regime for Weakly Interacting Massive Particles. In my talk, I will present a new analysis method that takes advantage of photon-count statistics to distinguish astrophysical point sources from a potential dark-matter signal. I will show results obtained by applying this technique to public data from the Fermi Large Area Telescope. Using these data-driven methods, we can start to resolve the diffuse emission from ~1--189 GeV. I will discuss the possible nature of these sources and the implications for dark matter.
CMB Lensing Measurements, Present and Future
Friday noon seminar
Blake D Sherwin, University of California, Berkeley
By directly probing the cosmic mass distribution, measurements of gravitational lensing in the CMB provide a wealth of information about neutrino masses, inflation, dark energy, and galaxy biases. In my talk, I will discuss current and future work in this new but rapidly advancing field. In particular, I will discuss current measurements of the CMB lensing power spectrum with the ACTPol experiment and future measurements with the CMB Stage-IV experiment, explaining the promise and challenges of upcoming ultra-high-precision studies of this lensing signal. Lensing is not only a signal, however, but also a source of noise that limits how much we can learn about the early universe via B-mode polarization. In my talk, I will explain why delensing - removing the lensing effect to reveal the primordial sky - is crucial for the future of CMB science and will discuss recent work in delensing theory and data analysis.
Higgs Relaxation Leptogenesis
Lauren M Pearce, University of Minnesota & Valpariaso
The recent discovery of the Higgs boson, with a mass of 125 GeV, raises interesting possibilities for early universe cosmology. Its relatively flat potential means that it will typically acquire a large vacuum expectation value during inflation, ushering in a post-inflationary epoch of relaxation. In this talk, I will explore the possibilities for baryogenesis during this epoch.
Towards precision cosmology with large scale structures: the halo model and perturbative approaches
Irshad Mohammed, Fermilab
The theoretical modeling of the statistical observables of the large-scale structures of the Universe, like galaxy clustering, weak lensing etc., is necessary in order to derive any constraints on the cosmological parameters. One of the most important ingredients of the theoretical model is the two-point correlation function, or its Fourier transform the matter power spectrum. I will discuss the precision in its calculations based on a modified halo model, and the systematic effects due to the baryonic processes. Further, I will also discuss the covariance matrix of the matter power spectrum and its estimators based on the halo model and the perturbation theory. We find the agreement with the simulations is at a 10% level up to k ∼ 1 h/Mpc. We show that all the connected components are dominated by the large-scale modes (k < 0.1h/Mpc), regardless of the value of the wavevectors of the covariance matrix. Finally, I will provide a prescription for how to evaluate the covariance matrix from small box simulations without the need to simulate large volumes.
Preparing for the 21cm future - lessons from the Bleien Galactic Survey project
Chihway Chang, UofC
HI intensity mapping is emerging as a new and promising cosmological probe for both the large-scale structure and the early Universe. In preparation for the many large radio projects that are coming online, we launched the Bleien Galactic Survey project as an exercise to test new (and fun) techniques that could develop into useful tools in future surveys. I will first introduce the background science and basic setup of the experiment, and then touch upon two particularly interesting ideas - calibrating the telescope beam using drones, and RFI mitigation with start-of-the-art deep learning algorithms.
The Dark Energy Survey and Gravitational Waves
Marcelle Soares-Santos, Fermilab
PDF | Video
In this talk I present recent results of the Dark Energy Survey (DES) searches for optical counterparts to Gravitational Wave (GW) events detected by the LIGO/Virgo Collaboration. Our program achieved greater sensitivity than any other optical facility last year. For the second observing campaign (Fall/2016-Spring/2017) our goals are to either make a detection or establish significant constraints on optical emission from such events. DES is the greatest optical imaging survey yet, aiming at percent-level precision measurements of cosmological parameters from a combination of probes such as type Ia supernovae, galaxy clusters, and weak gravitational lensing. These probes are limited by astrophysical systematics and new independent methods are required in order to beat systematic effects down to sub-percent levels. Standard sirens, events for which distances are determined from their gravitational wave signal, are one possible new method to meet that challenge. Our program will potentially have a great impact in our field by exploring this possibility from the observational perspective. In this talk I will also briefly discuss this exciting prospect for future observing campaigns.
Partially Acoustic Dark Matter, Interacting Dark Radiation, and Large Scale Structure
Yuhsin Tsai, University of Maryland
The standard paradigm of collisionless cold dark matter is in tension with measurements on large scales. In particular, the best fit values of the Hubble rate and the matter density perturbation inferred from the CMB seem inconsistent with the results from direct measurements. In this talk, I will discuss these issues and propose a solution to both problems from a dark sector that contains dark acoustic oscillations with dark fluid. Such a solution can be tested by future experiments designed to probe the CMB and large scale structure.
SPIDER: Exploring the dawn of time from above the clouds
Jeffrey P Filippini, University of Illinois, Urbana-Champaign
Inflation is thought to have seeded the cosmos with a hum of primordial gravitational waves - unique messengers from the universe's earliest moments. These ripples in spacetime should have left a unique "B-mode" signature on the polarization of the cosmic microwave background. SPIDER is a powerful balloon-borne instrument designed to tease out this polarization pattern in the presence of galactic foregrounds. I will give an update from SPIDER's successful long-duration balloon flight over the Antarctic ice in January 2015, including performance estimates and the current status of the analysis, as well as a status report on payload development for SPIDER's upcoming second flight.
The First Four Months of Gravitational Wave Astronomy
Ben Farr, Enrico Fermi Institute and KICP
PDF | Video
On September 14, 2015 LIGO made the first direct detection of gravitational waves, marking the beginning of gravitational wave astronomy. The LIGO instruments continued to take data over the next four months, completing their first observing run on January 19, 2016 with 51.5 days of coincident data. I will present results from advanced LIGO's first four months of operation, and what they have taught us thus far.
ADMX (Axion Dark Matter eXperiment)
Ian P Stern, University of Florida
Nearly all astrophysical and cosmological data point convincingly to a large component of cold dark matter (CDM) in the Universe. The axion particle, first theorized as a solution to the strong charge-parity problem of quantum chromodynamics, has been established as a prominent CDM candidate. Cosmic observation and particle physics experiments have bracketed the unknown mass of CDM axions between approximately a μeV and a meV.
The Axion Dark Matter eXperiment (ADMX) is a direct-detection CDM axion search which has set limits at the KSVZ coupling of the axion to two photons for axion masses between 1.9 and 3.7 μeV. ADMX has recently begun conducting searches with an upgraded detector, which will allow for detection at even the most pessimistic couplings within this mass range. In order to expand the mass reach of the detector, ADMX is conducting extensive research and development of microwave cavity technology. Status of the experiment, current research, and projected sensitivities will be presented.
Dark Matter Radio (Hidden Photons/Axions)
Arran Phipps, Stanford University
Determining the composition of dark matter is at the forefront of modern scientific research. There is compelling evidence for the existence of vast quantities of dark matter throughout the universe, however it has so-far eluded all direct detection efforts and its identity remains a mystery. While weakly interacting massive particles (WIMPs) are a favored candidate and have been the primary focus of direct detection for several decades, there has been recent interest in searching for ultra light field dark matter. The Dark Matter Radio is a tunable superconducting high-Q lumped-element resonator (read out with SQUIDs) being built to search for sub-eV hidden photon and axion dark matter. I will discuss the motivation, detection strategy, design, and current status of the DM Radio experiment.
New Directions in Searching for the Dark Universe
Surjeet Rajendran, UC Berkeley
PDF | Video
Observational bounds currently permit the existence of a large number of dark matter candidates, ranging from ultra-light axions with masses ~ 10^(-22) eV to MACHOs with mass as large as 10^(24) gm. It is important to develop experimental methods to constrain this vast range of parameters. In this talk, I will describe new experimental methods to probe a wide variety of dark matter candidates, ranging from ultra-light axions with masses ~ 10^(-22) eV to light WIMPs with mass in the keV - GeV range. A variety of precision measurement technologies such as optical/atomic interferometry and SQUID magnetometry can be applied to search for these particles. I will also discuss methods to search for the direction of the nuclear recoil induced by conventional WIMP scattering in detectors with solid state densities. These directional detectors may enable probes of conventional WIMP dark matter beyond the solar neutrino floor.
Cooling and AGN heating in cool-core galaxy clusters
Yuan Li, University of Michigan
The feedback from active galactic nuclei (AGNs) is widely considered to be the major heating source in cool-core galaxy clusters, preventing a classical cooling flow where the intra-cluster medium (ICM) cools at hundreds to a thousand solar masses per year. We perform adaptive mesh simulations using Enzo including both momentum-driven AGN feedback and star formation to study the interplay between ICM cooling, AGN heating and star formation over 6.5 Gyr in an isolated cool-core cluster. We find that AGN jets globally heat up the ICM via weak shock waves and turbulence. Locally, cold clumps can cool out of the ICM due to the non-linear perturbation driven by the AGN jets. These cold clumps feed both star formation and the supermassive black hole (SMBH), triggering an AGN outburst which increases the entropy of the ICM and reduces its cooling rate. When star formation completely consumes the cold gas, leading to a brief shutoff of the AGN, the ICM quickly cools and develops multiphase gas again, followed by another cycle of star formation/AGN outburst. The simulation reproduces a wide range of observed properties and naturally explain the variety of star forming clouds observed in the center of cool-core clusters.
Substantial Variation in the Stellar Halos of Spiral Galaxies
Allison Merritt, Yale University
The Dragonfly Telephoto Array, comprised of 48 individual Canon telephoto lenses operating together as a single telescope, is an innovative approach to low surface brightness imaging. Sub-nanometer coatings on each optical element reduce scattered light from nearby bright stars and compact galaxy centers -- typically a key obstacle for integrated light observations -- by an order of magnitude, and Dragonfly's large field of view (2 x 2.6 degrees for a single frame) provides a large-scale view of galactic stellar halos and satellite systems. Using extremely deep (>30 mag/arcsec^2) optical imaging in g and r bands from the Dragonfly Nearby Galaxies Survey (DNGS), we have characterized the stellar halos of a sample of nearby luminous galaxies. I will present measurements of the stellar halo mass fractions of an initial sample of spiral galaxies from the survey, and discuss these in the context of the assembly histories of individual galaxies.
Increasing Accuracy and Increasing Tension in H0
Wendy Freedman, KICP
PDF | Video
The accuracy in direct measurement of distances to galaxies has continued to improve dramatically over the past decade. Local measurements of the Hubble constant based on Hubble Space Telescope observations of astrophysical standard candles -- Cepheids and Type Ia supernovae -- have converged on a value of about 73 km/sec/Mpc with an uncertainty of 2-3%. At the same time, estimates assuming a Lambda-CDM standard model and fitting highly precise measurements of cosmic microwave background fluctuations have yielded a value of Ho = 67 km/sec/Mpc. The two methods disagree at approximately the 3-sigma level. The reason for this discrepancy is not understood at present, and new data have only increased the tension. If real, the disagreement could be signaling missing physics in the standard model; for example, additional dark radiation. Major efforts are ongoing to improve further the accuracy in the local measurements, including developing other techniques to test the Cepheid distance scale. In the near future JWST and Gaia will provide a path to measuring Ho to 1%, comparable to the precision in CMB measurements.
Solid-state imaging detectors for low-energy particle physics
Friday noon seminar
Alvaro Chavarria, The University of Chicago
The low noise, high spatial resolution and reliable performance of charge-coupled devices (CCDs) and complementary metal-oxide-semiconductor (CMOS) active pixel arrays have made them detectors of choice for digital imaging, from consumer electronics to state-of-the-art astronomical cameras. Although the slow time response of these devices has limited their application in high-energy particle physics, for the case of rare-event searches, where the particle interaction rate is extremely low, their properties can be fully exploited to build detectors that outperform in many aspects the traditional technologies of the field. I will present recent results from the DAMIC experiment, a low-mass dark matter search consisting of low-noise CCDs deployed in the SNOLAB laboratory. I will show how the exquisite spatial resolution of the detector allows for particle identification, and provides the unique capability to reject sequences of radioactive decay with utmost efficiency. These techniques can be extended to the field of neutrinoless double beta decay. I will present a recent proposal where we argue that a large array of amorphous Se-82 imagers based on CMOS technology could achieve the background requirements necessary to test if neutrinos are Majorana fermions even in the case of a normal hierarchy of neutrino masses.