April 6, 2016 | 3:30 PM | ERC 161 | Astronomy Colloquium Global Radiation MHD Simulations of Black Hole Accretion Disks James Stone, Princeton University
The inner regions of accretion flows in luminous sources such as AGN and X-ray binaries are radiation dominated. In the case of AGN, radiation and winds produced by such flows are thought to be an important feedback mechanism during galaxy formation in massive halos. I present new results from numerical studies of the magnetohydrodynamics of accretion this regime. These calculations use numerical methods based on a formal solution of the time-dependent radiation transfer equation, eliminating the need for approximate closures. We find that turbulent transport of radiation energy can be a significant contribution to the cooling rate in the disk, and this changes the global properties of the flow compared to standard slim-disk models. We describe new work to extend our calculations to full general relativity, in order to follow the dynamics in the innermost regions of the disk.
April 13, 2016 | 3:30 PM | ERC 161 | Wednesday colloquium 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.
April 20, 2016 | 3:30 PM | ERC 161 | Astronomy Colloquium The Dynamic Last Years in the Lives of Massive Stars Eliot Quataert, UC Berkeley
In the last few years of the lives of massive stars, fusion in the core of the star produces a nuclear power that greatly exceeds the Eddington luminosity. This drives vigorous convection in numerous core and shell burning phases. I describe the surprising effect that waves excited by such convection can have on the properties of massive stars in the years leading up to core collapse. Wave transport of energy into the stellar envelope can power prodigious mass loss in the last years of stellar evolution. Angular momentum transport by waves excited during shell burning phases may well determine the angular momentum of the pre-supernova core, setting the birth spins of compact objects.
April 27, 2016 | 3:30 PM | ERC 161 | Wednesday colloquium 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.
May 4, 2016 | 3:30 PM | ERC 161 | Astronomy Colloquium A data-driven model of stars David W Hogg, New York University
There is a lot of knowledge built in to our physical models of stars. But there is even more information in the sum total of all the data ever taken of stars (tens of thousands of pixels of spectral data on many hundreds of thousands of targets). The Cannon (named after Annie) uses a small amount of physical modeling and a huge amount of data to build very precise, predictive, probabilistic models of stellar spectra. I show that these data-driven models can be used to obtain extremely precise measurements of stellar parameters and detailed chemical abundances, substantially more precise even than the physical models used to generate "ground truth" inputs. Indeed, we believe that The Cannon working on SDSS-APOGEE data is delivering more precise chemical abundance measurements than any previous method for the analysis of stellar spectra. These results have implications for studies of exoplanets (which I won't discuss) and the Milky Way (which I might). Work in collaboration with Melissa Ness (MPIA), Andrew R. Casey (Cambridge), Anna Y. Q. Ho (Caltech) and Hans-Walter Rix (MPIA).
May 11, 2016 | 3:30 PM | ERC 161 | Wednesday colloquium 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.
May 18, 2016 | 3:30 PM | ERC 401 (Note new location) | Astronomy Colloquium The next frontier of Massive Galaxies and Quasars at the Cosmic Dawn Tiziana Di Matteo, Carnegie Mellon University
I will discuss recent progress in cosmological hydrodynamic simulations of galaxy formation at unprecedented volumes and resolution. I will focus on predictions for the first quasars and their host galaxies and their contribution to reionization from the BlueTides simulation. BlueTides is a uniquely large volume and high resolution simulation of the high redshift universe: with 0.7 trillion particles in a volume half a gigaparsec on a side. This is the first simulation large enough to resolve the relevant scales relevant to the formation of the first large galaxies and quasars. These massive objects at high redshifts will be investigated with the next generation telescopes (Euclid, JWST and WFIRST).
May 25, 2016 | 3:30 PM | ERC 401 | Astronomy Colloquium A single prolific r-process event preserved in an ultra-faint dwarf galaxy Anna Frebel, MIT, Kavli Institute for Astrophysics and Space Research
The heaviest elements in the periodic table are synthesized through the r-process, but the astrophysical site for r-process nucleosynthesis is still unknown. The major current candidates are ordinary core-collapse supernovae and neutron star merger. Ancient, metal-poor ultra-faint dwarf galaxies contain a simple fossil record of early chemical enrichment that provides the means to study clean signatures of nucleosynthesis events, and thus, can yield unique information on the origin of these processes. Previously, extremely low levels of neutron-capture elements were found in the metal-poor stars in ultra-faint dwarf galaxies which supported supernovae as the r-process site. Based on Magellan/MIKE high-resolution spectroscopy, we have determined chemical abundances of nine stars in the recently discovered ultra-faint dwarf Reticulum II. Seven stars display extremely enhanced r-process abundances, comparable only to the most extreme r-process enhanced metal-poor stars found in the Milky Way's halo. The enhancement is also 2-3 orders of magnitude higher than that of stars in any of the other ultra-faint dwarfs. This implies the neutron-capture r-process material in Reticulum II was synthesized in a single prolific event that is incompatible with r-process yields from ordinary core-collapse supernovae but consistent with that of a neutron star merger. This would be the first signature of a neutron star merger in the early universe which holds the key to finally identifying the r-process production site. Furthermore, such a single r-process event is a uniquely stringent constraint on the metal mixing and star formation history of this ultra-faint dwarf galaxy. (Alex Ji, Anna Frebel, Ani Chiti, Joshua Simon, 2016, Nature 531, 610, http://arxiv.org/abs/1512.01558)
April 1, 2016 | 12:00 PM | ERC 401 | Friday noon seminar Gravity at the horizon: from the cosmic dawn to ultra-large scales Miguel Zumalacarregui, Nordic Institute for Theoretical Physics
PDF 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.
April 8, 2016 | 12:00 PM | ERC 401 | Friday noon seminar 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.
April 15, 2016 | 12:00 PM | ERC 401 | Friday noon seminar 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.
April 22, 2016 | 12:00 PM | ERC 401 | Friday noon seminar 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.
May 6, 2016 | 12:00 PM | ERC 401 | Friday noon seminar 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.
May 13, 2016 | 12:00 PM | ERC 401 | Friday noon seminar 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.
May 20, 2016 | 12:00 PM | ERC 401 | Friday noon seminar CMB Lensing Measurements, Present and Future 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.
June 3, 2016 | 12:00 PM | ERC 401 | Friday noon seminar 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.
May 31, 2016 | 2:00 PM | ERC 445 | Open Group seminar Parametrizing general linear cosmological perturbations. Lagos A Macarena, Imperial College London
We have great certainty on how gravity works around our solar system: General Relativity (GR) has been found to be very accurate at these small scales. On large scales though, we still have a considerable lack of understanding about the evolution of the universe, and its constituents. While the LCDM model is in good agreement with cosmological data, this might change in the future. For this reason, we need to test GR on these scales.
There are a number of proposals on how to characterize deviations from GR on larges scales, by parametrizing different cosmological evolutions of the universe - the PPF, EFT and EA approaches. The objective is to use experimental data to constrain these parameters, and thus identify the most accurate cosmological model. In this talk I will show an alternative, systematic and general, parametrization method, in which we construct the most general quadratic action for linear cosmological perturbations, given some field content and gauge symmetries. I will show the example of linearly diffeomorphism-invariant scalar-tensor theories, in which case the parametrization encompasses well-known theories such as Horndeski and Beyond Horndeski. The method can straightforwardly be applied to any gravity theory with any fields and gauge symmetries.
April 5, 2016 | 12:00 PM | ERC 576 | Tuesday Seminar Modeling Galaxies in the Era of Precision Cosmology: A community-driven approach with Halotools Andrew Hearin, Yale University
Models of the galaxy-halo connection provide insight into galaxy formation physics and can be exploited to tightly constrain cosmology with observations of large-scale structure. However, theoretical predictions of conventional formulations of these models are plagued by persistent systematic errors, for example due to uncertainty associated with "assembly bias". As galaxy surveys continue to provide ever more precise information on large-scale structure measurements, these theory-level systematics will place a ceiling on the reliability of the conclusions that can be drawn from traditional galaxy-halo techniques. In this talk, I'll describe how the open source Halotools package provides an object-oriented python framework designed to help remedy assembly bias and other systematics associated with nonlinearities in structure formation. Halotools is analogous to Boltzmann codes such as CMBFAST, CAMB and CLASS, but instead provides an optimized pipeline for populating mock galaxy catalogs into both low- and high-resolution simulations. I will conclude by describing how Halotools can be used to provide robust constraints on galaxy formation and help prepare the field of cosmology for the arrival of Stage IV dark energy experiments. Following the talk there will be a tutorial on the Halotools package.
May 3, 2016 | 12:00 PM | ERC 161 | Tuesday Seminar Fire and Ice: The role of energetic processes in the cold chemistry of planet-forming circumstellar disks Ilse Cleeves, Harvard CfA
Planets form from the coldest, T<200 K, and densest parts of circumstellar disks around young stars. During this phase, the active nature of the star subjects the disk to relatively high fluxes of UV and X-ray photons and energetic particles. Simultaneously, the local star-forming environment may provide additional external UV and/or radioactive pollutants from recent massive stellar populations. These energetic agents play a vital role in 1) setting protoplanetary disks' turbulent and thermal physics, and 2) regulating the important gas and grain surface chemical reactions, impacting the overall disk molecular composition. Consequently, chemistry can be used to "map out" the important ionizing processes in disks using submillimeter molecular emission. Using data from the Submillimeter Array and Atacama Large Millimeter/Submillimeter Array, we have put strong constraints on the global ionization levels in the disk of TW Hya, constraining the cosmic ray rate to a value two orders of magnitude below the dense interstellar value. This result has major consequences for the active disk chemistry and, in particular, the chemistry of water and organic material. Finally, I will discuss future directions aiming to spatially resolve ionization structure with ALMA and its time variability.
May 24, 2016 | 12:00 PM | ERC 576 | Tuesday Seminar New Constraints on the Acceleration of AGN-driven Galactic Winds and on the Density Structure of the CGM Jonathan Stern, Max Planck Institute for Astronomy
A vital ingredient in our theoretical understanding of AGN-driven galactic winds is the nature of the physical mechanism which accelerates them, such as radiation pressure on dust grains or the ram pressure of an expanding hot gas bubble. I will demonstrate that the nature of the acceleration mechanism can be constrained from the ratios of emission lines which originate from the cool (T~10^4K) gas illuminated by the AGN. I will then show that observed line ratios in UV-selected quasars suggest that radiation pressure dominates at all scales (0.1 pc -- 10 kpc). I will demonstrate that this result is apparently in conflict with the large momentum outflow rates measured by many previous studies, and discuss how this conflict might be resolved. In the second part of my talk I will argue for a new phenomenological model for the T~10^4K circum-galactic medium (CGM), where the cool CGM spans a large range of gas densities, and small high-density clouds are hierarchically embedded in large low- density clouds. I will then constrain the relation between gas density and physical size, or the "density structure", using ionic column measurements from the COS-Halos survey of low-redshift ~L* galaxies. I will show that this density structure is inconsistent with self-gravity, and thus its physical origin is unclear. I will also present a 3D model of the cool CGM based on our results, which can provide a benchmark for the CGM structure in hydrodynamic simulations.
May 19, 2016 | 2:00 PM | ERC 401 | Special Seminar Some Like it Hot: What Observations can tell us about Coronal Heating Joan T. Schmelz, Arecibo Observatory; Solar Physics Lab, University of Memphis; Universities Space Research Association (USRA)
Using data from Hinode-XRT, Hinode-EIS, and SDO-AIA, we have found that our observations support the nanoflare storm model of coronal heating. In project 1, the target loop is overdense and cooling. The cross-field temperature is multithermal as the loop cools but is isothermal before it fades from view. If these multi-stranded, multithermal, cooling loops are widespread, they could resolve the original isothermal/multithermal coronal loop controversy. In project 2, we found the best DEM for a sample of XRT and EIS loops as well as loops from the literature. We found a strong correlation between the DEM width and the DEM-weighted temperature, where the hotter the loop, the broader the DEM required to model the data. In project 3, we investigated active region cores that were observed with XRT and AIA. We found the best DEM for the areas where the Be_thick signal was significant, then artificially truncated the hot plasma of the DEM model at 5 MK. About half of our regions required the hot plasma for a good DEM fit, indicating that the hot plasma is present, even if the precise DEM distribution cannot be determined with the data available. We conclude that reconnection was heating the hot plasma component of these active regions. In project 4, we characterized the cross-field temperature distribution of every loop visible in the 171-A AIA image of a target active region. Results from all four projects indicate that coronal loops are multi-thermal and multi-stranded and that hot (T > 5 MK) plasma appears to be common and widespread in active regions. All our results support the nanoflare storm model and provide observational constraints that any viable coronal heating models will need to explain.