January 11, 2017 | 3:30 PM | ERC 161 | Astronomy Colloquium Circumgalactic Precipitation Mark Voit, Michigan State University
Feedback from a central supermassive black hole is an essential component of galaxy evolution models. Without it, those models cannot produce realistic massive galaxies and galaxy clusters. However, the black-hole feedback mechanism remains mysterious. Somehow, accretion of matter onto the central black hole of a massive galaxy becomes precisely tuned so that it regulates radiative cooling and condensation of gas in a volume many orders of magnitude larger than the black-hole's gravitational zone of influence. I will discuss how the required coupling can arise through condensation and precipitation of cold clouds out of a galaxy's circumgalactic medium, and will show how a feedback mechanism that suspends the circumgalactic medium in a marginally unstable state can regulate star formation within galaxies.
January 18, 2017 | 3:30 PM | ERC 161 | Wednesday colloquium The Milky Way's Dark Companions Alex Drlica-Wagner, Fermilab
PDF | Video Our Milky Way galaxy is surrounded by a host of small, dark-matter-dominated satellite galaxies. Over the past two years, the Dark Energy Camera (DECam) has nearly doubled the number of known Milky Way satellite galaxies compared to the previous 80 years combined. While these discoveries continue to help resolve the "missing satellites problem", they have also raised new questions about the influence of the Magellanic Clouds on the Milky Way's satellite population. In the near future, the rapidly growing population of dwarf galaxies will be sensitive to deviations from ΛCDM at small scales, while definitively testing whether the annihilation of dark matter particles could be responsible for excess gamma-ray emission from the Galactic center. I will summarize recent results, outstanding questions, and upcoming advancements in the study of the Milky Way's dark companions.
January 25, 2017 | 3:30 PM | ERC 161 | Astronomy Colloquium The Yin and Yang of Slowly-Pulsating B Stars: Asteroseismology and Angular Momentum Redistribution Richard Townsend, University of Wisconsin-Madison
During their main-sequence evolution, almost all B-type stars will pass through a phase where they are unstable toward oscillation in one or more global internal gravity waves ('g modes'). The g modes, driven by iron and nickel opacity in the stars' outer envelopes, generate surface temperature and velocity changes with periodicities on the order of days. In the 'Yin' part of my talk, I'll discuss how time-series spectroscopy and photometry of these `slowly-pulsating B' (SPB) stars can be leveraged into asteroseismology --- probing the stars' interiors by careful analysis of their oscillation spectra. I'll highlight in particular how the Kepler mission, together with the MESA stellar evolution code and my GYRE stellar oscillation code, has allowed novel constraints to be established on the internal rotation and mixing physics of SPB stars. I'll then pivot to the 'Yang' part of my talk. Although we typically regard stellar oscillations as passive tracers of stellar structure, they can also modify this structure. I'll present recent work by my group exploring angular momentum redistribution by g modes. Modeling this process in SPB stars, we find that significant modification of internal rotation profiles can occur on timescales as short as centuries. This suggests that the g modes can impact the stars' life trajectories, a possibility that's been hitherto ignored in stellar evolution calculations.
February 15, 2017 | 3:30 PM | ERC 161 | Astronomy Colloquium Why interstellar grain align and why you should care B-G Andersson, SOFIA Science Center
More than 70 years after the discovery of interstellar polarization, we now have a quantitative, empirically tested, theory of grain alignment giving ride to the observed effect. This Radiative Alignment Torque (RAT) theory predicts that dust grains are spun up by an anisotropic radiation field, if the wavelength of the light is less than the grain diameter. If the grain is made of a paramagnetic material, it will then align with the magnetic field. A number of specific, observationally testable, predictions follow from the theory, many of which have already been addressed. With a full testing of the theory and quantification of its parameters, polarimetry has the promise to not only allow efficient and reliable tracing of interstellar and interplanetary magnetic fields, but also to provide new and unique probes of the dust and the interstellar environment. I will review RAT alignment and its observational testing, and discuss some of the probes of ISM environmental parameters and dust that the verified theory allows.
February 22, 2017 | 3:30 PM | ERC 161 | Astronomy Colloquium Space astrometry: the Hipparcos and Gaia missions Michael Perryman, Priceton
Alone amongst the space agencies, ESA made its entry into space astrometry with the adoption of the Hipparcos mission in 1981. Outside of the astrometric community, it was viewed at the time as fundamental if not particularly exciting, although Freeman Dyson described it as "... the first time since Sputnik in 1957 that a major new development in space science has come from outside the US". In his ASP Millennium Essay in 2001, Cavendish Professor Malcolm Longair stated that "It is invidious to single out surveys which I find particularly impressive, but I make an exception for the Hipparcos astrometric satellite". Hipparcos delivered its high-accuracy catalogue of 120,000 star distances and space motions in 1997. As a follow-up, ESA accepted the Gaia mission in 2000. Launched in 2013 and expected to operate into the next decade, Gaia will represent a revolution in its dynamical stereoscopic mapping of our Galaxy, promising a catalogue of more than a billion stars to 20 magnitude at microarcsec-level accuracy. The talk will provide a short historical context and describe the scientific motivation for these missions, outline the essential experimental principles which underpin their measurements, and give an overview of the science objectives, including Gaia's expected yield of many thousands of astrometrically-detected exoplanets. The Speaker: Michael Perryman obtained his PhD in 1980 (Cambridge, UK) and spent most of his subsequent career with the European Space Agency. He was project scientist for Hipparcos from adoption in 1981 to catalogue finalisation in 1997, holding the dual role of overall project manager (1989-1993) after the satellite failed to achieve its nominal geostationary orbit. With Lennart Lindegren (Lund, Sweden) he was the co-originator of Gaia, and responsible for driving many of its principal attributes. He was study scientist from the Gaia's origins in 1995 to mission adoption in 2000, and thereafter ESA project scientist until the Critical Design Review in 2008.
March 8, 2017 | 3:30 PM | ERC 161 | Astronomy Colloquium Young Star Fundamentals and Surprises Lynne Hillenbrand, Caltech
Young stars associated with regions of recent star formation are both predictably, and enigmatically, variable over much of the electromagnetic spectrum -- due to processes occurring on the stellar surface, within the disk-to-star accretion zone, in the inner circumstellar disk regions, and perhaps in the outflow. The talk will begin with an overview of the relevant young star phenomena, then proceed to discuss recent work on fundamental stellar parameters for young stars, including several young planet discoveries, and then to the revelations of circumstellar phenomena based on state-of-the-art time domain data sets.
January 20, 2017 | 12:00 PM | ERC 401 | Friday noon seminar Coherent neutrino-nucleus scattering: signal or background? Jayden Newstead, Arizona State University
The next generation of dark matter direct detection experiments will be sensitive to coherent nuclear scattering of solar neutrinos. This presents an irreducible background to dark matter searches, the so called 'neutrino floor'. However, this effect that has yet to be observed and so provides an opportunity for discovery. Dedicated experiments are racing to observe this effect and use it as a probe of new physics. In this talk I will discuss the neutrino floor and present some dark matter models where the prospects for discovery are not so grim. Then I will introduce the MINER experiment, a Texas A&M effort to observe coherent neutrino-nucleus scattering, and present its sensitivity to models of new physics.
January 27, 2017 | 12:00 PM | ERC 401 | Friday noon seminar From axion inflation to leptons, baryons and cosmological magnetic fields Evangelos Sfakianakis, UIUC
Axions are attractive candidates for theories of large-field inflation that are capable of generating observable primordial gravitational wave backgrounds. These fields enjoy shift-symmetries that protect their role as inflatons from being spoiled by coupling to unknown UV physics. This symmetry also restricts the couplings of these axion fields to other matter fields. At lowest order, the only allowed interactions are derivative couplings to gauge fields and fermions. These derivative couplings lead to the biased production of fermion and gauge-boson helicity states during and after inflation. I will describe some recent work on preheating in axion-inflation models that are derivatively coupled to Abelian gauge-fields and fermion axial-currents.
For an axion coupled to U(1) gauge fields it was found that preheating is efficient for a wide range of parameters. In certain cases the inflaton is seen to transfer all its energy to the gauge fields within a few oscillations. Identifying the gauge field as the hypercharge sector of the Standard Model can lead to the generation of cosmologically relevant magnetic fields.
Coupling the inflaton-axion to Majorana fermions leads to the biased production of fermion helicity-states which can have interesting phenomenological implications for leptogenesis.
February 3, 2017 | 12:00 PM | ERC 401 | Friday noon seminar Recent Advances & Current Challenges in Cosmology with Galaxy Clusters Douglas Applegate, The University of Chicago
Observations of galaxy clusters not only test the basic parameters of the Lambda-CDM universe, but also test new physics such as non-zero neutrino masses, evolving dark energy, and departures from General Relativity on large scales. In this talk, I will first motivate why clusters are so useful for cosmology and why we are confident that these measurements are robust. In particular, I will describe how gravitational lensing measurements have been essential to cosmological constraints with the Weighing the Giants project, which reported a 12% constraint on Omega_m and a 15% constraint on a constant dark energy equation of state (when assuming flatness) from cluster data alone. While currently competitive, control of systematic uncertainties in weak lensing measurements need to improve by an order of magnitude to fully realize the potential of cluster cosmology with Stage-IV dark energy experiments that are turning on in the coming decade. I will detail some of the ongoing work within the South Pole Telescope and Dark Energy Science collaborations that is moving us towards the 1% systematics level, and highlight some of the interesting challenges yet to be solved.
February 10, 2017 | 12:00 PM | ERC 401 | Friday noon seminar Testing neutrino properties with cosmological observables Elena Giusarma, Carnegie Mellon University
One of the great puzzles related to the ΛCDM model is the nature of the dark matter (DM) component. In standard cosmology, hot, thermal relics are identified with the three light, active neutrinos which are sub-eV elementary particles which, apart from gravity, only interact via weak interactions. From neutrino oscillation experiments we know that neutrinos have masses but they are not sensitive to the absolute neutrino mass scale. Cosmology provides an independent tool to tackle the absolute scale of neutrino and to study its properties. In this talk I will illustrate the implications of neutrino properties on cosmological observables, in particular on the Cosmic Microwave Background radiation and on Large Scale Structure, and I will explore the current constraints on neutrino masses focusing also on the dependence of the cosmological limits on the total neutrino mass under the assumption of different mass spectra.
February 17, 2017 | 12:00 PM | ERC 401 | Friday noon seminar Cosmological Gravitational Waves: Causal Structure And Memories Yi-Zen Chu, University of Minnesota Duluth
Despite being associated with particles of zero rest mass, electromagnetic and gravitational waves do not travel solely on the null cone in generic curved spacetimes. (That is, light does not always propagate on the light cone.) This inside-the-null-cone propagation of waves is known as the tail effect, and finding novel ways of understanding it in the strong field regime near a black hole may find applications for modeling the gravitational signals sought after by next-generation space-based detectors such as LISA. Motivated by these considerations -- and as a first step -- I have been exploring techniques to understand the causal structure of scalar, electromagnetic and gravitational waves in cosmological spacetimes. I will describe my efforts to date, which include how the gravitational wave memory effects in 4D asymptotically flat spacetime generalize to the cosmological case.
February 24, 2017 | 12:00 PM | ERC 401 | Friday noon seminar Standard Model Background of the Cosmological Collider Zhong-Zhi Xianyu, Harvard University
Primordial non-Gaussianities record interactions of fields in the early universe, which can be viewed as collision events in a "Cosmological Collider" with huge energy. In this talk, I shall introduce the workings of the Cosmological Collider as an explorer of new physics at very high scales, and describe the Standard Model spectrum during inflation and its "background signals" in Cosmological Collider. The nontrivial quantum correction during inflation plays a crucial role in this process, which I shall describe in detail.
March 3, 2017 | 12:00 PM | ERC 401 | Friday noon seminar Simulating Milky Way-like Galaxies with Realistic Satellite Populations Andrew Wetzel, Carnegie Observatories, Caltech, UC Davis
Low-mass 'dwarf' galaxies trace structure formation on the smallest cosmological scales and represent the most significant challenges to the cold dark matter (CDM) model. I will introduce the Latte simulations, a new suite of cosmological zoom-in baryonic simulations that model the formation of Milky Way-like galaxies at parsec-scale resolution, using the FIRE (Feedback in Realistic Environments) model for star formation and feedback. Using these simulations, I will discuss the roles of cosmic accretion and stellar feedback in driving the formation and structure of disk galaxies like the Milky Way. These simulations also self-consistently resolve the satellite dwarf galaxies that form around each host. I will discuss the impact of stellar feedback and MW-like environments on dark-matter subhalos and their connection to dwarf galaxies, demonstrating progress in addressing the 'missing satellites' and 'too-big-to-fail' problems of LCDM cosmology.
March 10, 2017 | 12:00 PM | ERC 401 | Friday noon seminar Precision searches for new physics using optically levitated microspheres David Moore, Yale University
I will describe the development of a new class of force sensors based on optically levitated dielectric microspheres in high vacuum, which allow the detection of sub-attonewton forces acting on micron sized objects. These force sensors can enable novel precision searches for new physics through the detection of weakly coupled or short range (<< 1 mm) interactions. Results from the initial application of these force sensors to search for millicharged dark matter particles bound in matter, and for interactions arising in certain screened scalar dark energy models, will be presented. Finally, I will discuss the expected sensitivity of these techniques to search for non-Newtonian or non-Coulombic forces at micron length scales, which can probe a variety of models of physics beyond the Standard Model.
March 17, 2017 | 12:00 PM | ERC 401 | Friday noon seminar Kinetic Inductance Detectors for 100 GHz CMB Polarimetry Amy Lowitz, University of Wisconsin - Madison
Kinetic inductance detectors (KIDs) are a promising detector technology across a broad range of wavelengths from mm-waves up to the soft X-ray regime. KIDs offer relatively simple, relatively inexpensive fabrication and straightforward passive frequency-domain multiplexing, which makes them an attractive solution for instruments requiring very high pixel-count arrays, including upcoming CMB polarimetry instruments. In this talk, I will describe a recent effort to produce a prototype array of direct-absorbing lumped element KIDs designed for CMB polarimetry at 100 GHz (3 mm) with the QUBIC telescope, including a discussion of design considerations, material development, and measured array performance.
February 22, 2017 | 2:00 PM | ERC 445 | Open Group seminar Measuring the peculiar acceleration of merging binary black holes with LISA Nicola Tamanini, IPhT CEA/Saclay
Multi-frequency gravitational wave (GW) observations are useful probes of the formation processes of coalescing stellar-mass binary black holes (BBHs). In this talk I will show how the cosmic acceleration of the universe and the peculiar acceleration of the center-of-mass of a merging BBH, distort the gravitational chirp signal by a phase drift in the GW inspiral waveform. The effect due to the peculiar acceleration can be much larger than the one due to the universe acceleration, strongly depending on the location where a BBH forms within a galaxy. For BBHs formed in dense nuclear star clusters or via compact accretion disks around a nuclear supermassive black hole, the phase drift can be large enough to be measured by LISA, allowing for an independent probe of the nuclear formation channels to the stellar-mass BBH population. I will present forecasts on the accuracy with which LISA will be able to measure this peculiar acceleration effect, with and without coincident observations by Earth-based interferometers.
March 9, 2017 | 2:00 PM | ERC 401 | Open Group seminar Weak lensing in the nonlinear regime Jia Liu, Princeton University
Within the next decade, galaxy and CMB lensing datasets of unprecedented precision will come online from large surveys (LSST, Euclid, WFIRST, AdvACT, SPT-3G, CMB-S4, etc.). These surveys will be sensitive to structure evolution in the strongly nonlinear regime, warranting the study of higher-order (non-Gaussian) statistics that contain information beyond the traditional second-order statistics. In this talk, I will present our recent study of one simple non-Gaussian statistic, the lensing peaks -- their origin (1606.01318), application to galaxy lensing data (1412.0757), and forecasts for CMB lensing (1608.03169).
January 10, 2017 | 12:00 PM | ERC 576 | Tuesday Seminar How do pulsars shine? Sasha Philippov, Princeton
The modeling of pulsar radio and gamma-ray emission suggests that in order to interpret the observations one needs to understand the field geometry and the plasma state in the emission region. In recent years, significant progress has been achieved in understanding the magnetospheric structure in the limit of abundant plasma supply. However, the very presence of dense plasma everywhere in the magnetosphere is not obvious. Even the region where the observed emission is produced is subject to debate. To address this from first principles, we constructed global kinetic simulations of pulsar magnetospheres using relativistic Particle-in-Cell codes, which capture the physics of plasma production and particle acceleration. In this talk I will describe how plasma is produced in magnetospheres of pulsars. I will present modeling of high-energy lightcurves, calculated self-consistently from particle motion in the pulsar magnetosphere. I will also show evidence that observed radio emission is powered by non-stationary discharge at the polar cap.
January 31, 2017 | 12:00 PM | ERC 576 | Tuesday Seminar Observing the faint universe in emission Erika Hamden, Caltech
In the last several years, groundbreaking instruments have detected significant Lyman-alpha emission from the circumgalactic media (CGM) of z>2 galaxies, providing an initial corroboration to results from years of absorption line studies. Taken together, these indicate the presence of vast reservoirs of gas that we are only just beginning to observe and understand. To probe when star formation declines throughout the universe, we need to conduct similar observations at lower redshifts, moving into the UV. The Faint Intergalactic medium Redshifted Emission Balloon (FIREBall-2) is a balloon-born UV multi-object spectrograph designed to detect Lyman-alpha emission from the circumgalactic medium (CGM) around z~0.7 galaxies. In this talk, I will discuss the science drivers for this mission and its current status as we prepare for a Fall 2017 flight. In addition to groundbreaking science, FIREBall-2 will flight test several new technologies in a balloon setting, including photon counting, high efficiency UV detectors. I will discuss these technologies in the context of their impact on future space missions.
February 14, 2017 | 12:00 PM | ERC 576 | Tuesday Seminar Status of, and science with, the Stratospheric Observatory For Infrared Astronomy (SOFIA) B-G Andersson, SOFIA Science Center
The SOFIA project flies a significantly modified Boeing 747SP, carrying a 2.7m telescope into the stratosphere 3-4 times a week, to perform astronomical observations primarily in the mid- to far-infrared. The observatory is just starting Cycle 5 of its general user observations, employing all the first and second generation instruments, and covering all areas of astronomy from Solar System studies to extra-galactic astronomy. The Cycle 6 Call for Proposals will be released at the end of April, offering about 500h of observing time to the astronomical community. I will review the status of the project and provide some science highlights from the first few observing cycles.
February 28, 2017 | 12:00 PM | ERC 576 | Tuesday Seminar Astronomical Transients that defy all classification schemes Raffaella Margutti, Northwestern
Observations are drawing a complex picture of the latest stages of massive stars evolution and their explosions. In this seminar I concentrate on two among the least understood aspects of stellar evolution, adopting an observational perspective: How do massive stars loose a significant fraction of their mass in the years preceding the explosion? What powers the most luminous stellar explosions in our Universe? I address these questions by taking advantage from panchromatic observations of two remarkable transients: (i) the "normal" envelope-stripped SN2014C, which experienced a dramatic metamorphosis and evolved from Type I into Type II supernova over a timescale of a few months, thus violating the supernova classification scheme that hat has existed for decades. (ii) I will then describe the recent results from our efforts to constrain the energy source of Super-Luminous SNe, with a case study of the "bactrian" transient ASASSN-15lh, which might be the first element of an entirely new class of transients.