Talks & Events
Astronomy Colloquia: 2015
Probing structure formation beyond LCDM
Neal Dalal, University of Illinois, Urbana-Champaign
Cosmological structure formation has long been recognized as a sensitive probe of fundamental physics, especially physics beyond the Standard Model, and recent years have seen tremendous progress in our understanding of structure formation, both from the observational and theoretical sides. In this talk, I will describe some of my group's work on this subject. First, I will discuss a novel method we have developed for numerically simulating nonlinear structure formation in cosmologies where traditional N-body simulations produce large errors. I'll present preliminary results of our simulations for cosmologies with massive neutrinos, and I will describe a new potential signature of neutrino mass in large-scale structure. Finally, I will describe how upcoming ALMA observations of sources from the South Pole Telescope will vastly improve our knowledge of small-scale cosmic structure, thereby constraining the physics of inflation and dark matter.
Frontiers in Cosmology and Radio Astronomy: 21cm cosmology as a probe of reionization and beyond
Adrian Liu, University of California, Berkeley
In recent years, 21cm cosmology has emerged as an exciting new way to map our Universe. By using the 21cm hyperfine transition as a tracer of neutral hydrogen, one is sensitive not only to the large scale distribution of matter, but also to the astrophysical conditions of the high-redshift intergalactic medium (IGM). The redshifted 21cm line is therefore particularly well-suited for understanding the as-yet unobserved Epoch of Reionization (EoR), a key part of our cosmic history when the first luminous objects were formed and systematically ionized the IGM.
In this talk, I will highlight recent progress in 21cm cosmology, including recent observations from the Precision Array for Probing the Epoch of Reionization (PAPER). These observations disfavor “cold reionization” scenarios, where early luminous sources did little to reheat the IGM. Along the way, I will discuss novel techniques that have been developed for moving beyond technical hurdles (such as foreground contamination) to a first detection of the cosmological 21cm signal. I will conclude by introducing the Hydrogen Epoch of Reionization Array (HERA), a recently commenced experiment that promises to make high signal-to-noise measurements of the power spectrum of 21cm emission. This will not only provide new and direct observational constraints on the EoR, but will also benefit other cosmological probes by reducing uncertainties on a key epoch of cosmic history, thus transforming 21cm cosmology from a promising theoretical idea to a practical way to probe our Universe.
The Circumgalactic and Interstellar Medium of Star-Forming Galaxies at 2<z<3
Gwen Rudie, Observatories of the Carnegie Institution for Science
The exchange of baryons between galaxies and their surrounding intergalactic medium (IGM) is a crucial but poorly-constrained aspect of galaxy formation and evolution. I will present results from the Keck Baryonic Structure Survey (KBSS), a unique spectroscopic survey designed to explore both the physical properties of high-redshift galaxies and their connection with the surrounding intergalactic baryons. The KBSS is optimized to trace the cosmic peak of star formation (z~2-3), combining high-resolution spectra of 15 hyperluminous QSOs with densely-sampled galaxy redshift surveys surrounding each QSO sightline. I will characterize the physical properties of the gas within the circumgalactic medium (CGM) through measurements of the spatial distribution, column densities, and kinematics of ~6000 HI absorbers surrounding ~900 foreground star-forming galaxies that lie within 50 kpc to 3 Mpc of a QSO sightline. This analysis provides clear evidence of gas inflow and outflow as well as accretion shocks or hot outflows from these forming galaxies. My ongoing study of metallic absorbers in these fields will provide detailed information about the enrichment patterns and overall abundance of metals as a function of distance and velocity, providing a high-fidelity probe of the nature and sphere of influence of galaxy-scale outflows at high-z. I will also discuss KBSS-MOSFIRE, a rest-frame optical spectroscopic survey of more than 800 galaxies in these same QSO fields. These data provide new insight into the physical properties of HII regions at high redshift which show remarkable differences in their ionization and excitation conditions compared to low-redshift star-forming regions. These results have significant implications for both diagnostics of the chemical abundances of high-z galaxies as well as our understanding of massive stars during the peak of cosmic star formation.
Origins and Demographics of Super-Earth and Sub-Neptune Sized Planets
Leslie Rogers, Caltech
Sub-Neptune, super-Earth-size exoplanets are a new planet class. Though absent from the Solar System, they are found by microlensing, radial velocity, and transit surveys to be common around distant stars. The nature of planets in this regime is not known; terrestrial super-Earths, mini-Neptunes with hydrogen-helium gas layers, and water-worlds with several tens of percent water by mass are all a-priori plausible compositions. Disentangling the contributions from each of these scenarios to the population of observed planets is a critical missing link in our understanding of planet formation, evolution, and interior structure. I will review individual highlights from the diverse complement of sub-Neptune-size planets discovered to date, and present statistical analyses constraining the nature and origins of short-period rocky planets. With the suite of space-based exoplanet transit surveys on the horizon (K2, TESS, CHEOPS and PLATO) and continuing development of ground-based spectrographs (e.g., MAROON-X and G-CLEF), the pace of exoplanet discovery and characterization is poised to continue accelerating. I will conclude by describing pathways forward to identify bulk composition trends in the growing census of known exoplanets and to connect these composition trends back to distinct planet formation pathways.
Triumphs and tribulations of near-field cosmology with wide-field surveys: a biased perspective
Beth Willman, Haverford College
Over the last decade, wide-field surveys have revolutionized our view of the Milky Way’s stellar halo and dwarf galaxy population. Much of this observational progress has been motivated by a series of apparent “crises” for our cosmological model: the missing satellites problem, too big to fail, and the apparent planar distribution of dwarf satellite galaxies. These challenges have effectively functioned as flashlights pointing us to interesting galaxy formation physics. I will highlight related observational progress in our understanding of galaxy formation using near-field observations. I will then focus on the limiting impacts of observational bias and ways that current and future surveys will be used to tackle these biases. In particular, I will present new predictions for the number of Milky Way dwarf galaxies expected to be discovered in DES and LSST, RR Lyrae stars as a tool to discover dwarf galaxies in previously unexplored territory, and the use of M giant stars to map the Milky Way’s halo beyond its virial radius.
Image credit: J. Bullock, M. Geha, R. Powell
Galaxies in the Reionzation Era
Daniel Stark, University of Arizona
Deep infrared images from the Hubble and Spitzer Space Telescopes have recently pushed the cosmic frontier back to just 500 million years after the Big Bang, delivering the first reliable census of galaxies in what is likely the heart of the reionization era. I will discuss implications of these results for the build-up of stellar mass in early galaxies. I will then present the latest results of a large ground-based spectroscopic program aimed at using the Lyman-alpha emission line as a probe of the ionization state of the IGM at z>7. The results indicate that Lyman-alpha is strongly attenuated in galaxies at z=7-8, as would be expected if the IGM is still partially neutral. Finally, I will introduce a new approach to the spectroscopic study of galaxies at z>7. I will demonstrate that various metal lines in the rest-UV are much stronger in early galaxies than we expected. I will discuss a large new observational campaign aimed at targeting these lines in the brightest known gravitationally-lensed galaxies with the goal of providing the first constraints on the metallicity, ionizing spectrum, and stellar populations of galaxies in the reionization era.
Studies of Star-Forming Galaxies in the Reionization Era
Richard Ellis, Caltech
Deep exposures with the Hubble Space Telescope (HST) have provided the primary evidence that star-forming galaxies were present in the first billion years of cosmic history. Sometime during this early period the intergalactic medium transitioned from a neutral gas to one that is fully ionized. How did this 'cosmic reionization' occur and were star-forming galaxies responsible? Recent imaging of deep fields with HST's Wide Field Camera 3 in conjunction with Spitzer photometry and Keck spectroscopy has provided important new insight into understanding when reionization occurred and the role of early galaxies in the process. Gravitational lensing by foreground clusters is providing complementary evidence. I will review this rapid progress in our understanding of what could be considered the last missing piece in our overall picture of cosmic history and discuss the remaining challenges ahead of future facilities such as TMT, GMT and JWST.
Primordial non-Gaussianity in the CMB and Large-Scale Structure
Kendrick Smith, Perimeter Institute
I'll give a pedagogical review of inflation and explain how its physics can be constrained by searching for "primordial non-Gaussianity", i.e. differences between the statistics of the initial curvature field in our universe and the statistics of an ideal Gaussian field. Then I'll talk about observational CMB constraints, including some new results from Planck. Finally I'll discuss future prospects for improving Planck constraints with large-scale surveys such as Euclid and LSST.
Probing the Nature of Inflation
Daniel Green, University of Toronto
The idea that the early universe included an era of accelerated expansion (Inflation) was proposed to explain very qualitative features of the first cosmological observations. Since then, our observations have improved dramatically and have lead to high precision agreement with the predictions of the first models of inflation, slow-roll inflation. At the same time, there has been significant growth in the number of mechanisms for inflation, many of which are qualitatively distinct from slow-roll.
Nevertheless, most of these ideas are also consistent with current data. In this talk, I will first review inflation and its current observational status. I will then discuss the important theoretical targets for the future and the prospects for achieving them.
Binary Black Hole Accretion
Andrew MacFadyen, New York University
Binary black hole mergers in the presence of gaseous accretion flows are prime candidates for simultaneous observations of both gravitational waves and electromagnetic signals. I will present the results of 2D hydrodynamical simulations of circumbinary disk accretion using the moving-mesh code DISCO. These simulations demonstrate that gas accretion is not impeded by binarity. Gas is efficiently stripped from the inner edge of the circumbinary disk and enters the cavity along accretion streams, which feed persistent “mini-disks” surrounding each black hole. I will discuss characteristic periodicity in the measured accretion rate onto each BH, with implications for the quasar PG 1302-102 which shows evidence for periodic variability, as well as the dependence of the accretion flow on the binary mass ratio. I will also discuss characteristic modifications to the spectrum which arise from shock heated gas inside the circumbinary cavity. Finally, I will discuss simulations which include binary inspiral and merger due to gravitational wave emission in order to track the changes in accretion and electromagnetic radiation as the orbit shrinks.
The Dark and Light Side of Galaxy Formation
Piero Madau, University of California Observatories
In recent years, precision measurements across cosmic time have led to a widely accepted cosmological paradigm for galaxy assembly and evolution, the cold dark matter (ΛCDM) model. Within this theory, galaxies form "bottom-up," with low-mass objects ("halos") collapsing earlier and merging to form larger and larger systems over time. Ordinary matter follows the dynamics dictated by the dominant dark matter until radiative, hydrodynamic, and star-formation processes take over. Although ΛCDM has had great success in explaining the observed large-scale distribution of mass in the universe, the nature of the dark matter particle is best tested on small scales, where its physical characteristics manifest themselves by modifying the structure of galaxy halos and their lumpiness. It is on these scale that detailed comparisons between observations and theory have revealed several discrepancies and challenged our understanding of the mapping between dark matter halos and their baryonic components. In this talk I will review some of the triumphs and tribulations of the theory. While the latter may indicate the need for more complex physics in the dark sector itself, emerging evidence suggests that a poor understanding of the baryonic processes involved in galaxy formation may be at the origin of these controversies.
Short-Duration Gamma-Ray Bursts and the Electromagnetic Counterparts of Gravitational Wave Sources
Edo Berger, Harvard University
Gamma-ray bursts are the most luminous and energetic explosions known in the universe. They appear in two varieties: long- and short- duration. The long GRBs result from the core-collapse of massive stars, but until recently the origin of the short GRBs was shrouded in mystery. In this talk I will present several lines of evidence that point to the merger of compact objects binaries (NS-NS and/or NS-BH) as the progenitor systems of short GRBs. Within this framework, the observational data allow us to determine the merger rate of these systems as input to Advanced LIGO, to infer the electromagnetic properties of gravitational wave sources, and to study r-process nucleosynthesis.
The shortest-period planets
Josh Winn, MIT
Short-period planets were a gift from nature that enabled the rapid acceleration of exoplanetary science. They are more readily studied than long-period planets, and their existence and orbital properties pose interesting questions. I will present the results of a search for the shortest-period transiting planets, using data from the Kepler spacecraft. The results show that 0.5% of Sun-like stars have orbiting "lava worlds": terrestrial planets with periods ranging from 4 hours to one day. The search also revealed a new class of objects that seem to be small rocky planets disintegrating in the blazing heat from their parent stars. Finally, I will describe an upcoming NASA mission, the Transiting Exoplanet Survey Satellite (TESS), which will identify thousands of short-period planets around the nearest and brightest stars in the sky.
Timing of Exoplanets: the Big, the Small, the Circumbinary
Dan Fabrycky, The University of Chicago
Since the beginning of the exoplanet era, the reality of planets was confirmed and their properties mapped out by their time-variable signals. From the first pulsar-timing system, PSR 1257+12, to the dynamically precessing Doppler system, GJ 876, some of the first examples of multi-planet systems were confirmed via Newtonian interactions among the planets. This business has gone industrial using the 4-year stare of the Kepler mission, with which we used transit timing variations to map out the mutual orbital perturbations, confirm systems of planets by the dozens and elucidating the class of sub-Neptune planets. Now we are using transit timing to determine masses and architectures of systems of temperate Jupiters, determine the eccentricity scale of terrestrial planets, and detect and measure masses of planets orbiting around short-period binary stars. While the dynamical calmness of this new crop of exoplanets is comparable to the Solar System, other aspects seem as weird as ever, a continued forcing function on planet formation theory.
Image Credit: NASA / Tim Pyle
The Continuing Mystery of the Anomalous Microwave Emission
Bruce T. Draine, Princeton University
"Anomalous Microwave Emission" (AME) refers to dust-correlated emission in the 15-50 GHz frequency range that is far stronger than had been expected from the low-frequency "tail" of the thermal infrared emission from dust grains at ~20K. Discovered in the course of CMB studies, it presents a significant "foreground" that CMB observers would like to remove.
In 1998 it was proposed that the AME was rotational emission from ultrasmall dust grains with rotational frequencies of 15-50 GHz. The population of polycyclic aromatic hydrocarbons (PAHs) responsible for strong infrared emission bands between 3.3 and 17um were natural candidates for the emitter. This "spinning dust" hypothesis appeared to be a very natural explanation for the AME, and was supported by upper limits on polarization of the AME.
I will describe recent observational efforts to demonstrate a link between the AME and the PAHs. Surprisingly, the results do not support the conjectured AME-PAH linkage. Other possibilities for the source of the AME will be discussed.
The figure shows the spectrum of a region in the Perseus molecular cloud with strong Anomalous Microwave Emission (Planck Collaboration et al 2011 Astronomy & Astrophysics 536, A20).
Statistical mechanics of self-gravitating N-body systems
Scott Tremaine, Institute for Advanced Study
There have been many attempts to apply the powerful tools of statistical mechanics to self-gravitating N-body systems such as star clusters, galaxies, and planetary systems. I will describe why this is difficult, some notable failures and successes, and recent work on two arenas where these tools offer new insight: the distribution of young stars in the central parsec of our Galaxy, and the distribution of orbits of exoplanets.
Numerical Simulations of Black Hole Accretion
Ramesh Narayan, Harvard University
Accreting black holes are observed in a large variety of systems in astronomy: active galactic nuclei, X-ray binaries, tidal disruption events, gamma-ray bursts. While analytical one-dimensional models have been enormously useful for understanding several aspects of accretion physics, other aspects such as the formation of jets and winds are beyond the scope of such models. Numerical general relativistic MHD simulations are able to include more physics than analytical models and are proving increasingly useful for studying the multidimensional gas dynamics and radiative properties of accretion flows. The talk will review current progress in this field.