List of Past Astronomy Colloquia : 01-Sep-2014 to 31-Dec-2014


Date:   Wednesday 10-Sep-2014
Speaker:   CANCELLED
Title:  CANCELLED

Due to illness, this colloquium has been cancelled.

[Dr. John Johnson (Center for Astrophysics-Harvard) "Hot on the Trail of Warm Planets Orbiting Cool Stars"]


Date:   Wednesday 17-Sep-2014
Speaker:   Dr. Hagai Netzer (Tel Aviv University)
Title:  Revisiting AGN accretion disks: Black hole spin and growth rate

Broad wavelength X-shooter observations of a unique AGN sample at z=1.55 were used to model the accretion disk spectrum with an unprecedented accuracy. This provides reliable black hole spin measurements for 80% of the sources. There is a clear signature of ``spin-up'' of the most massive black holes which indicates either unusual evolution or a previously unnoticed selection effect. This, combined with the analysis of large AGN samples, suggests that the most luminous AGNs at high redshift are powered by slim accretion disks with growth rates that can differ substantially from earlier estimates.


Date:   Wednesday 24-Sep-2014
Speaker:   Dr. Jillian Bellovary (Vanderbilt)
Title:  Scrutinizing the Relationship Between Galaxies and Supermassive Black Holes

Supermassive black holes (SMBHs) have a ubiquitous presence in massive galaxies, but their formation and evolutionary history remain mysteries. One of the strongest observed trends between SMBHs and their host galaxies is the tight relation between black hole mass and the velocity dispersion of the stellar spheroid (aka the M-Sigma relation). While this relation hints at a fundamental link between galaxy and SMBH growth, there are also some challenges regarding scatter and outliers. I will present evidence that the M-S relation is not the clear-cut trend it seems to be; for example, the orientation of galaxies on the sky affects the value of velocity dispersion by up to 30%. Additionally, the existence of SMBHs in dwarf galaxies and galaxies without spheroids challenges the standard paradigm of SMBH-galaxy coevolution. I will offer some alternative methods of forming and growing SMBHs which can provide explanations for these puzzling outliers.


Date:   Wednesday 01-Oct-2014
Speaker:   Dr. Mark Showalter (SETI Institute)
Title:  Order and Chaos in the Pluto System

Our view of the Pluto system has changed radically in the last decade. Four small moons have been discovered orbiting the central Pluto-Charon binary. As we gather new data in preparation for the New Horizons flyby of Pluto next July, some of the subtle implications of this configuration are coming to light. For example, it appears that the gravity field generated by Pluto and Charon drives the small moons into chaotic rotation. With improved orbital elements, it now appears that three of the moons--Styx, Nix and Hydra--are locked in a resonance similar to the Laplace resonance that connects Io, Europa and Ganymede. We are also learning surprising details about the shapes, sizes and surface properties of the four moons; these results challenge our ideas about how the system formed. The Kepler mission has shown that circumbinary planets are relatively abundant. As a result, the Pluto system now serves as a nearby, accessible laboratory for observing dynamical processes that are probably commonplace, although on larger scales, throughout the galaxy.


Date:   Wednesday 08-Oct-2014
Speaker:   Dr. Jason Kalirai (STsci)
Title:  The Stellar Graveyard of the Milky Way

In old stellar populations such as globular clusters and stellar halos, the bulk of the initial mass function beyond the current main-sequence turnoff resides on the white dwarf cooling sequence. This end state of stellar evolution is difficult to observe, given the intrinsic faintness of white dwarfs and their lack of nuclear energy sources. In this talk, I'll describe a new Hubble and Keck study to uncover and characterize faint populations of white dwarfs in a wide range of stellar systems with well established properties. In these environments, the masses and temperatures of the stellar remnants can be uniquely linked to the properties of their progenitors. This work has now led to the first global mapping of the initial-final mass relation - a fundamental input into our understanding of the stellar evolution process for low and intermediate-mass stars.

A robust initial-final mass relation enables many related astrophysical studies that were not previously possible. My team has used the relation to develop new and precise methods to date the Galactic halo and the oldest globular clusters from their cooling remnants, to calculate the lifetimes and luminosities of stars in key evolutionary phases such as the thermally-pulsing AGB, and to measure the mass loss and chemical yields from giant stars. In this talk, I will describe the past and future development of the initial-final mass relation, and how it has now provided answers to several outstanding astrophysical problems.


Date:   Wednesday 15-Oct-2014
Speaker:   Dr. Ralph Lorenz (Johns Hopkins APL)
Title:  Sailing the Seas of Titan

Saturn's giant moon Titan has been revealed to be remarkably Earth-like, with a landscape of vast dunefields, river channels and lakes under a smoggy sky punctuated by methane downpours. Titan serves as a frigid laboratory in which the same processes that shape our own planet can be seen in action under exotic conditions. Titan has a rich inventory of complex organic molecules that may provide clues how the building blocks of life are assembled. I will review telescopic observations of this dynamic world, and the latest findings from the multinational, multibillion Cassini-Huygens mission, in orbit at Saturn since 2004. I will also discuss prospects for future missions, and in particular the unique challenges and opportunities for exploring Titan's seas of liquified natural gas with submersibles or floating wind-blown capsules.


Date:   Wednesday 22-Oct-2014
Speaker:   Dr. Charlie Conroy (University of California Santa Cruz)
Title:  Extragalactic Archeology

One of the primary avenues for understanding the formation and evolution of galaxies is through studying their stellar populations. A new generation of population synthesis tools that we have been developing are now capable of measuring an unprecedented amount of information from high quality spectra of galaxies. In this talk I will present results from an ongoing program aimed at measuring the stellar initial mass function, ages, and detailed elemental abundance patterns of early-type galaxies over the interval 0


Date:   Wednesday 29-Oct-2014
Speaker:   Dr. Marcelo Alvarez (CITA)
Title:  Secondary CMB Anisotropies: The View from Full-sky Simulations

Secondary CMB anisotropies are both a rich source of information on cosmic structure, and a foreground for measurements of primary anisotropies at z~1100. Secondary anisotropies are generated as CMB photons are scattered by electrons, first during reionization, and then in galaxy clusters. Dust in star forming galaxies at high-redshifts is heated and glows at the same wavelengths as the CMB, creating even more anisotropies. I will review the recent observational results and theoretical models. To better confront theory with observation, we have developed a way to generate simulated full-sky multi-wavelength CMB maps, based on large scale structure realizations several Gpc across, containing millions of galaxies and clusters and including the effects of patchy reionization. I will explain how we make these maps and how they compare with the latest CMB observations with ACT, SPT, and Planck. Final large scale structure, like galaxy surveys and intensity mapping, to map out the cosmic web over a substantial fraction of the observable universe and understand how cosmological fluctuations were generated during inflation.


Date:   Wednesday 05-Nov-2014
Speaker:   Dr. Alice Quillen (University of Rochester)
Title:  Resonant chains and Three-body resonance, the Stability of multiple planet systems

The Kepler mission has recently discovered hundreds of compact multiple planet systems, including systems in chains of mean motion resonances. I explore theoretical attempts to understand numerically measured relations between planetary mass, spacing and orbital stability in closely-spaced planar multiple planet systems. Using the 13 moon inner Uranian satellite system as a testbed, I show that three-body resonances and chains of pairs of bodies in mean-motion resonances account for coupled behavior between nearby bodies. Lastly I will discuss approaches for predicting the diffusive-like behavior in these chaotic systems.


Date:   Wednesday 12-Nov-2014
Speaker:   Dr. Kristine Spekkens
Title:  Cold Dark Matter and the Interstellar Medium in Nearby Dwarf Galaxies

The interstellar medium (ISM) in nearby dwarf galaxies provides important insight into how all galaxies form and evolve within the standard cold dark matter cosmology. In this talk, I will highlight some of my group's recent efforts to use observations of the ISM in nearby dwarfs as a cosmological probe. First, I'll discuss our efforts to reconcile the rotation curves of nearby dwarf and low-surface brightness disk galaxies with the cusps predicted by collisionless simulations of halo assembly. I'll then present results from our ongoing deep search for radio synchrotron emission from Galactic dwarf spheroidal galaxies (dSphs), that constrains the properties of the particle dark matter that may make up their halos. Finally, I'll explain how upper limits that we derive on the atomic gas content of some dSphs, obtained during the course of our particle dark matter search, may elucidate key dynamical properties of both the Milky Way and its dSph satellites.


Date:   Wednesday 19-Nov-2014
Speaker:   Dr. Yuexing Li (Penn State)
Title:  Bridging the Gap between Theory and Observations

Over the past decade, we have witnessed an explosion of multi-wavelength surveys on galaxies and quasars across cosmic time. In order to understand the observations, a comprehensive framework that fully accounts for the formation, evolution and multi-band properties of galaxies is desperately needed. I will discuss recent developments in the modeling of structure formation that combine cosmological simulations with multi-wavelength radiative transfer. I will present new results from the Illustris Radiative Transfer Project on the cosmic reionization, the origins of extragalactic background lights, and detectability of the first galaxies with the next generation instruments such as JWST and ALMA.


Date:   Wednesday 26-Nov-2014
Speaker:   NO COLLOQUIUM
Title:  THANKSGIVING HOLIDAY


Date:   Wednesday 03-Dec-2014
Speaker:   Dr. Brandon Bozek, Dr. Brian Morsony & Dr. Hsiang-Yi Karen Yang (UMD-Astronomy)
Title:  Various

Dr. Hsiang-Yi Karen Yang: Challenges and Prospects of Modeling the Intracluster Medium

Modeling the intracluster medium (ICM) is crucial for precision cosmology; however, theoretical modeling of the ICM is challenging due to incomplete understanding of feedback from active galactic nuclei and plasma properties on small scales. In this talk, I will discuss the projects I am involved with and how one can gain insights from magnetohydrodynamic and/or cosmic-ray simulations.


Date:   Wednesday 10-Dec-2014
Speaker:   Dr. Jon Miller (University of Michigan)
Title:  New Views of Black Hole Disk Winds

High-resolution X-ray spectroscopy has been central to revealing the nature of X-ray winds from accreting massive black holes. These winds may ultimately have important effects on the evolution of host galaxies. However, early studies with Chandra may have given a partially skewed view of the radii at which such winds originate, and the mechanisms by which they are driven. With an eye toward the coming Astro-H era, I will briefly review past results, and then highlight some new and emerging views of black hole disk winds. Where possible, I will draw analogies between parallel investigations in accreting stellar-mass black holes. Overall, new results may suggest that disk winds may originate closer to the black hole, and carry more mass than previously appreciated.


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