List of Past Astronomy Colloquia : 01-Jan-2016 to 01-Jun-2016

Date:   Wednesday 03-Feb-2016
Speaker:   Dr. Josh Peek (STScI)
Title:  Edge of Tomography

Astronomy has always been a science of catalogs: catalogs of stars, galaxies, quasars, and planets. While most of the light in the Universe comes from these dense objects in the darkness, the contents of the universe are largely diffuse. Even setting aside dark energy and dark matter, plasma and gas make up the vast majority of the mass of the Universe and cannot be easily cataloged. If we want to understand how the objects in the universe came to be, we must understand the state the largely invisible diffuse phase that forms them.

I will argue in this talk that /tomography/, the process by which we take catalog data (quasars, galaxies, and stars) and reconstruct the intervening gas (intergalactic, circumgalactic, and interstellar medium), is poised on the edge of a revolution. I will describe advances of the last decade, powered by the industrial photometric precision of SDSS, and how much we have learned about the diffuse phase of the universe, especially in astrophysical dust. I will also give an introduction to a whole new subfield we are developing, Kinetic Tomography, which attempts to reconstruct not the 3D gas and dust distribution, but the 4D distribution, including velocity. With this 4D data we can crack longstanding problems in the formation and destruction processes that govern spiral arms, molecular clouds, and galaxies as whole.

Date:   Wednesday 10-Feb-2016
Speaker:   Dr. Jonathan Tan (University of Florida)
Title:  Massive Star Formation Through The Universe

Massive stars have played a dominant role in shaping our universe since its earl iest times, but there is no consensus on the mechanism by which they form. I rev iew the physical processes thought to be important in massive star formation, co ncentrating on a particular theoretical model, Turbulent Core Accretion. This as sumes the initial conditions are massive, turbulent, magnetized cloud cores of g as and dust that are reasonably close to virial equilibrium. We test this via th eoretical models and simulations of the physics and chemistry of the interstella r medium and observational searches for these cores. We next consider the protos tellar collapse phase as a massive star grows from the core. Various forms of fe edback become important in reducing the efficiency of accretion, although it is not clear if one particular mechanism operates to set a fundamental limit on the maximum stellar mass. Again, these theoretical ideas can be tested by observati ons of massive stars forming in our Galaxy today. Finally, I discuss an applicat ion of massive star formation theory to the early universe: how massive were the first stars and could they have been the progenitors of supermassive black hole s?

Date:   Wednesday 17-Feb-2016
Speaker:   Dr. Amy Simon (GSFC)
Title:  New Observations and Insights of the Giant Planets' Atmospheres

Recent observations of the outer planets, combined with older data sets, are beginning to give us new insights into the workings of their atmospheres. In this talk, I will present recent spacecraft observations of Jupiter, Saturn, Uranus and Neptune. Combined with data from Voyager, Cassini, and New Horizons, we are beginning to compile a more complete picture of the fluid dynamics and atmospheric structure of each of these unique planets. Future plans, and potential missions, will also be discussed.

Date:   Wednesday 24-Feb-2016
Speaker:   Dr. Ehud Behar (Isreal Institute of Technology)
Title:  An X-Ray Radio Connection Between Stellar Coronae and Black Hole Accretion Disk Coronae

Stellar coronae are known for their million degree plasma that is magnetically energized and emits bright X-rays. The X-ray source in black hole accretion disks is also referred to as a corona, although its observed properties, e.g., spectra, luminosity, temperature, and variability are significantly different from those of stellar coronae. In fact, the governing physical mechanisms of accretion disk coronae remain largely unknown. The colloquium will show that radio and mm observations of active galaxies (AGN) could be key to understanding the X-ray coronae of super-massive black holes, and they suggest that perhaps stellar and accretion-disk coronae have more in common than meets the eye (well, ... the telescope).

Date:   Wednesday 02-Mar-2016
Speaker:   Dr. Sarah Horst (JHU)
Title:  Understanding Haze Formation in Planetary Atmospheres: Lessons from the Lab

From exoplanets, with their surprising lack of spectral features, to Titan and its characteristic haze layer, numerous planetary atmospheres may possess photochemically produced particles or haze. With few exceptions, we lack strong observational constraints (in situ or remote sensing) on the size, shape, density, and composition of these particles. Photochemical models, which can generally explain the observed abundances of smaller, gas phase species, are not well suited for investigations of much larger, solid phase species. Laboratory investigations of haze formation in planetary atmospheres therefore play a key role in improving our understanding of the formation and composition of haze particles. I will discuss a series of experiments aimed at improving our understanding of haze in the atmospheres of Titan, the early Earth, exoplanets, and any other atmospheres composed of a combination of N2, CH4, CO, CO2, and/or O2. In particular, I will discuss investigations of the density of Titan aerosol analogues (“tholins”), the effect of energy source on incorporation of nitrogen into tholin, the effect of CO on haze formation, and the possible presence of haze during the rise of oxygen on the early Earth.

Date:   Wednesday 09-Mar-2016
Speaker:   Dr. Brett Denevi (JHU APL)
Title:  Exploring the Innermost Planet: The Geologic History of Mercury

The MESSENGER spacecraft orbited Mercury from 2011­2015, and provided the first complete view of our solar system's innermost planet. Nearly a year after the completion of the MESSENGER mission, we present a synthesis of our current understanding of the formation of Mercury's crust and its geologic history - a history that has been revealed to be quite different from previous expectations. Mercury's unusual composition appears to have resulted in the formation of an exotic early crust and voluminous volcanic eruptions that affected nearly all of the surface. Subsequent cooling led to global contraction and the end of widespread effusive volcanism, but geologic activity continued in the form of explosive eruptions, tectonic deformation, and surface modification that produced enigmatic "hollows". We explore these features and how Mercury is helping to shape our understanding of the variety of processes and styles of planetary evolution in our solar system.

Date:   Wednesday 16-Mar-2016

Date:   Wednesday 23-Mar-2016
Speaker:   Dr. Michael McDonald (MIT)
Title:  Galaxy Cluster Evolution Over the Past 10 Billion Years

In this talk, I will summarize recent results from the South Pole Telescope 2500 deg^2 survey. This mass-limited survey has discovered hundreds of new galaxy clusters at 0 < z < 1.7, allowing an unprecedented view of galaxy cluster evolution. Using follow-up observations from Spitzer, Hubble, Chandra, XMM-Newton, Magellan, VLT, ALMA, ATCA, and Gemini, we are able to study the evolution of the stars, gas, and dark matter in these massive systems. Based on these data, we constrain the evolution of cluster galaxies, the central AGN, the cooling ICM, the heavy metal abundance of the ICM, the dynamical state of the cluster, and various other cluster properties. Looking forward, I will present several new and ongoing surveys which will dramatically change the landscape of galaxy cluster research in coming years.

Date:   Wednesday 30-Mar-2016
Speaker:   Dr. Mateusz Ruszkowski (University of Michigan)
Title:  The role of cosmic rays in stellar and supermassive black hole feedback

I will discuss the role of cosmic rays in stellar and supermassive black hole feedback. I will argue that cosmic rays are likely to play a very important role across a large range of distance scales -- from the scales of individual galaxies to the scales comparable to those of cool cores of galaxy clusters. Regarding the galactic scale feedback, I will focus on supernova and cosmic ray driven winds but will also briefly discuss black hole feedback model for the Fermi bubbles. Galactic outflows play an important role in galaxy evolution and, despite their importance, a detailed understanding of the physical mechanisms responsible for the driving of these winds is lacking. Although cosmic rays comprise only a tiny fraction of interstellar particles by number, they carry energy comparable to that in the thermal gas. I will describe a suite of global 3D MHD numerical simulations that focus on the dynamical role of cosmic rays injected by supernovae, and specifically on the impact of cosmic ray streaming along the magnetic fields. Our results show that this microphysical effect can have a significant impact on the wind launching depending on the details of the plasma physics. Regarding the feedback on galaxy cluster scales, I will discuss recent 3D MHD simulations of black hole feedback in galaxy cluster cool cores. I will argue that cosmic ray heating of the intracluster medium is likely to be a very important heating mechanism. This mechanism can provide the bulk of the heating needed for the self-regulation of supermassive black hole feedback. Non-thermal particles may provide an adequate level of heating without violating observational constraints. While largely an unexplored territory in the context of galactic winds and AGN feedback, cosmic ray feedback is an important process facilitating launching and efficient driving of galactic-scale winds in starburst galaxies and heating of the intracluster medium, and remains the subject of intense research.

Date:   Wednesday 06-Apr-2016
Speaker:   TBA
Title:  TBA

Date:   Wednesday 13-Apr-2016
Speaker:   Dr. Shobita Satyapa (GMU)
Title:  The Search for Supermassive Black Holes in Low Mass and Bulgeless Galaxies

It is now known that supermassive black holes one million to a few billion times the mass of the sun lurk in the centers of most bulge-dominated galaxies in the local Universe and that their mass is strongly correlated with the galaxy's bulge mass. This discovery has launched numerous speculations that the formation and evolution of galaxies and supermassive black holes (SMBHs) are funamentally linked and that perhaps the presence of a bulge is necessary for a black hole to form and grow. While the SMBH and host galaxy properties in the high bulge mass regime have been studied extensively, very little is known about the existence and properties of SMBHs in galaxies with low masses and those with small bulges. This is a significant deficiency since the study of this population allows us to gain an understanding on the origin and growth efficiency of SMBH seeds, thought to have formed at high redshift.

In this talk, I will summarize our recent multiwavelength investigation of low mass and bulgeless galaxies that remarkably reveals that optically hidden AGNs possibly reside in a larger fraction of low mass and bulgeless galaxies than previously thought.

This study can place important constraints on the nature of SMBH seeds as well as refine our understanding of the importance of secular processes in black hole growth.

Date:   Wednesday 20-Apr-2016
Speaker:   Dr. Jim Braatz (NRAO)
Title:  A Direct Measurement of the Hubble Constant with H2O Megamasers

Water vapor megamasers provide the only direct means for mapping sub-parsec accretion disks in active galactic nuclei. In the best cases they trace exquisite Keplerian rotation profiles and provide the gold standard measurements of masses of SMBH. In a few galaxies, the masers have been used to determine geometric distances to the host galaxies, thereby providing a direct, one-step measurement of the Hubble Constant, independent of standard candles. In light of the discrepancy between the Planck prediction of H0 based on CMB measurements and the direct measurement of H0 from standard candles, an independent and geometric measurement is particularly valuable. I will discuss the Megamaser Cosmology Project, which is discovering disk masers and applying this distance measurement technique, with the goal of measuring H0 to 3%. If the technique can be pushed in the future to achieve even higher accuracy, it can also lead to a constraint on neutrino mass.

Date:   Wednesday 27-Apr-2016
Speaker:   Dr. Patricia Boyd (GSFC)
Title:  Pulling it all together: a new look at long-term time variability in compact binary star systems in the era of continuous, uninterrupted light curves.

The current decade is one of anticipation for what is just around the corner. The James Webb Space Telescope is coming together in Goddard’s cleanroom, the TESS mission is poised to identify planets around nearby stars, and the Large Synoptic Survey Telescope will soon truly usher in the era of Time Domain Astronomy. While we wait, the TDA renaissance is already well underway. The K2 mission is amassing an impressive collection of high precision, evenly sampled, long time baseline optical observations on a par to the original Kepler mission, and high-energy space-based all-sky monitors continue to watch virtually every bright X-ray source in the sky. With each passing orbit these datasets sample a more complete variability history of accreting compact binaries. Some show high-amplitude “superorbital” variability at many times the orbital timescale. This is sometimes ascribed to the motion of a warped, precessing accretion disk, but even the most well-behaved sources (such as SS 433 and Hercules X-1) show surprising deviations from strictly periodic variability. In fact, the typical superorbital variable shows atypical variability, which can be a challenge to characterize, beyond “not periodic”. This talk will explore the variety of non-periodic variations seen in these systems, and some of the analysis tools used when traditional power spectral analysis is not illuminating. Such tools include phase-space embedding and topological analysis used by the nonlinear dynamics community. As the time baseline grows, these tools become more powerful, giving us hints to the drivers and dampers of such variability. The rich and growing archive at our fingertips may allow us to take the next steps in understanding the dynamics of accretion disks, while also serving as a sandbox for observers and analysts to build and test the tools that will realize the full potential of time-domain astronomy for the next era of astrophysics, when the data floodgates open.

Date:   Wednesday 04-May-2016
Speaker:   Dr. Molly Peeples (STScI)
Title:  The Circumgalactic Medium

The circumgalactic medium (CGM) serves as a fuel supply, waste dump, and recycling center for galaxies' baryons and metals. Our empirical understanding of this vast reservoir of gas surrounding galaxies has been revolutionized in the seven years since the installation of the Cosmic Origins Spectrographic (COS) aboard the Hubble Space Telescope in 2009. I will review the observational landscape of the low-redshift CGM and how it is challenging prevailing theories of galaxy evolution. I will then delve in to how trying to understand the underlying physical conditions and relevant physical processes governing the observed CGM is uncovering more and more questions as to the origin and fate of circumgalactic gas.

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