List of Past Astronomy Colloquia : 01-Jan-2017 to 01-Jun-2017

Date:   Wednesday 01-Feb-2017
Speaker:   CANCELLED

Date:   Wednesday 15-Feb-2017
Speaker:   Dr. Hidenobu Yajima (Tohoku University)
Title:  High-redshift Lyman-alpha emitting galaxies and submillimeter sources in cosmological simulations

Lyman-alpha emitting galaxies (LAEs) are key objects to understand cosmic reionization and galaxy formation in the early Universe. Combining cosmological hydrodynamics simulations and multi-wavelength radiation transfer calculations, we investigate the formation mechanism of LAEs and their contribution to cosmic reionization. Our findings are as follows : (1) cold gas accretion produces bright LAEs via the excitation cooling process; (2) observed LAEs can keep the ionization of IGM at z < 7; (3) some of high-z LAEs evolve to local L* galaxies (Yajima et al. 2015, ApJ, 801, 52; Yajima et al. 2017 arXiv:1701.05571). In addition, interstellar dust absorbs UV continuum and Lyman-alpha photons, and releases the absorbed energy by their thermal emission.

In this talk, I will discuss high-z submillimeter sources by calculating the thermal emission from dust.

Date:   Wednesday 22-Feb-2017
Speaker:   Dr. Eliza Kempton (Grinnell College)
Title:  Revealing the Atmospheres of Extrasolar Super-Earths

Discoveries of extrasolar planets over the last two decades have reshaped our understanding of how planetary systems form. Super-Earths -- planets intermediate in size/mass between Earth and Neptune -- do not exist in our Solar System, and the discovery of such planets poses a challenge to theories of planetary formation and composition based on the Solar System paradigm. Through observations of the atmospheres of these planets, we can learn about their formation history, their climate, and in some cases their propensity to support life. This talk will focus on the modeling of super-Earth atmospheres as it relates to current and future observations. I will detail the current state of characterization efforts for super-Earth atmospheres, focusing on the challenges and successes in modeling and interpreting the early observations of these objects. I will conclude with a forward-looking view of super-Earth atmospheric studies over the next 5-10 years, in the era of JWST and 30-meter class ground-based telescopes.

Date:   Monday 27-Feb-2017
Speaker:   Dr. Ondrej Pejcha (Princeton)
Title:  The Rugged Landscape of Core-Collapse Supernova Explosions

The collapse of the core and the associated supernova explosion mark the end of life of most massive stars, but the mechanism of explosion is poorly understood and perhaps even unknown. Some of its puzzling features were recently observed in the statistics of supernova progenitors, explosion energies, nickel yields, and in the remnant neutron star and black hole mass functions. I will describe my theoretical studies of the supernova explosion mechanism, its dependence on the progenitor star structure, and the connection with observables. I will argue that successful explosions are intertwined with failures in a complex but well-defined pattern that is tied to the pre-collapse structure of the progenitor star. I will also present a new method to extract the supernova parameters from light curves and expansion velocities, and illustrate how to constrain the explosion mechanism in the future.

Date:   Wednesday 01-Mar-2017
Speaker:   Dr. Paul Torrey (MIT)
Title:  Unlocking the Mysteries of Galaxy Formation with Cosmological Simulations

Cosmological simulations are among the most powerful tools available to probe the non-linear regime of cosmic structure formation. They also provide a clear test-bed for understanding the impact that hydrodynamics and feedback processes have on the evolution of galaxies. I will present leading-edge galaxy formation simulations that couple a novel moving mesh computational method with explicit baryon feedback prescriptions. This approach results in detailed galaxy formation models that reproduce fundamental observations such as the galaxy stellar mass function, cosmic star formation rate density, and galaxy morphological diversity. In turn, we are able to leverage these simulations to derive physical knowledge on a wide range of science questions, including the importance of supernova and black hole feedback on galaxy growth, the impact of environment on galaxy properties, the origin of diverse morphological types, and much more. I will discuss ongoing efforts to improve the physical fidelity of our simulations, which will continue to play an increasingly large role in building a comprehensive theory for galaxy formation over the coming decades.

Date:   Monday 06-Mar-2017
Speaker:   Dr. Blakesley Burkhart (CFA-Harvard)
Title:  New Diagnostics of MHD Turbulence in the Multiphase Interstellar Medium

Our current view of the interstellar medium (ISM) is as a multiphase environment where turbulence affects many key processes. These include star formation, cosmic ray acceleration, and the evolution of structure in the diffuse ISM. It is therefore essential to study and quantify interstellar turbulence using the strengths of numerical simulations in concert with observational studies. In this talk, I shall discuss progress in the development of new techniques for comparing observational data with numerical MHD simulations in the molecular medium, in neutral gas as traced by HI, and warm ionized gas as traced by synchrotron polarization. I will show how a confluence of simulations and novel multiwavelength measurements have taught us that: 1) The ISM in our Galaxy and nearby galaxies is supersonic in both the diffuse and molecular media, 2) Turbulence is primarily driven at scales larger than 100 parsecs, and 3) the magnetic field is a critical regulator of star formation. I will demonstrate how these measurements open up new avenues for studying star formation, cosmic ray acceleration and the formation of molecules in the ISM.

Date:   Wednesday 08-Mar-2017
Speaker:   Dr. Hsiang-Yi Karen Yang (University of Maryland)
Title:  The Microphysics of AGN Feedback

Feedback from active galactic nuclei (AGN) is one of the most important processes governing the formation and evolution of galaxies and galaxy clusters. It is believed to be responsible for inhibiting the formation of massive galaxies and for solving the long-standing "cooling-flow problem" in galaxy clusters. A lot of understanding of AGN feedback has been gained using hydrodynamic simulations; however, some of the relevant physical processes are unresolvable or not captured by pure hydrodynamics, such as plasma effects and cosmic-ray (CR) physics. In this talk, I will present how we use simulations that incorporate this "microphysics" to understand how AGN jets feedback on galactic and cluster scales. Specifically, I will discuss the roles of thermal conduction and CRs in addition to purely hydrodynamic models. I will also talk about how we could use multi-messenger observations of the Fermi bubbles as a nearby aboratory for studying AGN feedback. Finally, I will conclude with open questions and future prospects of applying simulations beyond hydrodynamics to various interesting astrophysical systems.

Date:   Monday 13-Mar-2017
Speaker:   Dr. Federico Marinacci (MIT)
Title:  Forming realistic galaxies and magnetic fields in cosmological simulations

Cosmological simulations represent the most complete approach to investigate the complex interplay among the physical processes that shape the evolution of galaxies. In my talk I will present state of the art cosmological simulations performed with an advanced, moving-mesh treatment for hydrodynamics, and featuring a cutting-edge implementation of baryonic physics. I will highlight the tremendous progress made by simulations in producing galaxies that are in agreement with key observables. In particular, I will focus on the formation of realistic disk galaxies similar to our own Milky Way, which has notoriously been a difficult problem to solve in the context of LCDM. Going beyond simple hydrodynamics, I will also discuss evolution of magnetic fields on galactic and cosmic scales. Finally, I will describe current developments on the future generation of simulations, also discussing how more detailed comparisons of the simulation results to the observations may help us to build a fully predictive theory of galaxy formation.

Date:   Monday 27-Mar-2017
Speaker:   Dr. Amy Reines (NOAO)
Title:  The Origin of Supermassive Black Holes

The origin of supermassive black holes remains a major outstanding issue in modern astrophysics. These monster black holes reside in the nuclei of essentially every massive galaxy and power the most luminous quasars at the edge of the observable Universe. However, directly observing the first "seed" black holes in the early Universe - that can eventually grow to upwards of a billion solar masses - is not feasible with current telescopes. Present-day dwarf galaxies, on the other hand, are within observational reach and offer another avenue to learn about black hole seeds since low-mass galaxies can host relatively pristine black holes. In this talk, I will highlight some of my recent achievements in this field that have taken us from a few rare examples to large systematically-assembled samples of dwarf galaxies hosting nuclear black holes. I will also discuss how my work has implications for directly detecting black hole activity in the first galaxies at high redshift. Finally, I will describe my future plans to probe the origin of supermassive black holes with dwarf galaxies, and provide the much needed observational constraints on the otherwise theory-dominated work on the formation of the first black hole seeds.

Date:   Wednesday 29-Mar-2017
Speaker:   Dr. Brendan Bowler (University of Texas Austin)
Title:  Revealing the Formation and Atmospheres of Exoplanets with Direct Imaging

Finding and characterizing extrasolar planets has become one of the fastest-paced and most rapidly evolving fields in modern astronomy. Direct imaging— spatially resolving exoplanets from their host stars— is especially challenging but provides unique insight into the architectures, atmospheres, and formation of giant planets. By exploring planetary systems from the outside-in and directly detecting photons originating in their atmospheres, imaging complements other planet-finding techniques sensitive to smaller orbital separations and enables detailed studies of atmospheric structure and composition. In this talk I will review the field of high-contrast adaptive optics imaging with an emphasis on observational programs I am leading to test theories of planet formation, primarily by means of large surveys, planet population statistics, and near-infrared spectroscopy. I will also outline the long-term future of the field; imaging planets has been a consistent motivating factor for the next generation of telescopes like JWST, WFIRST, and the 30-m class telescopes. Eventually these facilities will pave the way for the ultimate objective: a dedicated space-based mission to image and characterize Earth analogs.

Date:   Monday 03-Apr-2017
Speaker:   Dr. Nicholas Stone (Columbia University)
Title:  Dynamics in Dense Stellar Systems: Tidal Disruption Flares & Other Transients

In galactic nuclei, the coexistence of a dense cusp of stars with a central supermassive black hole (SMBH) leads to a unique and dramatic phenomenon: tidal disruption events (TDEs). During a TDE, the star is ripped apart by the tidal field of a massive black hole, and the eventual accretion of the stellar debris powers a luminous flare brighter than most supernovae and visible across the entire electromagnetic spectrum. Dozens of TDE flares have been discovered at optical, UV, and X-ray wavelengths in the last decade, and upcoming time domain surveys (such as ZTF, LSST, and eROSITA) will discover thousands more. Properly interpreted, a large sample of these flares holds great promise for testing questions in accretion physics and measuring the demography of quiescent supermassive black holes. I will discuss the dynamical processes that set tidal disruption event rates and the leading order role general relativity plays in the light curves of TDE flares, as well as ways in which black hole spin may be imprinted into TDE observables. TDEs are, however, only one example of the high energy transients produced by collisional stellar dynamical processes in dense star clusters (both galactic nuclei and globular clusters). Tidal capture, binary-single scatterings, and dynamical triple formation are all examples of dynamical phenomena that only arise in dense star systems, and I will discuss how these processes lead to unique sources of electromagnetic and gravitational wave radiation.

Date:   Wednesday 05-Apr-2017
Speaker:   Dr. Xuening Bai (CFA-Harvard)
Title:  Protoplanetary Disks and Planet Formation: A Microphysical Perspective

Planet formation takes place in gas-rich protoplanetary disks (PPDs) orbiting newly born stars. The internal density and flow structures of PPDs, as well as their long-term evolution, play a crucial role in almost all stages of planet formation, yet they are far from being well understood largely due to the complex interplay among various microphysical processes. Such lack of understanding also leads to major uncertainties in interpreting disk observations and exoplanet discoveries. I will review the basic physical processes in PPDs that govern disk structure and evolution, highlighting the fundamental role of disk ionization and magnetic fields. I will then describe my effort of incorporating and understanding these processes, both analytically and (primarily) computationally, towards building up the most realistic global evolutionary picture of PPDs. In particular, these efforts have led to a paradigm shift in understanding PPD gas dynamics, with major implications on planet formation. Finally, I will discuss future directions, in particular how the advanced computational tools will transform the science on PPDs and the formation of (exo-)planetary systems.

Date:   Wednesday 12-Apr-2017
Speaker:   Dr. Jim Stone (Princeton University)
Title:  Super Eddington Black Hole Accretion Flows

Super Eddington accretion occurs in many systems, such as the inner regions of quasars and luminous AGN, ultra-luminous X-ray sources (ULXs), and tidal disruption events. Understanding such flows is important not only for interpreting the spectra and variability of these sources, but also to predict the rate of growth of black holes in the early universe, and to quantify energy and momentum feedback into the medium surrounding the black hole, a process likely to be important in controlling galaxy formation in the case of AGN. New results from a study of the magnetohydrodynamics of luminous accretion flows, in which radiation pressure dominates, will be presented. We have developed new numerical methods based on a formal solution of the time-dependent radiation transfer equations to study this regime. Our numerical simulations reveal new effects that require extension of standard thin-disk models. We discuss these results, and their implications for the astrophysics of accreting black holes.

Date:   Wednesday 19-Apr-2017
Speaker:   Dr. John Debes (STScI)
Title:  Taking scattered light imaging of young planetary disks into the time domain

As spatially resolved scattered light imaging of young planet-hosting disks becomes more routine and the number of spatially resolved disks explodes thanks to ground- and space-based high contrast imaging, the time has come to bring such imaging into the time domain. I review this new aspect of observing protoplanetary and debris disks, while describing how time domain imaging helps to elucidate planet formation and evolution. Applications of this work with an eye toward future space- and ground-based observatories is also explored.

Date:   Wednesday 26-Apr-2017
Speaker:   Dr. Pieter van Dokkum (Yale University)
Title:  "Exploring the low surface brightness sky with the Dragonfly Telephoto Array"

The talk will describe a new telescope concept, optimized for low surface brightness imaging. The Dragonfly Telephoto Array consists of 48 high-end telephoto lenses, creating the equivalent of an f/0.4 refractor with a 1m aperture. Early results from Dragonfly include a measurement of the stellar halos around nearby spiral galaxies down to ~32 mag/arcsec^2 and the identification of new satellite galaxies. Perhaps the most spectacular result so far is the discovery of a population of Milky Way-sized, very low surface brightness galaxies in rich clusters. A follow-up study of one of these "Ultra Diffuse Galaxies" shows that it is massive and extremely dark matter dominated, with a dark matter fraction of 98% inside its effective radius.

Date:   Wednesday 03-May-2017
Speaker:   Dr. Sangeeta Malhotra (Arizona State University)
Title:  “Lymna-alpha galaxies through cosmic time — from z=0.3 - 7$

Lyman-alpha photons are resonantly scattered by neutral hydrogen, to these photons. Yet we see a population of Lyma-alpha emitters at all redshifts. Is there anything different about these galaxies? What makes Lyman-alpha escape? I will try to address these questions based on recent (and older) observations. By understanding the Lyman-alpha escape from galaxies at lower redshifts we hope to fine-tune them to diagnose the neutral fraction of the intergalactic gas at the epoch of reionization.

Date:   Wednesday 10-May-2017
Speaker:   Dr. Alexander Rudolph (Cal Poly Pomona)
Title:  Cal-Bridge and CAMPARE: Engaging Underrepresented Students in Physics and Astronomy

The level of participation by underrepresented minority (URM) and female students in physics and astronomy PhD programs is shamefully low (2-4% for URM v. 30% in the general population; 20% for women v. 50% in the general population). I will begin by discussing research into why these participation rates are so low for these groups, highlighting role the physics and general GRE tests play in suppressing diversity in our field, while providing little to no benefit in helping predict long-term success. I will describe some alternative methods of conducting graduate admissions, including the role of programs like Cal-Bridge and CAMPARE.

I will then describe these two California-wide, multi-institutional programs, Cal-Bridge and CAMPARE, with the common mission of increasing participation of underrepresented minorities and women in astronomy and physics through summer research opportunities, in the case of CAMPARE, scholarships in the case of Cal-Bridge, and significant mentoring and professional development in both programs, leading to an increase in their numbers completing bachelor’s degrees, and successfully pursuing a PhD in these fields.

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