Astronomy Colloquia at Caltech for 2014-15

Colloquia are held every Wednesday during the academic year at 4pm in the Cahill Hameetman auditorium.
Wine and cheese will be served in the Cahill Foyer from 5-6pm.


Talk Title

Nicholas Law
Univ North Carolina

Host: Shri Kukarni

The Evryscope: the first full-sky gigapixel-scale telescope

Current wide-field time-domain sky surveys generally operate with few-degree-sized fields and take many individual images to cover large sky areas each night. I will discuss the Evryscope (“wide-seer”), which takes a different approach: using an array of small telescopes to form a single image covering every part of the accessible sky simultaneously and continuously. The Evryscope is a low-cost gigapixel-scale imager with a 10,000 sq. deg. field of view, and has an etendue three times larger than the Pan-STARRS sky survey. The system will search for transiting exoplanets around nearby bright stars, M-dwarfs and white dwarfs, as well as obtain minute-by-minute  imaging of microlensing events, nearby supernovae, and gamma-ray burst afterglows as they happen. We plan to deploy the system at CTIO in early 2015. I will present the project status and plans for science operations, as well as an update on the Evryscope prototype telescopes which we are operating near the North Pole (the first High-Arctic astronomical survey).


Shane Davis

Host: Phil Hopkins

How Black Holes Dine above the Eddington "Limit" without Overeating
or Excessive Belching

The study of super-Eddington accretion is essential to our understanding of the growth of super-massive black holes in the early universe, the accretion of tidally disrupted stars, and the nature of ultraluminous X-ray sources. Unfortunately, this mode of accretion is particularly difficult to model because of the multidimensionality of the flow, the importance magnetohydrodynamic turbulence, and the dominant dynamical role played by radiation forces. However, recent increases in computing power and advances in algorithms are facilitating major improvements in our ability to model the transport of radiation in numerical simulations of astrophysical plasmas.  I will briefly describe our new radiation transfer modules and discuss our efforts to model super-Eddington accretion flows.  I will focus on applications to ultraluminous X-ray sources, which must be radiating well above their Eddington luminosity unless they harbor intermediate mass black holes. I will argue that most of these sources can be (and likely are) "normal" ~10 solar mass black holes accreting and radiating with luminosities well above their Eddington "limit".


John Wise
Georgia Tech

Host: Christian Ott

The First Stars and Galaxies in the Universe
Cosmic structure forms hierarchically through smooth accretion and dark matter halo mergers.  As a consequence, all galaxies are the product of the dozens of mergers over billions of years.  However, one can ask, "What were the first stars and galaxies in the universe?"  I will review the current state-of-the-art simulations of early galaxy formation, starting with the formation of the first stars, which are initially devoid of elements heavier than lithium and are suggested to have a characteristic mass of tens of solar masses.  I will then present results from a suite of cosmological radiation hydrodynamics simulations that focus on the transition from the first stars to the first galaxies.  Each simulation captures the radiative and chemical
feedback from ~300 first stars, leading to the formation of a 10^9 solar mass dwarf galaxy 700 million years after the Big Bang.  I will show that momentum transfer from ionizing radiation plays an important role in providing turbulent support and mixing supernova ejecta, preventing the overproduction of stars and metals.  This results in a stellar population with a tight metallicity distribution function centered at 0.01 of solar metallicity, agreeing with the observed luminosity-metallicity relation in local dwarf galaxies.  I will also demonstrate that these faintest galaxies are the primary driver of the reionization of the universe, only to be suppressed by photo-heating at later times, perhaps evolving into a subset of dwarf galaxies in the local universe.


Todd Thompson
Ohio State

Host: Christian Ott

Preludes to a Theory and Phenomenology of Galactic Winds

Galactic winds are ubiquitous in rapidly star-forming galaxies at low and high redshift.  They shape the stellar mass function, enrich the IGM, and determine in part the evolution of the metal abundance of galaxies.  I will present new work constraining the physics of galactic outflows and feedback processes in these systems.   I will first focus on galactic superwinds driven by very hot gas generated by overlapping supernovae within the host galaxy, and argue that the X-ray observations strongly constrain the mass-loading efficiency, from dwarf starbursts to ultra-luminous infrared galaxies.  These constraints also limit the ability of a hot wind to accelerate cool gas clouds via ram pressure.  I will then show in what regimes hot winds become radiative on 1-100kpc scales and discuss the observational implications.  I will then focus on momentum injection by supernovae and radiation pressure of starlight on dust grains.  I will discuss new work on the phenomenology of high redshift galaxies and their generalized Eddington limit for expelling gas.  Finally, I will discuss a set of numerical experiments to understand the coupling between radiation and matter in highly optically-thick environments relevant for dense starburst nuclei and the self-gravitating disks that attend the fueling of active galactic nuclei.


Meredith Hughes

Host: Jessie Christiansen

Planet Formation through Radio Eyes

Circumstellar disks provide the raw material and initial conditions for planet formation.  Millimeter-wavelength interferometry is a powerful tool for studying gas and dust in planet-forming regions, and it is undergoing an immense leap in sophistication with the advent of the ALMA  interferometer that is now beginning operations.  I will discuss some ways in which millimeter-wavelength interferometry is being used to study the process of planet formation in circumstellar disks, with particular emphasis on the kinematics of turbulence in protoplanetary disks and the degree to which debris disk structure reflects the dynamics of embedded planetary systems.


Bekki Dawson

Host: Jessie Christiansen

Planetary Systems in 4D

Discoveries of exoplanets so different from those in our Solar System have called in question conventional theories for how planetary systems form and evolve. I will present recent progress in our understanding of the physical processes that drive the assembly of planetary systems and result in the surprising variety of orbital properties we observe today. I will focus on the orbital evolution of giant planets, including the origin of hot Jupiters and giant planets on elliptical and tilted orbits. I will conclude with pathways forward toward a blueprint for how planetary systems form and evolve, including connections between small and giant planets and the different initial conditions that lead to a diverse array of planetary orbits and compositions.


Amy Reines
Univ of Michigan

Host: Fiona Harrison

Probing the Origin of Supermassive Black Holes with Dwarf Galaxies

Supermassive black holes (BHs) live at the heart of essentially all massive galaxies with bulges, power AGN, and are thought to be important agents in the evolution of their hosts.  However, the origin of the first supermassive BH "seeds" is far from understood.  While direct observations of these distant BHs in the infant Universe are unobtainable with current capabilities, massive BHs in present-day dwarf galaxies offer another avenue to observationally constrain the masses, host galaxies and formation path of supermassive BH seeds.  Using optical spectroscopy from the SDSS, we have increased the number of known dwarf galaxies hosting massive BHs by more than an order of magnitude.  These dwarf galaxies have stellar masses comparable to the Magellanic Clouds and contain some of the least-massive supermassive BHs known.  I will present results from this study, and well as on-going efforts using radio and X-ray observations to reveal massive BHs in star-forming dwarfs that can be missed by optical diagnostics.


Jenny Greene

Host: Judy Cohen

MASSIVE Galaxies and Small Supermassive Black Holes

I will discuss MASSIVE, an ambitious new integral-field survey of the ~100 most massive galaxies within 100 Mpc.  Using integral-field spectroscopy covering 200 pc to 20 kpc scales, we are studying the assembly history of massive galaxies from the supermassive black holes
at the center to the dark matter halos on large scales. I will then discuss black hole scaling relations over a large range in galaxy mass, using MASSIVE observations at the high end and megamaser disk galaxies at low mass. If time permits, I will discuss progress on 
survey planning for the Prime Focus Spectrograph.


Konstantin Batygin

Host: Jessie Christiansen

The onset of large-scale dynamical instability in the Solar System

Over the last two decades, evidence has mounted that the centuries-old question concerning the dynamical stability of the solar system has a straight-forward, definitive answer: with a probability of ~1%, the inner solar system may gravitationally unravel on a timescale comparable to the remaining main-sequence lifetime of the Sun. Concurrently, as the orbital distribution of extrasolar planets began to surface, it had become clear that dynamical instability is a generic process that plays a central role in shaping the architecture of planetary systems. Despite its inherent significance, an unembellished qualitative description of the onset of orbital disorder is largely missing. In this talk, I will describe a purely analytical theory for the chaotic disintegration of planetary systems. Specifically, with an emphasis on the Solar System, I will delineate a perturbative model that broadly captures the onset of large-scale instability and use it to elucidate the source of Mercury's chaotic behavior, as well as estimate the corresponding Lyapunov and diffusion coefficients. Subsequently, I will present a framework for calculating the characteristic dynamical lifetime of the inner Solar System. The obtained results constitute an important step towards developing an intuitive view of the long-term evolution of planetary systems.


Eve Ostriker

Host: Phil Hopkins

[In]efficient Star Formation and the ISM

Galaxies are long-lived systems.  Disk galaxies like the Milky Way retain a massive reservoir of atomic and molecular gas over billions of years, converting it to stars only slowly.  Because gas consumption timescales far exceed both global and local dynamical timescales, star
formation is often characterized as being highly inefficient. The perspective shifts if we consider the effects of star formation on the interstellar medium (ISM) rather than its causes: without star formation feedback, the ISM could not exist as we know it.  Rapid losses of thermal energy by radiative cooling and turbulent energy by dissipation must be constantly replenished to maintain the observed state of the ISM, and feedback from short-lived massive stars -- including UV radiation and supernova blasts -- is crucial. In this talk, I will discuss theory and numerical hydrodynamic/RHD simulations that quantify the physics of feedback and star formation self-regulation at a range of scales.  We find that radiation forces
can be important to ejecting gas and limiting the efficiency of individual star-forming clouds. However, supernovae play the most important role in the ISM overall, because the momentum injected by Sedov-Taylor blast waves is an order of magnitude greater than other source terms.  Resolved simulations show that each supernova blast robustly provides momentum ~ 1-4 e5 Msun km/s to the ISM.  This momentum yield is just what is required to explain ISM properties and star formation rates as observed in diverse galactic environments, with the self-regulation model providing a unified theoretical framework and quantitative explanation for both Kennicutt-Schmidt and pressure-based empirical star formation laws.


Joshua Frieman
Fermilab and U. Chicago

Host: Richard Ellis

Probing Cosmic Acceleration with the Dark Energy Survey

The Nobel Prize in Physics for 2011 was awarded for the discovery that the expansion of the Universe is accelerating. Yet the physical origin of cosmic acceleration remains a mystery. The Dark Energy Survey (DES) aims to address the questions: why is the expansion speeding up? Is cosmic acceleration due to dark energy or does it require a modification of General Relativity? If dark energy, is it the energy density of the vacuum (Einstein's cosmological constant) or something else? DES is addressing these questions by measuring the history of cosmic expansion and of the growth of structure through four complementary techniques: galaxy
clusters, the large-scale galaxy distribution, weak gravitational lensing, and supernovae. The DES collaboration built a new, 570-megapixel, digital camera for the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory in Chile to carry out a deep, wide-area sky survey of 300 million galaxies and a time-domain survey that will discover 3500 supernovae. I will overview the DES project, which achieved `first light' in September 2012 and which is now in its second of five survey seasons,  and describe a number of early science results.


Kartik Sheth

Host: Gregg Hallinan

Pushing the Frontiers of Discovery with ALMA

The Atacama Large Millimeter / Submillimeter Array has been in operation for nearly three years and although it has not yet entered full science operations, has delivered incredible results changing the paradigms across the entire spectrum of astrophysics research.  There are also some important synergies between ALMA, JWST and the TMT. I will describe the science results and the synergies along with an update on ALMA today with the goal of inspiring the audience to use this transformational millimeter / submillimeter facility.  Time permitting I hope to also show our newest ALMA results on gas and dust content of nearby and high redshift galaxies.


Fabian Walter

Host: Phil Hopkins

2nd Annual Biard Lecture
What drives the Star Formation History of the Universe?

Understanding how galaxies form and evolve over cosmic time is one of the central topics in observational astrophysics and the last decade has seen dramatic advances in our understanding of cosmic structure formation. The cosmic star formation rate density (the ‘star formation history of the Universe’), and stellar mass build-up, have been quantified back to first light and cosmic reionization, within 1Gyr of the Big Bang. Studies of galaxy formation are now turning attention to the evolution of the cool gas, the fuel for star formation, in galaxies. Here, observations of the cool interstellar medium in distant galaxies, via molecular and atomic fine structure line emission, has gone from a curious look into a few extreme, rare objects, to a mainstream tool to study galaxy formation, out to the highest redshifts. I will highlight some of the recent results emerging from molecular gas studies of main sequence galaxies at the 'peak of galaxy assembly' at z~2 that form stars at much higher rates than in today's Universe. I will also present results based on a molecular line scan of the Hubble Deep Field North that provides first blind constraints on the evolution of the cosmic molecular gas density. I will close by highlighting how the Cosmic Microwave Background may hamper studies of the interstellar medium of the highest redshift, main sequence galaxies, even in the era of ALMA.


Marc Pinsonneault
Ohio State

Host: Judy Cohen

Precision Stellar Astrophysics: Asteroseismology and Large Spectroscopic Surveys

Large time domain and spectroscopic surveys are revolutionizing stellar astrophysics.  In this talk I will focus on the APOKASC project, involving a combination of asteroseismic and rotational data from the Kepler satellite and high-resolution H-band spectroscopy from APOGEE.  Asteroseismology gives precise surface gravities and evolutionary state measurements, and the frequency pattern combined with spectroscopic data can also be used to independently measure mass, radius, and age.  I critically assess the performance of asteroseismology against independent measurements, and  also discuss the prospects for detailed tests of stellar physics (helium, convection zone depth, core mass and rotation.)  Insights into stellar populations, such as an unexpectedly large fraction of young alpha-rich stars, will also be covered.  Future and ongoing missions will be reviewed, and synergies with Gaia will be discussed. 


Sander Weinreb

Host: Sterl Phinney

17th Annual Greenstein Lecture

Radio Astronomy – Past, Present, and Future

Radio Astronomy grew from an  accidental discovery in 1932, virtually ignored for years, to a prolific window to a previously invisible  universe,  producing four  Nobel Prizes.  A major new international instrument, the Square Kilometer Array, is now in the detailed design phase and  next steps for complementary instruments  in the US are being explored.   A theme of the lecture will be lessons-learned from past history and application  to the future.  Some of the  gems of history leading to the start of radio astronomy at Caltech and NRAO will be recalled in the contributions  of Jansky, Reber, Greenstein, and Heeschen. The current state-of-the-art and future directions of receivers will be briefly presented.   The evolution of the VLA will be described   along with the status of the  SKA and  plans for a next generation of the VLA.   Finally, I will present my views concerning some of the vital  global policies relevant to astronomy.


Ralf Klessen

Host: Christian Ott

Star Formation

Stars and star clusters are the fundamental visible building blocks of galaxies at present days as well as in the early universe. They form by gravitational collapse in regions of high density in the complex multi-phase interstellar medium. The process of stellar birth is controlled by the intricate interplay between the self-gravity of the star-forming gas and various opposing agents, such as supersonic turbulence, magnetic fields, radiation pressure, and gas pressure. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. This process is modified by the thermodynamic response of the gas, which is determined by the balance between various heating and cooling processes, which in turn depend on the chemical composition of the material. I will review the current status of the field and discuss a few examples of the recent progress in present-day star formation and speculate about the implications for the first and second generation of stars in the universe.


Daniel Stern

Host: Fiona Harrison

Surprising New Insights into Quasars from the WISE Satellite

We now believe that every large galaxy hosts a supermassive black hole at its core, with masses ranging from millions to billions of times that of our Sun.  At times, these black holes are actively accreting, causing the nuclei of the galaxies to shine brightly across the electromagnetic spectrum.  However, most quasars have obscuring material along the line of sight that shields the inner nucleus.  This obscuring material is heated, and emits strongly in the mid-infrared.  The Wide-field Infrared Survey Explorer, or WISE, has mapped the entire sky in mid-infrared light with exquisite depth and clarity.  WISE has allowed us to find luminous quasars across the whole sky due
to this heated material, more than tripling the number of quasars known.  I will discuss several surprising new insights into quasars that have come out of this work.  In brief, the dominant paradigms do not match our observations, with potentially important implications for the role of quasars in the growth of galaxies.  I will conclude by discussing how these studies will be further enabled by the Euclid and WFIRST satellites.


Kathryn Johnston

Host: Judy Cohen

Dark Matter and Stellar Halos:  Formation, Histories and Structure 

The existence of spectacular low-surface-brightness features - remnants of past mergers - surrounding many galaxies has been known about for many decades.  A major accomplishment for more recent, large scale stellar surveys of the Milky Way has been the discovery of a multitude of debris from dead and dying small satellites encircling our Galaxy.  This talk will look at three questions that can be addressed by the properties of such debris: what is the orbit distribution of in falling satellites? what is the mass, shape and radial profile of our dark matter halo? and does our halo contain any stars that originally formed in our Galactic disk?


Edwin Bergin
U. Michigan

Host: John Carpenter

Chemical Signatures of Planet Formation in Gas Rich Disks

In this talk I will focus on two aspects of planet formation that have potential observational signatures that should be present in molecular emission associated with gas-rich disks.  First, in the early stages of disk evolution, settling of ice-coated dust grains to the midplane should lead to the sequestration of the disk’s icy solidsinto the planet feeding zone.  Thus, beyond their snow lines volatiles (CO, H2O, N2) condense and become incorporated into any forming planetesimals.  We suggest that over time this process must deplete  the upper atmosphere of these important volatile reservoirs.  I will present evidence that this process might be active in a nearby disk. This work has broad implications for estimates of  disk gas mass and gas dissipation timescales from species such as CO will consequently be intertwined with the timescales of planet formation. In this second part of my talk, I will focus on exploring the possibility that forming gas giants might be detected via disk chemistry using ALMA.
Thus I will present a 3D physical/chemical model that includes two point sources: star and accreting protoplanet. With generic assumptions based on planet formation theory and observations, we find that the localized heating of an accreting protoplanet can alter the chemistry in its near vicinity by, for example, releasing volatiles that otherwise would
be frozen on grain surfaces. I will show that this effect is present and predicted to be detectable, perhaps opening up the submm/mm-wave to planet searches.


David Hogg

Host: Phil Hopkins

Data-driven models of stars

Despite incredible maturity, physical stellar models do not match all the detailed properties of stellar spectra, and different models compared to different sections of stellar spectra return different stellar parameter and abundance estimates.  At the same time, we have enormous data sets containing high-resolution, high signal-to-noise spectra of tens of thousands of stars.  I discuss ways to build data-driven stellar models, permitting stellar parameter and chemical abundance estimation without any explicit use of physical models.  I will show successes with the SDSS-III APOGEE data.  Time permitting, I will also talk about data-driven approaches in other areas, such as the search for exoplanets in Kepler and K2 data.


Dave Alexander

Host: Fiona Harrison

Active Galaxies in Cosmic X-ray Surveys: The Ecology of Distant AGNs

Extragalactic X-ray surveys over the past decade have dramatically
improved our understanding of the majority populations of active
galactic nuclei (AGNs) over most of the history of the Universe. In
this talk I will focus on the discoveries made from X-ray surveys on
the ecology of distant AGNs - the connection between the growing black
holes at the centre of galaxies and the larger-scale environment.


George Efstathiou

Host: Richard Ellis

  Cosmology with Planck 2015

The Planck satellite was launched by the European Space Agency in 2009 and mapped the sky in nine bands spanning the frequency range 30-857 GHz. Planck has given us the most accurate temperature and polarization measurements of the cosmic microwave background to date. The results are in beautiful agreement with simple single field inflationary models. The background cosmology is consistent, to high precision, with a spatially flat Universe dominated at the present day by weakly interacting cold dark and a cosmological constant. I will discuss the constraints on inflationary gravitational waves derived from the Planck temperature data and from a recent joint analysis of polarization measurements from BICEP/Keck Array and Planck. These results provide strong and reliable constraints on the physics that operated within 10^-35 seconds of the birth of our Universe.


Sean Couch

Host: Christian Ott

The Turbulent Frontier in Massive Stellar Death

Core-collapse supernovae are the luminous explosions that herald the death of massive stars. Neutron stars, pulsars, magnetars, and black holes are all born in these explosions. Supernovae are the drivers of galactic chemical evolution, being responsible for the synthesis of most of the heavy elements throughout the universe. Additionally, a Galactic supernova should be detectable by neutrino and gravitational wave detectors, opening entirely new windows on the observable universe. Despite the importance of CCSNe to our understanding of many aspects of astrophysics, the mechanism that reverses stellar core collapse and drives these explosions is not fully understood. I will discuss the revolution underway in supernova theory made possible by high-fidelity 3D simulations. In particular, I will focus on work going on right here at Caltech revealing the paradigm-shifting importance of turbulence in aiding neutrino-driven supernova explosions, and how this turbulence is influenced by realistic 3D progenitor structure as well as magnetic fields. These new developments at the frontier of core-collapse supernova theory may lead to a solution for the long-standing problem of how massive stars explode.


Stella Offner

Host: Phil Hopkins

The Impact of Stellar Feedback on Molecular Clouds

Low-mass stars influence their surroundings through a variety of feedback processes. However, because they are less energetic than high-mass stars, their contributions are often ignored. In this talk, I will discuss numerical simulations including protostellar outflows and winds from main sequence B-type stars. I will evaluate the role of these processes in the star formation process, and I will demonstrate that such feedback has important implications for the IMF, the star formation efficiency of dense gas, and cloud turbulence.


Charlie Conroy

Host: Evan Kirby

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 ages and detailed elemental abundance patterns of early-type galaxies over the interval 0<z<1.  Constraints on the abundances of the alpha, iron peak, and neutron capture elements offer the promise of reconstructing the detailed star formation histories of these now dormant galaxies.  By measuring the evolution of these quantities through cosmic time we are gaining fresh insights into the assembly histories of galaxies.  The techniques we are developing will enable `extragalactic chemical tagging' and, more generally, will open up the low resolution universe for detailed study.


Niel Brandt
Penn State

Host: Fiona Harrison

Exceptional X-ray Weak Quasars and Their Implications for
Accretion Flows, Broad Line Regions, and Winds

Actively accreting supermassive black holes are found, nearly universally, to create luminous X-ray emission, and this point underlies the utility of X-ray surveys for finding active galactic nuclei throughout the Universe. However, there are apparent X-ray weak exceptions to this rule that are now providing novel insights, including weak-line quasars (WLQs) and especially analogs of the extreme WLQ, PHL 1811. We have been systematically studying such X-ray weak quasars with Chandra and near-infrared spectroscopy, and I will report results on their remarkable properties and describe implications for models of the accretion disk/corona, emission-line formation, and quasar winds. We havefound evidence that many of these quasars may have geometrically thick inner accretion disks, likely due to high accretion rates, that shield the high-ionization broad line region from the ionizing continuum. Such shielding may, more generally, play a significant role in shaping the broad distributions of quasar C IV emission-line equivalent widths and blueshifts. Furthermore, I will report NuSTAR observations indicating that a significant fraction of BAL quasars are intrinsically X-ray weak, thereby promoting strong wind driving. I will end by discussing some promising ongoing studies that are extending these ideas.


Juna Kollmeier

Host: Phil Hopkins

Cosmological Calorimetry:  The nature of the intergalactic medium and
the photon underproduction crisis

The Lyman alpha forest remains one of the most robust predictions of cosmological hydrodynamic simulations.  Lyman alpha absorption lines have been used for decades to trace cosmic structures that -- only recently, with the exquisite sensitivity achievable with modern
instruments -- are beginning to be detected in emission as well. I will discuss the predicted morphology of the Lyman alpha forest and the possibility of revealing this structure with Lyman alpha imaging surveys.  The emission signal is dependent on the ionizing background
radiation which, at high redshift, is well-understood and constrained.   However, I will show that at low redshift there is a huge mismatch between our expectations and observations.  I describe a factor of 5 discrepancy between the value of the photoionization rate required to
match cosmological models of the z = 0 intergalactic medium to observations of the Lyman alpha forest and the value predicted by state-of-the-art models that account for the emissivity of stars and quasars over time.  Examining potential resolutions to this problem, I will demonstrate that solving it requires, at minimum, a major revision of our thinking about the low redshift universe.

Information for Speakers

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