Astronomy Colloquia at Caltech for 2015-16

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
Scott Ransom, Kingsley

Host: Shri Kulkarni

"But Wait!  There's more: A Wealth of Science from Millisecond Pulsars"

Pulsars are some of physics and astrophysics' most exotic objects, and they have already earned two Nobel Prizes.  We currently know of about 2500 of them in our Galaxy, but a small subset, the millisecond pulsars (MSPs), are truly remarkable.  These systems are notoriously hard to detect, yet their numbers have more than doubled in the past 4 years via surveys using the world's most sensitive telescopes, new instrumentation, and huge amounts of computing.  Specialized "timing" observations of these systems, accounting for each and every one of the billions of rotations of the stars, are providing fantastic results. In this talk I'll focus on the efforts to directly detect gravitational waves from super-massive black hole binaries and some surprising new physics opportunities from a unique triple system consisting of an MSP orbited by two compact white dwarf stars.


No Colloquium


Jennifer van Saders

Host: Jim Fuller

Tuning the Clock: Making Sense of Stellar Rotation with Kepler

Gyrochronology utilizes the spin-down of stars as a function of time as an indicator of stellar age. This technique has the potential to yield precise ages for large samples of stars, providing unprecedented chronological information for studies of the Milky Way and extrasolar planets. However, gyrochronology is in its adolescence: it has been tested under limited scenarios, but its weaknesses and limitations have hitherto been largely unexplored. With data from the Kepler mission we can address these gaps: we now have access to datasets of rotation periods for tens of thousands of stars, as well as independent asteroseismic ages and rotation periods for a few hundred old (main sequence) stars.  I will discuss my comparisons of theoretical rotation models to these Kepler data, which have yielded unexpected insights into the rotational lives of stars (and the Sun!), as well as a better understanding of the power and peril of gyrochronology as a tool.


Ben Mazin

Host: Dimitri Mawet

The Future of Astronomy is Super(conducting): Microwave Kinetic Inductance Detectors for UVOIR Astronomy

In the last five years we have made remarkable progress in turning superconducting lumped element microwave resonators into the most powerful UV, optical, and near-IR detectors in the world.  In this talk I will describe in detail the operating principles of these detectors, called Microwave Kinetic Inductance Detectors, and describe some of the exciting astronomy that we have done with them.  I will finish by discussing future possibilities of the technology, including the real possibility that these detectors will be the first ones to detect life on nearby planets.


George Djorgovski

"Big Data Meet Big Black Holes: Quasars in the "Time Doman"

Variability is one of the key observational properties of quasars, and it can be used as a probe of their fueling, physics, and evolution. A new generation of synoptic sky surveys, in combination with the novel data analytics tools, offers unprecedented data sets for the studies of quasars in the time domain.  I will illustrate this with examples from the Catalina Real-Time Transient Survey (CRTS), which has an open and growing archive of 500 million light curves, including 350,000 spectroscopically confirmed quasars, with the time baselines ranging from 10 minutes to 10 years.  This includes: a new approach to quasar discovery using a combination of variability and mid-IR colors from WISE, that will result in a catalog of at least a million new quasar candidates; the discovery of a characteristic time scale for a stochastic variability of quasars, that may probe the physics of their accretion disks; discoveries of “transgender” quasars that show strong spectroscopic changes on the time scales of years, coupled with an anomalous variability; and so on.  Perhaps the most interesting is the discovery of periodically variable quasars, which we interpret as a signature of close (milliparsec scale) supermassive black hole (SMBH) binaries en route to a merger.  Existence of such systems is expected from our understanding of hierarchical galaxy and SMBH assembly, and studies of this population can provide new insights into the final stages of SMBH mergers.  Long wavelength gravitational waves from them may be detectable with the pulsar timing arrays in the next decade.


Sean Andrews
CfZ Harvard

Host: A. Sargent

"Observing the Evolution of Solids in Protoplanetary Disks"

The canonical model for the formation of terrestrial planets and giant planet cores relies on an early and very efficient phase of planetesimal growth in a gas-rich circumstellar disk.  But, as theorists have known for decades now, there are some formidable obstacles to meeting that requirement.  Many of these problems, and potentially their solutions, are associated with the growth and migration of "pebbles" (mm/cm-sized particles) in the first few million years of a disk's lifetime.  That is fortuitous, since the continuum emission from these particles in nearby disks can be readily detected and resolved with long-baseline radio interferometers.  In this talk, I will describe what we are learning about the evolution of solids from such data, including: (1) the signatures of particle growth and radial drift; and (2) the mounting evidence that small-scale substructures in the (gas) disk play fundamental (perhaps mandatory) roles in the planet formation process.


Dennis Stello
U. Sydney

Host: Jim Fuller

The asteroseismic revolution of red giant stars: from stellar interiors to the structure of the Milky Way

In the past five years we have seen a number of breakthroughs in stellar astrophysics, sparked by seismic data from NASA's Kepler mission.  Very recent results (appearing in Nature and Science) suggest that this seismic revolution is still going strong.  With all the funded follow up space missions it seems there are great prospects for using asteroseismology to not only probe the interior of stars, but also to probe the structure and evolution of the Galaxy.  I will give an overview of recent breakthroughs and look at future prospects for the decade to come.


Jin Koda

Host: Vandana Desai

Evolution of the ISM in the Milky Way and Nearby Galaxies

The formation and evolution of molecular gas and clouds control star formation in spiral galaxies such as the MW. I will discuss two key topics on the evolution of the gas that leads to star formation: (1) the HI and H2 phase balance and evolution during a galactic rotation, and (2) the development of dense clumps within molecular gas during a spiral arm passage. The classic/textbook scenario of ISM evolution ties the gas phase evolution and star formation, positing a rapid gas phase transition from interarm HI, to giant molecular clouds (GMCs) and star formation in a spiral arm, and then back into HI by photodissociation. This HI-H2 phase transition, however, is observed only at the outskirts of galaxies, but not in their major inner parts. Instead, the gas resides largely in GMCs even in interarm regions, but stays inactive in star formation. Therefore, the mere presence of GMCs is not sufficient for star formation, and it must be triggered in pre-existing GMCs. I will also show evidence for the development of dense molecular clumps, the precursor of star formation, predominantly in molecular gas in spiral arms.


Ger de Bruyn
Univ of Groningen

Host: H. Vedanthan

The LOFAR EoR project: science goals, foregrounds, processing challenges and status

Abstract: The LOFAR EoR project set out to measure the spatial power spectrum (PS) of redshifted HI signals in the redshift range from z = 6 to 11.4. WMAP and Planck data suggest that this redshift range will cover the most interesting phases of the Epoch of Reionization, a key phase in the history of the Universe. Among the main questions we are after are the redshift evolution of the spatial PS and what it might tell us about the sources of reionization.

The data taking, in two windows centered on the NCP and the source 3C196, started in earnest in Dec 2012 and will soon embark on its fourth (Winter) observing season. Thusfar we have accumulated about 1500 h of data. The project faces a wide variety of challenges. Many of these had been expected and are well understood.  Among these are RFI, ionospheric effects, calibrating and processing a Petabyte of data, and dealing with (polarized) Galactic foregrounds.

Constructing an accurate high resolution sky model and solving for direction-dependent complex gains (due to time-varying station beams and ionospheric turbulence) have proved more difficult and time consuming.  I will describe and illustrate these challenges and how we are trying to solve them. In the mean time we are collecting interesting results on ancillary science related to Galactic and extragalactic foregrounds, especially as regards to their polarization. I will conclude with the project status and timeline.


Caitlin Casey
UT Austin

Host:Vandana Desai

The Universe's most extreme star-forming galaxies

Dusty star-forming galaxies host the most intense stellar nurseries in the Universe.  Their unusual characteristics (SFRs=200-2000Msun/yr) pose a unique challenge for cosmological simulations and galaxy formation theory, particularly at early times.  Although rare today, they were factors of 1000 times more prevalent at z~2-5, contributing significantly to the buildup of the Universe's stellar mass and the formation of high-mass galaxies. However, an ongoing debate lingers as to their evolutionary origins at high-redshift, whether or not they are triggered by major mergers of gas-rich disk galaxies, or if they are solitary galaxies continually fed pristine gas from the intergalactic medium.  Observational evidence has been mixed over recent years; some studies clearly point to chaotic kinematic histories and fast gas depletion times (~<100Myr), while other work may demonstrate secular (though active) disks can sustain high star-formation rates over long periods of time.  Similarly, some works argue such extreme star-formers contribute very little to cosmic star-formation, while others find quite the opposite.  Furthermore, their presence in early protoclusters, only revealed quite recently, pose intriguing questions regarding the collapse of large scale structure.  I will discuss some of the latest observational programs dedicated to understanding their origins and frequency at early times, their context in the cosmic web, and future long-term observing campaigns that will reveal their relationship to `normal’ galaxies, thus teaching us valuable lessons on the physical mechanisms of galaxy growth in an evolving Universe.


Joshua Winn

Host: L. Hillenbrand

The shortest-period planets

Short-period planets are a gift from nature that allowed the rapidacceleration 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.


Jason Kalirai

Host:Evan Kirby

Stellar Astrophysics: 100 Years after Russell

Since the pioneering work of Henry Norris Russell 100 years ago, the study of nearby stellar populations has served as a foundation for our quest to understand the nature of galaxies.  Today, studies of resolved stellar populations constrain fundamental relations -- such as the initial mass function of stars, the time scales of stellar evolution, the timing of mass loss and amount of energetic feedback, the color-magnitude relation and its dependency on age and metallicity -- that represent key ingredients in our prescription to interpret light from the Universe and to measure the physical state of galaxies.  In this presentation, I'll describe new, high-precision measurements of some of the most important relations that provide this mapping between resolved and unresolved light, and discuss new opportunities for stellar population studies given an unprecedented suite of astronomical tools that are now on our horizon - GAIA, JWST, LSST, WFIRST, 30-meter telescopes.


Tommaso Treu

Host: Phil Hopkins

Highlights from the Grism Lens-Amplified Survey from Space (GLASS)

The Grism Lens-Amplified Survey from Space (GLASS) is a large HST cycle-21 program targeting 10 massive galaxy clusters with extensive HST imaging from CLASH and the Frontier Field Initiative. The program consists of 140 primary and 140 parallel orbits of near-infrared WCF3 and optical ACS grism observations, which result in spatially resolved spectroscopy of thousands of galaxies.  I will present some of the first highlights from GLASS: 1) a census of lya emission at z>6, and resulting constraints on the epoch of reionization; 2) spatially resolved star formation and metallicity gradients at 1.3<z<2.3 as probes of the physics of gas accretion, star formation, and outflows. 3) The discovery of the first multiply-imaged supernova and the consequent first blind prediction of the appearance of a supernova in the sky.


Pascal Oesch

Host: Tom Soifer

Probing the Dawn of Galaxies

Understanding when and how the first galaxies formed and what sources reionized the universe are key goals of extragalactic astronomy. Over the last few years great progress has been made by pushing the observational frontier of galaxies out to z~9-12, only ~450 Myr from the Big Bang. Thanks to several large surveys with the Hubble Space Telescope (HST) we have now identified almost 1000 galaxies at z~7-8, in the heart of the cosmic reionization epoch, and we are now building up the sample sizes at z~9-11. These detections allow us to directly track the evolution of the cosmic star-formation rate density over 96% of the age of the universe. Additionally, the combination of ultra-deep HST and Spitzer/IRAC datasets has proven to be extremely powerful leading to the first measurement of the stellar mass density out to z~10. In this talk I will highlight recent progress in exploring the build-up of the first generations of galaxies down to the peak of cosmic star-formation at z~2 based on deep HST and Spitzer surveys, and I will provide a future perspective how this field will be revolutionized in the JWST era and beyond.


Quinn Konopacky

Host: Kirby/Mawet

Orbits and Atmospheres of Directly Imaged Exoplanets

Directly imaged exoplanets offer a new window into the rapidly evolving field of planet formation and evolution. The ability to separate the light of widely separated planets (~5 - 100 AU) from their host stars is extremely advantageous for studying Jovian planets. The combination of dynamical and atmospheric characterization can give essential clues about how these planetary systems form. To demonstrate this new insight, I will present results from an ongoing monitoring campaign of the HR 8799 directly imaged multi-planet system using Keck Observatory adaptive optics system. High precision astrometry has provided constraints on the orbital properties of the four HR8799 planets. Moderate resolution (R~4000) spectroscopy has given precise estimates of the planets' effective temperature, surface gravity, and chemical composition. By analyzing the implied atmospheric chemisty, we have found tantalizing clues about the possible formation pathway for this planetary system.  The next wave of direct imaging discovery has now begun with the successful commissioning of new instruments such as the Gemini Planet Imager (GPI).  I will highlight initial results from the Gemini Planet Imager Exoplanet Survey (GPIES), which has now completed one year of its three year campaign.


Yacine Ali-Haimoud
John Hopkins

Host: L. Hillenbrand

New frontiers in Cosmology

Exquisitely sensitive measurements of Cosmic Microwave Background (CMB) fluctuations have made it possible to determine with great precision the Universe’s inventory, as well as basic properties of its initial conditions. This required a highly accurate theoretical modeling of the underlying physics, in particular of the cosmological recombination process, which I will describe in the first part of this talk. A wealth of new and complementary data will be collected in the decades to come, promising spectacular advances in our yet very incomplete understanding of the cosmos. In the remainder of this talk, I will highlight some upcoming cosmological probes and illustrate fundamental questions they will help us answer. I will first discuss CMB spectral distortions, a powerful probe the Universe’s early thermal history, and illustrate how they can be used to probe, for instance, the properties of the dark matter particle. I will then describe how large-scale structure can be used to measure the yet unknown neutrino mass. I will end by describing the exciting prospects of high-redshift 21-cm cosmology, in particular for characterizing the Universe’s initial conditions.


Jonathan Fortney

Host: Kulkarni/Mawet

Toward an Understanding of Exoplanetary Composition

Nearly all of the narratives about understanding the composition of planets come from our understanding of the solar system.  However, this is a terribly small sample size.  The era of exoplanet characterization allows us to understand planets with a tremendously larger scope.  In this talk I will cover several topics that shed light on planetary composition.  In the realm of transiting planets, planetary structure models, compared to observations from Kepler and
ground-based surveys, show our solar system’s population of rocky planets is potentially unusual since they lack thick hydrogen atmospheres. However, our two gas giants, Jupiter and Saturn, fit well within the population of transiting giant exoplanets, in that all these planets show similar enrichments in "metals" compared to their parent star’s composition. Next, we are obtaining spectra of transiting planet atmospheres with Hubble to constrain atmospheric molecular abundances, which also point toward metal-enrichment for gas giants.  Finally, we are developing new theoretical tools to extract constraints on atmospheric abundances from the spectra of brown dwarfs and giant planets to look for composition clues that distinguish these two populations. As a whole these advances will yield fundamentally new insights on planetary formation and evolution.



Jessica Lu
U. Hawaii

Host: T. Soifer


Alexie Leauthaud
Kavli IPMU, Tokyo

Host: Phil Hopkins



Anastasia Fialkov

Host: J. Cohen



William McKinnon
Washington U, St. Louis

Host: K. Batygin

"New Horizons at Pluto"

Special Monday March 28, 2016
Joint GPS/AY Colloquium, Arms 155 at 4PM. 

Refreshments at 3:45


Kevin Bundy
Kavli IPMU, Tokyo

Host: Phil Hopkins



Rachel Bean

Host: Vandana Desai



Marcia Rieke

Host: T. Soifer

Inaugural Neugebauer Lecture



David Nidever
LSST Tucson

Host: Evan Kirby



Ryan Foley
U. Illinois

Host: S. Kulkarni



Jo Bovy
U. Toronto

Host: J. Cohen

Baird Lecturer





Blakesley Burkhart
cfa Harvard

Host: Phil Hopkins



Lars Bildsten

Host:S. Phinney

Greenstein Lecturer                                                                                                                                                    

Information for Speakers

Previous Astronomy Colloquia: