Astronomy Colloquia at Caltech for 2018-19

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 5-5:30pm.

Month
Date
Speaker

Talk Title
OCTOBER
3

Speaker: David Charbonneau
Institute: Harvard
Host: D. Mawet

The Terrestrial Planets of Other Stars

For the next decade, the only opportunity to study the atmospheres of terrestrial exoplanets will be scrutinize these worlds when they transit nearby small stars. There are 417 mid-to-late M-dwarfs within 15 parsecs, yet we know surprisingly little about them, let alone their attendant planets. I will discuss recent findings from the MEarth Project and TESS Mission, which seek to discover the most spectroscopically accessible terrestrial exoplanets. Our recent discovery of a several rocky worlds transiting the nearby small star LHS1140 provides an unprecedented opportunity to detect the molecules present in the atmosphere of a terrestrial exoplanet. In our planetary quest, MEarth observations have sharpened our understanding of the evolution of these low-mass stars, and hence the stellar environment in which these planets may wither or thrive.

10

Speaker: Caroline Morley
Institute: UT Austin
Host: Eve Lee

From Exotic to Familiar: Observing Exoplanet Atmospheres in the Coming Decade

Observations of exoplanets to date have used the Hubble and Spitzer Space Telescopes to reveal exotic exoplanet atmospheres, including significant effort to characterize planets with radii between Earth's and Neptune's -for which we have no counterparts in the solar system-that are accessible to current telescopes. Observations of their transmission spectra reveal a diversity of worlds, some shrouded in clouds and others with molecular features. I will discuss the types of clouds and hazes that can obscure transmission spectra, and show the effect that these thick hazes have on the thermal emission of small exoplanets. I will discuss some of my plans for new observations during the first cycle of JWST to measure the thermal emission of these planets. The new frontier of exoplanet atmosphere studies is characterizing the atmospheres of planets more familiar to Earthlings: cold gas giants and temperate Earths. I will discuss my current work to reveal the atmosphere of a cold free-floating giant planet, and my plans for detecting a host of interesting molecules in its atmosphere with JWST. We will soon be able to access Earth-sized, temperate worlds for the most favorable 3 systems orbiting the small stars. I will discuss the recent discoveries of Earth-sized planets around bright M dwarfs and how we might use JWST to detect their atmospheres. Lastly, I will discuss prospects with current and upcoming ground-based telescopes to detect exoplanet atmospheres, including some steps to take preceding the launch of JWST.

17

Speaker:  J. Xavier Prochaska
Institute: UC Santa Cruz
Host: Chris Martin

The Wolfe Disk: ALMA Discoveries of Distant, HI-selected Galaxies

I will review our series of successful programs to dissect the interstellar medium of distant, star-forming galaxies with the Atacama Large Millimeter Array (ALMA).  In particular, I will discuss surveys of the set of HI-selected galaxies known as the damped Lya systems (DLAs).  We resolve, in part, a decades old struggle to identify the galactic counterparts of these DLAs and thereby place them firmly in the modern picture of galaxy formation.  I will also highlight high spectral and spatial resolution observations of the Wolfe Disk,a z~4 galaxy with a Milky Way-like rotation curve.

24
Speaker: Brad Cenko
Institute: NASA
Host: M. Kasliwal

Recent Observational Puzzles from Tidal Disruption Flares: Towards Viable Probes in the LSST Era

I will present an overview of efforts across the electromagnetic spectrum to identify and study tidal disruption flares (TDFs), when a star wanders too close to a super-massive black hole and is torn apart by tidal forces. In particular I will focus on three unexpected surprises that challenge the most basic picture of these events: 1) large inferred radii for the optical/UV-emitting material, indicating either circularization of the bound debris at large distances and/or significant reprocessing of the radiation from the inner accretion disk; 2) the ubiquity of outflows, detected at radio, X-ray, and UV wavelengths, ranging from speeds of 100 km/s to near the speed of light; and, 3) the peculiar atomic abundances observed in the UV and optical spectra of these objects. Understanding the nature of the broadband emission will be critical if we wish to ultimately utilize these events as probes of black hole mass in distant quiescent galaxies in the LSST era. Finally I will provide an introduction to upcoming and proposed wide-field surveys that will help us address these issues in the coming years.

31
Speaker: Jason Rhodes
Institute: JPL
Host: D. Mawet

NASA's Next Astrophysics Flagship: The Wide Field Infrared Survey Telescope (WFIRST)

The top recommendation for a large space mission in the US 2010 Decadal Survey was the Wide Field Infrared Survey Telescope (WFIRST).  Similarities in hardware requirements between proposed dark energy, exoplanet microlensing, and near infrared surveyor missions allowed for a single mission that would accomplish all three goals.  The gift of an existing 2.4 meter telescope to NASA by another US government agency allowed for the addition of a coronagraph that will take images and spectra of nearby exoplanets; this instrument will be a technological stepping stone to imaging other Earths in the 2030s.  I will give an overview of WFIRST's proposed instrumentation, science goals, and implementation plan. I will delve into the details of a number of scientific and technical efforts in Pasadena that are paving the way for this ambitious and exciting mission.


NOVEMBER
7
Speaker: Decker French
Institute:  Carnegie Observatories
Host: A. Faisst

Bold Nuclei in Old Galaxies

Galaxies evolving through the post-starburst (or E+A) phase are in the midst of drastic changes in their stellar populations, morphologies, and gas content. They are likely experiencing periods of AGN-driven feedback, alongside a high rate of tidal disruption events (TDEs), and are thus good laboratories to explore how these nuclear processes depend on and affect the rest of the galaxy. While post-starburst galaxies have stopped forming new stars, we have discovered that many have significant reservoirs of molecular gas remaining, which are depleted only after the starburst had already ended. Young post-starburst galaxies have molecular gas reservoirs similar to normal star-forming galaxies, which decline to the levels of early type galaxies within 1-2 Gyr. This rate of gas depletion is too rapid to be explained by the low star formation rates in these galaxies, so AGN feedback may be responsible. Recent ALMA observations show these galaxies have low dense gas fractions, and the mechanisms which deplete the gas after the starburst may be the same as those which suppress the collapse of gas into denser states. We have also found that post-starburst galaxies host a disproportionate number of TDEs, in which a star is accreted onto the black hole. I will present recent work on what the cause of the TDE rate enhancement during this phase may be and how this host galaxy preference can be used as a tool for identifying new TDEs.

14
Speaker: Tamas Budavari
Institute: John Hopkins Univ
Host: M. Graham

Computational Optics for Astronomy Surveys

We will discuss recent efforts to extract more information from time-domain imaging surveys. Our approach is to model the latent image behind the turbulent atmosphere which blurs all observations to varying degrees - making the stars twinkle. If we can infer the image of the sky behind the atmosphere, all observational studies will simultaneously benefit. Advanced statistical modeling and computational methodology promise reliable solutions. In simulations using the software stack of the Large Synoptic Survey Telescope, we study the subtle prism effect of the atmosphere which shifts sources around depending on their spectral energy distribution and the angle of the telescope's pointing. Expected improvements include not only subband color estimates within the broadband filters, but also better determination of sky coordinates and shape measurements, which ultimately yield better maps of the Universe.

28

Speaker: Ken Shen
Institute: Berkeley
Host: J. Fuller

A Revolution in Our Understanding of Type Ia Supernovae

Despite the wide-ranging role Type Ia supernovae (SNe Ia) play throughout astrophysics, we lack a firm theoretical understanding of how they arise.  For decades, the "single-degenerate" scenario, in which an accreting white dwarf reaches the Chandrasekhar mass, held sway.  However, spurred by recent observational and theoretical advances, researchers have begun exploring alternative progenitor channels.  In this talk, I will show why some of us were drawn to the "dynamically driven double-degenerate double-detonation" (D6) scenario, and how it makes unique predictions for surviving hypervelocity companions that were confirmed by the Gaia satellite earlier this year.  These stars, perhaps the fastest unbound stars in the Milky Way, represent the first ever detections of SN Ia survivors and have brought the D6 scenario to the forefront of the field as the only known SN Ia progenitor scenario to succeed in nature.

DECEMBER
5
Speaker: Jessie Christiansen
Institute: Caltech
Host: D. Mawet

Ten Thousand Pieces of Blue Sky: Building towards the complete picture of exoplanet demographics

The NASA Kepler mission has provided its final planet candidate catalogue, the K2 mission has contributed another four years’ worth of data, and the NASA TESS mission has just started producing planet candidates of its own. The demographics of the exoplanet systems probed by these transiting exoplanet missions are complemented by the demographics probed by other techniques, including radial velocity, microlensing, and direct imaging. I will walk through the progress of the Kepler occurrence rate calculations, including some of the outstanding issues that are being tackled. I will demonstrate how K2 and TESS are able to push the stellar parameter space in which we can explore occurrence rates beyond that examined by Kepler, and progress to that end. Finally, I will highlight some of the pieces of the larger demographics puzzle - occurrence rate results from the other techniques that probe different stellar and exoplanet regimes - and how we can start joining those pieces together.

12

Speaker: Smadar Naoz
Institute: UCLA
Host:  Jim Fuller

Mergers and Disruptions in Extreme Gravitational Potentials

Nuclear star clusters around supermassive black holes are likely the most collisional stellar systems in the Universe and are also embedded in extremely deep gravitational potential. Consequently, unique stellar dynamical processes and interactions are expected to take place. For example, collisions and mergers between stars and compact objects are likely to happen in this environment. I will explore these collisions and mergers and their product and will connect between them and some of the observed puzzles in galactic nuclei and in particular our own Galactic Center. Specifically, I will offer possible connections between those merger products and (1) the perplexing population of young stars that are isotropically distributed (S-stars) in a region that is hostile to star formation, (2)  the new class of cold stars in this same region that are two orders of magnitude larger than typical stars (e.g., the "gas-like cloud" G2), (3) stellar black hole-black hole binary mergers and LIGO observations, and (4) supermassive black-hole merger with a stellar-mass compact objects and future LISA observations. Recent developments in our understanding of the underlining physics of three- to few- body dynamics offer the opportunity to address puzzles at these extreme places in our Universe.

JANUARY 9

Speaker:
Institute:
Host:

No Colloquium - AAS week

16

Speaker: Jenny Greene
Institute:  Princeton
Host: P. Hopkins

Dwarf Galaxies and Their Black Holes

Because they are dark-matter dominated, dwarf galaxies provide some of
the most stringent tests of our cold dark matter model. We continue to
debate whether the number densities and distributions of low-mass
galaxies match theoretical expectations. I will describe how the advent 
of deep and wide-area surveys like the Hyper-Suprime Camera (HSC) Survey 
(and eventually LSST) are changing our ability to find and characterize 
low-mass, low surface-brightness populations. First, I will present a 
new calibration of surface-brightness fluctuations to the blue, which will 
provide a powerful new tool to determine satellite luminosity functions 
around nearby galaxies. Second, I will talk about our ongoing search for 
low-surface brightness galaxies is all environments with the HSC survey. 
Finally, if time permits, I will discuss the massive black hole populations 
in low-mass galaxies.

23

Speaker: Alex Szalay
Institute: John Hopkins
Host: G. Djorgovski

The Era of Surveys and the Fifth Paradigm of Science

Starting with the Sloan Digital Sky Survey and the Hubble Deep Field, astronomy has entered the Era of Surveys. Today we have covered a substantial fraction of the sky in multiple wavelengths. Much of this data is now available on-line, as an easy-to-use virtual telescope. The data sets are interoperable and it is easy to cross-correlate between surveys. Astronomers became proficient in databases, and today they use these not as tools but rather like musical instruments. Over the centuries science has gone through several paradigms, starting with the "empirical", followed by "theoretical" and "computational" approaches to science. Today, the large surveys have led us to the so-called Fourth Paradigm of Science, where discoveries are "data-driven". Astronomers were early adopters, as we can only observe the sky, but cannot undertake experiments which change the behavior of celestial objects. This data-intensive approach to astronomy has resulted in disruptive changes, both technological and sociological. This talk will discuss the journey over the last 20 years, and where these changes have led us, and what may lie ahead. The Large Synoptic Survey Telescope, LSST, will open up the time domain and will produce the largest dataset astronomers will encounter. Such data sets will bring new challenges, as systematic errors will increasingly dominate over statistical noise. We already see how machine learning is turning new detections into discoveries. But the most interesting changes are still ahead: just as in self-driving cars, algorithms are making the decisions, and soon we will see AI tools setting adaptive choices about survey strategies, like target selection. This may be the beginning of the Fifth Paradigm of Science, where computers decide objectively which experiments will yield the biggest gain in our knowledge. Finally, I will also discuss structural and organizational changes that should happen, to make sure that legacy data sets, which have cost hundreds of millions to acquire, can be safely preserved and analyzed throughout their useful lifetime. This will require a fresh look at long term data curation - how to be FAIR (Findable, Accessible, Interoperable, Reusable) and how to be open, free and sustainable, all at the same time.

30

Speaker: Yanqin Wu
Institute: Univ of Toronto
Host: Eve Lee

A uniform population of planets

A large number of close-in super-Earths have now been discovered.
How they form, and how they relate to other classes of planets, remain 
unsolved. In this talk, I will discuss recent progress regarding their 
masses, their composition, and their companion planets, leading to the 
conclusion that super-Earths make up  a surprisingly uniform population.
This uniformity is unpredicted, and brings new insights to the theory of 
planet formation. As an aside, the census also indicates that planetary 
systems like our  own, with low-mass terrestrials inside and cold giants 
outside, are likely rare in the Milky Way.

FEBRUARY
6

Speaker: Suvi Gezari
Institute: Univ of Maryland
Host: M. Graham

A Systematic Exploration of Extragalactic Nuclear Transients with the Zwicky Transient Facility

The Zwicky Transient Facility (ZTF) is performing a multi-band optical time domain survey of the entire Northern Sky that is enabling a comprehensive study of transients associated with supermassive black holes in the nuclei of galaxies.  I will present exciting results from our systematic selection, classification, and multiwavelength characterization of nuclear transients in the first year of the ZTF alert stream, including the discovery of tidal disruption events (TDEs), changing-look active galactic nuclei (CLAGN), and candidate recoiling supermassive black holes (rSMBHs).  Our nearly complete classification of bright (r < 19 mag) nuclear transients in the ZTF survey with the Palomar 60in SEDM low-resolution spectrograph will provide an excellent training set for optimizing photometric selection of TDEs in the LSST era.  Indeed, I will conclude with strong recommendations for the LSST Wide-Fast Deep Survey cadence based on our ZTF discoveries so far.

13

Speaker: Sandro Tacchella
Institute: Harvard
Host: A. Faisst

Building quiescent galaxies

Today's galaxy population shows a large structural diversity that depends on stellar mass, star-formation activity, and environment. Even quiescent galaxies that have little star formation display a large range of morphological properties, indicating several different formation mechanisms at work. The peak of cosmic star formation rate density at redshifts of 1 to 3 is thought to be the epoch of the major buildup of these massive spheroids. By combining ground-based SINFONI integral-field unit observations with Hubble Space Telescope imaging data, I will show how these early galaxies grow on spatially resolved scales, and how they transition to the quiescent population. In particular, I will discuss the connection (or lack thereof) between quenching and morphological transformation. Furthermore, I will highlight how such galaxies grow in a self-regulated equilibrium and thereby building up their central bulge component. I will show the importance of understanding the variability of the star formation rate on different timescales to constrain numerical simulation. Finally, I will end by an outlook of the key questions that we can achieve with the next generation of telescope such as the James Webb Space Telescope and the 30m-class telescopes.

20

Speaker: Eliot Quataert
Institute: UC Berkeley
Host: M. Kasliwal

What Happens When a Massive Star Fails (Sort of) to Explode?

There are observational and theoretical reasons to suspect that up to 10s of percent of massive stars that undergo core-collapse at the end of their lives fail to explode in a canonical energetic supernova explosion.    In this talk I will describe what transpires in such nominally failed supernovae and how they may manifest themselves observationally in time-domain surveys.

27

Speaker: Felix Aharonian
Institute: MPIK, Germany
Host: Gregg Hallinan

TBD

MARCH
6

Speaker: Ariel Goobar
Institute: Stockholm University
Host: M. Kasliwal

TBD

13

Speaker: Maxwell Moe
Institute:Univ of Arizona, Tucson
Host: J. Fuller

TBD

20

Speaker: Matthew Povich
Institute: Cal Poly Pomona
Host: Matthew Graham

TBD

27

Speaker:
Institute:
Host:

No Colloquium - Spring Break

APRIL
3

Speaker: Boris Gaensicke
Institute: Univ of Warwick
Host:

Kingsley

10

Speaker: Jocelyn Bell Burnell
Institute: Oxford University
Host: Mansi Kasliwal

Greenstein


17

Speaker: Dale Frail
Institute: NRAO Socorro
Host:

Biard

24

Speaker: Adam Burrows
Institute: Princeton
Host: M. Kasliwal

TBD

MAY
1

Speaker:
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8

Speaker:
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15

Speaker:
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22

Speaker: Ignas Snellen
Institute: Leiden Observatory
Host:



29

Speaker:
Institute:
Host:



JUN
5

Speaker:
Institute:
Host:




12

Speaker: France Cordova
Institute: NSF
Host:

Neugebauer

Information for Speakers

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