Astronomy Colloquia at Caltech for 2016-17

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.


Talk Title
Rob Simcoe
MIT-Kavli Institute &
Radcliff Institute
for Advanced Study

Host: A. Sargent

Heavy Element Enrichment in Early Circumgalactic and Intergalactic

Using the FIRE spectrometer at Magellan, we have gathered deep IR spectra of 40 quasars at z > 5.8 and over 100 at z > 4, providing at last a statistically significant pathlength for studying intervening
heavy-element absorption systems near the epoch of reionization.  I will discuss the status of these absorption measurements, with particular emphasis on detections at z > 5.5.  These relatively rare
absorbers have the potential to place joint bounds on heavy element pollution and the ionizing background radiation field at early times.  I will describe how high resolution optical and IR measurements may be combined to constrain these systems' gas-phase metallicity, even when Gunn-Peterson absorption saturates the HI profiles into an unusuable condition.  I will close with some discussion of future prospects for
this work using instruments for existing telescopes and ELTs.


No Colloquium

Julie Comerford
U. Colorado

Host: R. Chary

AGN Triggering in Galaxy Mergers as Traced by Dual AGN and Offset AGN

Galaxy mergers drive central inflows of gas, which are an important triggering mechanism for AGN.  Kpc-scale separation supermassive black hole pairs in ongoing galaxy mergers are ideal tracers of this link between galaxy mergers and AGN.  In dual AGN systems both black holes are fueled as AGN, whereas in offset AGN systems only one of the black holes is an AGN.  I will present multiwavelength approaches to building catalogs of dual AGN and offset AGN, and show the results of our observing campaigns with HST, Chandra, VLA, and Keck.  Finally, I will discuss what our results show about whether galaxy mergers preferentially fuel the most luminous AGN, which black hole in a merger is more efficient at accreting gas, and where in a merger the AGN fueling occurs.

Kevin France
U. Colorado

Host: G. Hallinan

“The MUSCLES Treasury Survey: An X-ray to IR Spectral Survey of Low-Mass Exoplanet Host Stars”

High-energy photons (X-ray to NUV) from exoplanet host stars regulate the atmospheric temperature profiles and photochemistry on orbiting planets, influencing the long-term stability of planetary atmospheres and the production of potential “biomarker” gases.  However, relatively few observational and theoretical constraints exist on the high-energy irradiance from typical (i.e., weakly active) M and K dwarf exoplanet host stars.  In this talk, I will describe results from a panchromatic survey (Chandra/XMM/Hubble/ground) of M and K dwarf exoplanet hosts. The MUSCLES Treasury Survey (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) combines UV, X-ray, and optical observations with reconstructed Lyman-alpha and EUV (10-90 nm) radiation to create 5 Angstrom to 5 micron stellar irradiance spectra that are available as a High-Level Science Product on STScI/MAST.
I will present an overview of the project and focus on three main results – 1) the F(FUV)/F(NUV) flux ratio increases by ~3 orders of magnitude as the star’s habitable zone moves inward from 1 to 0.1 AU, and I will describe implications for the possible abiotic production of the suggested biomarkers O2 and O3, 2) we develop scaling relations to infer the high-energy particle fluxes from these stars based on solar UV flare/particle flux measurements and to calibrate visible-wavelength proxies for the high-energy irradiance, and 3) to characterize the UV variability and flare frequency of “optically inactive” M dwarfs.

Vikram Ravi


Fast radio bursts from across the Universe?

Surveys of the GHz radio sky with sub-millisecond time resolution have revealed a population of astrophysical fast radio bursts (FRBs), lasting up to a few milliseconds and with peak flux densities rivaling the brightest quasars. Their cold-plasma dispersion delays, and levels of pulse-broadening due to multi-path propagation, are significantly in excess of expectations from the Milky Way along their sightlines. They are thus identified as coherent outbursts from extragalactic compact objects. This, however, remains the extent of our knowledge. We do not know from which objects in which galaxies, and at what distances, they originate. I will discuss possible insights into these basic properties from analyses of observed FRBs, and also provide a status update on an FRB localization machine being built at Caltech's Owens Valley radio observatory.  In addition to representing a new, likely exotic form of radio source, FRBs provide the opportunity for tomographic studies of the density, turbulence, and magnetization of plasma in the interstellar media of distant galaxies, in galactic halos, and in the intergalactic medium.

Ondrej Pejcha

Host: J. Fuller

Cool and Luminous Transients from Merging Binary Stars

The discovery of V1309 Sco, a contact binary with rapidly decreasing orbital period followed by an outburst, established a connection between astrophysically critical, catastrophic phases of
binary star evolution and a group of transients characterized by their red color and the luminosity in the gap between classical novae and supernovae. However, many properties of these transients, including
their connection to the binary evolution theory, remain largely unexplained. I will present an exploration of the dynamics of outflows from mass-losing binary stars and the associated variety of transients. Using my simulation results, I will interpret the unprecedentedly detailed pre-merger data on V1309 Sco and argue that these transients can provide a fresh observational input into some open problems of binary star evolution.

Erika Hamden

Host:A. Sargent

Observing the faint universe in emission

In the last several years, groundbreaking instruments have detected significant Lyman-alpha emission from the circumgalactic media (CGM) of z>2 galaxies, providing an initial corroboration to results from years of absorption line studies. Taken together, these indicate the presence of vast reservoirs of gas that we are only just beginning to observe and understand. To probe when star formation declines throughout the universe, we need to conduct similar observations at lower redshifts, moving into the UV. The Faint Intergalactic medium Redshifted Emission Balloon (FIREBall-2) is a balloon-born UV multi-object spectrograph designed to detect Lyman-alpha emission from the circumgalactic medium (CGM) around z~0.7 galaxies. In this talk, I will discuss the science drivers for this mission and its current status as we prepare for a Fall 2017 flight. In addition to groundbreaking science, FIREBall-2 will flight test several new technologies in a balloon setting, including photon counting, high efficiency UV detectors. I will discuss these technologies in the context of their impact on future space missions.

Andrew Fabian

Host: D. Stern

AGN coronae in the NuSTAR era, evidence for hybrid plasma?

The properties and origin of the powerful, hot, X-ray emitting corona at the heart of most AGN has remained an enigma for decades. NuSTAR has now measured the spectrum of a good number of AGN, revealing temperatures of 30-100 keV with a tail to higher values. I discuss how X-ray reflection
and reverberation measurements with NuSTAR and XMM have pinned down the size of coronae and  revealed them to be radiatively compact.   Pair production within the corona provides an upper temperature bound and, if the plasma is a hybrid mix of thermal gas with a small nonthermal
component, then it is possible for pairs to act as a thermostat determining the coronal temperature, as suggested back in the 1980s.


No Colloquium: Thanksgiving Holiday
Nick Scoville

Host: P. Goldreich

ALMA Observations of Galaxy Evolution and Arp 220

The new ALMA observatory has factors of 20 increase in both resolution and sensitivity.
It is revolutionizing the study of high redshift galaxies and forming planetary systems.
I will provide a brief introduction to its capabilities and then describe in detail two projects: 1) observations of 600 galaxies at z = 0.3 to 4 for which we measure the ISM gas contents and observations at 0.1 arcsec resolution of the local ULRIG galaxy Arp 220.  In the first project, we analyze the evolution of ISM contents, star formation rates and efficiencies and galaxy accretion rates with cosmic time and galaxy properties for the first time. We also derive the overall cosmic evolution of gas in galaxies for comparison with existing data on the star formation rate density. In the second project, we resolve
both nuclei of the late stage merging galaxies and find spectacular properties for the two galactic nuclei.


No Colloquium: Christmas Holiday

No Colloquium: AAS meeting


Elisabeth Newton

Host: G. Hallinan

Spin and Magnetism in Small Stars

Main sequence stars with masses below approximately 0.35 solar masses (red dwarfs, or M dwarfs) are fully-convective, and are expected to have a different type of dynamo mechanism than solar-type stars. These low-mass stars are the most common type of star in the galaxy, but a lack of observational constraints at ages beyond 1 Gyr has hampered studies of rotational evolution and magnetic activity. To address this, we have made new measurements of rotation and magnetic activity in nearby, field-age M dwarfs. I will discuss the relationships we see between age, rotation, and activity, and what they mean for rotational evolution, starspot properties, and the magnetic dynamo. Upcoming ground-based instruments and space-based surveys offer exciting prospects for continuing the study of cool stars, as well as the exoplanets that orbit them.


Joanna Dunkley

Host: R. Chary

The polarized microwave background: ACTPol and beyond

Measurements of the Cosmic Microwave Background radiation have taught us a great deal about the origins and content of the universe, but there is still more information to be extracted, teaching us about the energetic moments in the early universe, about neutrino physics, and the later-time expansion of the Universe. I will show new results from the Atacama Cosmology Telescope Polarimeter in northern Chile, an experiment measuring the polarization of the CMB. I will then talk about the future: plans for new measurements from the ground from the Simons Observatory and the CMB-S4 experiment, and from space with the PIXIE satellite.


Branimir Sesar

Host: J. Cohen

"Unraveling the Mystery of the Ophiuchus Stellar Stream"

"Stellar streams are remnants of dwarf satellite galaxies and globular clusters that were accreted by the Milky Way. The streams are promising tools for constraining the properties of the
Galactic gravitational potential, but first we need to precisely measure their properties and understand their dynamical evolution. In this talk, I will present a followup study of the Ophiuchus stellar stream, a mysteriously short stream located about 5 kpc from the Galactic Center.  I will show how a probabilistic approach and a rich data set can be used
to tightly constrain i) the distance, ii) the 3D kinematics, iii) the chemical abundance, iv) and the orbit of the Ophiuchus stream, and how these characteristics help unravel the mystery of the Ophiuchus stream."


Gerry Gilmore

Host:E. Kirby

Gaia - one billion objects in 3+ dimensions

Gaia is the European Space Agency mission which is revolutionising our knowledge of our Milky Way Galaxy, providing a census of positions, motions, colours, and properties of 1.5billion stars. Gaia's data will revolutionising much of astronomy, from near-Earth asteroids, through
stellar evolution, the structure, formation and evolution of our Milky Way Galaxy, the distribution of Dark Matter in the Milky Way, the number of planetary systems around other stars, the cosmological distance scale, and fundamental tests of General Relativity.  In addition to the wealth of position data Gaia's camera repeatedly scanning the sky discovers variable and new sources. These are published for follow up by professional astronomers and by amateur astronomers and school classes, using remotely controlled telescopes across the world.
Gaia's first major data release happened on Sept 14 2016. You can learn more, follow the mission, and download the app at 

Further information is available at the ESA website


Alan Weinstein

Host: M. Kasliwal

Gravitational Waves Observed by LIGO

We describe some of the results from the first observing run of Advanced LIGO, including the discovery of the binary black hole mergers GW150914, GW151226 and LVT 151012, and what we have learned from studying them.


Matthew Graham

Host: G. Djorgovski

Understanding extreme quasar optical variability

Quasars are known to be an optically variable population but the physical mechanisms behind this remain unclear. Variability provides information that is complementary to the studies of their spectral energy distributions, and can thus lead to new insights into the physics of AGN. The availability of large collections of astronomical time series, e.g., from CRTS, PTF, and PAN-STARRs, is now supporting systematic analyses of both the normal stochastic variability of quasars, as well as of the rare and extreme behaviors.
It is still unclear, however, whether these represent the tail of general quasar variability or a different class of phenomena but both have implications for quasar models.
In this talk I will review the different extreme categories we have identified so far: close (subparsec) supermassive black hole binaries, flaring sources suggestive of explosive stellar-related activity in the accretion disk, microlensed sources, and dramatic spectral variability coupled with strong, persistent changes in luminosity. I will also consider upcoming possibilities for further detection, characterization and modeling of these objects.


Nick Stone

Host: J. Fuller

Tidal Disruption Events: Theory Confronts Observation

Tidal disruption events (TDEs) are luminous bursts of electromagnetic radiation emitted from galactic nuclei when stars are ripped apart by supermassive black holes.  In the last decade, dozens of candidate flares have been seen by time-domain efforts such as Pan-STARRS, PTF, ASASSN, ROSAT, and the XMM-Newton Slew Survey.  Although the locations, energetics, and durations of these flares are generally compatible with theoretical expectations, many surprises have emerged from our growing observational sample.  I will focus on two in particular.  First, the
observed TDE sample is preferentially concentrated in a very rare galaxy subtype.  Post-starburst, or E+A, galaxies make up ~0.2% of all low redshift galaxies but host an order unity fraction of optically-selected tidal disruption flares.  I will present preliminary observational evidence that this preference is due to centrally concentrated star formation during the starburst, and the creation of an overdense galactic nucleus.  Second, it is increasingly clear that only a small fraction of TDEs launch collimated relativistic jets (Swift J1644+57 being the most famous example of this).  I will present recent theoretical work translating nondetections of radio afterglows around thermally-selected TDEs into upper limits on the energetics of off-axis relativistic jets.  We found that <10% of all TDEs launch jets comparable to those of Swift
J1644+57, and I will discuss possible theoretical explanations for this, which connect to open questions in jet launching physics.


Paul Groot
Radbound (Nijmegen)

Host: A. Sargent

From ultracompact binaries and transient surveys to gravitational wave astronomy

As gravitational wave astronomy is taking off it is clear that to understand events detected with Advanced LIGO/Virgo and, in the future, eLISA, we need to build up a better understanding of their source populations: ultracompact binaries containing two stellar remnants. For the bulk of the white dwarf populations, the main source for eLISA, this is done be only through studies in the Milky Way Galaxy. For short-lasting transient phases in these systems synoptic surveys of
the Local Universe can shed light on their frequency and environment.  For Advanced LIGO/Virgo sources the final stages of massive stars require a far better understanding. The challenges of developing gravitational wave astrophysics therefore combine the fields of
(Galactic) population surveys, synoptic transient surveys such as performed with PTF/ZTF and BlackGEM/MeerLICHT, as well as an understanding of core-collapse supernovae and the physics of fallback accretion disks as well as transistional disks as found in kilonovae events.


Norbert Langer
Univ of Bonn

Host: M. Kasliwal

The stellar Eddington limit

The well-known Eddington limit for stars is thought to correspond to the maximum luminosity which a star may have in hydrostatic equilibrium. We show that in our galaxy, stars above 40 Msun reach or even exceed this limit. Stellar models show that these stars do not become unstable, but inflate their radius. We discuss the observational evidence for this picture, its relation to the truely unstable so called Luminous Blue Variable stars and to dynamical
pulsations, and derive consequences for massive single and binary star evolution and for the early stages of supernova explosions.


Saul Teukolsky

Host: A. Sargent

The Coming Revolution in Computational Astrophysics

Large-scale computer simulations are increasingly crucial in explaining astrophysical phenomena. Just one recent example is the stunning agreement of the full 3-d solution of Einstein's equations for colliding black holes with the observed signal from LIGO. For the past 50 years, the dominant computer method for solving these kinds of equations has remained essentially unchanged. To keep up with continuing advances in observation, simulations will require more fidelity and higher accuracy.  One might think that with exascale machines becoming available in the next 5 years, this will be easy. This is not true: I will explain why Moore's Law is broken, and how the next generation of supercomputers will instead get their power by having millions of processors. Current codes will not be able to use these machines efficiently. I will describe new methods for harnessing the power of such exascale computers to solve some of the largest problems in astrophysics.


David Hogg

Host: T. Soifer

Neugebauer lecturer- Making the Most Accurate Possible Measurements with Telescopes

The discoveries of all extra-solar planets, our percent-level knowledge of the cosmological parameters, and the measurements of proper motions for millions of stars all depend on exceedingly good calibration of astronomical instruments. Gerry Neugebauer cared deeply about accurate calibration, and he was the US lead of IRAS, which was one of the first large sky surveys from space. These large sky surveys have transformed how we think about making accurate measurements; the most precise calibration of telescopes no longer comes from calibration programs or standard stars; it comes from the science data themselves. Indeed, GXN might have been upset to learn that we now often advise new projects not to take any calibration data whatsoever! Self-calibration capitalizes on causal structure in our generative model of the data set, and therefore there are important design choices to be made for an astronomical project that maximize the evidence for this causal structure in the data. The CMB and astrometry communities have understood this for decades, and it is now becoming true in a much wider range of areas. I will give unsolicited advice for the designers of any new instruments, telescopes, spacecraft, or surveys. I will comment a bit on the differences between accuracy and precision, on linear algebra (!), and on what self-calibration can and cannot do. I will bring examples from SDSS, Kepler, HST, Gaia, radio interferometry, coronography, stellar spectroscopy, and CMB polarimetry.


Stan Whitcomb

Host: G. Djorgovski

Greenstein Lecture - An Idiosyncratic Look at LIGO's Detections and Prospects

It is nearly impossible to find anyone in astronomy at Caltech who has not heard a pretty complete account of LIGO's first detection of gravitational waves from the merger of two black holes.  In an effort to find something new to say about it, I will review some aspects of this event and the other events from LIGO's first observing run, with the aim to addressing some misconceptions or to providing alternative ways to view it.  There will be a few facts, and a lot of opinion.   I will finish up by sharing some thoughts about the challenges for the future.


Eduardo Banados

Host: S. Golwala


Gongjie Li

Host: L. Hillenbrand


Rychard Bouwens

Host: N. Scoville

Young Galaxies Forming in the High-Redshift Universe

Over the last few years, enormous progress has been made in studying galaxies in the first two billion years thanks to the incredible capabilities of the Hubble and Spitzer Space Telescopes.  Already, more than 1500 probable galaxies are known at redshifts above z~6, and now the current frontier is at z~9-10, with 50 plausible galaxy identifications to date, and a spectroscopic redshift measurement to z=11.1.  Noteworthy advances are also being made in characterizing
the physical properties for these distant galaxies, with probes of the nebular emission lines and specific star formation rates to z~8.5 and new constraints on dust-enshrouded star formation at z>~2 from ALMA.  One area where there has been particularly exciting activity is in the study of ultra-faint galaxies in the early universe with the Hubble Frontier Fields (HFF) program, combining the power of long exposures with Hubble and Spitzer with gravitational lensing by massive galaxy clusters.  In this colloquium, I survey these and other highlights of current research on high redshift galaxies, while looking forward to future work with JWST.


Pieter van Dokkum

Host: G. Djorgovski

Biard Lecture


Eliot Quataert

Host: L. Hillenbrand


Maryam Modjaz

Host: M. Kasliwal

Information for Speakers

Previous Astronomy Colloquia: