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.

Month
Date
Speaker

Talk Title
SEPTEMBER 28
Rob Simcoe
MIT-Kavli Institute &
Radcliff Institute
for Advanced Study

Host: A. Sargent

Heavy Element Enrichment in Early Circumgalactic and Intergalactic
Environmnents

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.

OCTOBER 5



No Colloquium

12
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.

19
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.

26
Vikram Ravi
Caltech

Host:

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.

NOVEMBER
02
Ondrej Pejcha
Princeton

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.

09
Erika Hamden
Caltech

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.

16
Andrew Fabian
Cambridge

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.

23


No Colloquium: Thanksgiving Holiday
30
Nick Scoville
Caltech

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.

DECEMBER 28


No Colloquium: Christmas Holiday
JANUARY 4



No Colloquium: AAS meeting

11

Elisabeth Newton
MIT

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.

18

Joanna Dunkley
Princeton

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.

25

Branimir Sesar
MPIA

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."

FEBRUARY
1

Gerry Gilmore
Cambridge

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 https://gaia.ac.uk. 

Further information is available at the ESA website
http://www.cosmos.esa.int/web/gaia/home.

8

Alan Weinstein
Caltech

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.

15

Speaker
Institute

Host:



22

Nick Stone
Columbia

Host: J. Fuller



MARCH
1

Speaker
Institute

Host:



8

Speaker
Institute

Host:



APRIL
5

Speaker
Institute

Host:



12

David Hogg
NYU

Host: T. Soifer

Neugebauer lecturer
19

Stan Whitcomb
Caltech

Host: G. Djorgovski

Greenstein Lecture

26

Speaker
Institute

Host:



MAY
3

Gongjie Li
Harvard

Host:



10

Rychard Bouwens
Leiden

Host: A. Sargent



17

Pieter van Dokkum
Yale

Host: G. Djorgovski

Biard Lecture

24

Speaker
Institute

Host:



31

Maryam Modjaz
NYU

Host: M. Kasliwal










Information for Speakers

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