Astronomy Colloquia at Caltech for 2017-18

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 20

Speaker: Chris Kochanek
Institute: Ohio State
Host: M. Kasliwal

Dust Formation Is Not Voodoo

Dust formation is generally viewed as an act of magic or voodoo, to be viewed
with the same level of disbelief as invocations of rotation or magnetic fields.
But the formation of dust is actually a fairly simple problem that has been
obscured by invocations of the problems in classical nucleation theory. While
classical nucleation theory has many problems, they are not generally
relevant to observational problems in astrophysics.                                                 
When a transient source ejects mass, dust formation becomes inevitable if
the density is sufficiently high and the radiation environment is benign.
High density means that the condensible species forming the dust have a
significant collision rate.  A benign radiation environment means that
UV photons cannot destroy fragile, small grains before they can grow.
All the problems in nucleation theory boil down to modest and relatively
uninteresting shifts in when and where the dust forms.  Once
dust forms, the future evolution is well-specified and provides strong,
quantitative constraints on the ejecta.  When the dust optical depth is high,
the mid-IR emission constrains the luminosity while the evolution of
the optical emission constrains the ejecta mass.   As the optical
depth drops, the roles of the wavelengths reverses. I will discuss these
issues in the context of a range of astrophysical sources

27

Speaker: Jamie Bock
Institute: Caltech
Host: M. Kasliwal

SPHEREx:  An All-sky Infrared Spectral Survey Explorer Satellite

SPHEREx, a mission in NASA's Medium Explorer (MIDEX) program that was selected for a competitive Phase A in August 2017, is an all-sky survey satellite designed to address all three science goals in NASA's astrophysics division, with a single instrument, a wide-field spectral imager.  We will probe the physics of inflation by measuring non-Gaussianity by studying large-scale structure, surveying a large cosmological volume at low redshifts, complementing high-z surveys optimized to constrain dark energy. The origin of water and biogenic molecules will be investigated in all phases of planetary system formation - from molecular clouds to young stellar systems with protoplanetary disks - by measuring ice absorption spectra. We will chart the origin and history of galaxy formation through a deep survey mapping large-scale spatial power in two deep fields located near the ecliptic poles. Following in the tradition of all-sky missions such as IRAS, COBE and WISE, SPHEREx will be the first all-sky near-infrared spectral survey.  SPHEREx will create spectra (0.75 - 4.2 um at R = 40, and 4.2 - 5 um at R = 135) with high sensitivity using a cooled telescope with a wide field-of-view for large mapping speed.  During its two-year mission, SPHEREx will produce four complete all-sky maps that will serve as a rich archive for the astronomy community.  With over a billion detected galaxies, hundreds of millions of high-quality stellar and galactic spectra, and over a million ice absorption spectra, the archive will enable diverse scientific investigations including studies of young stellar systems, brown dwarfs, high-redshift quasars, galaxy clusters, the interstellar medium, asteroids and comets.  SPHEREx is a partnership between Caltech, JPL, Ball Aerospace, and the Korea Astronomy and Space Science Institute.

OCTOBER 4

Speaker: Juna Kollmeier
Institute: Carnegie
Host: J. Cohen

AS4:  Pioneering Panoptic Spectroscopy

I will describe the current plans for a Next Generation Sky Survey that will begin After SDSS-IV --- AS4.  AS4 will be an unprecedented all-sky spectroscopic survey of over six million objects. It is designed to decode the history of the Milky Way galaxy, trace the emergence of the chemical elements, reveal the inner workings of stars, and investigate the origin of planets. It will also create a contiguous spectroscopic map of the interstellar gas in the Milky Way and nearby galaxies that is 1,000 times larger than the state of the art, uncovering the self-regulation mechanisms of our Galactic ecosystem. It will pioneer systematic, spectroscopic monitoring across the whole sky, revealing changes on timescales from 20 minutes to 20 years.   The project is now developing new hardware to build on the SDSS-IV infrastructure, designing the detailed survey strategy, and is actively seeking to complete its consortium of institutional
and individual members.

11
Speaker: Eve J. Lee
Institute: Caltech
Host: Phil Hopkins

Planets Close-in and Far-out

The data-rich Kepler mission provided an unprecedented view of the demographics of planetary systems. Close to the star (orbital periods shorter than ~100 days), super-Earths (~1--4 Rearth) and Earth-sized planets dominate. These small planets are evenly distributed in log orbital period down to ~10 days, but dwindle in number at shorter periods. I will demonstrate that both the break at ~10 days and the slope of the occurrence rate down to ~1 day can be reproduced if planets form in situ in disks that are truncated by their host star magnetospheres at co-rotation. Planets can be brought from disk edges to ultra-short (<1 day) periods by asynchronous equilibrium tides raised on their stars. Close-in super-Earths are massive enough to trigger runaway gas accretion, yet they accreted atmospheres that weigh only a few percent of their total mass, keeping their size below that of the Neptune. This puzzle is solved if super-Earths formed late, in the inner cavities of transitional disks. Over a wide range of nebular depletion histories, super-Earths can robustly build their ~1% by mass envelopes. Super-puffs present the inverse problem of being too voluminous for their small masses. I will show that super-puffs most easily acquire their thick atmospheres as dust-free, rapidly cooling worlds outside 1 AU, and then migrate in just after super-Earths appear. Small planets may remain ubiquitous out to large orbital distances. The variety of debris disk morphologies revealed by scattered light images can be explained by viewing an eccentric disk, secularly forced by a planet of just a few Earth masses, from different observing angles. The farthest reaches of planetary systems may be perturbed by eccentric super-Earths.

18
Speaker: Robyn Ellyn Sanderson
Institute: Caltech
Host: Phil Hopkins

Insights into dark matter from the stellar halos of galaxies

Cosmological simulations can now make specific and detailed predictions for the shapes, masses, and substructure fractions in galactic dark matter halos that depend on the dark matter model assumed. Comparing these predictions to the observed mass distributions of galaxies should in principle lead to constraints on the nature of dark matter, but observable dynamical tracers can be scarce in regions where the dark matter distribution is best able to discriminate between models. One such region is the distant outskirts of galaxies, where the influence of baryonic matter on the dark matter halo is limited and the effect of dark substructures most prominent. New surveys of Milky Way stars like Gaia, alongside next-generation instruments and giant telescopes, will for the first time provide accurate positions, velocities, and abundances for large numbers of stars in faint tidal streams: remnants of tidally-disrupted satellite galaxies that trace out the mass distribution in the distant reaches of galaxy halos. I will show how state-of-the-art simulations play a crucial role in interpreting and analyzing this wealth of new information about stellar halos, and how stellar halo observations over the next decade will characterize the dark matter distribution in galaxies, test theories of the nature of dark matter, and illuminate the role of dark matter in galaxy formation.


25
Speaker: Meredith MacGregor
Institute: Harvard
Host: D. Mawet

Debris Disks as Probes of Planetary System Evolution

At least 20% of nearby main sequence stars are surrounded by disks of dusty material resulting from the collisional erosion of planetesimals, larger bodies similar to asteroids and comets in our own Solar System.  The resulting dust can be observed via scattered light at visible to near-infrared wavelengths or thermal emission at mid-infrared to millimeter wavelengths.  Since the dust-producing planetesimals are expected to persist in stable regions like belts and resonances, the locations, morphologies, and physical properties of dust in these debris disks provide probes of planet formation and subsequent dynamical evolution.  Observations at millimeter wavelengths are especially critical to our understanding of these systems, since the large grains that dominate emission at these long wavelengths do not travel far from their origin and therefore reliably trace the underlying planetesimal distribution.  I will present ongoing work that uses observations of the angularly resolved brightness distribution and the spectral dependence of the flux density to constrain both the structure and grain size distribution of nearby debris disks.  In particular, I will show new ALMA observations that place constraints on the position, width, surface density gradient, and any asymmetric structure of several well-known debris disks (including the Fomalhaut system).  Together these results provide an exciting foundation to investigate the dynamical evolution of planetary systems through multi-wavelength observations of debris disks.

NOVEMBER
1
Speaker: Peter Jonker
Institute:  SRON-NL
Host: S. Phinney

Stellar-mass black holes: X-ray binaries vs LIGO/Virgo?

The existence of stellar-mass black holes is now well established but the recent LIGO/Virgo results suggest that the black hole mass distribution of stellar-massblack holes is more complex than that derived so far from measurements usingX-ray binaries. I will explain how dynamical mass measurements have been obtained for the black holes in these single-lined spectroscopic binaries anddiscuss several biases that may help explain the differences between masses from LIGO/Virgo events and X-ray binaries. The goals are to investigate how black holes forms and, in particular, to investigate if intermediate-mass black holes exist. Intermediate-mass black holes may well be necessary to explain the presence of super-massive black holes when the universe was less than 1 Gyr old.I will finish by showing results from our recent attempts to find intermediate-mass black holes using geometrical constraints.

8
Speaker: Krista Smith
Institute: KIPAC Stanford
Host: G. Hallinan

A New Regime of Optical Variability in AGN: the Kepler Light Curves

The optical light curves of AGN provide a unique window into the conditions and behavior within the accretion disk. The development of a specialized pipeline for AGN science with the unparalleled photometry of exoplanet-hunting satellites allows us to explore new optical variability phenomena. Among the insights from these new light curves are bimodal flux distributions, power spectral slopes that depend on luminosity, characteristic variability timescales, and more. Such data provide an opportunity for direct comparison with X-ray light curves, and promise to inform models of both accretion physics and the relationship between X-ray and optical emitting regions in the central engine. These data will be critical in learning how to interpret AGN light curves from upcoming large variability surveys like LSST. Finally, exoplanet mission data have enormous future promise for a multifaceted understanding of accretion processes, including blazar jets and quasi-periodic oscillations.

15

Speaker: Lia Corrales
Institute: Univ of Wisconsin
Host: M. Kasliwal

An X-ray View of the Dusty Universe

A significant fraction of the heavy elements produced by stars spend some time in the interstellar medium as dust grains.  These heavy metal transporters influence gas cooling during star formation, eventually becoming the seeds for planet formation.  Observations of X-ray bright Galactic compact objects can yield key insights to the mineralogy and evolution of dust grains in the Milky Way.  With high resolution X-ray spectroscopy, we can directly measure the state of metals and the mineral composition of dust in the interstellar medium.  In addition, dust scattering produces a diffuse halo image around bright X-ray objects, revealing information about dust grain sizes and their spatial distribution.  I will review the most recent exciting dust scattering discoveries, which draw on multi-wavelength observations. I will show how X-ray studies of the ISM are important for interpreting accretion by compact objects and the supermassive black hole at the center of our galaxy. Finally, I will discuss open questions regarding our X-ray view of the dusty Universe that can be addressed with future X-ray observatories.

29
Speaker: Paola Caselli
Institute: MPI
Host: N. Scoville

Astrochemistry at the dawn of star and planet formation in the ALMA Era 

Molecules are unique tracers of the dynamical and chemical evolution of star and planet forming regions. Thus, astrochemistry is crucial to test theories and shed light on our origins. Stars and planetary systems in our Galaxy form within dense (n(H2) ~ 100,000 cm-3) and cold (T ~ 10 K) fragments of interstellar molecular clouds, called pre-stellar cores. Important chemical processes take place at this early stage, such as isotope fractionation, production of complex organic molecules and growth of thick icy mantles onto dust grains, where water and organics are stored, and which boost dust coagulation. These processes can affect later phases of star and planet formation, which can now be traced with powerful interferometers such as ALMA and NOEMA. In this talk I shall review the chemical and physical structure of pre-stellar and protostellar cores, as well as theoretical work on prototostellar disk formation and early evolution. Links to protoplanetary disks and our Solar System will be made.


JANUARY 3

Speaker:
Institute:
Host:

No Colloquium

10

Speaker: Erik Petigura
Institute:  Caltech
Host: D. Mawet

A Bird's Eye View of Extrasolar Planets

One startling result from the Kepler mission was that nearly every Sun-like star has a planet between the size of Earth and Neptune. Given the lack of such planets orbiting the Sun, Kepler has demonstrated that the Solar System is not a typical outcome of planet formation, in at least that one key respect. Therefore, to build a complete understanding of the processes that form planets, we must look to extrasolar planets. I will present some new insights into the physics of planet formation, made possible by advances in the exoplanet census and our understanding of planet host stars. This bird's eye view sheds light on where planets form, the speed at which they are assembled, and how they are sculpted by high-energy radiation from their host stars.

17

Speaker: Jennifer Barnes
Institute: Columbia
Host: G. Hallinan

Welcome to the multi-messenger era: a report on the first binary neutron star merger detection

On August 17th, the gravitational wave detectors LIGO and Virgo observed for the first time the signature of a binary neutron star merger. Roughly two seconds later, the Fermi satellite detected a short gamma-ray burst whose location was consistent with the position of the gravitational wave source. These signals triggered an electromagnetic follow-up campaign by dozens of groups around the world, who quickly identified an electromagnetic counterpart, which was observed over the next several weeks at energies ranging from the x-ray to the radio. These observations allowed astronomers to construct a detailed picture of an event that had previously been studied only theoretically, and to test key theories about the nature of neutron star mergers. Among these is whether mergers are the astrophysical site of r-process nucleosynthesis, which produces roughly half of elements heavier than iron. I will give 
​​
 an overview of the electromagnetic observations of this system, with an emphasis on the optical and infrared emission (the "kilonova") powered by the radioactive decay of elements synthesized in the merger. I will outline how recent theoretical advances allowed us to interpret kilonova observations and decode signs of heavy element production.

24

Speaker: Scottt Tremaine
Institute: Princeton
Host: S. Phinney

Comets and the outer fringes of the solar system

Comets have inspired awe since prehistoric times, but their nature and origin have only been investigated in the last few decades. The statistical analysis of a few thousand comets with well-determined orbits implies that there are two distinct sources of comets: the Oort cloud,
containing over 100 billion comets at 5,000 to 50,000 times the Earth-Sun distance; and the Kuiper belt outside Neptune's orbit. I will review our current understanding of the formation of the Oort cloud and Kuiper belt, our successes and failures in explaining the properties of comets and their orbits, and the relation between comets and possible undiscovered planets in the outer solar system.

31

Speaker: Carl Rodriguez
Institute: MIT
Host: M. Kasliwal

Forging binary black holes in dense star clusters

 Since the first detection two years ago, gravitational waves have promised to revolutionize our understanding of astrophysics.  But to understand what the gravitational waves are telling us, we need to understand how these relativistic systems formed in the first place.

I will describe how binary black holes form in the cores of dense star clusters by simple, Newtonian gravitational interactions.  I will demonstrate how these dynamically-formed binary black holes can easily explain most of the signals that LIGO has detected so far, and what distinguishes them from similar systems formed by the evolution of binary stars.  Finally, I will discuss how we can potentially discriminate between different formation scenarios, and what this can tell us about astrophysics.


FEBRUARY
7

Speaker: Jessica Werk
Institute: Univ Washington
Host: P. Hopkins

Circumgalactic Matter Matters for Galaxy Evolution

The circumgalactic medium (CGM; non-ISM gas within a galaxy virial radius) regulates the gas flows that shape the assembly and evolution of galaxies. It most likely contains enough material to harbor most of the metals lost in galaxy winds and to sustain star-formation for billions of years.  Owing to the vastly improved capabilities in space-based UV spectroscopy with the installation of HST/COS, observations and simulations of the CGM have emerged as the new frontier of galaxy evolution studies. In this talk, I will describe observational constraints we have placed on the origin and fate of this material by studying the gas kinematics, metallicity and ionization state of gas 10 - 200 kpc from galaxies' stars. I will conclude by introducing several exciting new techniques for resolving the gaseous structures in the CGM, and by posing unanswered questions about the CGM that will be addressed with future survey data and hydrodynamic simulations in a cosmological context.

14

Speaker: Richard Ellis
Institute: Univ College London
Host: P. Hopkins

Early Galaxies and Cosmic Reionization: Progress & Challenges

The birth of galaxies represents the last unexplored frontier of cosmic history and it is commonly believedsuch early systems led to the transformation of neutral gas in the intergalactic medium into its presentfully-ionized state. Some progress has been made in charting the demographics of early galaxiesinto the era when reionization is thought to occur, but little is known about their nature of their stellar populations,
the possible role of active nuclei and whether galaxies are capable of generating sufficient ionizing radiation. Spectroscopy holds the key to addressing these questions, targeting both individual sources at high redshiftas well as carefully-chosen analogs at intermediate redshift. I will describe the recent progress and challengesas we anticipate the launch of JWST and the arrival of next-generation large telescopes.


21

Speaker: Marc Pinsonneault
Institute: Ohio State
Host: L. Hillenbrand

Diversity Revealed: Stellar Rotation and the Time Domain Revolution

Stellar rotation is intimately connected to some of the most challenging problems in stellar astrophysics: the star and planet formation processes, the origin and generation of magnetic fields, and the transport of angular momentum and associated mixing in stellar interiors.  Nearly complete surveys of rotation periods are now available for star clusters and star forming regions down to much lower masses than were available before, and they are challenging our ideas about angular momentum evolution. We have strong evidence that rapid rotation in M dwarfs, traditionally attributed to weak torques, is imprinted early in their evolution, while higher mass stars are born rotating more slowly.  I will also present evidence for differences between the rotation of (non-synchronized) binary stars and single stars, and environmental effects on the distribution of stellar rotation rates.  Star spots also appear to have a significant impact on stellar structure, inducing significant changes in stellar radii.  For the Kepler field, there is evidence for both a maximum age for rotation as a population diagnostic and a possible transition in stellar dynamos below a critical rotation rate.  Older stars also appear to be able to spin down to very long rotation periods much earlier than anticipated, with a possible change in behavior near the fully convective boundary.  I will close by reviewing the massive data sets that should be available soon and the wide range of potential applications that they will have.

28

Speaker: Abigail Vieregg
Institute: U. Chicago
Host: R. Patterson

Discovering the Highest Energy Neutrinos Using a Radio Phased Array

Ultra-high energy neutrino astronomy sits at the boundary between particle physics and astrophysics. The detection of high energy neutrinos is an important step toward understanding the most energetic cosmic accelerators and would enable tests of fundamental physics at energy scales that cannot easily be achieved on Earth.  IceCube has detected astrophysical neutrinos at lower energies, but the best limit to date on the flux of ultra-high energy neutrinos comes from the ANITA experiment, a NASA balloon-borne radio telescope designed to detect coherent radio Cherenkov emission from cosmogenic ultra-high energy neutrinos.  The future of high energy neutrino detection lies with ground-based radio arrays, which would represent an large leap in sensitivity.  I will discuss a new radio phased array design that will improve sensitivity enormously and could push the energy threshold for radio detection down to overlap with the energy range probed by IceCube.

MARCH
7

Speaker: Cara Battersby
Institute: U. Conn
Host: P. Hopkins

The Milky Way Laboratory

Our home Galaxy, the Milky Way, is our closest laboratory for studying physical processes that occur throughout the Universe.  Submillimeter observations of the cool, dense gas and dust in our Milky Way provide insights on universal processes including how stars form in both 'regular' and 'extreme' environments and how gas is organized on galactic scales.  On a tour through our Milky Way Laboratory, I will discuss 1) how we can use dense, filamentary molecular clouds, potential "Bones of the Milky Way," to trace our Galaxy's spiral structure, and 2) how observing our extreme, turbulent Galactic Center (the Central Molecular Zone) can help us learn more about how gas is converted into stars during the peak epoch of cosmic star formation. I will also briefly discuss the Origins Space Telescope, a NASA mission concept study for the 2020 Decadal survey, opening up 2-4 orders of magnitude of discovery space on science from first stars to life.

14

Speaker: Rachel Shuchter Bezanson
Institute: Univ of Pittsburgh
Host: Phil Hopkins

The Surprisingly Complex Lives of Massive Galaxies

 Massive galaxies reside in the densest and most evolved regions of the Universe, yet we are only beginning to understand their formation history. Once thought to be relics of a much earlier epoch, the most massive local galaxies are red and dead ellipticals, with little ongoing star formation or organized rotation. In the last decade, observations of their assumed progenitors have demonstrated that the evolutionary histories of massive galaxies have been far from static. Instead, billions of years ago, massive galaxies were morphologically different: compact, possibly with more disk-like structures, and on-going star formation. The details of this observed evolution can place constraints on the physical processes that have driven massive galaxy evolution through cosmic time. I will discuss on-going observational studies of the structure, dynamics, and compositions of massive high-redshift galaxies. Specifically, I will highlight preliminary results from the LEGA-C spectroscopic survey of 0.6 < z < 1 massive galaxies. Finally, I will outline prospects for further understanding of the history of these intriguing objects with next generation observatories and instruments.

21

Speaker: Maryam Modjaz
Institute: NYU
Host: M. Kasliwal

Stellar Forensics with the Most Powerful Explosions in the Universe

Supernovae (SNe) and Gamma-ray Bursts (GRBs) are exploding stars and constitute the most powerful explosions in the universe. Since they are visible over large cosmological distances, release elements heavier than Helium, and leave behind extreme remnants such as black holes, they are fascinating objects, as well as crucial tools for many areas of astrophysics, including cosmology. However, for many years the fundamental question of which stellar systems give rise to which kinds of explosions has remained outstanding, for both Type Ia SNe used for cosmology as well as for Superluminous SNe and long-duration GRBs that must originate from special kinds of massive stars. I will discuss the exciting recent progress that we have made on this question in key areas by publishing and thoroughly analyzing the largest data sets in the world. I will conclude with an outlook on how the most promising venues of research - using the existing and upcoming innovative large 
time-domain surveys will shed new light on the diverse deaths of stars.

28

Speaker:
Institute:
Host:

No Colloquium

APRIL
4

Speaker: Jim Fuller
Institute: Caltech
Host: P. Hopkins

Surprising Impacts of Gravity Waves

Gravity waves are low frequency fluid oscillations restored by buoyancy forces in planetary and stellar interiors. Despite their ubiquity, the importance of gravity waves in evolutionary processes and asteroseismology has only recently been appreciated. In stars and planets, orbital resonances with standing gravity waves in the stellar/planetary interior can accelerate tidal evolution, especially when such resonances are maintained by a resonance locking mechanism. This process can explain observations of tidally excited pulsations in heartbeat stars and the rapid observed migration of Saturn's moons. In the late phase evolution of massive stars approaching core-collapse, vigorous convection excites gravity waves that can redistribute huge amounts of energy and angular momentum within the star. I will present preliminary models of this process, showing how wave energy redistribution can drive outbursts and enhanced mass loss in the final years of massive star evolution, with important consequences for the appearance of subsequent supernovae. 

11

Speaker: Judd Bowman
Institute: ASU
Host: G. Hallinan

The Dawn of 21cm Cosmology with EDGES

After stars form in the early Universe, their ultra-violet light alters the excitation state of the 21cm hyperfine line of neutral hydrogen gas in the intergalactic medium.  This initially causes the gas to absorb photons from the cosmic microwave background (CMB).  Later, energy deposited into the gas by the ultra-violet and X-ray emission from these early stars and their remnants heats the gas and eventually ionizes it.  These effects produce spectral features in the CMB observable today at frequencies below 200 MHz.  Our team recently reported the first detection of this redshifted 21cm signal in all sky radio observations acquired by our Experiment to Detect the Global EoR Signature (EDGES).  EDGES measures the all-sky radio spectrum between 50 and 200 MHz in the Outback of Western Australia.  We have found a flattened 21cm absorption profile in the sky-averaged radio spectrum centered at a frequency of 78 MHz with full width at half maximum of 19 MHz and an amplitude of 0.5 K.  The observed profile contains attributes that are both expected and surprising.  The low-frequency edge of the profile indicates that stars existed and created a strong background of Lyman-alpha photons by 180 million years after the Big Bang (z~20).   The high-frequency edge indicates that the hydrogen gas was heated above the radiation temperature by 270 million years (z~15).   These times are roughly consistent with astrophysical models of early star formation.  However, the best-fit amplitude of the observed profile is more than a factor of two greater than the largest standard predictions and suggests that the gas was either significantly colder than expected or the background radiation temperature was hotter than expected.  I will present the measurements along with results from instrumental verification tests that provide evidence that the observed feature is from the astronomical sky.


18

Speaker: Hans-Walter Rix
Institute: MPIA
Host: G. Djorgovski

 Biard Lecture - "Mass Migration: a Galactic, not just an Earthly Phenomenon"

The Milky Way can indeed serve as a Galaxy model organism to tell us which physical processes shape the current structure and stellar content of galaxies: what sets the overall radial profile of the disk, which the present-day orbital of any star, and how much formation memory does the Milky Way's disk retain?  We can now draw on global Galactic stellar surveys that constrain orbits, abundances and ages. I will show how modelling these data now shows that global radial orbit migration is a very strong effect that decisively shapes the structure of the Milky Way's disk. If the Milky Way is typical in this respect this explains why galaxy disk profiles are exponential. And I will sketch how Gaia is about to tell us in far  more detail the mechanisms that drive this orbit evolution.

25

Speaker: Gregg Hallinan
Institute: Caltech
Host: P. Hopkins

Imaging All the Sky All the Time in Search of Radio Exoplanets


 All the magnetized planets in our solar system, including Earth, produce bright emission at low radio frequencies, predominantly originating in high magnetic latitudes and powered by magnetospheric currents. It has long been speculated that similar radio emission may be detectable from exoplanets orbiting nearby stars, potentially providing the first direct confirmation of the presence, strength and topology of exoplanet magnetospheres, and informing on their role in shielding the atmospheres of potentially habitable exoplanets. Despite 4 decades of searching, no exoplanet radio emission has been detected. Surprisingly, however, brown dwarfs have been found to produce both radio and optical emissions that are strikingly similar to the auroral emissions from solar system planets, albeit 100,000 times more luminous.  I will discuss the radio emission from exoplanets and brown dwarfs with particular focus on the OVRO-LWA, a low frequency radio astronomy array located in the Owens Valley, California, that will simultaneously monitors 4000 nearby stellar systems in the search for radio emission from exoplanets.


MAY
2

Speaker: Bjorn Emonts
Institute: NRAO, Charlottesville
Host: Phil Appleton

The Cold Molecular Medium around Distant Galaxies: Light up the Darkness!

The evolution of galaxies is tightly linked to processes that occur in the circum-galactic medium (CGM). Unfortunately, most of the baryons in the CGM are too faint to be easily detected. At high-z, we view glimpses of dark baryonic halos through quasar absorption lines, or cooling-radiation emitted as Ly-alpha. However, a direct connection with the stellar growth of massive galaxies has long remained missing, because we had yet to identify the ultimate reservoir of halo gas that has sufficient mass to fuel widespread star-formation, namely the cold molecular gas (~10-100 K). I will present sensitive low-surface-brightness CO and [CI] observations of the massive Spiderweb Galaxy (z=2), which reveal a very extended (~70 kpc) reservoir of cold molecular star-forming gas. This cold CGM has a carbon abundance and excitation conditions similar to the ISM in starforming galaxies, implying that massive high-z galaxies grown not directly through accretion of pristine gas from the Cosmic Web, but from recycled gas in the CGM. Based on this and several other examples, I will explain the technical challenges involved in lighting up these hitherto hidden reservoirs of cold molecular CGM. This leads to the question of how much molecular gas in the Universe we are still missing? I will show how low-surface-brightness observations with future radio interferometers, with particular emphasis on the Next-Generation VLA, promise to revolutionize our view of the molecular Universe.

9

Speaker: Mike Liu
Institute: Univ Hawaii
Host: D. Mawet

Young Gas-Giant Planets and their Brown Dwarf Cousins: 
From 1 Square Arcsecond to 30,000 Square Degrees

The year 1995 was the annus mirabilis for substellar astronomy, with the
discovery of both brown dwarfs and gas-giant exoplanets. In the 20+years since, study of both classes of objects have flourished, with the discovery of thousands of these objects and a concomitant leap in characterizing their properties. Direct imaging has become a key method to study gas giants in their youth and has opened a new window to deepen our knowledge of both the exoplanet and brown dwarf populations.  I discuss how direct imaging studies have challenged our conceptions of gas-giant formation and properties, as well as enriched our knowledge of substellar evolution and atmospheres.  I also show how our understanding of low-mass objects in the inner square arcsecond around bright stars is being advanced by wide-field surveys that mine the entire sky.  Finally,
I present high-precision astrometry results that measure these objects' fundamental properties (luminosities, temperatures and masses) and thereby test the theoretical models common to all studies in the substellar regime.

16

Speaker: Patrick Ogle
Institute: STScI
Host: P. Appleton

Super Spirals. What the most massive, giant disks in the universe tell us about galaxy evolution
23

Speaker: Scott Gaudi
Institute: Ohio State
Host: Dimitri Mawet

TBD

30

Speaker: Tom Soifer
Institute: Caltech
Host: P. Hopkins

Neugebauer Lecture

JUN
6

Speaker: John Tonry
Institute: IFA Hawaii
Host: G. Djorgovski

Greenstein Lecture








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