Astrophysics Colloquium

Colloquium Meetings are held in the Physics and Astronomy Building (PAB) in Room 1-434A from 3:30-4:30pm every Wednesday of the Academic Year.

Coordinator: Jean Turner

Fall 2019 Schedule

Date Speaker Title
10/2/2019 Juna Kollmeier (Carnegie) SDSS-V:  Pioneering Panoptic Spectroscopy
10/9/2019 Lars Bildsten (UCSB)

Hearing the Stars: New Insights into Stellar Interiors from Asteroseismology

10/16/2019 Michael Johnson (CfA) Imaging the Shadow of a Supermassive Black Hole with the Event Horizon Telescope
10/23/2019 Hagai Perets (Technion - Israel Institute of Technology) 

 

The origins of type Ia supernovae

10/30/2019 Anna Ciurlo (UCLA) Gas dynamics in the Galactic Center
11/6/2019 Lena Murchikova (IAS)  
11/13/2019 Erika Hamden (Arizona)

The Life Cycle of Hydrogen

11/20/2019 Brendan Bowler (UTA)  
11/27/2019 Paul Robertson (UCI)  
12/4/2019 Andrew MacFadyen (NYU)  

Colloquium Archive:

Spring 2019 Schedule

Date Speaker Title 

04-3-19

Anna Nierenberg (JPL)

Testing the nature of dark matter with narrow-line lensing

The abundance of low mass dark matter halos (M_vir <10^8 M_sun) provides key insight into the nature of dark matter, as this abundance depends on the freestreaming length of dark matter at early times and thus its particle properties. Measuring the abundance of low mass halos is difficult as stars become increasingly poor tracers of structure on these scales owing to the complex and not yet well understood physics of star formation in these systems. I will present two complementary approaches to resolving these issues. First I will present measurements of the properties of faint satellite galaxies at a range of redshifts and around a variety of host types and demonstrate how these place strong new constraints on theoretical models of star formation in low mass halos. Secondly, I will present a novel approach to gravitational lensing which makes it possible directly measure the subhalo mass function to masses well below the mass scale of the missing satellite problem in a much larger sample of systems than previously possible. I will conclude by discussing future prospects for these programs given the next generation of ground and space based facilities.

04-10-19

Dimitri Veras (Univeristy of Warwick)

Transformative advances in post-main-sequence planetary system science

Connecting planetary systems at different stages of stellar evolution helps us understand their formation, evolution, and fate, and provides us with exclusive and crucial insights about their dynamics and chemistry. Post-main-sequence white dwarf and giant branch stars host planetary systems which include a variety of observed objects and phenomena, such as planetary debris discs, disintegrating and embedded asteroids, exo-comets, and photospheric metal pollution. Here, I provide a review of both our current knowledge of these systems and models which have been used to explain them. I also highlight the transformative advances expected in upcoming years with the current and next generation of ground-based and space-based initiatives. Looming orders-of-magnitude increases in available data must be accompanied by novel theories and simulations in order to understand the results from this interdisciplinary and expanding research field.

04-17-19

Simon Birrer (UCLA)

Probing Dark Matter and Dark Energy with strong gravitational lensing

Evidence from different cosmological probes have lead to the establishment of the dark matter and dark energy paradigm. Dark matter dominates the matter budget at cosmological scales and drives the formation of structure and dark energy is responsible for the late time acceleration of the universe. To date, these phenomenas have purely been described as gravitational effects and no other evidence have provided convincing evidence for their underlying physical nature - remaining one of the most striking mysteries the universe has not yet revealed. In my talk, I will describe the phenomena of strong gravitational lensing and how we can use this unique probe to constrain the physical nature of dark matter and dark energy. I am going to highlight recent results in both domains with a particular focus on the work done at UCLA in the last two years in collaboration with graduate students. I will then look in the near future and highlight the prospects of this technique with increasing sample size, analysis techniques and instrumentation.

04-24-19

Caroline Morley (University of Texas at Austin)

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.

 

05-01-19

Joe Silk (IAP, JHU, Oxford)

The Limits of Cosmology

One of our greatest challenges in cosmology is understanding the origin of the structure of the universe, and in particular the formation of the galaxies. I will describe how the fossil radiation from the beginning of the universe, the cosmic microwave background, has provided a window for probing the initial conditions from which structure evolved and seeded the formation of the galaxies, and the outstanding issues that remain to be resolved, including the nature of dark matter and dark energy. I will address our optimal choice of future strategy in order to make further progress towards understanding our cosmic origins.

 

05-08-19

Jo Dunkley

Weighing neutrinos: using the universe as our cosmic laboratory

Cosmological measurements of the Cosmic Microwave Background radiation, and of the large-scale distribution of galaxies, have taught us a great deal about the origins and content of the universe. In the next decade we anticipate using new microwave background data, and new measurements of the positions, masses and gravitational distortions of galaxies and galaxy clusters, to measure the mass of neutrino particles. I will discuss our path to making this indirect detection using astronomical data, which will complement direct laboratory measurements. This will progress from using current data, including the Planck satellite, to new measurements coming from Chile from the early 2020s with the Simons Observatory and the Large Synoptic Survey Telescope.

05-15-19

Elena D'onghia

 

A few important issues in Galactic Dynamics

The Gaia satellite is currently mapping the phase-space of a few million stars in the solar neighborhood showing time-varying phenomena.  About 350,000 stars within 200 pc of the Sun are identified in streams, bundles of stars that move together in the same direction with a velocity that is distinct from neighboring stars.  I will present a set of N-body simulations of the Milky Way disk that shows the role of a long stellar bar in understanding the kinematics of the solar vicinity.  Finally I will describe an unsupervised approach applied to Gaia DR2 data to re-build the nearby Orion complex with the discovery of 10 new stellar groups.

05-22-19

Jorge G. Moreno

CANCELED

05-22-19

Frank van den Bosch

New Insight into Cosmology and the Galaxy-Halo Connection from Non-Linear Scales

In our LCDM paradigm, galaxies form and reside in dark matter halos.  Establishing the (statistical) relation between galaxies and dark matter halos, the `Galaxy-Halo connection', therefore gives important insight into galaxy formation, and also is a gateway to using the distribution of galaxies to constrain cosmological parameters.  After a brief introduction to how clustering and gravitational lensing can be used to constrain the galaxy-halo connection, I show that several independent analyses all point towards a significant tension in cosmological parameters compared to the recent CMB results from the Planck satellite.  I discuss the potential impact of assembly bias, and present satellite kinematics as a complementary and competitive method to constrain the galaxy-halo connection.  After a brief historical overview of the use of satellite kinematics, I present two new analyses, and show how they can be used to improve our knowledge of the galaxy-halo connection.

05-22-19

Doug Lin


Stellar Rejuvenation in Galactic Center and AGNs: Analog of Planet Formation in Gravitational Wave Sources
 
The prevalence of massive black holes and nuclear stellar clusters implies some stars may be trapped in accretion disks around active galactic nuclei.  We determine the star trapping rate and we show that the trapped stars rapidly accrete gas which leads to a top heavy initial mass function similar to that found in the Galactic Center. These massive stars undergo supernova explosion, pollute the accretion disks around high-redshift quasars to produce super solar metallicity. They also produce single stellar-mass black hole remnants which gain mass through accretion and capture companions.  Binary seed black holes lose angular momentum to the surrounding gas to tighten their separation.  They generate intense gravitational wave when they coalesce. We provide analysis on the distribution of their masses, mass ratio, spin rate, binary orbit-spin obliquity and red-shift-dependent occurrence rate as observable predictions. I will describe some relevant mechanisms which are analogous to the astrophysics of planet formation and the implications of this scenario in the context of 1) coeval population and kinematic properties of disk and S stars around the Galactic center,  2) super solar metallicity, 3) duty cycle of AGN active phase, and 4) the rapid growth of their central massive black holes.
Date Speaker Title 

01-09-19

No Colloquium

N/A

N/A

01-16-19

Omer Blaes (UCSB)

Radiation Magnetohydrodynamics of Accretion Disks: From Dwarf Novae to Quasars 

Just as in many other branches of theoretical astrophysics and cosmology, modern supercomputer simulations have been having an enormous impact on our understanding of the physics of accretion onto compact objects. It is now possible to incorporate magnetohydrodynamics and radiation transport (both optically thick and thin) together to produce dynamically and thermodynamically self-consistent simulations. The latter in particular is now allowing us to tackle and perhaps solve long-standing problems between theory and observation in a variety of systems, from enhanced stresses in outbursting white dwarf systems to variability in black hole sources. It is also suggesting that accretion disks can be structurally different in surprising ways from that predicted by classical accretion disk theory.

01-23-19

Andrew Vanderburg (CfA/UT)

Preparing for Earth 2.0

Over the past thirty years, astronomers have made extraordinary progress in detecting planets around other stars. We now know that stars commonly host planets with a wider range of physical properties and system architectures than exist in our own solar system, and that planets likely outnumber stars in our galactic neighborhood. Now, planet detection technologies have advanced to the point where the direct detection of Earth-like exoplanets to search for biosignatures is within reach, and NASA is studying two space mission concepts with these goals in mind to potentially launch in the late 2030s. However, before this can happen, significant gaps in our knowledge of exoplanets must be covered so that these missions can be designed and their data can be interpreted. In my talk, I will describe work to fill in these gaps in our understanding of exoplanets. In particular, I will show how measurements of the bulk densities of small planets can constrain the planets' bulk composition and the presence of thick hydrogen/helium atmospheres. I will show how observations of white dwarf stars can reveal the elemental composition of rocky planets and the path towards using white dwarfs to learn about the compositional diversity of planets around other stars. Finally, I will show how modern artificial intelligence techniques can help measure precisely how common Earth analogs are, a crucial input parameter for designing missions to characterize Earth analogs.

 

01-30-19

Jenny Greene (Princeton)

Dwarf Galaxies and Their Black Holes 

The advent of deep and wide optical imaging surveys like the Hyper-Suprime Camera (HSC) Survey, and eventually LSST, are illuminating the low surface brightness universe. I will describe our efforts to determine the space density of low-mass galaxies, including the lowest surface brightness population. Then I will show how surface brightness fluctuations may revolutionize our understanding of dwarf satellite populations, and our tests of cold dark matter predictions. Finally, I will describe our ongoing efforts to quantify the massive black hole population in present-day low-mass galaxies, with the hope of understanding the first black hole seeds.

 

02-06-19

Cliff Will (University of Florida)

Is Einstein Still Right?

Einstein formulated general relativity just over 100 years ago. Although it is generally considered a great triumph, the theory's early years were characterized by conceptual confusion, empirical uncertainties and a lack of relevance to ordinary physics. But in recent decades, a remarkably diverse set of precision experiments has established it as the "standard model" for gravitational physics. Yet it might not be the final word. We review a century of measurements that have verified general relativity, including the recent detections of gravitational waves, and describe some of the opportunities and challenges involved in testing Einstein’s great theory in new regimes of strong fields and gravitational radiation.

 

02-13-19

Carl Rodriguez (MIT)

From Stellar Dynamics to Gravitational Waves: How to Merge Two Black Holes 

Since the first detection three years ago, gravitational waves have promised to revolutionize our understanding of compact objects, binary evolution, general relativity, and cosmology. But to make that a reality, we need to understand how and where these relativistic binaries form. In this talk, I will describe the various astrophysical pathways for creating the binary mergers detected by LIGO/Virgo, and how specific features of the gravitational waves (such as the binary eccentricities and black hole spins) can shed light on the formation of these dark remnants. I will show how simple gravitational dynamics in the centers of dense star clusters, particularly globular clusters, is uniquely efficient at producing merging binaries. Finally, I will describe how black holes can undergo multiple mergers in clusters, creating a second generation of black holes more massive (and with potentially greater spins) than those formed through the collapse of isolated stars.

 

02-20-19

Todd Thompson (OSU)

TBD 

TBD

02-27-19

Steve Finkelstein (UT Austin)

TBD 

TBD

 

03-06-19

Katey Alatalo (Space Telescope Science Institute)

The life-cycle of gas in dying galaxies 

Modern day galaxies populate a bimodal distribution, in both morphology and color space. Their morphological and color properties are also inter-related, with lenticular and elliptical galaxies usually exhibiting red colors and spiral galaxies usually exhibiting blue colors. In color space, there is a genuine dearth of intermediate colored galaxies, suggesting that the transition a galaxy undergoes to transform must be rapid, and quenching galaxies, rare. Gas - its presence, absence, and mechanics - serves as the anchor of a galaxy's transformation from blue to red. I will discuss the nature of gas in transitioning and transitioned galaxies through two lenses: (1) How a galaxy transition is able to impact the behavior of molecular gas, and (2) how new observations of molecular gas in quenching and quenched galaxies has recast our understanding of how they ultimately metamorphose from blue, star-forming spirals into red, quiescent ellipticals and lenticulars.

 

03-13-19

Allison Kirkpatrick (University of Kansas)

Dusty Galaxies in the Distant Universe 

Ten billion years ago, the universe was an exciting place, forming 50% of the stars we see today and concurrently feeding the supermassive black holes that reside at the center of every large galaxy. Yet, the most active galaxies were shrouded in dust, obscuring their newly formed stars and active supermassive black holes. In fact, these galaxies have 10 times more dust and are significantly colder than galaxies of the same luminosity in our local universe. I use Herschel and Spitzer observations to explore the reasons for this "cooling" of galaxies and it's implication for how they evolve. I also discuss how, in unresolved observations of galaxies, we can distinguish between young stars and an active supermassive black hole as the primary heating mechanism of the interstellar medium. I use infrared techniques to quantify how many galaxies in the distant universe are harboring hidden black holes, and I discuss what effect these black holes can have on the interstellar medium in their host galaxies.

 

Date Speaker Title 

10-03-18

Mike Rich (UCLA)

Our complicated Galactic Bulge

The central bulge of the galaxy has long been viewed as a kind of mini-elliptical galaxy, especially since it hosts a supermassive black hole at its center.  The bulge appears instead to be dominated by a bar structure that very likely originated from the dynamical evolution of a massive disk. I will review how the composition of bulge stars indicates a different history of enrichment from the thin and thick disk and discuss new findings in the inner nucleus and the nuclear star cluster.  There are some controversies: The main sequence turnoff population suggests that the bulge must be old, while other lines of evidence appear to require a substantial young component. I will describe results from three surveys: BRAVA-RR, a study of the 10 Gyr old RR Lyrae population near the center, the Blanco DECam Bulge Survey (BDBS), a survey of 200 sq. deg. of the Southern bulge in SDSS ugrizY, and the Bulge Asymmetry and Dynamics Experiment (BAaDE) that uses SiO masers to discover and characterize the properties of asymptotic giant branch stars in the Galactic bulge.  The emerging picture of the bulge is a concentrated barred population that formed rapidly, yet possibly also contains a substantial fraction of recently formed stars. I will discuss challenges for the era of ELTs and large surveys.

10-10-18

Dave Sand (U of A)

Unveiling the Physics and Progenitors of Cosmic Explosions with a Sub-Day Cadence Supernova Search

Supernovae (SNe) are a linchpin for understanding the chemical evolution and star formation history of the Universe. Despite progress, some of the most basic questions about SNe persist, and we seek to answer the question: What are the explosion mechanisms and progenitor star systems of SNe? In the early hours to days after explosion, SNe provide clues to how they explode, and what their progenitor star systems were. Observing these ephemeral signatures requires a fast search cadence and immediate spectroscopic response, a dual feat which has been difficult to achieve. Motivated by the need to discover, and study, SNe within the first day of explosion, we have begun a sub-day cadence SN search of nearby galaxies (D<40 Mpc; also known as the DLT40 Survey) with twin small telescopes in Chile and Australia, directly tied to the robotic FLOYDS spectrographs, a pair of instruments that I helped to develop. Here I will highlight our team's initial discoveries, with an eye towards what will be achievable with future time domain surveys -- perhaps including nearly automated follow-up of LSST transients and Advanced LIGO gravitational wave events with the suite of Steward Observatory's small telescopes. .

10-17-18

Eric Becklin (UCLA)

SOFIA/USRA Chief Scientific Advisor

My adventures in infrared astronomy started when I was a grad student  in 1965 with the discovery of an infrared-bright object (now known as the Becklin-Neugebauer Object) in the Orion Nebula.  In 1966, I made the first measurements of the infrared radiation from the center of the Milky Way Galaxy. I was fortunate enough to be able to take advantage of the 2.2 micron sky survey carried out by Neugebauer and Leighton (1969), which produced many remarkable discoveries, the most spectacular being the heavily dust-embedded carbon star IRC+10216, the brightest object in the sky at 5 microns outside the solar system.   In the 1970’s there was a growth in Infrared astronomy with the availability of many new facilities such as the Kuiper Airborne Observatory, (KAO) which I used extensively with Mike Werner and Ian Gatley for many unique observations. In 1977, I moved to Hawaii to work on the NASA IRTF 3- meter telescope. Many discoveries were made, including the first L dwarf star around a white dwarf (with Ben Zuckerman). In the 1980’s the introduction of large format arrays changed the way we did infrared astronomy.   With Ian McLean, I moved to UCLA in 1990 to start the IR lab and get involved in Keck development and science. In 1995, Andrea, Ghez, Mark Morris and I started looking for evidence of a possible massive Black Hole in the Galactic Center. Spectacular observations using the Keck 10 meter telescopes with large format near-infrared arrays and adaptive optics led to the confirmation of the presence of such a black hole and an estimate of its mass (4xE6 M (Sun)). In 1996, I began working on the Stratospheric Observatory For Infrared Astronomy (SOFIA) and I will finish my talk by discussing SOFIA observations of the ring of dust and gas orbiting the massive black hole in the center of our Galaxy and other recent discoveries.

 

10-24-18

Ilse Cleeves (U. Virginia)

X-ray Driven Time-Domain Astrochemistry During Planet Formation

The chemistry of protoplanetary disks sets the initial composition of newly formed planets and may regulate the efficiency by which planets form. Disk chemical abundances typically evolve over timescales spanning thousands if not millions of years. Consequently, it was a surprise when ALMA observations taken over the course of a single year showed significantly variable emission in H13CO+ relative to the otherwise constant thermal dust emission in the IM Lup protoplanetary disk. HCO+ is a known X-ray sensitive molecule, and by using simple time-evolving chemical models including stellar activity, we demonstrate that stellar X-ray flares are a viable explanation for the observed H13CO+ variability. If this link between chemistry and stellar activity is confirmed, simultaneous observations can provide a new tool to measure (and potentially map) fundamental disk parameters, such as electron density, as the light from X-ray flares propagates across the disk.

 

10-31-18

Adriano Fontana (INAF Osservatorio Astronomico di Roma)

Dead galaxies, ionized bubbles and other weird beasts in the Early Universe.

The search of high redshift galaxies is motivated not only by the desire to explore the Universe in its earliest epochs, but (mainly) by the attempt to build a consistent picture of the physical processes that shaped the Universe since its early phases.

I will present in my talk a few recent observational results concerning the nature and properties of high redshift galaxies that constrain and sometime challenge the physical theories that have been developed so far.

 

One result concerns the search of passive galaxies (i.e. galaxies with negligible amount of on going star-formation rate) at z>3. We show that the combination of deep  HST/ground based and ALMA data enables us to demonstrate for the first time that a sizable population of these objects exist at high redshift, implying a very efficient mechanism to form Milky-Way sized galaxies in a short time after the Big Bang. The existence of these objects is a major challenge for most theoretical models of galaxy formation and may require significant changes to the their prescriptions.

 

At even higher redshift we are investigating the process of reionzation at z>7, in an attempt to identify the sources responsible for this transition. I will report the study of the first “overdense and re-ionized” region identified so far at z>7, i.e. at an epoch when the Universe was still partly neutral. I will report the results of a deep VLT spectroscopic program and discuss how they challenge the simplest interpretations of the reionization process.

 

11-07-18

Sarah Brough (UNSW)

Environment or Mass: A Spatially-Resolved Perspective on Galaxy Evolution

The question of whether a galaxy's mass or environment have driven its current observed properties is a vexed one. I will present results from the low-redshift Galaxy And Mass Assembly (GAMA) and SAMI integral field Galaxy Surveys that suggest that once the spatially-resolved properties of a galaxy or it's environment are examined, environment is found to play more of a role in galaxy properties than has previously been found in the local Universe. .

 

11-14-18

Gongjie Li (Georgia Tech)

A Song of Ice and Fire Dynamics of Planets Hot and Cold

The unexpected diversity of planetary systems has posed challenges to our classical understanding of planetary formation. For instance, Jupiter sized planets have been detected with short orbital periods of a few days in misaligned orbits with respect to the spin-axis of their host stars. I will first describe the statistical implication of these misaligned planets, and then illustrate that dynamical interactions between an outer perturber and the inner planet can naturally lead to the formation of such objects. Next, I will discuss a similar dynamical process in the outer Solar System, far away from our Sun, which could cause the observed clustering of extreme trans-Neptunian objects. This can constrain properties of a possible outer planet, Planet Nine, in our own Solar System.

11-21-18

HOLIDAY

No Talk

11-28-18

Nick Stone (Columbia)

TBD

12-05-18

Mark Vogelsberger

Simulating Galaxy Formation: Illustris, IllustrisTNG and beyond

Cosmological simulations of galaxy formation have evolved significantly over the last years. In my talk I will describe recent efforts to model the large-scale distribution of galaxies with cosmological hydrodynamics simulations. I will focus on the Illustris simulation, and our new simulation campaign, the IllustrisTNG project. After demonstrating the success of these simulations in terms of reproducing an enormous amount of observational data, I will also talk about their limitations and directions for further improvements over the next couple of years.

 
Date Speaker Title 
4-4-18

Adam Riess (Johns Hopkins University)

TBD

4-18-18

Jessica Werk(U. Washington)

Why 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.

5-2-18

Jonathan Fortney(UCSC)

Population-Level Analysis of Giant Planet Composition and Evolution

We live in a unique time with the dramatic advancement of exoplanetary science. Beyond population-level analyses of planetary frequency, we can now study some aspects of the physics of the planetary population. In particular, hundreds of transiting gas giant planets with well-determined masses are known. I will discuss what we have learned regarding the composition and metal-enrichment of giant exoplanets from modeling the structure and evolution of the planets as a population, and what it means for our understanding of planet formation. Furthermore, additional insights are enabled by this work on the population, including two novel assessments of the long-sought-after hot Jupiter "radius inflation" mechanism. We can now constrain the magnitude of this inflation mechanism across planetary temperature and stellar age.

5-9-18

Jordan Mirocha (UCLA)

New Directions in Galaxy Formation and Cosmology Following the First High-z 21-cm Detection

On March 1st, the EDGES collaboration reported the detection of a sharp absorption signal in the all-sky radio spectrum at ~80 MHz. This frequency is roughly consistent with early theoretical predictions for the global 21-cm signal, a sky-averaged spectral signature of neutral hydrogen atoms in the intergalactic medium before cosmic reionization. However, the reported amplitude is ~2.5 times larger than the strongest possible global 21-cm signal in standard cosmological models. This startling feature of the EDGES signal has led to a variety of exotic explanations, including milli-charged dark matter and as-yet-unidentified radio backgrounds in the early Universe, both of which can amplify 21-cm absorption signals relative to standard expectations. In this talk, I will first highlight the flurry of ideas that have arisen in the last two months to explain the anomalous amplitude of the EDGES signal. Then, I will turn to an under-appreciated aspect of the signal: its timing is not consistent with empirically-calibrated models of high-z galaxies. If confirmed, the EDGES signal may thus be evidence of new physics and new astrophysics.

5-16-18

Matt McQuinn (U. Washington)

Wacky Applications of Perturbation Theory in Cosmology

Perturbation theory provides a largely controlled way of solving problems in which the fluctuations are small. Large-scale structure provides this setup, where we know the matter fluctuations were a part in a thousand or so at z~1100 and still are small when smoothed over 10 Mpc scales today. Yet, in cosmology we have only recently understood how to formulate beyond-linear-order perturbation theory for the late time matter distribution. I will discuss a cute 1D example of perturbation theory for the matter distribution, which demonstrates the superiority of recent effective formulations over the historical ones. Then, I will proceed to apply effective perturbation theory to what may be viewed as the most nonlinear cosmological process ever, reionization, showing that over many of the scales that 21cm observatories are forecast to be sensitive, perturbation theory describes well the signal seen in complex radiative transfer simulations of reionization. This theory decomposes the 21cm signal into a few intuitive shapes whose coefficients have physical meanings.

5-23-18

Joe Hennawi (UCSB)

How Long do Quasars Shine?

Luminous quasars are believed to be the progenitors of the supermassive black holes observed ubiquitously at the centers of all massive galaxies. But half a century after their discovery, and decades after tight scaling relations between supermassive black holes and their host galaxies were uncovered, we are still in the dark about how these black holes actually formed. Our ignorance largely results from the long characteristic timescale for supermassive black hole growth, known as the Salpeter time, of 45 million years -- far longer than humans have been conducting astronomical observations. A holy grail would thus be a direct measurement of the lifetimes of luminous quasars, shedding light on the structure of black hole accretion disks, how gas funnels to the centers of galaxies triggering quasar activity, and feedback that might influence galaxy formation. Analogous to the way O-stars transform their nearby ISM, the enhanced UV radiation field in the vicinity of luminous quasars dramatically alters the physical state of the surrounding intergalactic medium. I will show how observations of diffuse gas in the environs of luminous quasars can be used to chronicle the history of quasar emission on timescales from kiloyears to gigayears. I will also discuss how these same observations can be used to constrain the reionization history of the Universe.

5-30-18

Brian Keating (UCSD)

Losing the Nobel Prize: A Cosmological Tale

Brian Keating tells the inside story of BICEP2's detection and the ensuing scientific drama.Building on lessonslearned from BICEP2, Keating and his colleagues have begun construction of the Simons Observatory, the most ambitious ground-based Cosmic Microwave Background experiment ever funded. Keating will describe the design goals and status of this exciting observatoryto be sited at in the Atacama Desert of Northern Chile.

6-6-18

Lorenzo Sironi (Columbia)

Fast and furious: magnetic reconnection in relativistic jets and black hole coronae

Relativistic blazar jets and black hole coronae routinely display fast and bright flares of non-thermal emission. By means of fully-kinetic particle-in-cell (PIC) simulations, we show that magnetic reconnection in the relativistic regime appropriate for blazar jets (i.e., at magnetizations sigma>>1) can satisfy all the basic conditions for the emission: extended non-thermal distributions of accelerated particles (with power-law slope between -4 and -1), efficient dissipation and rough equipartition between particles and magnetic field in the emitting region. In addition, we show that ultra-relativistic plasmoids generated by reconnection can power the ultra-fast bright flares observed from a number of TeV blazars. We also discuss electron heating and non-thermal acceleration in the trans-relativistic regime sigma~1 appropriate for the magnetized coronae of collisionless accretion flows, like Sgr A* at the center of our Galaxy.

 
Date Speaker Title 
1-17-18

Anna Pancoast (Harvard/CfA)

Probing the Structure of Active Galactic Nuclei Using Light Echoes 

Echoes from the broad line region in active galactic nuclei (AGN) allow for the measurement of supermassive black hole masses outside the local Universe. However, the detailed structure of the broad line region is difficult to constrain due to the very small scales involved. With a new generation of high-quality reverberation mapping datasets, we can substitute time resolution for spatial resolution and begin to model echoes from the broad line region directly. I will discuss the development of a direct modeling approach for reverberation mapping data capable of measuring the absolute black hole mass and constraining the geometry and dynamics of the broad line region. We find that the broad line region emission comes from gas in a thick disk that can be either inflowing or outflowing. Finally, I will discuss how this work can improve AGN black hole mass measurements at all redshifts.

1-24-18

Raffaella Margutti (Northwestern University)

Astronomical Transients in the New Era of Multi-Messenger Astrophysics

New and improved observational facilities are sampling the night sky with unprecedented temporal cadence and sensitivity across the electromagnetic spectrum. This exercise led to the discovery of new types of astronomical transients and revolutionized our understanding of phenomena that we thought we already know. In this talk I will concentrate on two recent remarkable discoveries: (i) The “normal” envelope-stripped supernova SN2014C, which experienced a dramatic metamorphosis and evolved from Type I into Type II supernova over a timescale of a few months, thus violating the supernova classification scheme that hat has existed for decades. (ii) I will then present the results from the first successful hunt for radiation from the gravitational wave source GW170817 focusing on major still-open questions in the field: what is the nature of the fastest ejecta? Did GW170817 launch a relativistic jet?

1-31-18

Mike Cooper (UC Irvine)

Constraining the Physics of Satellite Quenching 

Despite remarkable success at modeling the evolution of massive galaxies over cosmic time, modern hydrodynamic and semi-analytic models of galaxy formation fail to reproduce the properties of low-mass galaxies. This shortcoming in our theoretical picture is largely driven by an inability to understand the physical mechanisms by which star formation is suppressed (or “quenched”) in satellite galaxies. In an effort to address this shortcoming, I will present recent work to measure the efficiency of satellite quenching over more than 7 orders of magnitude in satellite stellar mass at z ~ 0. In particular, our work utilizes observations of galaxy groups identified in the Sloan Digital Sky Survey as well as detailed studies of dwarfs in the Local Volume to constrain the timescale upon which satellite quenching occurs following infall (and thus the physical mechanisms at play). By bringing together multiwavelength data across a broad range in satellite and host mass, our analysis has established a coherent physical picture of satellite quenching that addresses the most glaring deficiencies of current galaxy formation models.

2-7-18

Ralph Bird (UCLA)

Our Very Energetic Universe

Very high energy gamma rays provide a window to some of the most extreme places in the universe, such as the jets of active galactic nuclei, supernova remnants, binaries and potentially dark matter. They allow us to probe particle acceleration and interaction up to the highest energies, to study the physics of these objects and shed light on the origin of cosmic rays. In this talk I look at gamma ray emission in the Cygnus region of the galaxy. A gas rich, local region of star formation with multiple gamma ray sources, it provides an ideal location to study these emitters and how their relationship with their environment. I discuss the latest results on these sources and what they can tell us about the extremes of our universe.

2-14-2018

Maria Drout (Carnegie Observatories)

The Evolution, Influence, and Ultimate Fate of Massive Stars: Transient Phenomena and Stellar Astrophysics in the Era of Wide-Field Surveys

An improved understanding of the lifecycle of massive stars benefits every subfield in astrophysics. Through their ionizing radiation, powerful stellar winds, nucleosynthesis, and deaths as supernova (SN) explosions, massive stars give birth to black holes and neutron stars, while stoking the dynamical and chemical evolution of the universe. Although the study of massive stars is one of the oldest subfields in astronomy, the recent advent of wide-field time-domain surveys has launched an upheaval in field of stellar evolution. In this talk I will highlight on-going efforts to constrain the evolution, influence, and ultimate fate of massive stars, using observations of both transient phenomena and resolved massive star populations in local group galaxies. Within this context I will also discuss several aspects of the recent discovery of an electromagnetic counterpart to the neutron star merger identified by LIGO/Virgo.

2-21-18

Erik Petigura (Caltech)

TBD

TBD

2-28-18

Kelsey Johnson (University of Virginia)

How were the most ancient objects in the universe formed? 

Ancient remnants from the early universe surround our galaxy. These relics, known as “globular clusters” have the potential to provide insight into the physical conditions that prevailed during an epoch that cannot be directly observed. We now know that globular clusters can form during extreme episodes of star formation in the relatively nearby universe, but the actual physical conditions that give rise to globular clusters have vexed both observers and theorists for decades. With the new capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) we can probe the natal environments of these ancient objects for the first time. This talk will give an overview of progress that has been made in understanding globular clusters, and highlight the importance of using chemistry to understand physical conditions in space.

3-7-18

Yuri Levin (Columbia)

Dynamics of strongly magnetic neutron stars 

Magnetars produce beautiful fireworks in x-rays and gamma-rays: small, large, and stupendous flares, fast quasi-periodic oscillations (QPOs), as well as transient increases of their luminosities on timescales of months to years. How the release of magnetic energy leads to this rich variability is not precisely understood, but the processes responsible for it may well be hidden under the surfaces of the stars, in their solid crusts and super-fluid cores. The pulsars' magnetic dynamics is less dramatic, yet the evolution of the pulsars' magnetic fields with age is an important unsolved theoretical and observational problem. In this talk I will discuss a set of theoretical ideas for how the magnetic dynamics is likely to work. 1. I will demonstrate a new type of failure that is likely to occur in magnetars' crusts , a thermo-plastic wave. These do not have direct terrestrial analogues, and behave qualitatively like deflagration fronts. Coupling to Hall waves leads to avalanches of thermoplastic waves, and I will argue that these avalanches are responsible for magnetar outbursts. 2. I will show that the coupling between elastic waves in the crust and the Alfven waves in the core leads to rich dynamics and phenomenology of magnetar oscillations, but leave un-answered questions for how these relate to the observed QPOs 3. I will show a series of numerical experiments on the magnetic field evolution in pulsars that model the coupling between the crust and the core and, with significantly more uncertainty, between the superfluid vortices and superconducting flux tubes in the core.

3-14-18

Edo Berger (Harvard/CfA)

Rattle and Shine: Joint Detection of Gravitational Waves and Light from the Binary Neutron Star Merger GW170817

The much-anticipated joint detection of gravitational waves and electromagnetic radiation was achieved for the first time on August 17, 2017, for the binary neutron star merger GW170817. This event was detected by Advanced LIGO/Virgo, gamma-ray satellites, and dozens of telescopes on the ground and in space spanning from radio to X-rays. In this talk I will describe the exciting discovery of the optical counterpart, which in turn led to several detailed studies across the electromagnetic spectrum. The results of the observations carried out by our team include the first detailed study of a "kilonova", an optical/infrared counterpart powered by the radioactive decay of r-process nuclei synthesized in the merger, as well as the detection of an off-axis jet powering radio and X-ray emission. These results provide the first direct evidence that neutron star mergers are the dominant site for the r-process and are the progenitors of short GRBs. I will also describe how studies of the host galaxy shed light on the merger timescale, and describe initial constraints on the Hubble Constant from the combined GW and EM detection.

 
Date Speaker Title 
4-5-17

Jennifer van Saders, Carnegie

Making Sense of Stellar Rotation Observed with Kepler: Gyrochronology, Magnetism, and a Sun in Transition

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

4-12-17

Aomawa Shields, UCLA

Ice, Light, and Company: Radiative and Gravitational Effects on the Habitability of Planets Orbiting Low-mass Stars Aomawa Shields

The recent discovery of numerous potentially habitable planets orbiting low-mass stars signals a major planetary population that may be the primary environment explored in the search for life beyond the Solar System. However, many factors and processes can affect planetary climate and habitability, including the unique interaction between the spectral energy distribution of the host star and the atmospheres and surfaces of orbiting planets. Additionally, as lower-mass stars often host multiple rocky planets, gravitational interactions among planets can have significant effects on climate and habitability over long timescales. To identify habitable worlds beyond our Solar System, it is important to understand how both orbital, surface, and atmospheric properties affect the climate of exoplanets, and how these climatic effects might change for different stellar and planetary environments. I will share results from work performed using a hierarchy of models to simulate planets orbiting stars of different spectral types and with varied orbital architectures, and discuss the implications of these results for planetary climate and habitability.

4-19-17

Assaf Horesh, Hebrew University

Forensics of Stellar Death Through Radio-Wave Eyes

The dynamic radio sky is seen as a frontier area in astrophysics, ripe for discovery, as recent technological advancements enable the comprehensive study of cosmic explosions. By comparison to observations in other wavelengths, radio observations offer unique diagnostics, as they trace high-energy particles, magnetic fields - and reveal fast and relativistic outflows, which otherwise remain hidden. In the case of supernovae, the complex process of mass-loss, a key ingredient in the last stages of stellar evolution, can be illuminated by thoroughly mapping the circumstellar medium, around the progenitor. Radio observations can also help identify the physical process responsible for launching fast outflows and/or relativistic jets such as in a stellar disruption by a supermassive black hole. More importantly, whenever a new class of transients has emerged, radio observations played a key role in unveiling their nature. While the field of radio time-domain astronomy has seen a tremendous progress over the last decade, just now we are on the verge of a revolution as (a) next generation synoptic surveys searching for cosmic explosion are becoming available, (b) the radio astronomy field is experiencing a renaissance with new and improved facilities, and (c) the first detection of a gravitational wave opened a new window for discoveries.

4-26-17

Ben Shappee, Carnegie

The All-Sky Automated Survey for Supernovae

For the first time, the entire visible sky is being surveyed for the violent, variable, and transient events that shape our universe. To accomplish this, my collaborators and I built the All-Sky Automated Survey for Supernovae (ASAS-SN), which is a long-term project to monitor the whole sky, at a high cadence, using a global network of robotic telescopes. The primary goal of ASAS-SN is to find the closest and brightest supernovae (SNe) with an unbiased search: ASAS-SN now discovers about two-thirds of all bright (V<17 mag) supernovae. These nearby supernovae are critical in studying the physical nature of their progenitor systems because we can study them in unprecedented detail across the electromagnetic spectrum which cannot be done for their more distance counterparts. However, this systematic all-sky technique also allows ASAS-SN to discover many other interesting galactic and extragalactic transients. During this talk, I will give an overview of the ASAS-SN survey and highlight some of our more interesting discoveries. These discoveries include ASASSN-15lh, likely the most luminous supernovae ever discovered; ASASSN-14lp, one of the earliest observed Type Ia supernovae; ASAS-SN16ae, the largest (Delta V > 11 mag) and second-ever L-dwarf flare; and ASASSN-14ae, ASASSN-14li, and ASASSN-15oi, the three brightest tidal disruption events discovered in the optical. These discoveries, however, are just the beginning. In 2017 ASAS-SN will more than double in size, allowing us to survey the visible entire sky with better than a daily cadence while being more resistant to weather.

5-3-17

Dan Marrone, University of Arizona

The Cosmic Abundance of Molecular Gas: Intensity Mapping the Faint Universe

The cool, molecular phase of the interstellar medium is the fuel that enables the formation of new stars. In the early universe, large gas reservoirs dominated the baryonic mass of galaxies and enabled a cosmic star formation density that peaked at 10 times the current value. While the light from young stars has made it possible to trace the star formation itself, observing the molecular gas itself is much more difficult. This phase, typically traced by CO emission, has only been observed at high redshift in the most massive objects, while normal galaxies are nearly inaccessible to even the most sensitive radio telescopes. The technique of “intensity mapping,” which measures the aggregate molecular emission from the three-dimensional distribution of galaxies, provides a tool to detect the faint signal of the molecular ISM and chart its history across cosmic time. I will describe a staged program of intensity mapping that targets CO emission from the peak of cosmic star formation to the present. The first phase, the CO Power Spectrum Survey (COPSS), used archival and targeted observations with the CARMA interferometer to constrain the CO power spectrum at z~3 for the first time. I will review these measurements and their implications for the distribution of molecular gas in the universe. This work is now transitioning into new phases in which we use archival and new data from the most sensitive radio telescopes to further explore the growth of the ISM, and ultimately deploy a purpose-built instrument to Kitt Peak to survey the universe from z~0 into the epoch or reionization.

5-10-17

Ranga-Ram Chary, Caltech/IPAC-Planck

Unexpected Spectral Variations in the Cosmic Microwave Background: Constraining the Alternate Universe Hypothesis

The fine-tuning of initial conditions required to reproduce our present day Universe and the value of the vacuum energy density therein, suggests that our Universe may merely be a region within an eternally inflating super-region. Many other regions could exist beyond our observable Universe with each such region governed by a different set of physical constants than the ones we have measured for our Universe - this is the alternate Universe hypothesis. Some groups think that testing the existence of alternate Universes and thereby the theory of eternal inflation is an impossible goal. However, if chance collisions occur between these regions, they should leave signatures of anisotropy in the cosmic microwave background. I will present our analysis of the Planck data which has led to the detection of at least one statistically significant spectral anomaly at the location of a high-latitude CMB cold spot. The intensity of this anomaly, if foregrounds are ruled out, would favor a collision with an alternate Universe, inducing compensated isocurvature fluctuations with a much different baryon to photon ratio than in our own Universe. This would suggest an anthropic explanation for the value of the cosmological constant and provide an astrophysical way to test string theory. Future, observational tests of this hypothesis will also be discussed.

5-17-17

Rychard Bouwens, Leiden Observatory

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 of 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.

5-24-17

Phil Marshall, Stanford/KIPAC

Time Delay Cosmography with LSST

Strong gravitational lenses have become an important astronomical tool: they allow us to make accurate measurements of galaxy masses, they provide a magnified view of the distant universe, and they allow us to constrain cosmological parameters. In particular, the time delays in multiply-imaged quasar systems enable measurements of distance in the Universe each with around 5% precision. I will review recent measurements of time delay distance in galaxy-scale lens systems in the context of a longer term vision of how we can realize the potential of this cosmological probe, by increasing the size of our lens sample, and continuing to improve the accuracy of its analysis. More broadly, I'll use the example of time delay cosmography to introduce the LSST, its Dark Energy Science Collaboration, and what it will take to do accurate cosmology with its Big Data set.

5-31-17

Guinevere Kauffmann, MPA

Observations and modelling of gas in and around galaxies

This talk will focus on gas galaxies in the low redshift universe. I will review recent advances in quantifying scaling relations for atomic hydrogen in galaxies using radio telescopes, as well as the diffuse gas around galaxies using Hubble Space Telescope UV spectroscopy of quasars whose sight-lines pass through the halos of galaxies similar to our own Milky Way. I will show how such observations constrain galaxy formation models, in particular the physical processes that regulate the heating and cooling of gas as a function of cosmic epoch.

6-7-17

Zach Berta-Thompson, CU Boulder

Small Planets Transiting Nearby Small Stars

When you stand on a mountain admiring a colorful sunset, some light from the Sun passes over your shoulder and continues on through our atmosphere and out into space. As that light journeys out to distant stars, it carries with it the spectral fingerprint of our atmosphere’s molecules and aerosols. Astronomers have found thousands of small planets transiting other stars, many of them small enough to be rocky. In principle, we could observe the transmission spectrum of these planets’ atmospheres and use these observations to illuminate how these often-weird planetary atmospheres work. In practice, such observations are possible for small planets only when then transit very nearby, very small stars. I will present our efforts with the MEarth Project to find these systems, including our recent discovery of the transiting, terrestrial, potentially habitable planet LHS 1140b. I will also present our efforts to observe exoplanet atmospheres with multiobject spectroscopy from the ground, and I will discuss the landscape for small planet characterization in the TESS and JWST era.


Date Speaker Title (mouseover for abstract)
1-11-17

Dan Weisz, UC Berkeley

The Lowest-Mass Galaxies In the Early Universe: Insights from the Local Group

The Local Group is home to ~100 galaxies less massive than the Small Magellanic Cloud (10^8 Msun).  Such low-mass galaxies have become increasingly relevant to a broad range of astrophysics from cosmic reionization to deciphering the nature of dark matter.  Yet, they are simply too faint to be directly detected at any appreciable redshift, compromising our ability to place them into a cosmological context. In this talk, I will describe how observations of resolved stellar populations in Local Group galaxies enable the measurement of detailed star formation histories, which provide the only avenue for tracing the evolution of low-mass galaxies across cosmic time. I will review our current knowledge of low-mass galaxy evolution over 6 decades in stellar mass, with a particular emphasis on the very early Universe.  I will illustrate how local and high-redshift galaxy observations can be used in tandem to improve our understanding of cosmic reionization, and will conclude by discussing prospects for increased synergy between near-field and far-field galaxy studies in the JWST era.

1-18-17

Andy Skemer, UCSC

Characterizing the Coldest Exoplanets

Temperature, rather than mass, is the dominant factor in determining the appearance of gas-giant planets, and the diversity and complexity of worlds both increase at cold temperatures.  The coldest known exoplanets are still much hotter than the gas giant planets in our own Solar System.  Pushing to colder temperatures requires imaging in the thermal infrared (3-5 microns) where self-luminoous gas-giants peak in brightness.  I will present observational studies characterizing the atmospheres of the coldest exoplanets and brown dwarfs, down to a temperature of 250K.  Additionally, I will describe a new instrument that can obtain thermal infrared spectroscopy of directly imaged planets for the first time.

1-25-17 Massimo Stiavelli, STScI

Space Telescope Science Institute, USA

I will review the capabilities of the James Webb Space Telescope to study the era of the first stars, particularly focussing on the study of lensed or unlensed primordial star clusters or dwarf galaxies. I will discuss possible avenues of discovery of pair-instability supernovae using JWST or other telescopes, and how JWST could follow them up. Finally, I will provide a brief status on the mission and on the upcoming opportunities to learn more about JWST and propose for time.

 

2-1-17 Andrey Kravtsov, U Chicago 

Towards understanding the inefficiency of star formation in galaxies

Understanding how galaxies form is one of the main unsolved problems in astrophysics. One of the long-standing puzzles is the global inefficiency with which galaxies convert baryonic matter available to them into stars. This inefficiency is manifested in 1) the fact that ratio of baryon mass observed within galaxies to the total inferred mass of their host halos is much smaller than the universal baryon fraction and 2) the fact that galaxies convert their observed gas into stars on ~1-2 Gyr time scale (aka depletion time), which is much longer than any dynamical time scale within galaxies. I will review recent progress in galaxy formation simulations due to improvements in treatment of stellar feedback and star formation, which sheds light into the 1st aspect of inefficiency. I will highlight the key role that modelling of star formation and stellar feedback play in setting the basic properties of galaxies, such as stellar mass (and global baryon mass fraction), size, and morphology using specific examples of recent galaxy formation simulations. I will present a new model, in which local star formation efficiency is modelled "on the fly" using a turbulence-based subgrid model based on results of high-resolution simulations of molecular clouds. The model predicts a wide variation of star formation efficiency per free fall time at odds with the usual assumption of constant efficiency. At the same time, our model predicts distribution of star formation rates in broad agreement with observations of both local and resolved extragalactic GMCs. I will show that with realistic implementation of stellar feedback this modelling can reproduce the basic properties of star formation and the Kennicutt-Schmidt relation in galaxies, such as the Milky Way. Finally, using insights from such simulations I will present a simple model explaining why star formation in galaxies is inefficient and depletion times are long.

2-8-17 Fred Rasio, CIERA

Dense Star Clusters as LIGO Source Factories

Theoretical predictions for compact binary mergers from field populations of binary stars are extremely sensitive to the assumptions of stellar evolution, leading, for example, to predicted merger rates for binary black holes that span several orders of magnitude. But in dense stellar environments such as globular clusters, binary black holes form by well-understood gravitational interactions. In this talk I will present an overview of recent theoretical work on the dynamical formation of black hole binaries based on realistic N-body simulations of globular clusters. By calibrating theoretical models against observed Milky Way and extragalactic globular clusters, we find that the mergers of dynamically formed binaries could eventually be detected by Advanced LIGO at a rate of ~ 100 per year, potentially dominating the overall detection rate of gravitational wave sources. Dynamical processes in globular clusters can also form very naturally the more massive black hole binaries like the one that produced GW150914, the first merger signal detected by LIGO.

2-15-17 James Guillochon, CfA

 The Impact of the Debris from Stellar Tidal Disruptions on their Environment

Abstract: When a star is destroyed by tides from a supermassive black hole, the star is stretched into a long, extraordinarily thin debris stream that extends from the star's original periapse (about an AU from the black hole) to distances of tens of parsecs. As the stream extends, it simultaneously cools, clumps, magnetizes, and slams into the ambient medium, delivering a supernova's worth of kinetic energy into the surrounding gas. In my talk, I will describe the predicted observable phenomena arising from this debris, and the impact it can have in the centers of galaxies.
2-22-17 Jennifer Lotz, STScI

 Galaxy Assembly over Cosmic Time

Abstract: Deep HST observations have revealed galaxies fainter than ever seen before, at look-back times when the universe was less than a billion years old. These first dwarf galaxies grow throughout cosmic time via the accretion of gas and dark matter, and via mergers with other galaxies. The detailed structures of galaxies provide direct insight into their most recent assembly events. From large high-resolution imaging surveys, we now have a broad-brush picture of how galaxy shapes and sizes have evolved over the past 10 billion years. But the role of galaxy mergers in galaxy evolution is poorly understood, particularly at early times. More subtle morphological tracers are needed to track the complex processes responsible for the transformation of galaxies. I use new machine learning classifications of galaxy morphology at 0 < z < 3 to identify galaxy mergers and galaxies transitioning to today's Hubble types. Numerical simulations are used to inform the interpretation of these systems. I track the evolution of galaxies as a function these new measures, and discuss the role of mergers in the size growth of galaxies. Finally, I discuss the prospects for studying galaxy assembly in the coming decade.

3-1-17 Emily Rauscher, U. Michigan

The Diversity and Complexity of Exoplanet Atmospheres

We are now in the era of exoplanet characterization.  Over a decade ago the first exoplanet atmosphere was detected and since then we have been gathering compositional and temperature information for the brightest targets, primarily "hot Jupiters".  Recent technical advances are enabling measurements that contain more complex information about exoplanet physical properties; however, that additional complexity also makes interpretation of the data more difficult.  I will discuss the extra boons and challenges that come with these newer measurements, and present my own work on using three-dimensional atmospheric circulation models to guide and interpret observations.  In particular, I will show how we can combine different types of measurements in order to robustly measure, or at least constrain, exoplanet physical properties such as: wind speeds, magnetic field strengths, rotation rates, or obliquities.  As exoplanet missions identify more bright targets for atmospheric characterization, we will be able to apply these techniques to planets beyond hot Jupiters, in our inevitable march toward identifying potentially habitable worlds

3-8-17

 Keren Sharon, U. Michigan

 The Universe, Magnified: The Power of Gravitational Lensing

Abstract: When did the Universe form its first galaxies? What do galaxies look like at the epoch when the Universe formed most of its stars? Some of the answers to those questions (and others) await a new generation of large ground and space based telescopes. In the meanwhile, strong gravitational lensing has become a useful tool to boost the power of present day telescopes, enabling detailed studies of galaxies that are otherwise either too dim or have too small of an angular size on the sky.

3-15-17 Maryam Modjaz NYU

 Stellar Forensics with the Most Powerful Explosions in the Universe

Abstract: Supernovae and Gamma-ray Bursts 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. 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, such as the Large Synoptic Survey Telescope - will shed new light on the diverse deaths of stars.

09-28-2016

Avery Broderick (Waterloo/Perimeter Institute)

The Current and Future View from the Edge

10-05-2016

Tom Greene (NASA Ames)

Characterizing Exoplanets with JWST

10-12-2016 Ciska Kemper (ASIAA)

The production of dust by evolved stars in the Magellanic Clouds and other galaxies

10-19-2016 George Becker (UCR)

The Intergalactic Medium Near Reionization

10-26-2016 Quinn Konopacky (UCSD)

Constraining Planet Formation with Directly Imaged Exoplanets

11-02-2016 Ryan Foley (UCSC)

Continuing the Legacy of Supernova Cosmology

11-09-2016 Rachel Somerville (Rutgers)

The Connection between Quenching and Galaxy Structure

11-16-2016 Chuck Steidel (Caltech)

Reconciling the Stellar and Nebular Spectra of High Redshift Galaxies

11-23-2016 NO COLLOQUIUM Thanksgiving Day Observance
11-30-2016 Sarah Ballard (MIT)

The Grand Planetary Ensemble

 

 

Date Speaker Title (mouseover for abstract)
03-30-2016

Bryan Gaensler                           (Dunlap Institute)

Radio Polarimetry and Cosmic Magnetism

04-06-2016

Joel Primack (UCSC)

New Insights on Galaxy Formation from Observations and Simulations

04-13-2016

Kevin Schawinski (ETH)

The galaxy-black hole connection

04-20-2016 Pascal Oesch (Yale)

Galaxy Build-up at Cosmic Dawn: Hubble's Lasting Legacy

04-27-2016 Laura Lopez (OSU)

Observational Assessment of Stellar Feedback in Star-Forming Regions

05-04-2016

Nitya Kallivayalil                          (U. of Virginia)

Probing the Dark Halo of the Milky Way

05-11-2016 Scott Gaudi (OSU)

Gravitational Microlensing Surveys for Exoplanets: A Watershed

05-18-2016 Jay Strader (MSU)

Black Holes in Globular Clusters

05-25-2016

Rennan Barkana (TAU)

Novel Measurements of Starlight from Cosmic Dawn to the Present

06-01-2016 Daryl Haggard (McGill)

Interpreting Sgr A*'s Most Luminous X-ray Flares

 

 

 
Date Speaker Title (mouseover for abstract)
01-06-2016

 No Colloquium

No Colloquium (AAS)

01-13-2016

 Dan Stark (U of Arizona)

Galaxies in the Reionization Era

01-20-2016

 Alyssa Goodman              (Harvard)

The Intricate Role of Cold Gas and Dust in Galaxy Evolution at Early Cosmic Epochs

01-27-2016  Mark Swain (JPL)

Exoplanet Transit Spectroscopy Surveys:Present and Future

02-03-2016  Avi Shporer (JPL)

Science with orbital phase curves in the space age

02-10-2016  Alice Shapley (UCLA)

The MOSFIRE Deep Evolution Field (MOSDEF) Survey: Insights into the Evolving Physical                                                    Conditions in Star-forming Regions at High Redshift

02-17-2016  No Colloquium

Cancelled

02-24-2016  Sarah Tuttle (UT Austin)

Alt-Instrumentation – From Ground to Space

03-02-2016

 David Spergel (Princeton)

From '~' to precision science: Cosmology from 1995 to 2025

03-09-2016  Kevin Bundy (IPMU)

Galaxy Death and the Role of 'Red Geysers'

Date Speaker Title (mouseover for abstract)
10-28-2015

 Paul Schechter (MIT)

The measurement of stellar masses in <z> = 0.5 galaxies using the micro-lensing of quasars

11-04-2015

 Alycia Weinberger (Carnegie DTM)

Tracing the Formation of Planetary Systems

11-11-2015

 NO COLLOQUIUM

Veteran's Day
11-18-2015

 Courtney Dressing (Harvard/CalTech)

The Frequency and Composition of Small Exoplanets

11-25-2015

 NO COLLOQUIUM

Thanksgiving Day Observance
12-02-2015

 Karin Oberg (Harvard)

The Chemistry of Planet Formation


Date Speaker Title (mouseover for abstract)
04-01-2015  Ji Wang (Yale)

Planet Formation Under Different Environments

04-08-2015  Gwen Rudie (Carnegie/Princeton)

Observing the Baryon Cycle: The Circumgalactic and Interstellar Medium of Galaxies at 2<z<3

04-15-2015  Ann-Marie Madigan (UC Berkeley)

From the Solar System to the Galactic center: unstable disks and infalling clouds

04-22-2015  Nir Shaviv (IAS Princeton/ Hebrew University of Jerusalem)

Cosmic rays and the structure of the Milky Way’s Disk: From the Pamela Anomaly to Paleoclimatology

04-29-2015  Dimitri Mawet (Caltech)

The future of exoplanet imaging and spectroscopy at Keck

05-06-2015  Eric Ford (Penn State)

Characterizing the Distribution of Planetary Architectures with Kepler: The Formation of Systems with Tightly-packed Inner Planets (STIPs)

05-13-2015  Dimitrios Psaltis (U. of Arizona)

Testing General Relativity with the Event Horizon Telescope

05-20-2015  Daniel Stern (JPL)

Surprising New Insights into Quasars from the WISE Satellite

05-27-2015  Drew Newman (Carnegie/Princeton)

Observing the Assembly of Massive Galaxies

06-03-2015  Fabienne Bastien (Penn State)

Convection in Cool Stars, as Revealed through Stellar Brightness Variations