MN Institute for Astrophysics Colloquium

semester, 2019

Friday, January 25th 2019
There will be no colloquium this week. (Faculty candidate presentation on Thursday).

Thursday, January 31st 2019
2:30 pm:
Untitled in Physics Tate 301-20
Speaker: Valerie Fox, CalTech
Subject: TBD

Friday, February 1st 2019
There will be no colloquium this week

Wednesday, February 6th 2019
1:25 pm:
Speaker: Richard Anthony D'Souza, University of Michigan
Subject: Unravelling the Andromeda Galaxy's most important merger

The Andromeda Galaxy (M31), our nearest large galactic neighbour, offers a unique opportunity to test how mergers affect galaxy properties. M31's stellar halo caused by the tidal disruption of satellite galaxies is the best tracer of the galaxy's accretion history. Despite a decade of effort in mapping out M31's large stellar halo, we are unable to convert M31's stellar halo into a merger history. Here we use cosmological models of galaxy formation to show that M31's massive and metal-rich stellar halo containing intermediate age stars implies that it merged with a large (M* ~ 2.5 x 10^10 M_sun) galaxy ~2 Gyr ago. The simulated properties of the merger debris help to interpret a broader set of observations of M31's stellar halo and satellites than previously considered: its compact and metal-rich satellite M32 is the tidally-stripped core of the disrupted galaxy, M31's rotating and flattened inner stellar halo contains most of the merger debris, and the giant stellar stream is likely to have been thrown out during the merger. This accreted galaxy was the third largest member of the Local Group. This merger may explain the global burst of star formation ~2 Gyr ago in the disk of M31 in which ~1/5 of its stars were formed. Moreover, M31's disk and bulge were already in place before its most important merger, suggesting that mergers of this magnitude do not dramatically affect galaxy structure.

Faculty Host: Evan Skillman

Friday, February 8th 2019
There will be no colloquium this week. (Faculty candidate presentation on Thursday).

Friday, February 15th 2019
There will be no colloquium this week. (Faculty candidate presentation on Thursday).

Friday, February 22nd 2019
2:30 pm:
Speaker: Anna Williams, Macalester University
Subject: Exploring the coevolution of magnetic fields and galaxies in different environments

Galaxies are permeated with magnetic fields at all scale lengths--from protostellar disks to spiral arms. But how galaxies first acquired magnetic fields, and, in turn grow and sustain large-scale magnetic structures is not well understood. One way to unravel this problem is by observing magnetic fields in a variety of galaxy environments. Luckily, new and upgraded radio telescopes are providing a new window to the polarization universe, and greatly enhancing our ability to probe astrophysical magnetic fields. I will present the results of three observational studies focused on the coevolution of magnetic fields and galaxies in different environments: (1) a nearby spiral galaxy, NGC 6946, (2) a loose galaxy group, NGC 2563, and (3) distant disk-like galaxies at z~0.5.

Friday, March 1st 2019
2:30 pm:
Speaker:  Dale Gary, New Jersey Institute of Technology
Subject: A Breakthrough View of Solar Flares from Radio Imaging Spectroscopy

Solar flares are the result of explosive release of stored magnetic energy in the Sun's corona. The detailed physical processes that underlie the rapid conversion of such energy to other forms, particularly the acceleration of electrons and ions to relativistic energies, remain mysterious, partly because direct measurements of the coronal magnetic field and the spatial and energy distribution of the particles have been difficult or impossible. One emission mechanism that is sensitive to the coronal magnetic field in the flaring region is radio emission, but until recently the instrument capabilities needed to exploit that sensitivity have not been available. However, this has changed with the completion of a new, solar dedicated radio interferometer array, the Expanded Owens Valley Solar Array (EOVSA), that has the required combination of spatial, spectral, and temporal resolution to make these breakthrough measurements. In this talk, I illustrate the new capabilities and their implications using observations of a showcase solar flare that occurred on 2017 September 10. This event is a textbook example of the "standard solar flare model" eruptive event, with a clearly visible reconnecting current sheet connecting an erupting flux rope with a growing arcade of newly formed "post-flare" loops. The comparison of microwave diagnostics of high-energy electrons with those from hard X-rays seen by the RHESSI spacecraft show that, while they are fully consistent, the microwaves reveal that the coronal volume containing high-energy particles is much larger and more widespread than would have been deduced from hard X-rays alone. Even more important, however, are the quantitative measurements of the spatially and temporally resolved magnetic field strength, which, if our interpretation is correct, directly reveal the conversion of magnetic energy over a large volume into high-energy charged particles.and turbulent plasma. The new observations present both a challenge and an opportunity for further theoretical understanding of the processes occurring in solar flares.

Faculty Host: John Wygant

Friday, March 8th 2019
2:30 pm:
Speaker: Marc Pinsonneault, The Ohio State University
Subject: 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.

Faculty Host: Evan Skillman

Friday, March 15th 2019
2:30 pm:
Speaker: Emil Rivera-Thorsen, University of Oslo
Subject: A Series Of Fortunate Events: The discovery of a unique, lensed Lyman-continuum leaking galaxy at z=2

"The brightest known gravitationally lensed galaxy is arguably also among the most interesting.
Discovered as part of the Planck Foreground Project and included in the Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA), it is a young, dusty, and very bright starburst at z=2.4. Due to a fortunate alignment of lensing cluster galaxies, it is found in no less than 12 image-plane copies along four major arc segments. A theoretically predicted, but not previously observed, triple-peaked Lyman-alpha emission line profile has revealed that a narrow, empty channel through the neutral medium is aligned with the line-of-sight to Earth, giving us a so far unique, direct view at the young OB stars in the galaxy's most luminous cluster complex. The clear line of sight to the young star clusters has allowed the brightest and best resolved detection of escaping, ionizing Lyman-continuum radiation yet observed at redshifts beyond 0.5, even rivaling some of the local-Universe detections in brightness. The lines of sight traced by the multiply imaged ionizing radiation also provides a direct probe into intergalactic HI on transverse scales at least an order of magnitude smaller than so far probed by close quasar pairs.
In this talk, I will present our findings so far, discuss some of the implications for our knowledge of the radiative transfer and escape of Lyman-alpha and ionizing photons, and show some further puzzling aspects of the galaxy and how they may point us to further research."

Friday, March 22nd 2019
2:30 pm:
There will be be no colloquium this week due to spring break

Friday, March 29th 2019
2:30 pm:
Speaker: Vuk Mandic
Subject: Observing the Universe with Gravitational Waves

Since 2015 the LIGO and Virgo gravitational-wave detectors have observed 10 collisions of black hole pairs and one merger of two neutron stars. In addition to enabling unprecedented tests of the theory of General Relativity, these discoveries introduced a new way of observing the universe. While the traditional telescopes observe the universe using electromagnetic waves (i.e. light), LIGO and Virgo do so by measuring tiny ripples in space-time produced by accelerating massive objects moving at speeds near the speed of light. Using the two types of observations together, as was done in the case of the binary neutron star merger, opens a variety of new possible studies ranging from formation mechanisms for the heaviest elements in the periodic table to novel measurements of the expansion rate of the universe. I will describe how these discoveries were made, and I will discuss their implications as well as the exciting future prospects in the field of gravitational waves.

Faculty Host: Evan Skillman

Friday, April 5th 2019
2:30 pm:
Speaker: Rob Kennicutt, Texas A&M University
Subject: 2020 Decadal Survey

Thursday, April 11th 2019
7:00 pm:
Speaker: Eliot Quataert, UC Berkeley
Subject: Cosmic Gold: Neutron Star Mergers, Gravitational Waves, and the Origin of the Elements

Scientists have recently developed a new way to `see' the universe, using gravitational waves predicted by Einstein nearly a century ago. These waves can teach us about some of the most exotic objects known, including dead stars known as black holes and neutron stars. Remarkably, they have also helped solve a longstanding puzzle about where in the Universe some of the elements we know and love here on Earth are produced, including uranium, platinum, and even gold. Eliot Quataert will describe the exciting and remarkable new results coming out of our first steps into this new era.

Friday, April 12th 2019
2:30 pm:
Speaker: Eliot Quataert, UC Berkeley
Subject: What Happens When a Massive Star Fails (Sort of) to Explode?

There are observational and theoretical reasons to suspect that up to
10s of percent of massive stars that undergo core-collapse at the end
of their lives fail to explode in a canonical energetic supernova
explosion. In this talk I will describe what transpires in such
nominally failed supernovae and its importance for understanding the
masses and spins of black holes (e.g., detected by LIGO). I will also
describe how 'failed' supernovae may manifest themselves
observationally in time-domain surveys.

Friday, April 19th 2019
2:30 pm:
Speaker: Peter Garnovich, University of Notre Dame
Subject: The Spin Cycle: Rapidly Rotating Magnetized White Dwarfs in Close Binaries

The most dynamic types of cataclysmic variable stars are 0nes where the accreting white dwarf is magnetized and its rotation is not locked the binary orbital period. These are often called intermediate polars (IPs) or asynchronous polars (APs) depending on the white dwarfs field strength. I will discuss two of the most exciting and nearby polars and their recent activities. FO Aquarii has been dubbed the "King of the IPs" because of the large amplitude variations produced by the 20 minute spin period of its white dwarf. Recently it has gone into a series of low states that reveal a variety of periodicities. The low states may be related to the long-term evolution of the spin rate of its white dwarf. The second star, AR Scorpii, is a one-of-a-kind white dwarf pulsar. This unique system was identified only four years ago, and the source of its light variations are still in dispute.

Faculty Host: Patrick Kelly

Friday, April 26th 2019
2:30 pm:
Speaker: There will be no colloquium today
Faculty Host: Charles E. Woodward

Friday, May 3rd 2019
2:30 pm:
There will be no colloquium this week

Friday, May 10th 2019
To be announced.

Friday, May 17th 2019
There will be no colloquium this week

Friday, September 6th 2019
No colloquium today - 1st week of school

Friday, September 13th 2019
2:30 pm:
There is no MIFA colloquium this week

Friday, September 20th 2019
2:30 pm:
Speaker: Colby Haggerty, University of Chicago
Subject: Hybrid Plasma Simulations of the Acceleration and Back-Reaction of Cosmic Rays on Astrophysical Shocks

High energy low density cosmic rays (CR) are a ubiquitous feature of astrophysical systems and make up a sizable fraction of the energy budget in the interstellar medium. The shock waves associated with supernova remnants are believed to be the principal accelerators of galactic CRs, through a collisionless acceleration process referred to as diffusive shock acceleration (DSA). We present simulations of collisionless plasma shocks performed with the first hybrid code to include relativistic ion dynamics (dHybridR). In these simulations, we show evidence of modifications to the fluid shock jump conditions caused by the CR pressure. The rapid transition to CR modified shocks occurs soon after the onset of the shock. CR modified shocks are expected to have a harder power law slope, however we find a significantly steeper spectral index of nearly p^-5 for early times and hardening as the simulation progresses to approximately p^-4.3 as the compression ratio saturates. To understand these simulation results we present a non-linear theory of DSA which includes considerations for both the magnetic field and the CR modified jump conditions. The steep spectra is shown to be caused by the enhanced magnetic field downstream of the shock. This magnetic field was originally generated from the CR streaming instability upstream of the shock, and thus the compressed/enhanced magnetic field acts to regulate the energetic run away problem that CR modified shocks presents for non-linear DSA.

Faculty Host: Thomas W. Jones

Friday, September 27th 2019
2:30 pm:
Speaker: Patrick Wilcox, University of Minnesota
Subject: Observation of Pulsar Wind Nebula DA 495 at Very High Energies Using VERITAS

The constant flux of cosmic rays that bombard Earth from within our own galaxy are understood to come from both shell-type supernova remnants and pulsar wind nebulae (PWNe). Since we cannot directly trace the charged cosmic rays to their source, we must rely on multiwavelength study of these objects to help us understand the conditions within. The generation of very energetic particles also leads to very energetic emission processes, and as such, gamma-ray emission is key to understanding and identifying the types of cosmic rays that are generated by these extreme astrophysical laboratories; studies which cannot be done at lower-energies alone. DA 495 was recently identified as a TeV source and was observed for about 70 hours with the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a TeV gamma-ray observatory located in Southern Arizona. Radio observations of DA 495 indicate that it is nearby (~1kpc) "Crab-like" PWN with unusually strong (~mG) magnetic fields throughout the nebula. Even though PWN are often TeV gamma-ray sources, the strong magnetic fields made it an unlikely candidate for detection at TeV energies since it would be expected that the highest energy particles would quickly “burn off” in the strong fields and be unable to generate TeV gamma rays through inverse-Compton scattering. For this study, TeV results are combined with radio and X-ray spectral information to allow for detailed astrophysical modeling of the region that allows for both leptonic and hadronic emission scenarios to be evaluated. Hadronic scenarios instill doubt on the pure PWN interpretation and favor a previously undetected shell-type remnant being present.

In this colloquium, I will discuss how TeV gamma rays are detected from the ground using VERITAS and how we use can use the results to model the particle populations at the source of cosmic ray generating objects within our galaxy. Further, I will put DA 495 into context with its over 50 year observational history, and show how the mystery of the strong magnetic fields deepens with the observations made by VERITAS and how they could be solved using future gamma-ray observatories.

Faculty Host: Lucy Fortson

Thursday, October 3rd 2019
6:30 pm:
Public Lecture in Physics Tate B-50
Speaker: Claudia Scarlata, University of Minnesota
Subject: The end of the Dark Age

Leonardo created artworks that showed knowledge of aerodynamics, optics, physics, astronomy, and mechanical drawings for futuristic technology, including the first premodern telescope. 500 years later the NASA is launching the new James Webb Space Telescope, which will be a huge leap forward for space telescopes.

Prof. Claudia Scarlata will introduce the upcoming observatory peering toward the end of the Cosmic Dark Ages.
After a glowing beginning, the Universe went through a phase of darkness that lasted for millions of years. This phase, known as the Dark Ages, ended with the formation of the first stars and galaxies. These objects were very different from the stars and galaxies we see in the Universe today, after its 13.8 billion years of life. Come and learn about the impact that these objects had on the Universe, and what we have learned and will learn in the near future from upcoming facilities such as the new James Web Space Telescope.

Afterwards, if the weather allows, participants will be invited for telescope viewing on the patio.

Friday, October 4th 2019
2:30 pm:
Speaker: Robert Benjamin, University of Wisconsin-Whitewater
Subject: Discovery of Optical Emission Lines from the Inner Milky Way: Evidence for LI(N)ER-like gas

The first detection optical emission lines from the inner regions of a galaxy was in 1909 by E.A. Fath (a professor at Carleton College from the 1920s to 1950). But it was not until 1936 that a spectroscopic campaign at Mt Wilson and Lick Observatories began to turn up evidence for diffuse ionized gas in the inner parts of some galaxies using the [O II] 3727 line. Measurements of the [N II]/H-alpha ratio by M. Burbidge and G. Burbidge in the early 1960s showed that this gas was characterized by unusually high line ratios that could not be explained as dilute HII regions. Many of these systems were dubbed LINERS (low ionization nuclear emission line regions) in 1980; the ionization levels were neither consistent with a star-forming radiation field nor the radiation field of an Active Galactic Nuclus (AGN). The nearest LINER sytem known was M31. I will describe the Wisconsin H-alpha Mapper discovery of a reservoir of diffuse ionized gas in the inner Milky Way between Galactocentric radius R_G=0.25 to 1.5 kpc and the models developed to interpret these observations. We estimate a total mass ionized mass of 12±4 x 10^6 solar masses, finding that the atomic gas is at least 50% ionized. The [N II]/H-alpha ratio, which increases from 0.3 to 2.5 with Galactocentric radius is unlike anywhere else in the Milky Way, but typical of LINER systems. The ionizing flux required to maintain this gas is 10 times the estimated flux of Lyman continuum photons in the solar neighborhood (if ionized from the outside) or 5-10% of all Lyman continuum photons from the CMZ (if ionized from the inside). I will describe the implication of these results for understanding the nature of LINER systems.

Faculty Host: Thomas W. Jones

Friday, October 11th 2019
2:30 pm:
Speaker: Steve Rodney, University of South Carolina
Subject: Supernova Cosmology, Present and Future

The use of Type Ia supernovae (SNIa) as standardizable candles has been one of the pillars of modern cosmology for several decades. The luminosity distance-redshift relation for SNIa–which led to the discovery of cosmic acceleration at the turn of the millenium–is now being used to measure the dark energy equation of state. SNIa are also a player in the roiling crisis over precision measurements of the Hubble-LeMaitre constant, H0. Measurements of H0 from the early universe can not be reconciled with nearby constraints. This may be a chink in the foundations of the Lambda-CDM "concordance cosmology" model, and possibly hints at new physics in the early universe. We will review some of the recent developments in SNIa cosmology, incorporating wide-area surveys such as Pan-STARRS and deep infrared campaigns using the Hubble Space Telescope. We will look ahead to the paradigm-shifting surveys on the near horizon, including LSST and WFIRST, which will increase our samples by several orders of magnitude. Finally, we will touch on some of the new tools for SN cosmology that will soon enter the fray, including time delay cosmography using gravitationally lensed SNe.

Faculty Host: Patrick Kelly

Friday, October 18th 2019
2:30 pm:
No Colloquium. Physics Colloquium speaker Thurs: Charles M. Marcus, Niels Bohr Institute

Friday, October 25th 2019
2:30 pm:
Speaker: Pepi Fabbiano, Smithsonian Astrophysical Observatory
Subject: 20 Years of X-ray Observations with Chandra

Launched on the 23rd of July 1999 as the NASA Great Observatory for the X-ray band (~0.1-8 keV), Chandra has opened up a new age of discovery, culminating a 40-year development in the observations of the sky in the X-ray band. What makes the Chandra telescope unique is its sub-arc-second angular resolution. Combined with the moderate spectral resolution and timing of its most used focal plane imager, this imaging capability has opened a discovery space encompassing the whole of astronomy, from stars to high redshift quasars and large structures in the universe. Chandra observations provide a unique insight into the high-energy, hot universe, complementing the views from other wavebands.

After giving a general overview of Chandra’s results, in this talk I will discuss some new developments in the study of AGN-galaxy interaction that exploit to the fullest the imaging capability of Chandra. I will also briefly introduce the new release of the Chandra Source Catalog (CSC2.0) and conclude with a look to future developments.

Friday, November 1st 2019
2:30 pm:
No Colloquium. Physics Colloquium speaker Thurs: Rudolph Tromp, Leiden University

Friday, November 8th 2019
2:30 pm:
Speaker: Randolf Klein, SOFIA Science Center
Subject: SOFIA: Recent Results and Upcoming Opportunities in Airborne Astronomy

SOFIA, the only facility currently allowing access to the full infrared wavelength range, is in its 7th observing cycle and can look back on an impressive list of discoveries and research conducted with this airborne observatory. I will present recent results and some classics to showcase SOFIA’s capabilities and scientific productivity. The science topics range from planetary science and circum-stellar disks over astro-chemistry and processes in the interstellar medium to star formation in our and other galaxies and active galactic nuclei.

The capabilities of SOFIA are growing over time as new instruments are commissioned. The newest instrument, the far infrared camera and polarimeter HAWC+, has already published significant findings on magnetic fields in a wide range of objects.

Also the SOFIA data archive is growing with regular and science demonstration observations and SOFIA Legacy Programs. Access will become easier through IRSA and the SOFIA Science Center is improving support for archive research.
The presentation will wrap up with an overview over the capabilities of SOFIA making these discoveries possible pointing out that these capabilities are available to the community through the annual proposal calls.

Friday, November 15th 2019
2:30 pm:
Speaker: Yan Song, University of Minnesota
Subject: How Do Cosmic Plasmas Create High-Energy Particle Accelerators in Space?

Interaction between magnetized cosmic plasmas and electromagnetic fields and waves can lead to various collective behaviors, including particle acceleration, explosive energy conversion and structure formation. The acceleration of charged particles to high energy often occurs in space, solar and cosmic plasmas. Among other mechanisms for particle acceleration, parallel electrostatic fields related to charge separation are the most powerful means to directly and efficiently accelerate particles.
We have proposed a dynamical theory of the generation of parallel electric fields (Song and Lysak, 2006), and pointed out that the E||-generation is described by the displacement current term in the parallel component of Ampere’s law. The generation of these fields is favored by a low plasma density, enhanced magnetic stress and strong Alfvenic interaction. In this talk, I will review advances in developing the theory of the E||-generation. I will show that, the displacement current term provides a new mechanism of the generation of powerful high energy electrostatic fields, i.e., charge separation. In fact, the discovery of the role of the displacement current in particle acceleration has opened the door to understand the mechanism of high energy particle acceleration in cosmic plasmas.
We also realize that once the parallel electric fields are produced, they will disappear right away due to the high mobility of charged particles, unless the electric fields can be continuously generated and sustained for a fairly long time. Thus, the generation of a long-lasting parallel electrostatic field is needed for the acceleration of auroral particles to high energy. We have developed a preliminary theory to explain the formation of Alfvenic Double Layers (DL) which are electrostatic plasma structures, and serve as high-energy particle accelerators in auroral acceleration regions. I will describe the properties and characteristics of such high energy particle accelerators.
The Alfvenic DL consists of localized electrostatic fields related to charge separation embedded in low density cavities surrounded by enhanced reactive stresses. The enhanced magnetic shear carrying free energy serves as the local dynamo. The generated electrostatic fields will quickly deepen previously produced weak low density cavities, which can further enhance the generation of stronger electrostatic fields, causing auroral particle acceleration. The Poynting flux carried by Alfven waves is required to continuously supply energy to the auroral acceleration region, supporting Alfvenic DLs, leading to strong long-lasting electrostatic fields. The Alfvenic electrostatic plasma structure acts as powerful high energy particle accelerators for the formation of quasi-static and Alfvenic auroras, covering small to large scales.
In addition, I will briefly discuss some problems related to magnetic reconnection concept, which has been considered as one of the mechanism to cause charge particle acceleration to high energy, and to suggest using fundamental physical laws and principles to study “reconnection” related physical processes.

Friday, November 22nd 2019
2:30 pm:
No Colloquium. Physics Colloquium speaker Thurs: Dimitar Sasselov, Harvard University

Friday, November 29th 2019
2:30 pm:
No Colloquium this week due to Thanksgiving

Friday, December 6th 2019
2:30 pm:
Speaker: Larry Rudnick, University of Minnesota
Subject: The Dance of Two Plasmas: the future of radio galaxy/intracluster medium interactions

It is tempting to look back at 45 years of studying radio galaxies in clusters, but I will limit the nostalgia. What lies ahead, given the explosion of information from new radio surveys by SKA precursors, is much more exciting. The intracluster medium (ICM), carrying most of the cluster baryons, is visible in X-rays and through the Sunyaev-Zeldovich effect. Isolated glimpses of the dynamics of the ICM are visible in the form of contact discontinuities, weak shocks, and other temperature variations. But the tangled and filamentary structure of the frozen-in magnetic fields, and the turbulence on a variety of scales, are elusive. Radio galaxies, with their relativistic plasmas buffeted by the ICM, provide a way of probing those interactions, and I will describe a new schema for exploiting this tool. This new schema requires more sophisticated physical "cartoons", and numerical simulations, as well, so that we can understand the underlying physics. I will show the still-embargoed results from some of the new telescopes, to give a flavor of what's to come.

Friday, December 13th 2019
2:30 pm:
No colloquium this week

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