MN Institute for Astrophysics Colloquium

semester, 2019

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

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

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