ASD Colloquium Series - Fall 2021

ASD Colloquium Series - Fall 2021

The Astrophysics Science Division colloquia occur virtuallly on Tuesdays at 3:45 pm. Schedules from past colloquium seasons are available.

Contact: Knicole Colon


Sep 14 Virtual Colloquium
Daniel Castro (Center for Astrophysics - Harvard & Smithsonian) - Particle Acceleration in the Remnants of Supernova Explosions
Sep 21 No Colloquium
Sep 28 No Colloquium


Oct 5 Virtual Colloquium
Brandon Hensley (Princeton University) - Rethinking the Nature of Interstellar Dust
Oct 12 Virtual Colloquium
Oct 19 Virtual Colloquium
Tim Hamilton (Shawnee State University) - Using an Unusual Gravitational Lens to Measure Dark Matter Clumping
Oct 26 Virtual Colloquium
Andreea Petric (STScI) - Obscured Quasars and the Need for Optical to NIR, Massively Multiplexed, Spectroscopic Facilities


Nov 02 Virtual Colloquium
Eric Bellm (University of Washington) - Preparing for Rubin Observatory Time-Domain Science with the Zwicky Transient Facility
Nov 09 Virtual Colloquium
Melinda Soares-Furtado (University of Wisconsin-Madison) - Searching for Planet Ingestion Events in Open Clusters
Nov 16 Virtual Colloquium
Nov 23 No Colloquium - Thanksgiving Week
Nov 30 Virtual Colloquium
May Gade Pedersen (University of California Santa Barbara)


Dec 07 Virtual Colloquium
Eli Dwek (NASA Goddard Space Flight Center)
Dec 14 Virtual Colloquium

Particle Acceleration in the Remnants of Supernova Explosions

Daniel Castro

Center for Astrophysics - Harvard & Smithsonian

Tuesday, Sep 14, 2021


Supernova remnants (SNRs) and pulsar wind nebulae (PWNe) have great impact on the energy density and evolution of the galaxies where supernovae (SNe) take place. SNR shocks interact with the surrounding medium, compressing and heating it, as well as accelerating particles to cosmic ray (CR) energies. Most of the energy carried away by CRs is available to drive outflows in the interstellar medium (ISM), and thus they are a significant factor in galactic formation and evolution. The study of SNRs allows for understanding issues of great relevance in astrophysics, like stellar collapse and particle acceleration mechanisms. Furthermore, some core-collapse SNe leave behind rapidly spinning neutron stars (pulsars) as compact remnants of the progenitor systems, many of which create relativistic particle nebualae. PWNe studies provide us with information on particle acceleration mechanisms at relativistic shocks, on the evolution of the pulsar spin down and, at later phases, on the ambient interstellar gas. While the basic understanding of SNRs and PWNe has been developed, we still lack detailed knowledge about the characteristics of the relativistic particle populations in these systems, and particularly how efficient are SNR shocks at accelerating CRs. Moreover, the evolution of SNRs and PWNe under different conditions and how that is reflected in their high-energy γ-ray emission is yet to be well determined. To address these outstanding questions, we have undertaken an ambitious program of observational studies of SNRs and PWNe in the gamma-ray and X-ray bands, in combination with multi-wavelength observations and modeling. I will discuss our latest results in this presentation.

Rethinking the Nature of Interstellar Dust

Brandon Hensley

Princeton University

Tuesday, Oct 05, 2021


Observations of the highly polarized emission from Galactic dust have challenged our basic assumptions about the makeup of interstellar grains. In the first part of this talk, I will introduce a new model of interstellar dust that, unlike most dust models of the last four decades, posits that the silicate and carbonaceous materials largely reside on the same grains, an idealized mixture we term "astrodust." I will demonstrate the compatibility of the astrodust-based model with existing observations from the UV to the microwave and highlight how it can be tested with future data such as from CCAT-Prime, the Simons Observatory, and PIXIE. In the second part of the talk, I will discuss new insights into the relationships between Galactic dust, gas, and magnetic fields, in particular how HI emission can be used to predict dust extinction, emission, and polarization in 3D. I will emphasize throughout the importance of, and implications for, Cosmic Microwave Background polarization experiments, which produce exquisite maps of dust polarization in pursuit of inflationary B-modes.

Using an Unusual Gravitational Lens to Measure Dark Matter Clumping

Tim Hamilton

Shawnee State University

Tuesday, Oct 19, 2021


For a century, gravitational lensing has been used as a tool in astronomy, from testing General Relativity to probing the structure of quasars. In this talk, I'll describe an unusual gravitational lens discovered in a galaxy cluster. The source galaxy straddles the cluster's "caustic curve," which folds its image and greatly increases the magnification. Because we do not have any other confirmed multiple images in this cluster, we cannot do a reconstruction of the entire lensing region. So we instead reconstruct the source using a local method, which I will describe. With our reconstruction, we are able to constrain the clumping of dark matter, finding that it is smooth down to a scale of 6 kpc across the source galaxy. This represents an improvement in measurements over non-folded gravitational lenses.

Obscured Quasars and the Need for Optical to NIR, Massively Multiplexed, Spectroscopic Facilities

Andreea Petric


Tuesday, Oct 26, 2021


Most bulge-dominated galaxies host black holes with masses that tightly correlate with the masses of their bulges. This may indicate that the black holes may regulate galaxy growth or vice versa, or that they may grow in lock-step. The quest to understand how, when, and where those black-holes formed motivates much of extragalactic astronomy. Here we focus on a population of galaxies with active black holes in their nuclei (active galactic nuclei or AGN), that are fully or partially hidden by dust and gas: the emission from the broad line region is either completely or partially obscured with a visual extinction of 1 or above. This limit, though not yet precise, appears to be the point at which the populations of AGN may evolve differently. We highlight the importance of finding and studying those dusty AGN at redshifts between 1 and 3, the epoch when the universe may have gone through its most dramatic changes. Modern wide-field imaging surveys performed by Roman Space Telescope and Vera Rubin Observatory's Large Synoptic Survey will open a new window on the Universe, enabling the discovery and study of AGN hosts too faint to be studied previously. To realize the full scientific potential of these surveys, we must examine those objects using spectroscopic techniques. Specifically, we will need large multiplexed spectroscopic instruments that can perform dedicated surveys in the optical and NIR to pin down the demographics of such objects and study their reddening properties, star-formation histories, and excitation conditions. These key studies will shed light on the role of black holes in galaxy evolution during the epoch of peak growth activity.

Preparing for Rubin Observatory Time-Domain Science with the Zwicky Transient Facility

Eric Bellm

University of Washington

Tuesday, Nov 02, 2021


The Zwicky Transient Facility (ZTF) is an optical time-domain survey at Palomar Observatory currently in operation. Thanks to its 47 square degree field of view and fast readout time, ZTF images the entire visible Northern sky every two nights. Since 2018, ZTF has accumulated hundreds to thousands of epochs of each field it observes, enabling searches for rare and fast evolving transients, variable stars, and solar system objects. ZTF's imaging pipelines release near-real-time alerts for transient sources which presage those anticipated from the Vera C. Rubin Observatory. These alerts have seeded a rich public ecosystem of community alert brokers prepared for Rubin Observatory's Legacy Survey of Space and Time. I will present a technical and scientific overview of ZTF and its surveys and discuss major science highlights. I will emphasize ongoing searches for optical variability signatures from X-ray binary systems as well as prospects for discoveries with the Rubin Observatory.

Searching for Planet Ingestion Events in Open Clusters

Melinda Soares-Furtado

University of Wisconsin-Madison

Tuesday, Nov 09, 2021


The blending of stellar light in densely populated fields poses a formidable challenge in the generation of high-precision light curves. To mitigate such concerns, we produced an image subtraction pipeline that reveals a diverse population of periodic variables in open clusters NGC 6791 and NGC 6819, which were observed as part of the original Kepler mission. These data are revealing new candidate exoplanets, eclipsing binaries, rapidly rotating stars, and pulsational variables in these systems. While many cluster members are well-described by standard stellar evolutionary models, rotational and chemical outliers present a rare opportunity to investigate important divergences from this framework. Such divergences may be induced by the ingestion of substellar companions, which have been shown to impart changes to the angular momentum and chemical abundance of these cannibal hosts. The detection of planetary engulfment sites within an open cluster would offer an invaluable opportunity to probe the bulk composition of substellar companions, providing much-needed insight into the formation of these systems. In this talk, I outline our search for planetary ingestion events in open clusters using photometric and spectroscopic techniques, showcasing some of the ancillary science goals that accompany a close open cluster investigation.

Past Colloqia Schedules

2021: Spring
2020: Fall, Spring
2019: Fall, Spring
2018: Fall, Spring
2017: Fall, Spring
2016: Fall, Spring
2015: Fall, Spring
2014: Fall, Spring
2013: Fall, Spring, Summer
2012: Fall, Spring
2011: Fall, Spring
2010: Fall, Spring

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