List of Past Astronomy Colloquia : 01-Sep-2020 to 31-Dec-2020

Date:   Wednesday 09-Sep-2020
Speaker:   Dr. Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics)
Title:  "Testing the Massive Black Hole Paradigm and GR with Infrared Interferometry"

The GRAVITY near-IR beam combiner allows very sensitive (K~19), phase-referenced milliarcsec K-band imaging and polarimetry, 20-100 micro-arcsecond broad-band astrometry, and micro-arcsecond differential spectro-astrometry with the combined 4 UT telescopes of the Very Large Telescope of the European Southern Observatory. GRAVITY is a game changer in studying the massive black hole in the Galactic Center, and in active galactic nuclei. I will summarize the highlights of the last two years and report new results in using GRAVITY for tests of General Relativity near a massive black hole.

Date:   Wednesday 16-Sep-2020
Speaker:   Dr. Mark Reid (Harvard-Smithsonian Center for Astrophysics)
Title:  "The Spiral Structure of the Milky Way"

The Bar and Spiral Structure Legacy (BeSSeL) Survey uses the Very Long Baseline Array to observe masers associated with massive young stars and provide trigonometric parallaxes across the Milky Way. The Survey is named for Friedrich Bessel, who in 1838 measured the first stellar parallax. I will show Bessel's original data and comment on its accuracy.

There are now about 200 maser parallaxes and these are accurately tracing spiral structure of the Milky Way, the distance to its center, its rotation curve, and the location of the Sun. We have developed a Bayesian approach to leverage these results to estimate distances to large numbers of sources from surveys based only on Galactic coordinates and velocities. Using this program we can make a realistic visualization of the Milky Way.

Additionally, our results make strong predictions for the distance of the Hulse-Taylor binary pulsar, assuming its orbital decay from gravitational radiation follows General Relativity, and for the proper motion of Sgr A*, if indeed it is a supermassive black hole.

Date:   Wednesday 23-Sep-2020
Speaker:   Prof. Ruth Murray-Clay (UCSC)
Title:  "Origins of Structure in Inner Planetary Systems"

Why do many stars host close-in chains of super-Earths? Why are eccentric gas giants found in some inner planetary systems? What determines which of these outcomes will occur around a particular star? I will present a possible framework for answering these questions that appeals to the “flow isolation mass,” a limiting mass for pebble accretion. Flow isolation occurs when small particles, coupled to the gas, are pulled around a growing planet along gas streamlines. Its consequences are highly dependent on the particle sizes present in the planet’s natal disk. In inner planetary systems, if fragmentation limits “pebble” sizes to Stokes numbers approaching one, then flow isolation yields limiting masses similar in scale to the thermal mass, comparable to the distinct “pebble isolation mass.” At larger orbital separations or if Stokes numbers are smaller, these processes diverge. I will present work showing that flow isolation can yield systems of super-Earths comparable to those observed. I will then discuss how a giant impact phase for giant planets results if multiple gas giants are instead produced in inner planetary systems, yielding a population of gas giants that well matches the data and in particular explaining why higher-mass giants are more likely to have high eccentricities. Finally, I will connect these two ideas to demonstrate that for a reasonable distribution of disk parameters, we can reproduce the relative frequency of super-Earth and gas giant systems.

Date:   Wednesday 30-Sep-2020
Speaker:   Prof. Raffaella Margutti (Northwestern University)
Title:  "The Renaissance of Astrophysics: a landscape of opportunities in the era of Time Domain Multi-Messenger investigations"

Astronomical transients are signposts of catastrophic events in space, including the most extreme stellar deaths, stellar tidal disruptions by supermassive black holes, and mergers of compact objects. Thanks to new and improved observational facilities we can now sample the night sky with unprecedented temporal cadence and sensitivity across the electromagnetic spectrum and beyond. This effort has led to the discovery of new types of astronomical transients, revolutionized our understanding of phenomena that we thought we already knew, and enabled the first insights into the physics of neutron star mergers with gravitational waves and light. In this talk I will review some very recent developments that resulted from our capability to acquire a truly panchromatic view of transient astrophysical phenomena. I will focus on two key areas of ignorance in the field: (i) What are the progenitors of stellar explosions and what happens in the last centuries before death? (ii) What is the nature of the compact objects produced by these explosions and what happens when compact objects merge? The unique combination of Discovery Power (guaranteed by planned transient surveys like LSST, combined with efforts in the realm of artificial intelligence) and Understanding (enabled by multi-messenger observations) is what positions time-domain astrophysics for major advances in the near future.

Date:   Wednesday 07-Oct-2020
Speaker:   cancelled
Title:  "cancelled"

Date:   Wednesday 14-Oct-2020
Speaker:   Prof. Nahum Arav (Virginia Tech)
Title:  "The Contribution of Quasar Absorption Outflows to AGN Feedback"

Determining the distance of quasar absorption outflows from the central source and their kinetic luminosity are crucial for understanding their contribution to AGN feedback. Here we summarize the results for a sample of nine luminous quasars that were observed with the Hubble Space Telescope. We find that the outflows in more than half of the objects are powerful enough to be the main agents for AGN feedback. The sample is representative of the quasar absorption outflow population as a whole and is unbiased towards specific distance ranges or kinetic luminosity value. Therefore, the analysis results can be extended to the majority of such objects, including broad absorption line quasars (BALQSO).

Date:   Wednesday 21-Oct-2020
Speaker:   Prof. Sharon Fries-Britt (University of Maryland)
Title:  "Fortifying Black Student Success in STEM"

I plan to share key themes from my own research over the years on student's experiences in the classroom, interacting in the department and science identity. I also plan to share findings from the new American Institute of Physics (AIP) TEAM Up report that help to amplify many of the issues that I have found as I have worked with my research team.

Date:   Wednesday 28-Oct-2020
Speaker:   Prof. Joanna Dunkley (Princeton University)
Title:  "The Millimeter Sky from the Atacama Cosmology Telescope"

I will show new maps of the Cosmic Microwave Background made by the Atacama Cosmology Telescope, and describe how they are advancing our knowledge of cosmology and complementing optical large-scale structure surveys. I will show how the new data let us weigh in on the LCDM cosmological model, the Hubble constant and the spatial geometry of the universe, as well as providing a sample of thousands of galaxy clusters, and revealing the distribution of gas around groups of galaxies. These new arcminute-resoution millimeter maps of the sky, and future refinements with upcoming observations, promise to teach us not only cosmology but also about Galactic science, active galactic nuclei and dusty high-redshift galaxies.

Date:   Wednesday 04-Nov-2020
Speaker:   Prof. James Lloyd (Cornell University)
Title:  "Cooling Radiation From the First Protostars - A New Cosmic Background Signal"

There are universal astrophysical processes that produce important universal cosmic backgrounds such as the cosmic microwave background, cosmic infrared background and epoch of reionization signal. As these backgrounds produce radiation that fills the Universe, they may be detected as a surface brightness signal, which fills the beam of a telescope of any size and therefore may potentially be detectable even with a small telescope. This talk will consider the possibility of detection of a new cosmic background associated with the formation of the first protostars, arguing that this background is likely to appear as emission in the far infrared (20-100 micrometers) with a unique spectral and spatial signature that may be within the resolution and sensitivity of modest instruments.

Date:   Wednesday 11-Nov-2020
Speaker:   Dr. Scott Bolton (SWRI)
Title:  "The New Jupiter as Revealed by Juno: Science meets Art"

No abstract available

Date:   Wednesday 18-Nov-2020
Speaker:   Prof. Carrie Nugent (Olin College)
Title:  "A NEATer asteroid survey: Reprocessing the NEAT dataset"

An asteroid impact is the only natural disaster we have the technology to prevent. But prevention takes time, meaning that we need to discover and track near‐Earth asteroids now. This talk will give an overview on asteroids and my asteroid discovery research with undergraduates at Olin College and Dr. Gerbs Bauer at UMD.

Currently, we are working with archived data from the Near-Earth Asteroid Tracking (NEAT) survey. NEAT operated from 1995 to 2007, discovered 41,227 minor planets, and reported observations of 258 comets. Along with Spacewatch, NEAT pioneered techniques used by major asteroid surveys today. To date, only three projects (Linear, Spacewatch, and the Mt. Lemmon Survey) have discovered more minor planets, according to the Minor Planet Center.

However, NEAT operated within the technological constraints of its time, and in the intervening years there have been significant advancements in computer hardware and data analysis tools. We apply these improvements to extract new detections from the dataset. When complete, we estimate we will double the number of detections from the NEAT data. Processing is done using the Deep Thought supercomputer at Olin College.

Date:   Wednesday 02-Dec-2020
Speaker:   Dr. Fabian Walter (Max Planck Institute for Astronomy)
Title:  "ASPECS - The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field"

I will report on some of the results emerging from the ALMA large program ASPECS ( ASPECS obtained deep imaging in the 1mm and 3mm bands of the Hubble Ultra-Deep Field (H-UDF) through frequency scans. The observations provide a full census of dust and molecular gas in the H-UDF, down to masses that are typical of main-sequence galaxies at redshifts 1-4. The resulting data products enable a great range of studies, from the characterisation of individual galaxies, capitalizing on the unique multi-wavelength dataset of the H-UDF, to CO excitation studies to constrain the gas properties of the distant galaxies. A 3D stacking analysis using precise redshifts from major VLT/MUSE initiatives on the field helped in recovering additional emission of galaxy samples that are too faint to be detected individually. Stacking in both the continuum and line (capitalizing on 100s of spectroscopic redshifts from major VLT/MUSE initiatives on the H-UDF) pushed the detection limits further. The nature of the observations (full spectral scans) provides a census of dust and molecular gas in the cosmic volume defined by the H-UDF. The resulting cosmic molecular gas density as a function of redshift shows an order of magnitude decrease from z=2 to z=0. This is markably different from independent measurements of the atomic gas phase that shows a rather flat redshift dependence. These measurements can be used to put new constraints on the gas accretion process that is needed to explain the build-up of stellar mass in galaxies through cosmic history.

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