MIT Astrophysics Colloquia - Fall 2020

(To be revised for virtual format.)
Tuesdays at 4:00 PM in the Marlar Lounge, Room 37-252
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
(unless location otherwise noted)
Refreshments are served at 3:45 PM.

Sponsored by
the Astrophysics Division of the MIT Department of Physics and
the MIT Kavli Institute for Astrophysics and Space Research.
MKI logo

Tuesday 08 September :
A New Perspective on the Assembly of Mass
Danail Obreschkow
University of Western Australia
Host: Paul Schechter

NOTE SPECIAL TIME: 10AM EDT

Abstract: Linking observable properties of galaxies to the assembly history of their host haloes is one of the great challenges in cosmology. In this colloquium, I will address this challenge from the view-point of the ``tree entropy'', an interesting mathematical way to characterize the structure merger trees. The tree entropy puts every halo merger tree on a scale between a ``minimal tree'', grown entirely by smooth accretion, and a ``maximal tree'', grown exclusively by equal-mass major mergers. I will try my best to provide an intuitive geometric way to think about this entropy. I will then discuss the tree entropies found in a LCDM cosmology and elaborate on how the properties of simulated galaxies relate to their tree entropies. The talk will end with an inspiring illustration of how the tree entropy of the Milky Way, inferred from GAIA data, can help us understand the Galaxy's morphology.

Tuesday 15 September :
How do galaxies form? New insights from the FIRE simulations
Claude-Andre Faucher-Giguere
Northwestern University
Host: Rob Simcoe



Abstract: Galaxies are remarkably diverse in their properties, ranging from irregular to disky to elliptical in morphology, and from blue to red in color. At the same time, when analyzed systematically, galaxy populations exhibit striking regularities, with clear trends with mass and redshift. How does this `regular complexity' emerge from the hot Big Bang? I will present results from the FIRE simulations which shed some light onto the processes that shape galaxies. The FIRE zoom-in simulations resolve the multiphase interstellar medium of galaxies and model several different feedback processes (including Type II/Ia supernovae, stellar winds, and radiation) while including the cosmological environment. I will highlight recent results on the formation of galactic disks, the `burstiness' of star formation, galactic winds, and the growth of supermassive black holes. Our new results indicate that transitions in the properties of each of these important phenomena can be simultaneously explained by a phase transition (virialization) in the inner circumgalactic medium, and arise from the interplay between feedback energy produced on small scales and the physics of halo gas on larger scales.

Tuesday 22 September :
Planets in a Bottle: Exploring Planetary Atmospheres in the Lab
Sarah Horst
Johns Hopkins University
Host: Natalia Guerrero



Abstract: From exoplanets, with their surprising lack of spectral features, to Titan and its characteristic haze layer, numerous planetary atmospheres may possess photochemically produced particles of `haze'. With few exceptions, we lack strong observational constraints (in situ or remote sensing) on the size, shape, density, and composition of these particles. Photochemical models, which can generally explain the observed abundances of smaller, gas phase molecules, are not well suited for investigations of much larger, solid phase particles. Laboratory investigations of haze formation in planetary atmospheres therefore play a key role in improving our understanding of the formation and composition of haze particles. I will discuss a series of experiments aimed at improving our understanding of the physical and chemical properties of planetary atmospheric hazes on Titan, Pluto, super-Earths, and mini-Neptunes.

Tuesday 29 September :
Dark matter heats up in dwarf galaxies
Justin Read
University of Surrey
Host: Tracy Slatyer



Abstract: Abstract: Pure dark matter structure formation simulations in LCDM predict that dwarf galaxies should inhabit dark matter halos that have steeply rising central density "cusps". However, observations of nearby dwarf irregulars favour instead constant density dark matter "cores". Many solutions to this cusp-core problem have been proposed, from modifications to the nature of dark matter to dark matter being kinematically "heated up" by bursty star formation. In this talk, I present the first observational evidence for dark matter being heated up at the centres of nearby dwarf galaxies due to baryonic processes. I show that the inner dark matter density of these dwarfs is anti-correlated with their star formation histories. Dwarfs that have undergone a Hubble time of star formation have a lower central density than those whose star formation shut down long ago. This result is challenging to understand in theories that attempt to solve the cusp-core problem by modifying dark matter or the cosmological model, because these theories predict that dark matter cores should be ubiquitous. Our results suggest that dark matter is, to a good approximation, a cold, collisionless fluid that can be heated up and moved around.

Tuesday 06 October :
Insights into the binary black hole population from an independent analysis of LIGO data
Tejaswi Venumadhav
UC Santa Barbara
Host: Scott Hughes



Abstract: Within the last few years, gravitational wave astronomy has progressed from notable single detections to a steady stream of events that provide a continually deeper view into the population of merging compact binaries in the universe. These detections are only possible due to sophisticated analyses of noisy interferometer strain data that were historically conducted within the LIGO-VIRGO collaboration (LVC). We developed an entirely independent analysis of LIGO data that improved its reach by identifying and rigorously accounting for systematics, and thereby identified several new binary black-hole mergers within. In this talk, I will provide a birds-eye view of the analysis, touch upon our improvements to the process of detecting as well as characterizing the events, and summarize the insights into the source population that we found. Our expanded catalog is broadly consistent with that of the LVC but with improved constraints on population parameters, and hints of a more complex distribution in the intrinsic parameters of binary black-holes. Time permitting, I will also describe an improved search for lensed binary black hole events in the O2 run of advanced LIGO-VIRGO.

Tuesday 13 October :
Hidden Friends To Gravitational Wave Sources
Smadar Naoz
UCLA
Host: Salvatore Vitale



Abstract: The recent gravitational wave detections by LIGO/Virgo revolutionized the way we sense our Universe. These detections have resurfaced a long-standing question about the formation channels of merging black holes and raised new challenges about forming gravitational sources in general. Motivated by these challenges, I will suggest how few-body gravitational interactions in a dense environment can alleviate some of the difficulties in merging compact objects. I will particularly focus on the dense stellar clusters surrounding supermassive black holes at the center of galaxies. I will show how this channel can leave a clear signature on the gravitational-wave signals, allowing differentiation between different merger mechanisms. The Laser Interferometer Space Antenna (LISA) can potentially be used to distinguish between channels.

Tuesday 20 October :
The Renaissance Of Astrophysics: A Landscape Of Opportunities In The Era Of Time Domain Multi-Messenger Investigations
Raffaella Margutti
Northwestern University
Host: Deepto Chakrabarty



Abstract: 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.

Tuesday 27 October :
High Redshift Supernovae: Beyond The Epoch Of Dark Energy
Lou Strolger
STScI
Host: Kiyoshi Masui



Abstract: For over two decades, the Hubble Space Telescope has been heavily used to locate supernovae in high redshift environments, with the primary goal of improving constraints on the nature of dark energy. Along the way we have made surprising observations on the nature of supernovae themselves, and clues to their elusive progenitor mechanisms, some of which are difficult to reconcile with observations at much lower redshift. From complete volumetric supernova rate histories, that now extend to z > 2, we find type Ia supernova delay-time distributions are consistent with a power law of index -1, but with the fraction of prompt (t_d < 500 Myr) much less than expected from various ground-based surveys. Core collapse supernova rates trace the cosmic star formation rate history, but require stellar progenitors more massive than have been seen in deep studies of nearby events. I will detail our current campaigns on clusters of galaxies, where gravitational lens magnification provides a real potential for locating the first, primordial supernovae, while also providing useful constraints on the mass models of the foreground gravitational lenses.

Wednesday 28 October :
Policies and Machine Learning Processes Which Reduce Bias in HST Time Allocation
Lou Strolger
STScI
Host: Kiyoshi Masui

SPECIAL SEMINAR ON WEDNESDAY AT 4PM EDT

Abstract: Most resource allocation for observatory time (or money) is done through peer review processes that can be time consuming and prone to bias. STScI has implemented new policies, tools, and streamlining mechanisms to reduce bias. This talk will showcase these changes, and share some preliminary results on their effects from the most recent HST Cycles.

Tuesday 03 November :
TBA
Kirpal Nandra
Max Planck Institute for Extraterrestrial Physics
Host: Erin Kara

NOTE SPECIAL TIME: 10AM EST

Abstract: TBA

Tuesday 10 November :
The Short-Timescale Universe According to CHIME
Ingrid Stairs
University of British Columbia
Host: Kiyoshi Masui



Abstract: The CHIME telescope, originally planned for Hydrogen intensity mapping, has two instruments sensitive to rapidly varying sources: a pulsar backend which processes tracking beams produced by the correlator, and a Fast Radio Burst (FRB) detector searching for these still-mysterious millisecond signals. I'll present some of the early results from both instruments, including the many things CHIME has already taught us about FRBs, and the planned contributions of the Pulsar instrument to the NANOGrav gravitational-wave background search.

Tuesday 17 November :
TBA
John Belcher
MIT
Host: TBA



Abstract: TBA

Tuesday 01 December :
Theoretical Modeling Of Cosmic Reionization And Its Observability With The 21cm Line From HI
Benedetta Ciardi
MPE Garching
Host: Christina Eilers

NOTE SPECIAL TIME: 10AM EST

Abstract: Cosmic reionization is the last major phase transition undergone by our Universe. Although most studies agree on the general characteristics of H reionization (for example that it is driven by stars and it is mostly if not fully complete by z~6), its details are still largely unknown, among which the contribution from and role played by more energetic sources. In this talk I will discuss the ingredients needed for a correct modeling of cosmic reionization and present results from recent radiative transfer simulations accounting for a variety of source types (such as stars, quasars, X-ray binaries). I will also discuss the observability of reionization in terms of the 21cm signal from neutral hydrogen and present the latest results from the LOFAR radio telescope.

Tuesday 08 December :
How do galaxies form? New insights from the FIRE simulations
Claude-Andre Faucher-Giguere
Northwestern University
Host: Rob Simcoe



Abstract: Galaxies are remarkably diverse in their properties, ranging from irregular to disky to elliptical in morphology, and from blue to red in color. At the same time, when analyzed systematically, galaxy populations exhibit striking regularities, with clear trends with mass and redshift. How does this `regular complexity' emerge from the hot Big Bang? I will present results from the FIRE simulations which shed some light onto the processes that shape galaxies. The FIRE zoom-in simulations resolve the multiphase interstellar medium of galaxies and model several different feedback processes (including Type II/Ia supernovae, stellar winds, and radiation) while including the cosmological environment. I will highlight recent results on the formation of galactic disks, the `burstiness' of star formation, galactic winds, and the growth of supermassive black holes. Our new results indicate that transitions in the properties of each of these important phenomena can be simultaneously explained by a phase transition (virialization) in the inner circumgalactic medium, and arise from the interplay between feedback energy produced on small scales and the physics of halo gas on larger scales.

This page is maintained by Erin Kara and Kiyoshi Masui