Colloquia Archives

Feng Long, SMA Fellow, Center for Astrophysics View(active tab)
Thursday, October 29, 2020
3:45 p.m.
Zoom
Title:
The ALMA view of protoplanetary disks: mass, size, and morphology
Abstract:
Planets are assembled from the gas and dust in the disks orbiting around young stars. How these disks evolve from primordial gas and dust into planetary systems like our own solar system is still not well understood. With the powerful Atacama Large Millimeter/submillimeter Array (ALMA), we are now able to study the planet-forming-disks in greater detail and towards larger samples. These observations are transforming our view of disks and offering new insights in planet formation. In this talk, I will share results from ALMA disk surveys, which reveal the general disk properties including masses and sizes, and the detailed dust grain distributions. The implications of the assembly and early evolution of planetary systems will be discussed based on these results.
Daniel Anglés-Alcázar (CCA Flatiron/UConn)
Thursday, October 22, 2020
3:45 p.m.
Zoom
Title:
Multi-scale cosmological simulations of massive black hole growth and feedback
Abstract:
Supermassive black holes play a key role in galaxy evolution and large-scale structure but the physical processes driving black hole growth and feedback into the surrounding medium remain a major uncertainty in current models. In this talk, I will present recent progress in understanding the nature of black hole-galaxy co-evolution by means of (1) the SIMBA large volume cosmological hydrodynamic simulations, (2) FIRE cosmological zoom-in simulations of individual galaxies with resolved multi-phase interstellar medium, and (3) new hyper-refinement simulations that for the first time resolve explicitly the transport of gas down to sub-pc scales in the nuclear regions of massive galaxies in a full cosmological setting. I will highlight some of the lessons learned from these multi-scale simulation techniques, with important implications including black hole-galaxy scaling relations, AGN demographics and the nature of luminous quasars, the star formation-AGN connection, and galaxy quenching. Looking forward, I will outline two opposite but complementary approaches under current development: the Interscale Galactic NucleI Simulations (IGNIS) to explicitly resolve black hole growth and feedback and significantly increase the predictive power of current models, and the Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) project containing thousands of cosmological volumes with feedback parameter variations to marginalize over uncertainties in baryonic effects and maximize the science return of next generation cosmological experiments.
Sandro Tacchella (CfA)
Thursday, October 15, 2020
3:45 p.m.
Zoom
Title:
The Diversity of Building up the Quiescent Sequence at z~
Abstract:
Abstract: I present evidence for a diversity of pathways for building up the quiescent galaxy population at early cosmic times. Specifically, I will present observational constraints on star-formation histories and quenching timescales by combining Keck DEIMOS spectroscopic data with >10-band photometry. I will discuss how one can self-consistently fit both photometric and spectroscopic data together with the tool Prospector, which allows fitting for non-parametric star-formation histories and complex stellar, nebular, and dust physics. Although the apparent diversity, we find that the most massive, compact galaxies have formed their stars the earliest and most rapidly. Finally, I will relate these findings to numerical simulations (in particular IllustrisTNG), putting forward that most galaxies not change their morphology significantly during quenching.
Zheng Zheng (University of Utah) View(active tab)
Thursday, October 8, 2020
3:45 p.m.
Zoom
Title:
The Curious Case of Lyman-alpha Emitting Galaxies Abstract:
Abstract:
Lyman-alpha Emitting galaxies or Lyman-alpha emitters (LAEs), star-forming galaxies selected through their strong Lyman-alpha emission, are becoming an important probe of galaxy formation, cosmic reionization, and cosmology. I will present our recent and ongoing work towards understanding properties of those galaxies based on Lyman-alpha radiative transfer modeling, discussing the role of the anisotropic gas distribution around galaxies and the origin of the extended Lyman-alpha emission around LAEs. I will then talk about the anisotropic clustering of LAEs induced by the radiative transfer effect and the current observational status. During the talk, I will also discuss a few puzzles in the study of LAEs.
Zhiyuan Ma (UMass Amherst)
Thursday, October 1, 2020
3:45 p.m.
Zoom
Abstract:
TolTEC is a new camera that will be mounted on the Large Millimeter Telescope (LMT). Once there, it will provide simultaneous, polarization-sensitive imaging at 2.0, 1.4, and 1.1mm wavelengths through its ~7000 Lumped Element Kinetic Inductance Detectors (LeKIDs). The TolTEC data analysis software stack (TolTECA) is developed to facilitate all the data related tasks including instrument diagnosing, data reduction, data product management, and visualization. TolTECA is used both at the observing time to provide the quasi real-time feedback ("quick look") for the observer, and at later times at the data analysis facilities (high performance clusters) to produce science-ready data products. Moreover, it provides a web-based solution to host interactive science-oriented services like S/N calculators, observation simulators, and SZ effect simulators - all designed to facilitate the exploration of the science cases enabled by TolTEC. The two key components of TolTECA are the Python package tolteca, and the highly optimized data reduction engine citlali written in C++. Tolteca implements the general data management and handling submodules that serve at the highest levels, while citlali is the high performance data reduction engine that carries out the the time-ordered data (TOD) reduction and mapmaking. In this talk, I will first cover the overall architecture of TolTECA, and then describe some of its more novel components in detail, which includes the LeKIDs data modeling, the highly configurable parallelization framework that we used in citlali, as well as the tolteca data analysis workflow and data product management. The second part of the talk will focus on some of our on-going efforts in using simulations to explore the science cases that are enabled by TolTEC.
Dara Norman (NSF NOIRLab)
Thursday, September 24, 2020
3:45 p.m.
Zoom
Title:
Advancing The Inclusion Revolution
Abstract:
The field of Astronomy and Astrophysics has seen major changes in the last couple of decades. There have been discoveries that have evolved our understanding of the Universe. The development of new methods and gathering of datasets have expanded topical areas of the field in profound ways. We have even seen the community begin to recognize and understand that the health and well-being of the workforce cannot be ignored if we intend to continue with scientific breakthroughs. However, advancing this understanding requires the evolution our scientific culture. In this talk, I will highlight some growing trends toward more diversity and inclusion in the field, the importance of access to decision making and research opportunities to promote these trends, as well as, the structural changes needed to usher in an Astronomical inclusion revolution.
Brian Svoboda (NRAO)
Thursday, September 17, 2020
3:45 p.m.
Zoom
Title:
Towards a Multi-Scale View of Dense Gas in the Milky Way
Abstract:
High-mass stars are key to regulating the interstellar medium, star formation activity, and overall evolution of galaxies, but their formation remains an open problem in astrophysics. Current theories of star formation disagree on the timing, initial masses of dense cloud-cores, and the role of gravity versus turbulence during the nascent phases of star cluster evolution. These long-standing uncertainties are now being addressed with the statistical power of multi-wavelength Galactic Plane surveys and the unprecedented sensitivity of ALMA. In this talk I shall review current surveys of the dense, star-forming gas of the Milky Way and discuss studies we have carried out to understand the physical conditions during the earliest phases of high-mass star formation. I shall further describe a novel Bayesian analysis framework to process large gas-kinematic surveys and discuss the results of an application to nearby clouds. In closing, I shall discuss how the wide-field capabilities of both current and next-generation cm/mm-wavelength facilities, such as the LMT, GBT, ALMA, and J/ngVLA, will expand our understanding of star formation in the Milky Way.
Astro Jamboree Week 2
Thursday, September 3, 2020
3:45 p.m.
Zoom
Title:
Meet & Greet Part 2
Astro Jamboree Week 1
Thursday, August 27, 2020
3:45 p.m.
Zoom
Title:
Meet & Greet Part 1
CANCELLED Amanda Kepley (NRAO)
Thursday, April 30, 2020
3:45 p.m.
LGRT 1033
Title:
TBD
CANCELLED Zhiyuan Ma (UMass)
Thursday, April 23, 2020
3:45 p.m.
LGRT 1033
Title:
TBD
CANCELLED JJ Hermes (BU)
Thursday, April 16, 2020
3:45 p.m.
LGRT 1033
Title:
TBD
CANCELLED Katharine Reeves (CfA)
Thursday, April 9, 2020
3:45 p.m.
LGRT 1033
Title:
TBD
CANCELLED Anglés-Alcázar Daniel (Flatitron)
Thursday, April 2, 2020
3:45 p.m.
LGRT 1033
Title:
TBD
CANCELLED Sandro Tacchella (CfA)
Thursday, March 26, 2020
3:45 p.m.
LGRT 1033
Title:
TBD
CANCELLED Jason Young (Mt. Holyoke College)
Thursday, March 12, 2020
3:45 p.m.
LGRT 1033
Title:
The Life Story of Low Surface Brightness Spirals
Abstract:
Low surface brightness spiral galaxies are paradoxical in that they are gas rich but have low star-formation rates. Combined ground-based/space-based data let us determine the spatially resolved star-formation histories of low surface brightness spirals. We aim to use these histories in tandem with velocity fields and metallicity profiles to determine the physical mechanism(s) that give these faint but numerous galaxies low star-formation rates despite ample gas supplies.
Brian Svoboda (NRAO)
Thursday, March 5, 2020
3:45 p.m.
LGRT 1033
Nir Mandelker (Yale)
Thursday, February 27, 2020
3:45 p.m.
LGRT 1033
Title:
The Multiphase Circum- and Intergalactic Media at the Nexus Between Galaxy Formation and Cosmology
Abstract:
Galaxies are not closed boxes. Rather, it has become clear in recent years that it is the flow of gas into and out of galaxies that shapes their evolution. Cycles of gas accretion, star-formation, galactic outflows and reaccretion, intimately link galaxies to the circumgalactic medium (CGM, gas outside galaxies but within dark matter halos) and the intergalactic medium (IGM, gas outside dark matter halos). While the diffuse gas in these regions has traditionally been very difficult to study, recent advances in both observations and numerical simulations are now providing a wealth of information on the gas around galaxies, revealing complex, multiphase and multiscale structure. I will describe my ongoing efforts to study the phase-structure of the C/IGM by combining novel cosmological magnetohydrodynamic moving-mesh simulations that I modified to overcome difficulties in resolving the diffuse gas in these regions, with analytic modeling and idealized numerical experiments that address detailed questions about physical processes affecting multiphase gas in the C/IGM. I will present two main results from these studies. I will first show how non-linear thermal instabilities cause hot gas to "shatter" into small-scale clouds of cool and dense gas. I will then present a model for the interaction of cold accretion streams with the ambient hot CGM by considering the Kelvin-Helmholtz Instability in a dense, supersonic, self-gravitating, and radiatively cooling cylinder. These have helped us to understand how the C/IGM are shaped by a complex interplay of hydrodynamical, thermal, and gravitational instabilities which we are beginning to put together in a coherent framework.
Paul Torry (Florida State)
Thursday, February 20, 2020
3:45 p.m.
LGRT 1033
Title:
Probing Galaxy Formation with Modern Cosmological Simulations
Abstract:
Cosmological simulations are among the most powerful tools available to probe the non-linear regime of cosmic structure formation. They also provide a clear test-bed for understanding the impact that hydrodynamics and feedback processes have on the evolution of galaxies. I will present an overview of modern galaxy formation simulations that couple a novel moving mesh computational method with explicit baryon feedback prescriptions. This approach results in detailed galaxy formation models that reproduce fundamental observations such as the galaxy stellar mass function, cosmic star formation rate density, and galaxy morphological diversity. I will briefly discuss the key physical model ingredients and explore in detail the cosmic coevolution of galaxies and their metals including specifically the mass-metallicity relation, fundamental metallicity relation, and metallicity gradient evolution. I will argue that metallicity observations may allow us to discriminate between bursty and non-bursty feedback models in the future with JWST.
Cassandra Hall (Smith College Faculty Candidate)
Thursday, February 13, 2020
3:45 p.m.
LGRT 1033
Title:
Gravitational Instability and Substructure in Protoplanetary Discs
Abstract:
In the ALMA era of observational astronomy, we are availed of a plethora of spatially resolved images of protoplanetary discs, the site of exoplanet formation. Significant substructure, such as spirals and ring-like gaps, has proved to be the norm, rather than the exception, in these systems. There is now a consensus that much of this substructure is caused by forming gas-giant exoplanets. However, this is problematic for our understanding of planet formation. These protoplanetary discs are typically a factor of 10 too young to have formed such massive planets in the standard core accretion paradigm, but it is also understood that planets do not form directly through gravitational collapse. When protoplanetary discs are very young, they are very massive relative to their host star, and therefore pass through a period of gravitational instability. This instability may accelerate the earliest stages of planet formation in the standard core accretion paradigm, offering a solution to the planet formation timescale problem. I discuss my research into gravitational instability and detection of substructure, and discuss future research plans which will explore the formation of exoplanets in dynamically evolving protoplanetary discs.
Kimberly Ward-Duong (Smith College Faculty Candidate)
Thursday, February 6, 2020
3:45 p.m.
LGRT 1033
Title:
Low-mass Stars and Substellar Systems as Planet Hosts and Planet Analogs
Abstract:
The overwhelming majority of the galactic stellar population consists of low-mass M-dwarf stars. Their abundance makes them attractive targets for exoplanet searches and a critical regime of exploration at the low-mass end of the initial mass function. In this talk, I will describe my work using adaptive optics imaging, submillimeter observations, and high-resolution spectroscopy to understand the planet-forming potential of these stars and their binary and disk environments. Extending to even lower masses beyond the hydrogen burning limit, substellar objects and their atmospheres currently serve as our best and most characterizable analogs for exoplanet systems, illuminating our understanding of how brown dwarfs and giant planets form and evolve. Studies of these systems require innovative instrumentation at the modern limits of high contrast imaging and high spectral resolution, and I will discuss how these efforts will help inform next generation approaches to directly image (sub)-Jovian and terrestrial planets.
Jennifer Andrews (Smith College Faculty Candidate)
Thursday, January 30, 2020
3:45 p.m.
LGRT 1033
Title:
Supernovae as Forensic Labs for Massive Stars
Abstract:
Core collapse supernovae (CCSNe) represent the final evolutionary stage of stars more massive than 8 M☉. Just like their massive star progenitors, CCSN explosions are far from homogenous. The photometric and spectral evolution zoo of CCSNe can be better understood when the mass-loss histories of their progenitors are taken into context. We can now reconstruct the mass-loss history and physical parameters of the massive star progenitor with observations of SNe in the hours to years after explosion, without the need for rare pre-explosion Hubble Space Telescope imaging. Mass-loss rates and eruptive events in evolved massive stars impact the behavior of the terminal CCSN, including red supergiants, yellow hypergiants, and luminous blue variables. Red supergiants and yellow hypergiants have slower wind velocities and lower mass-loss rates than luminous blue variables, creating very different explosion environments, and by extension, very different observational characteristics of the supernova. I will present how I use these extremely bright and violent explosions to understand more about the final years of massive star evolution, and what tools are being used and developed to discover these SNe so rapidly in what is becoming the golden age of transient astronomy.
Miao Li (Flatiron)
Thursday, January 23, 2020
3:45 p.m.
LGRT 1033
Title:
The Nature and Impact of Supernovae Feedback in Galaxy Formation
Abstract:
Circumgalactic Medium (CGM), where cosmic inflows interact with galactic outflows, bears critical information for galaxy formation. Current cosmological simulations can model cosmic inflows but their galactic outflows usually rely on ad hoc sub-grid models. The path forward requires better understanding the feedback physics and using physically-based models in large-scale simulations. In this talk, I will first summarize the recent progress of small-box hydrodynamical simulations focusing on the nature of supernovae (SNe) feedback; particularly, the hot outflows are much more powerful than cool outflows while also appearing very simple. Then, I will introduce our galactic scale simulations, which adopt the outflow models from the small-box simulations. For a Milky Way-mass galaxy, when the star formation surface density is low, hot outflows form large-scale fountains in the halo; when it is high, the outflows are bipolar in shape and can escape from the dark matter halo. I will discuss the resultant multiphase CGM from 10^4-7 K and how they compare to multi-wavelength observations. I will also discuss the implications to the missing baryon and the missing metals problems.
Kiyoshi Masui (MIT)
Thursday, December 5, 2019
3:45 p.m.
LGRT 1033
Title:
Mapping the Universe with CHIME - a Digital Radio Telescope
Abstract:
A powerful tool for understanding the Universe is the distribution of matter on very large scales, and a number of upcoming surveys will map the large-scale structure at radio wavelengths using the 21 cm line from neutral hydrogen. I will describe the Canadian Hydrogen Intensity Mapping Experiment (CHIME), which will make the largest-ever three-dimensional map of the Universe. This map will enable precise measurements of the expansion of the Universe to understand its anomalous acceleration and the dark energy hypothesized to be driving it. In addition, I will show how the digital design of the CHIME telescope allows it to simultaneously search for a new class of radio transient, fast radio bursts, which provide another probe of the Universe. CHIME belongs to a new class of radio telescope which are remarkably scalable, opening a new frontier in observational cosmology to understand both the evolution of the Universe and fundamental physics.
Erica Nelson (Harvard)
Thursday, November 21, 2019
3:45 p.m.
LGRT 1033
Title:
The emergence of galactic structure
Abstract:
Abstract: Using a combination of the Hubble Space Telescope, the Keck telescopes, and the ALMA millimeter interferometer, we have begun to piece together a picture of the emergence of galactic structure: how the Universe evolved from its uniform state shortly after the Big Bang to the rich diversity of galaxies today. In this talk, I will discuss recent observations and the impact they have had on our understanding of the formation and evolution of galaxies. This includes a method to directly image the emergence of galactic structure in the distant past using Hubble, a spatially resolved census of star formation at early times, and new findings about the enigmatic galaxies that ruled the early universe. Finally, the impending launch of the James Webb Space Telescope will revolutionize our understanding of early galaxy formation. I will conclude with a discussion of where the field is moving and the rich discovery space in this new era of extragalactic astrophysics.
Meng Gu (Princeton)
Thursday, November 14, 2019
3:45 p.m.
LGRT 1033
Title:
Stellar Populations and Assembly Histories of Galaxies in Nearby Massive Clusters
Abstract:
Understanding how massive galaxies form and grow is important to galaxy evolution and cosmology. In this talk, I will present the stellar population study of massive galaxies and their surrounding environments in nearby galaxy clusters, with particular focus on the scaling relations between stellar mass and galaxy central stellar population properties, as well as the radial profiles where their stellar population bear the imprint of the assembly history. I will begin by introducing the `coordinated assembly` picture for massive galaxies with a case study of an ongoing brightest cluster galaxy assembly at z=0.1. I will then present our `Deep Coma` project in the SDSS IV/MaNGA collaboration. The deep integral field spectroscopy and full spectral modeling method enabled detailed analysis at the low surface brightness regime. We are therefore able to demonstrate the stellar populations of various types of targets in the Coma Cluster. Last, I will briefly discuss the `coordinated assembly` picture in the IllustrisTNG simulations.
Lars Hernquist (Harvard)
Thursday, November 7, 2019
3:45 p.m.
LGRT 1033
Title:
The IllustrisTNG Project
Abstract:
A predictive theory of galaxy formation remains elusive, even after more than 50 years of dedicated effort by many renowned astrophysicists. The problem of galaxy formation is made difficult by the large range in scales involved and the many non-linear physical processes at play. This talk describes a new generation of numerical models that are designed to overcome these difficulties based on nove schemes for solving the fluid equations on a moving mesh. In particular, I will describe an ongoing project that extends results from the Illustris simulation by employing refined models for feedback from stars and supermassive black holes. Several applications will be described, including the color evolution of galaxies, low surface brightness galaxies, and the statistics of large-scale structure, pertaining to high precision cosmological surveys.
Song Huang (Princeton)
Thursday, October 31, 2019
3:45 p.m.
LGRT 1033
Title:
The Bright and Dark Sides of Massive Galaxies from the Hyper Suprime-Cam Survey
Abstract:
Abstract: Massive galaxies are valuable targets for studying cosmology and galaxy-halo connection as they help us locate massive dark matter halos across the history of the universe. Although the general behavior of stellar-halo mass relation (SHMR) is reasonably constrained at low-redshift, there is still much to learn about the connection between the assembly of massive galaxies to the growth of their host dark matter halos. Using deep images from the Hyper Suprime-Cam (HSC) survey and taking advantage of its unprecedented weak lensing capabilities, we reveal a remarkably tight connection between the stellar mass distribution of massive central galaxies and their host dark matter halo mass. Massive galaxies with more extended stellar mass distributions tend to live in more massive dark matter haloes. We explain this connection with a phenomenological model, showing that halo mass varies significantly at fixed total stellar mass (as much as 0.4 dex) with a clear dependence on stellar mass within 10 kpc. This two-parameter model provides a more accurate picture of the galaxy–halo connection at the high-mass end than the simple SHMR and opens a new window to connect the assembly history of halos with those of central galaxies. I will also discuss potential applications of our model on topics from the assembly of massive galaxies to cosmology using massive clusters.
Karin Sandstrom (UCSD)
Thursday, October 24, 2019
3:45 p.m.
LGRT 1033
Title:
Interstellar Dust at Low Metallicity
Abstract:
Dust plays critical roles in many of the processes occurring in the interstellar medium and dust’s infrared emission serves as a tracer for the ISM and star formation from the nearby universe out to high redshift. While most of our knowledge of dust is built from observations of the local area of the Milky Way, it is clear that dust properties change dramatically in low metallicity conditions which may be prevalent at high redshift and in nearby dwarf galaxies. I will discuss what we know about how dust properties change with metallicity and how this can impact physical processes occurring in the ISM. I will also present new results studying low-metallicity dust in the Small Magellanic Clouds and other nearby galaxies.
Colloquium Jamboree 3: New Postdocs
Thursday, October 17, 2019
3:45 p.m.
LGRT 1033
Title:
New Postdoc Research
Abstract:
Becky Arnold Stars, Stats, Software I will present a brief outline of my research to date. I will describe my work using N-body simulations to study the evolution of star clusters with a focus on binary clusters. I will then discuss statistical methods for quantifying and comparing spatial-velocity substructures. Finally I will go into my interest in good software development practice for rigorous and reproducible research. Chris Faesi PHANGs: Connecting cloud-scale star formation to galaxy evolution Star formation drives the secular evolution of galaxies across cosmic time. However it is not yet understood how this key process, which occurs on a localized basis within molecular clouds (10s of pc scales), couples to the kpc scale processes and environments of galaxies. The Physics at High Angular Resolution in Nearby Galaxies (PHANGs) collaboration is undertaking a comprehensive series of observational and modeling efforts to systematically explore the cloud-scale star-forming interstellar medium (ISM) in the nearby universe. With ongoing sub-arcsecond resolution ALMA, VLT/MUSE, and Hubble large programs mapping full disks of dozens of nearby galaxies, we are poised to revolutionize our understanding of molecular clouds, HII regions, star clusters, star formation efficiency, and feedback, as well as to explore their environmental dependences. I will highlight some first PHANGs science results and outline the exciting path forward in the coming years, which includes many opportunities for students to get involved. As one example, I am leveraging the MUSE data to map the Balmer decrement-derived dust extinction at 50 pc scales and comparing to ALMA CO emission to understand the cloud-scale distribution of dust and gas across galaxy disks. Matteo Messa Title: Young Star Cluster and Clumps in the local (and far) Universe Abstract: I will briefly go through what I have done, I am doing, and I would like to do, concerning the optical-NIR study of clumpy star formation. This includes: the effects of galaxy environment on the formation and evolution of star clusters; new machine-learning methods to fully exploit the large set of clusters’ observations; the clumpiness of nearby starburst galaxies and their relation with high-redshift galaxies; the use of lensed galaxies to test resolution-effects on the properties of massive clumps at high-redshift. Basically, I will try to convince you (in ~10 min.) that star clusters and star-forming clumps are powerful tools to study star-formation across space and time. Mimi Song Title: Tracing galaxy growth over cosmic time with galaxy stellar mass functions Abstract: Over the last decade the advent of the Hubble Space Telescope (HST) Wide Field Camera 3 has enabled us to build statistically significant samples of galaxies out to z=8. We have subsequently witnessed remarkable progress in our understanding of galaxy evolution in the early universe. However, our understanding of the galaxy stellar mass growth in this era has been limited due to the lack of rest-frame optical data at a comparable depth as the HST data. In this talk, I will first talk about observationally constraining the galaxy stellar mass function at z = 4–8 using deep Hubble and Spitzer data in the CANDELS GOODS fields. Then, I will present an empirical model for galaxy evolution out to z = 8 by combining the dark matter halo growth history extracted from cosmological simulations with the the observed galaxy stellar mass functions over cosmic time.

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