Colloquia Archives

Alyson Brooks, Rutgers
Thursday, March 21, 2019
3:45 p.m.
LGRT 1033
Title:
TBD
SPRING RECESS - NO COLLOQUIM
Thursday, March 14, 2019
3:45 p.m.
LGRT 1033
Katherine Whitaker, University of Connecticut
Thursday, March 7, 2019
3:45 p.m.
LGRT 1033
Title:
The Cosmic Life Cycle of Massive Galaxies
Abstract:
Over the last few decades, astronomers have progressed from archeological studies of nearby galaxies to direct observations of the early universe. We have uncovered billions of years of cosmic growth that present new challenges to galaxy formation theories. In this talk, I will review the recent innovative techniques developed to probe the distant universe, and the key observations constraining the formation histories of galaxies over the past 11 billion years. We have discovered a population of surprisingly compact and massive “red and dead” (quiescent) galaxies that are no longer actively forming stars. The physical mechanisms responsible for shutting down star formation and the subsequent buildup of this quiescent population at such early times is one of the most outstanding questions in astrophysics today. I will present promising paths forward towards solving this puzzle that leverage strong gravitational lensing and the capabilities of the Hubble Space Telescope, as well as a look toward the future with exciting upcoming public facilities.
Sandro Tacchella, Harvard/CfA
Thursday, February 28, 2019
3:45 p.m.
LGRT 1033
Title:
TBD
Abstract:
TBD
Prof. Massimo Della Valle, INAF-Observatory of Naples, Italy
Thursday, February 21, 2019
3:45 p.m.
LGRT 1033
Title:
The Empirical Ground Of The Supernova-GRB Connection
Abstract:
We review the status of the Supernova/Gamma-Ray Burst connection. Several pieces of evidence suggest that long duration Gamma-ray Bursts (GRBs) are associated with type Ic Supernovae (SNe). Current estimates of SN and GRB rates show that only a tiny fraction of massive stars, likely less than 3%, are able to pruduce GRBs.
Topic TBD
Thursday, February 14, 2019
3:45 p.m.
LGRT 1033
Title:
TBD
Abstract:
TBD
Anne Jaskot, UMass Amherst
Thursday, February 7, 2019
4:00 p.m.
LGRT 1033
Title:
How to Reionize the Universe - Clues from Green Pea Galaxies
Abstract:
The reionization of the intergalactic medium at z>6 is one of the major transformations in the universe’s history, and yet, we know little about how it occurred. The most likely explanation is that Lyman continuum (LyC) radiation escaped into the intergalactic medium from early star-forming galaxies. However, most star-forming galaxies show no signs of LyC escape. Recently, the “Green Pea” galaxies have emerged as the first known star-forming galaxy population where strong LyC escape is common. I will discuss what we are learning from the Green Peas about ways to identify LyC-leaking galaxies, the role of feedback in LyC escape, and the properties of LyC-emitters.
Todd Tripp, UMass Amherst
Thursday, January 31, 2019
4:00 p.m.
LGRT 1033
Title:
Strange Things in the Dark Backwaters: Toward a More Complete Understanding of Galaxies
Abstract:
During the past decade, observational and theoretical studies have increasingly indicated that the extended circumgalactic medium (CGM), i.e., the gaseous halos extending several hundred kpc out from regions where stars are found, is an important component of galaxies that regulates their evolution. The CGM is likely the dominant reservoir of ordinary matter in galaxies at all epochs, and gas flows to and from the CGM surely have profound effects on the tip-of-the-iceberg stellar regions that most people think of as a “galaxy”. The light emitted by the CGM is extremely faint, so the most practical technique for exploring these dark backwaters is to study the absorption lines that they imprint on the spectra of background continuum sources. This talk will review several recent absorption-line studies of the CGM, which turns out to harbor some strange and difficult-to-explain entities. For example, while the CGM plasma is detected in species such as O VI and Ne VIII and is typically highly ionized, it is also profoundly cold – very high-resolution data show that upper limits on the gas temperature are a factor of 10-20 below the temperature where O VI is expected to exist. Moreover, disparate ions ranging from Mg I (ionization potential = 7.6 eV) up to Ne VIII (IP = 239.1 eV) are kinematically very well aligned and clearly have a strong physical relationship. Standard collisional or photoionization models are hard-pressed to explain these data, and more complex physics will probably need to be considered. This talk will also (briefly) present data that suggest some tantalizing connections between CGM and the star-formation properties of galaxies, and some remarks will be made on the type of instruments that will be needed in the future to obtain a more complete understanding of galaxies including the difficult-to-observe components.
Kartik Sheth, NASA HQ
Thursday, December 6, 2018
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
TBA
Elena D'Onghia, University of Wisconsin
Thursday, November 29, 2018
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
TBA
Joshua Peek, STScI
Thursday, November 15, 2018
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
TBA
Amy Jones, University of Alabama
Thursday, November 8, 2018
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
TBA
Chiara Feruglio, Harvard & INAF - Osservatorio Astronomico Trieste
Thursday, November 1, 2018
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
TBA
Kelley Hess, University of Groningen, The Netherlands
Thursday, October 18, 2018
3:45 p.m.
LGRT 1033
Title:
Gas processing in the group environment and the coming radio revolution
Abstract:
The environment plays a key role in driving galaxy evolution. The most dramatic examples of this are seen in galaxy clusters, but galaxy groups are where the impact of environment is first felt. This has been teased out from large optical spectroscopic surveys of galaxies, but the atomic neutral hydrogen gas, observed through the 21 cm line in the radio, can reveal and provide insight to ongoing and past interactions in ways that are impossible to know from just observing the stellar content. I will set the stage by discussing how targeted HI observations combined with blind HI surveys such as ALFALFA have led us understand gas processing in groups and clusters. However, we are also on the verge of a new revolution in radio astronomy with several new facilities coming online in the next year. I will present an overview and new data from APERTIF, which will ultimately generate the largest collection of resolved HI galaxies in the northern hemisphere, and will provide an invaluable data set for revealing the nuanced impact of environment on the gas content of galaxies which is the fuel for star formation. By resolving thousands of galaxies in HI across different environments, and combining it with dedicated optical IFU spectroscopic follow-up, we will identify the physical mechanisms in individual galaxies which directly impact the optical tracers through which we infer galaxy evolution.
David Ballantyne, Georgia Institute of Technology
Thursday, October 11, 2018
3:45 p.m.
LGRT 1033
Title:
Nuclear Starburst Disks around Supermassive Black Holes: the Key to AGN obscuration?
Abstract:
Most of the AGNs in the Universe are obscured by significant columns of gas and dust. The origin of the obscuring gas is not understood, but it appears that the fraction of obscured AGN increases at higher z. This implies that processes within the galaxy are connected to producing the absorbing gas. Here, I present a model for AGN obscuration that may be relevant at z~1 and above -- nuclear starburst disks. I'll describe the physical conditions of these very compact and intense star-forming regions, present 1D and 2D models of their properties, and show how they can account for a large fraction of the shape of the X-ray background. I will also describe how they might be a potential origin for some nuclear star clusters seen in the centers of low redshift galaxies.
Michael Fall, STScI
Thursday, October 4, 2018
3:45 p.m.
LGRT 1033
Title:
GALAXY FORMATION, ANGULAR MOMENTUM, AND THE HUBBLE SEQUENCE
Abstract:
This talk emphasizes the importance of angular momentum in understanding the formation of galaxies of different morphological types (spirals, lenticulars, ellipticals). Angular momentum, for example, is crucial in determining the sizes of galaxies. The observed distribution of galaxies in a plot of specific angular momentum (J/M) against mass (M) provides an objective, physically motivated alternative to subjective classification schemes such as the Hubble sequence. This way of looking at galaxies also links nicely with the theory of tidal torques and galaxy formation in the standard cold dark matter cosmology. Much of the recent progress in this field has come from observations of galaxies at high redshifts and from hydrodynamical simulations of forming galaxies. One of the main lessons from the simulations is that feedback from young stars and possibly active galactic nuclei is crucial to the retention of most of the angular momentum imparted by tidal torques and thus to the formation of large galactic disks. This talk – aimed at a general astrophysics audience -- provides an overview of theory, simulation, and observation in this field, and presents several new results along the way.
Min Yun, UMass Amherst
Thursday, September 27, 2018
3:45 p.m.
LGRT 1033
Title:
The Most Luminous Galaxies in the Universe
Abstract:
I will discuss the motivation and results from the ongoing study of the most luminous galaxies discovered by the Planck, including the nature of their powering sources. I will contrast these results with the discussion of the new ultra-high resolution (20 to 50 milli-arsecond) ALMA observations of gas and dust in the two z=4 submillimeter galaxies that are also among the most luminous known so far. The mass, size, and dynamical state of their gas disks offer an interesting challenge to the more conventional view of gas and galaxies seen at Cosmic Morning Coffee Break.
Antara Basu-Zych, GSFC/Univ. of Maryland
Thursday, April 26, 2018
3:45 p.m.
LGRT 1033
Title:
Young galaxies in an old universe: A multiwavelength study into the misfits of our local universe
Abstract:
The mode of star formation in the early Universe was very different on average than star formation observed in the local Universe. Local galaxies, especially those with high star formation rates (SFRs), are generally relatively dusty, whereas galaxies at high redshifts have relatively low dust attenuations and metallicities. One advantageous approach to studying the high redshift Universe is to observe local (z=0.1-0.3) analogs, whose relative proximities allow for more detailed observations (i.e., higher spatial resolution and sensitivity to low surface brightness features) and access to individual galaxies, which may be unattainable at great distances. While there are many populations of local analogs, I will introduce one specific class: the Lyman break analogs (LBAs). Selected by their rest-frame ultraviolet luminosities and surface brightnesses, the LBAs resemble z~3 Lyman break galaxies — the key galaxy population from which the early star-formation history of the Universe has been constrained. I will discuss some of the insights gained by multi-wavelength studies of this rare breed of galaxies. In particular, the X-ray emission from LBAs might offer unique constraints on the heating of the intergalactic medium in the early universe and the expected frequency of gravitational wave events.
Susan Kassin, STScl
Thursday, April 19, 2018
3:45 p.m.
LGRT 1033
Title:
Toward a New Undertanding of Disk Galaxy Formaiton
Abstract:
One of the most important open issues in astronomy is the assembly of galactic disks. Over the last decade this has been addressed with large surveys of internal galaxy kinematics spanning the last 10 billion years of the universe. I will discuss recent results from my group that show the kinematic assembly of disk galaxies since a redshift of 2. Our results strongly challenge traditional analytic models of galaxy formation and provide an important benchmark for simulations. Furthermore, I will discuss our plans for using the multi-object spectrograph on JWST to enrich our understanding of galaxy kinematics at intermediate redshifts, and to extend our measurements to potentially unvirialized systems in the much earlier universe. From mock JWST observations of zoom-in simulations of galaxies, we are finding that interpreting these observations is not necessarily straightforward.
Regina Jorgenson, Maria Mitchell Association
Thursday, April 12, 2018
3:45 p.m.
LGRT 1033
Title:
From the Shadows to the Light: Revealing the Mysteries of Galaxy Formation and Evolution using Damped Lyman alpha Systems
Abstract:
Since their discovery over 40 years ago, the true nature of damped Lyman alpha systems (DLAs) has remained a mystery. Notoriously difficult to detect directly in emission, DLAs are typically identified by their large equivalent width absorption features in the spectra of background quasars. Thanks to the Sloan Digitized Sky Survey (SDSS), several thousand DLAs are now known and their absorption line properties well studied. Given their large column densities of neutral hydrogen gas, DLAs are believed to be the reservoirs of neutral gas for star formation across cosmic time. Evolution in DLA metallicity with redshift as well as the inferred star formation rates measured in roughly half of the DLA population indicate the presence of on-going star formation. However, numerous efforts to directly detect the galaxies that host these neutral gas-rich absorbers have failed. I will discuss the difficulties of this endeavor and the recent technological advances, namely Laser Guide Star Adaptive Optics (LGSAO) and the Keck/OSIRIS Integral Field Spectrograph, that have made possible significant strides in this field. I will present the first spatially resolved, direct detection of a DLA host galaxy and discuss the implications for the field of galaxy formation and evolution. Combining the power of absorption line and emission line diagnostics has the potential to yield unprecedented insight into the physics of high redshift galaxy formation. I will conclude by looking ahead to what advances near-future observational facilities such as JWST and ALMA will bring.
Mark Vogelsberger, MIT
Thursday, April 5, 2018
3:45 p.m.
LGRT 1033
Title:
Simulating Galaxy Formation: Illustris, IllustrisTNG and beyond
Abstract:
Cosmological simulations of galaxy formation have evolved significantly over the last years. In my talk I will describe recent efforts to model the large-scale distribution of galaxies with cosmological hydrodynamics simulations. I will focus on the Illustris simulation, and our new simulation campaign, the IllustrisTNG project. After demonstrating the success of these simulations in terms of reproducing an enormous amount of observational data, I will also talk about their limitations and directions for further improvements over the next couple of years.
Sarah Sadavoy, CfA
Thursday, March 29, 2018
3:45 p.m.
LGRT 1033
Title:
Polarization and Protostars: Evidence of a magnetized disk around a young protostar
Abstract:
Magnetic fields are expected to impact the formation of protostars and their planet-forming disks. We typically use observations of dust polarization to infer the plane-of-the-sky magnetic field, but these observations are nontrivial on the scales of protostellar disks. In particular, there are several alternative mechanisms that produce detectable dust polarization signatures in disks and previous observations appear to favor these mechanisms over magnetic fields. In this talk, I present evidence of a magnetized, Keplerian disk around VLA 1623-A using new ALMA Band 6 dust polarization observations at ~30 au resolution. VLA 1623-A is a young protostellar source with a large Keplerian disk. The ALMA data show highly ordered dust polarization, with two distinct polarization structures between the inner and the outer regions of the large disk. We find that that the inner dust polarization is well matched by models of dust self-scattering, whereas the polarization in the outer regions are well fit by a flux-frozen magnetic field disk model. Thus, the VLA 1623-A Keplerian disk has evidence of magnetization due to a poloidal magnetic field aligned with the collimated bipolar outflow. As a consequence of this magnetic field, we propose that either turbulence or non-ideal MHD processes are necessary to circumvent magnetic braking and produce the disk around VLA 1623-A.
Hui Li, MIT
Thursday, March 22, 2018
3:45 p.m.
LGRT 1033
Title:
Star cluster formation in cosmological simulations
Abstract:
Stars are mainly formed in a cluster environment and star clusters are the building blocks of the stellar component of galaxies. Current cosmological hydrodynamic simulations, though not be able to resolve individual stars, can reach the spatial and mass resolutions that are comparable to individual giant molecular clouds (GMCs). In my talk, I will present a new star formation prescription, continuous cluster formation, by considering star clusters as the unit of star formation and growing individual cluster particle continuously within single cluster formation period. The mass growth of cluster particles is resolved with high time-resolution and is terminated automatically by their own stellar feedback processes. We implement this prescription in the Adaptive Refinement Tree code to study the properties of simulated clusters in Milky Way-sized galaxies and find that it can successfully reproduce various observables. We find that the global properties of galaxies are sensitive to the feedback intensity but not sensitive to the choice of the local star formation efficiency per freefall time, eps_ff. However, on small scales, eps_ff can dramatically change the properties of model clusters. Giving that these young massive clusters form at high-z can be considered as the progenitors of the globular clusters at present, our simulations provide some interesting implications on the origin of globular clusters.
Keith Hawkins, Columbia University
Thursday, March 8, 2018
3:45 p.m.
LGRT 1033
Title:
Galactic Archaeology in the Gaia Era
Abstract:
One of the key objectives of modern astrophysics is to understand the formation and evolution of galaxies. In this regard, the Milky Way is a fantastic testing ground for our theories of galaxy formation. However, dissecting the assembly history of the Galaxy, requires a detailed mapping of the structural, dynamical chemical, and age distributions of its stellar populations. Recently, we have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys have been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.
Ryan Hickox, Dartmouth College
Thursday, March 1, 2018
3:45 p.m.
LGRT 1033
Title:
The Hidden Monsters: New Windows on the Cosmic Evolution of Supermassive Black Holes
Abstract:
At the heart of essentially every large galaxy in the Universe lies a supermassive black hole. In the past decade, surveys of the extragalactic sky have made great progress in understanding the cosmic growth of these black holes, as they "eat" surrounding material and radiate as active galactic nuclei (AGN). However, our picture of black hole evolution has remained incomplete, due to the challenges of detecting black holes that are highly obscured by gas and dust or hidden beneath thelight of their host galaxies. With the advent of new resources including hard X-ray observations from NuSTAR, mid-infrared data from WISE, and new insights from theoretical models, we can now identify millions of these “hidden” growing black holes across much the sky, and characterize the nature of their obscuration and their role in the formation of galaxies. I will describe recent efforts to characterize these "hidden” black holes, particularly highlighting work by our group at Dartmouth, and will present evidence that (at least some) powerful obscured AGN represent an evolutionary phase in the evolution of their host galaxies. Finally, I will point to the exciting potential for future of AGN population studies with the next generation of extragalactic surveys, including with NASA's Lynx concept X-ray mission.
Alex Szalay, Johns Hopkins University
Thursday, February 22, 2018
3:45 p.m.
LGRT 1033
Title:
New Challenges in Astrophysics with Large Datasets
Abstract:
The talk will present the new emerging challenges in the Era of Surveys. As these surveys produce billions of objects, the usual statistical errors are no longer the bottleneck, but we have to address a new problem of systematic errors. Supercomputer simulations are also emerging as new instruments, capable of generating petabytes of data. These Big Data challenges require new skills and techniques. As a result, the next generation of astronomers have to be equally at home in data science and astrophysics.
Gregory Laughlin, Yale University
Thursday, February 15, 2018
3:45 p.m.
LGRT 1033
Title:
'Oumuamua!
Abstract:
A rapid accumulation of observations and interpretation have followed in the wake of 1I ‘Oumuamua’s passage through the inner Solar System. We outline the consequences that this first detection of an interstellar asteroid implies for the planet-forming process, and we assess the near-term prospects for detecting and observing (both remotely and in situ) future Solar System visitors of this type. Using ‘Oumuamua as a proof-of-concept, we assess the prospects for missions that intercept ISOs using conventional chemical propulsion.
Rachel Friesen, NRAO
Thursday, February 8, 2018
3:45 p.m.
LGRT 1033
Title:
The Green Bank Ammonia Survey: Probing the evolution of star-forming regions from filaments to cores
Abstract:
The conversion of gas into stars is a key process driving the evolution of observable structures in the universe. Recent surveys of dust continuum emission of Galactic star-forming regions have revealed the ubiquity of high column density filamentary structures within molecular clouds, raising the tantalizing possibility that the star formation efficiency is strongly dependent on how these dense structures form and evolve. I will show how the combined analysis of gas dynamics and chemistry in star-forming regions is essential for understanding mass accretion onto molecular filaments, and the stability and fragmentation of filaments and embedded star-forming dense cores. Large-scale molecular line surveys are thus sorely needed. In particular, I will present science results from the Green Bank Ammonia Survey (GAS; co-PI), in which we have mapped the dense molecular gas of all the major star-forming molecular clouds within 500 pc with the GBT.
Betsy Mills, Boston University
Thursday, February 1, 2018
3:45 p.m.
LGRT 1033
Title:
Journey to the center of the Galaxy: following gas accretion from hundreds of parsecs to the black hole
Abstract:
The central 300 pc of the Milky Way is a reservoir of hot and turbulent dense gas that surrounds, and may in the future feed, a quiescent supermassive black hole. Fully constraining the physical conditions of this gas is critical for understanding how this central gas concentration will evolve, and influence future nuclear activity. I will present the results of recent work that follows the changes in physical properties of this gas as it approaches the black hole; increasing in temperature, density, and turbulence, while largely resisting the onset of star formation. One of the greatest challenges in relating the physical conditions of the gas with its location in the central potential is our edge-on view of this region, which complicates the determination of 3D positions. I will highlight the development of proxies for Galactocentric distance in this environment, and how these can be used to test of current orbital models. Our current best understanding of the 3D gas distribution indicates that there are currently several bottlenecks to accretion, where gas accumulates into dense rings. I will address the evidence for these being persistent features, and prospects for observations to identify and measure the gas flow through these boundaries. Finally, I will preview the advances in our understanding of gas accretion in nuclei that are now possible via comparisons to high resolution ALMA observations of the center of NGC 253, a galaxy with an order of magnitude more star formation and molecular gas, where this gas is not only accreting but also outflowing.
Edo Berger, Harvard University
Thursday, January 25, 2018
3:45 p.m.
LGRT 1033
Title:
Rattle and Shine: Joint Detection of Gravitational Waves and Light from the Binary Neutron Star Merger GW170817
Abstract:
The much-anticipated joint detection of gravitational waves and electromagnetic radiation was achieved for the first time on August 17, 2017, for the binary neutron star merger GW170817. This event was detected by Advanced LIGO/Virgo, gamma-ray satellites, and dozens of telescopes on the ground and in space spanning from radio to X-rays. In this talk I will describe the exciting discovery of the optical counterpart, which in turn led to several detailed studies across the electromagnetic spectrum. The results of the observations carried out by our team include the first detailed study of a "kilonova", an optical/infrared counterpart powered by the radioactive decay of r-process nuclei synthesized in the merger, as well as the detection of an off-axis jet powering radio and X-ray emission. These results provide the first direct evidence that neutron star mergers are the dominant site for the r-process and are the progenitors of short GRBs. I will also describe how studies of the host galaxy shed light on the merger timescale, and describe initial constraints on the Hubble Constant from the combined GW and EM detection.

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