Colloquia Archives

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.
Jens Kauffmann, MIT Haystack Observatory
Thursday, December 14, 2017
3:45 p.m.
LGRT 1033
Title:
Cosmic Star Formation: The View from the Milky Way
Abstract:
The formation of galaxies, stars, and planets over cosmic time is still only partially understood. What are the agents that impede the collapse of some dense molecular clouds? What are the star formation rates of these clouds? Did the star formation rate per unit cloud mass change over time? Does star formation in extreme environments, such as galaxy centers and starbursts, proceed differently from what is seen under average conditions? Here I present work that explores these questions in the Milky Way and that develops tools that allow to apply these lessons to other galaxies. In particular, I explore the density structure and the gravitational binding of molecular clouds in the disk of the Milky Way and its center. This is in part based on a new large survey (~30h) of the Central Molecular Zone with ALMA. I also discuss what molecular line emission and line ratios measured in the Milky Way can tell us about the gas conditions in other galaxies. For this work I use a new wide–field legacy survey (~600h) of galactic molecular clouds obtained at IRAM’s 30m–telescope.
Qizhou Zhang, Harvard-Smithsonian Center for Astrophysics
Thursday, December 7, 2017
3:45 p.m.
LGRT 1033
Title:
Fragmentation and protocluster formation
Abstract:
Most stars in our Galaxy are born in clusters. How molecular clouds collapse and fragment to give rise to a cluster of stars with a range of masses remains an open question. The physical conditions in clouds harboring protoclusters limit the Jeans mass to about 1 Msun. This creates a puzzle for massive star formation since dense cores much greater than 1 Msun tend to further fragment into lower mass cores. Recent sensitive and high angular resolution observations of cluster forming clouds begin to unravel the complex process of fragmentation. Turbulence support appears to play a significant role in the early formation of massive dense cores. However, turbulence alone is not sufficient to balance gravity and bring massive dense cores to a state of virial equilibrium. This challenges the theoretical picture of equilibrium star formation. My talk will review the recent progress in fragmentation studies and discuss new challenges ahead.
Ming Sun, University of Alabama in Huntsville
Thursday, November 30, 2017
3:45 p.m.
LGRT 1033
Title:
Study Baryon Physics with Galaxy Groups and Clusters
Abstract:
Galaxy groups and clusters are the least massive systems where the bulk of baryons are accounted for and also the most massive systems that are gravitationally bound. Baryons locked into stars and baryons remaining diffuse provide orthogonal constraints on cosmic structure formation, which makes groups and clusters ideal systems to study baryon physics. In this talk, I will summarize new results on two projects on baryon physics in galaxy groups and clusters. The first one will focus on X-ray scaling relations of local galaxy groups. I will also discuss our recent results on stacking. In the second part, I will focus on ram pressure stripping of cluster galaxies. The last decade has witnessed a burst of new discoveries about ram pressure stripping emerging from multi-wavelength, multi-scale observations. Stripped tails have emerged as another kind of ideal targets in clusters to study multi-phase media, along with X-ray cool cores. New results on optical, CO and X-rays will be discussed.
Aleksandar Diamond-Stanic, Bates College
Thursday, November 16, 2017
3:45 p.m.
LGRT 1033
Title:
Extreme Feedback and the Gas Around Galaxies
Abstract:
Our understanding of galaxy evolution centers around questions of how gas gets into galaxies, how it participates in star formation and black hole growth, and how it is returned to its galactic surroundings via feedback. I will present observational results on the geometry and kinematics of outflowing and inflowing gas around galaxies, including measurements of ejective feedback that is capable of shutting down star formation by removing the cold gas supply. These results have broader implications for how gas is consumed and expelled at the centers of galaxies and for the limits of feedback from stellar processes. I will also discuss prospects for characterizing the physical properties of gas in and around galaxies using multi-wavelength spectroscopy with existing and future facilities.
Vivienne Baldassare, Yale University
Thursday, November 9, 2017
3:45 p.m.
LGRT 1033
Title:
Characterizing Active Galactic Nuclei in Dwarf Galaxies
Abstract:
Massive black holes at the centers of dwarf galaxies provide our best constraints on the masses of black hole "seeds" in the early universe. However, until recently, there were very few dwarf galaxies known to contain central massive black holes. Large scale surveys have made it possible to search for signs of black hole accretion in large samples of dwarf galaxies, leading to an order of magnitude increase in the number of dwarf galaxies known to contain active galactic nuclei (AGN) in the last several years. These objects comprise a new population of AGNs. It is important to characterize this population to understand how they compare to more massive AGN, as well as to determine which factors influence the presence of AGNs in dwarf galaxies. I will discuss my multi-wavelength analysis of dwarf galaxies with AGN signatures, including their black hole masses and accretion properties, as well as discussing whether they follow well-known scaling relations between black hole mass and host galaxy properties.
Sean Andrews, Harvard-Smithsonian Center for Astrophysics
Thursday, November 2, 2017
3:45 p.m.
LGRT 1033
Title:
Observing the Evolution of Solids in Protoplanetary Disks
Abstract:
The canonical model for the formation of terrestrial planets and giant planet cores relies on an early and very efficient phase of planetesimal growth in a gas-rich circumstellar disk. But, as theorists have known for decades now, there are some formidable obstacles to meeting that requirement. Many of these problems, and potentially their solutions, are associated with the growth and migration of "pebbles" (mm/cm-sized particles) in the first few million years of a disk's lifetime. That is fortuitous, since the continuum emission from these particles in nearby disks can be readily detected and resolved with long-baseline radio interferometers (e.g., ALMA, VLA). In this talk, I will describe what we are learning about the evolution of solids from such data, including: (1) the signatures of particle growth and migration; and (2) the mounting evidence that small-scale substructures in the (gas) disk play fundamental - and perhaps mandatory - roles in the planet formation process.
Jiangtao Li, University of Michigan
Thursday, October 26, 2017
3:45 p.m.
LGRT 1033
Title:
The Hot Circum-Galactic Medium Around Massive Isolated Spiral Galaxies
Abstract:
Galaxies are not isolated. They are constantly exchanging matter, metal, and energy with their environment via the circum-galactic medium (CGM). Massive isolated spiral galaxies provide the cleanest cases in studying the very extended CGM, without strong contamination from the metal-enriched galactic feedback material or the intra-cluster/inter-galactic medium (ICM/IGM). I will present our recent X-ray studies of the hot CGM around a sample of massive isolated spiral galaxies, based on an XMM-Newton large project and archival observations. I will compare these massive spiral galaxies to other galaxy samples, and further discuss the energy budget, heating source, dynamical state, and baryon budget of the hot CGM. These X-ray observations of the hot CGM around massive spiral galaxies help us to understand some key questions affecting the co-evolution of galaxies and their environments, such as the missing galactic feedback problem and the missing baryon problem.
Paul Goldsmith, Jet Propulsion Laboratory
Thursday, October 19, 2017
3:45 p.m.
LGRT 1033
Title:
Fine Structure Lines Rediscovered
Abstract:
Fine structure lines have been employed as probes of the interstellar medium for more than 40 years, but it is really only with the extensive, high-sensitivity observations carried out with the Herschel Space Observatory and with the SOFIA airborne observatory that submillimeter fine structure transitions are being widely utilized as probes of conditions in the interstellar medium and of star formation on a range of spatial scales. These investigations extend from the boundaries of clouds in the Milky Way to ultra luminous galaxies at high redshifts. In this talk I will review some of the results from these varied programs, with an emphasis on modeling of emission from the most widely observed species, C+ and N+. The [CII] fine structure line at 158 microns wavelength is being imaged over relatively large areas, giving us new information on the “CO-dark molecular gas,” which adds substantially to the molecular mass of the Milky Way, since hydrogen there is molecular. It also probes the evolution of atomic to molecular clouds as well as the feedback produced by massive young stars. The 205 micron and 122 micron lines of ionized nitrogen arise in fully ionized regions. Comparing [CII] and [NII] emission allows determination of the fraction of [CII] coming from photon dominated regions, and better understanding of the relationship between [CII] emission and the rate of star formation. Modeling efforts have been significantly enhanced by improved calculations of collisional excitation rates. With the availability of multi-pixel high spectral resolution systems we can look forward to even more extensive imaging in these (and other) fine structure lines from suborbital as well as future space missions.
Caroline Morley, Harvard Center for Astronomy (CfA)
Thursday, October 12, 2017
3:45 p.m.
LGRT 1033
Title:
From Hot Neptunes to Temperate Earths: Observing the Atmospheres of Small Planets
Abstract:
Vast resources have been dedicated to characterizing the handful of planets with radii between Earth’s and Neptune’s that are accessible to current telescopes. Observations of their transmission spectra reveal a diversity of worlds, some shrouded in clouds and others with molecular features. I will discuss the types of clouds and hazes that can obscure transmission spectra, and show the effect that these thick clouds have on the thermal emission and reflected light spectra of small exoplanets. I present a path forward for understanding this class of planets: by understanding the thermal emission and reflectivity of small planets, we can potentially break the degeneracies and better constrain the atmospheric compositions. With future telescopes, we will also be able to access even smaller, more temperate worlds. I will discuss the recent discoveries of Earth-sized planets around small bright M dwarfs. These atmospheres may have atmospheres observable with JWST; I will explain how we might detect those terrestrial atmospheres in the coming years.
Kimberly Ward-Duong
Thursday, October 5, 2017
3:45 p.m.
LGRT 1033
Title:
Companions and Environments of Low-Mass Stars: From Star-Forming Regions to the Field
Abstract:
Comprising over 70% of the Solar Neighborhood, the vast majority of the nearest stars are M-dwarfs. Given their large number and relative ease of planet detection, M-dwarfs form the target samples for large upcoming ground- and space-based exoplanet searches. Together with the lower mass brown dwarf population, the lowest mass stars are prime systems for detailed study with high-contrast adaptive optics imaging and submillimeter interferometry. In this talk, I will describe the companion properties and environments of low-mass systems from The M-dwarfs in Multiples (MinMs) Survey, a volume-limited survey of 245 M-dwarfs within 15 pc, and the Taurus Boundary of Stellar/Substellar (TBOSS) Survey, an ongoing study of disk properties for low-mass members within the Taurus star-forming region. Direct imaging of M-dwarfs is a sensitive technique to identify low-mass companions over a wide range of orbital separation, and the high proper motion of nearby M-dwarfs enables rapid confirmation of new multiple stars. The new and archival observations of low-mass stars from the MinMs survey demonstrate lower binary frequency and closer orbital separations in comparison to those of solar-mass stars. From the TBOSS project, 885µm ALMA continuum observations of Taurus disks enable measurements of submillimeter emission from dust grains around hosts spanning the stellar/substellar boundary. The TBOSS results show a decrease in disk dust mass over the span of ~1 to ~10 Myr, and decreasing disk dust mass for lower host star mass, consistent with low incidence of giant planet detections around M-dwarfs.
Marcel Agueros
Thursday, September 28, 2017
3:45 p.m.
LGRT 1033
Title:
Setting Stellar Chronometers: The PTF(+) Open Cluster Survey
Abstract:
> While we have known for 40 years of the existence of a relation between a solar-mass star's age, rotation, and magnetic activity, observational limitations have hampered the assembly of uniform samples of rotation and activity measurements for stars spanning a wide range of ages and masses. We are still far from being able to describe fully the evolution of either rotation or activity for low-mass stars, or from being able to use rotation or activity measurements to estimate accurately the ages of isolated field stars. I will describe results from our efforts to assemble a complete sample of rotation and activity measurements for low-mass stars in six nearby open clusters ranging in age from ~100 Myr to ~3 Gyr. I will focus on our recent results for the benchmark clusters Praesepe and the Hyades, on new results for NGC 752, and on tests of models of rotational evolution that these data have enabled.
Matt Bayliss
Thursday, September 21, 2017
3:45 p.m.
LGRT 1033
Title:
The Distant Universe in High Definition
Abstract:
The most highly magnified, strongly lensed sources will forever constitute a unique and finite number of opportunities for probing the astrophysics of galaxy evolution and star formation in the distant universe. Wide-field surveys reveal on the order of a hundred strongly lensed galaxies that are magnified to have AB magnitude ~<21; these exceptional sources reveal the complexity of star formation on sub-galaxy scales. I will summarize several recent results of strong lensing-assisted studies from the Sloan Giant Arcs Survey (SGAS), with an emphasis on measurements that are uniquely enabled by the highest-magnification systems. I will also discuss the first results from a new deep spectroscopic follow-up program targeting highly magnified galaxies. The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA) is comprised of high signal-to-noise, moderate resolution rest-frame UV spectra of 15 of the brightest known lensed galaxies, at redshifts of 1.7<z<3.6. The individual MEGaSaURA spectra reveal a wealth of spectral diagnostics: absorption from the outflowing wind; photospheric absorption lines and P Cygni profiles from the massive stars that power the outflow; and faint nebular emission lines from the HII regions produced by those stars. The stacked MEGaSaURA data form the best spectrum yet obtained for star-forming galaxies at these redshifts, surpassing previous data in both wavelength coverage and spectral resolution. This stack reveals numerous rest-UV spectral diagnostics, is ideal for refining our toolkit for deducing the properties of the first galaxies and stars in the Universe with the next generation of observational facilities (e.g., JWST, 30m-class telescopes).
Jerry Ostriker
Thursday, September 14, 2017
3:45 p.m.
LGRT 1033
Title:
AGN Feedback and the Evolution of Massive Galaxies
Abstract:
Black holes, resident in the centers of galaxies, will be fed by accretion of ambient gas whenever gas reaches those central regions. This can be due to mergers, but even without mergers the evolution of the stellar populations of normal galaxies provides very large amounts of gas, as stars pass through the planetary nebula stage, the total mass release being greater than 1011 Msolar for massive ellipticals. Much of that gas will cool and fall to the centers of the systems, where it will induce starbursts and accretion events onto the central black holes with resultant AGN outbursts. The mass, momentum and energy in these outbursts can have dramatic consequences regulating the growth of the BH and quenching star formation in the ambient galaxy. Most AGN feedback treatments do not include the mass and momentum components. We follow these events with 1D, 2D and 3D hydrodynamic codes. BH growth is similar to what has been found by others, but the momentum driving produces much more energetic winds than does thermal feedback, reducing star formation and thermal X-ray emission. Observable consequences include the narrow line AGN absorption lines, shock accelerated synchrotron emitting particles and wind driven bubbles in the IGM. In addition, we find that the feedback strongly inhibits inflow, causing episodic accretion and a low “duty cycle”. The simulations help us to understand many phenomena including the black hole stellar mass relation, “quenching” of the mass growth, the X-Ray luminosity of ellipticals, the incidence of the “E+A” phenomena and the observed fact that most of the black holes found in galactic centers are found in the “off” state.
Anna Rosen, CfA-ITC
Thursday, September 7, 2017
3:45 p.m.
LGRT 1033
Title:
An Unstable Truth: How Massive Stars get their Mass
Abstract:
Massive stars play an essential role in the Universe. They are rare, yet the energy and momentum they inject into the interstellar medium with their intense radiation fields dwarfs the contribution by their vastly more numerous low-mass cousins. During their formation, the radiation pressure exerted by massive stars on the gas and dust around them can become stronger than their gravitational attraction, thereby inhibiting their growth by accretion. Therefore, detailed simulation of the formation of massive stars requires an accurate treatment of radiation. For this purpose, I will present a new, highly accurate radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. With this new tool, we performed a set of three-dimensional radiation- hydrodynamic simulations of the collapse of massive pre-stellar cores with laminar and turbulent initial conditions. We find that mass is channeled to the stellar system via gravitational and Rayleigh-Taylor (RT) instabilities through nonaxisymmetric disks and filaments that self-shield against radiation pressure while allowing for radiation to escape through optically thin regions. Furthermore, we find that turbulence and RT instabilities enhance the development of optically thick filaments that accrete onto massive stars. Our results suggest that RT features are significant and should be present around accreting massive stars throughout their formation.
Alberto Bolatto
Thursday, May 4, 2017
3:45 p.m.
LGRT 1033
Title:
Gone with the Wind? A Close Look at a Starburst-Driven Molecular Superwind
Abstract:
Galactic winds, due either to massive star formation or to active galactic nuclei, are one of the favored mechanisms necessary to regulate star formation activity and explain several aspects of present-day galaxies. In particular "cold" galactic winds, where the main ejecta are atomic or molecular, enable the cycling of large amounts of matter in and out of galaxies and provide explanation to a number of observations. Our understanding of how gas is launched and how much mass is involved in these winds is, however, rudimentary. I will present observations of the archetypal nuclear starburst galaxy NGC 253 by ALMA and HST, and show how we are starting to decipher the launching processes and mass loss rates in this example of a starburst-driven galaxy wind. Specifically, I will motivate the importance of the topic and discuss the morphology, mass, and acceleration of material in the molecular outflow. I will also show some spectacular 0.1" resolution
Aaron Lee, UMass Amherst
Thursday, April 27, 2017
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
TBA
Houjun Mo, UMass Amherst
Thursday, April 20, 2017
3:45 p.m.
LGRT 1033
Title:
Reconstructing the formation history of the local Universe
Abstract:
I will describe a method that can be used to reconstruct the initial conditions of the local universe accurately. The method is applied to the SDSS volume. High-resolution simulations of the reconstructed initial conditions have been carried to recover of the formation histories and structures observed in the local universe. I will also describe how the results can be used to study the formation and structure of the local cosmic web and of galaxies that are embedded in it.

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