Colloquia Archives

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.
Liz McGrath, Colby College
Thursday, April 13, 2017
3:45 p.m.
LGRT 1033
Title:
Massive Quiescent Disks in the Early Universe
Abstract:
Observations in the local Universe suggest that the mechanism responsible for quenching star formation in galaxies may be intimately linked to their structural transformation from disks to spheroids. In order to test quenching scenarios, however, it is vital to look beyond the local Universe and identify the first generation of quiescent galaxies at high redshift. Using CANDELS, we have examined the rest-frame optical morphologies for a sample of massive, quiescent galaxies at z>1 and find that a significant fraction (~30%) have morphologies dominated by exponential disks. The persistence of massive disks, long after star formation has ceased, implies that in at least some cases quenching precedes morphological transformation. I'll examine what constraints these observations place on the mechanisms responsible for quenching star-formation in the first generation of quiescent galaxies at z~2 and discuss them in context with an emerging picture of massive galaxy formation and evolution.
Nia Imara, CfA
Thursday, April 6, 2017
3:45 p.m.
LGRT 1033
Title:
Star Formation Then and Now
Abstract:
Understanding the initial conditions of star formation persists as one of the leading challenges of astrophysics today. How do molecular clouds, the sites of star formation, form and evolve? How do molecular clouds acquire their dense, star forming gas? In this presentation, I present new results on both the large-scale, diffuse gas, and small-scale, dense gas in molecular clouds. In one study on the atomic gas associated with Galactic molecular clouds having a range of star formation rates, I discuss how a simple statistical tool can be used to characterize some important physical properties of the atomic gas envelopes. In another study, multiwavelength radio and infrared observations are used to examine the role of filamentary structure and molecular outflows in an early stage of stellar evolution. Finally, I will discuss recent theoretical predictions on the consequences of star formation in the early universe. In particular, I will show that dust, the byproduct of massive star formation, may pervade the intergalactic medium and impact measurements of cosmological parameters.
Ilse Cleeves, CfA
Thursday, March 30, 2017
3:45 p.m.
LGRT 1033
Title:
From Disks to Planets Through the Astrochemical Lens
Abstract:
During the first few Myr of a young star's life, it is encircled by a disk made up of molecular gas, dust, and ice, materials that form the building blocks for future planetary systems. Improvements in observational spatial resolution and sensitivity have allowed us to characterize the protoplanetary disk environment in great detail. Recent observations with the Atacama Large Millimeter/Submillimeter Array (ALMA) have shed light on the particularly key role of the differential evolution of the gas and dust disk disks' chemical composition and the structure of their rocky/solid and gaseous components, which together feed young terrestrial and gas giant planets. I will discuss recent results and new puzzles regarding our understanding of protoplanetary disk chemical and structural evolution, along with future avenues to detect individual young planets forming in situ.
Anna Frebel, MIT
Thursday, March 23, 2017
3:45 p.m.
LGRT 1033
Title:
Observing the signature of a single prolific r-process event in an ultra-faint dwarf galaxy
Abstract:
The heaviest chemical elements in the periodic table are synthesized through the rapid neutron-capture (r-) process but the astrophysical site where r-process nucleosynthesis occurs is still unknown. The best candidate sites are ordinary core-collapse supernovae and mergers of binary neutron stars. Through their stars, 13 billion year old ultra-faint dwarf galaxies preserve a "fossil" record of early chemical enrichment that provides the means to isolate and study clean signatures of individual nucleosynthesis events. Until now, ultra-faint dwarf galaxy stars displayed extremely low abundances of heavy elements (e.g. Sr, Ba). This supported supernovae as the main r-process site. But based on new spectroscopic data from the Magellan Telescope, we have found seven stars in the recently discovered ultra-faint dwarf Reticulum II that show extreme r-process overabundances, comparable only to the most extreme ancient r-process enhanced stars of the Milky Way's halo. This r-process enhancement implies that the r-process material in Reticulum II was synthesized in a single prolific event. Our results are clearly incompatible with r-process yields from an ordinary core-collapse supernova but instead consistent with that of a neutron star merger. This first signature of a neutron star merger in the early universe holds the key to finally, after 60 years, identifying the cosmic r-process production site, in addition to being a uniquely stringent constraint on the metal mixing and star formation history of this galaxy from the early universe.
Mark Gurwell, CfA
Thursday, March 9, 2017
3:45 p.m.
LGRT 1033
Title:
The Submillimeter Array: Past, Present, & Future
Abstract:
The Submillimeter Array (SMA), located just below the summit of Mauna Kea on the Big Island of Hawaii, is a pioneering radio interferometer designed for arc-second imaging in the submillimeter spectral range. Designed initially for imaging molecular lines and dust continuum in cold interstellar clouds, the SMA has also found application in a wealth of scientific fields, including star formation, disk studies, solar system observations, nearby galaxies, high-z galaxies (including lensing systems), observations of high energy phenomena (black holes, gamma-ray bursts, supernovae, and blazars), and polarized emission from aligned dust grains in a range of environments. In addition to its outstanding record in astronomical research, the SMA is a world leader in the design of wide-bandwidth, high-frequency radio receivers for astronomy. To leverage this expertise, the SMA just commissioned a next generation correlator which vastly increases total bandwidth (to 8 GHz/sideband per polarization) while retaining high spectral resolution (140 kHz) across the entire processed spectral range. I will discuss the SMA's enhanced science capabilities, as well point toward even further upgrade plans.
Christina Williams, University of Arizona
Thursday, March 2, 2017
3:45 p.m.
LGRT 1033
Title:
The nature of compact quenched galaxies: new perspectives from current surveys, and future science with JWST
Abstract:
The cessation of star-formation in galaxies remains a poorly understood process, despite being one of the most influential events in the evolution of galaxies. It is now known that high stellar density, or compactness, is strongly associated with this process, although the nature of this association is also poorly understood. I will review recent progress on understanding compact and quenched galaxies at high-redshift, and present new results that help constrain the reason for the association between compactness and quenching. Understanding the early evolutionary development of these first quenched galaxies is a major science goal of the James Webb Space Telescope, and I will discuss how future surveys with this facility will resolve outstanding questions about the nature of quenched galaxies. Finally, I will describe the plans for the Guaranteed Time Observations with this new groundbreaking facility.
Kate Follette, Amherst College
Thursday, February 23, 2017
3:45 p.m.
LGRT 1033
Title:
High-Contrast Imaging of Extrasolar Planets and Circumstellar Disks
Abstract:
Of the thousands of known extrasolar planets, why are the dozen or so directly imaged exoplanets among the most important despite their apparently anomalous properties within the general exoplanet population (>10AU, >2MJ)? What are the prospects for (and recent successes in) detecting younger, lower mass and/or closer-in planets via direct imaging? I will discuss the current state of the art in the field of high contrast imaging of extrasolar planets and circumstellar disks, with a particular emphasis on a subset of objects that host both disks and (likely) planets - the so-called “transitional disks”. These young circumstellar disks are almost certainly actively undergoing planet formation, and yet the presence of disk material complicates our ability to isolate light from planets and/or protoplanets embedded within them. I will discuss my recent experiences “killing” one exoplanet candidate lying at/inside a transitional disk gap, and confirming another.
Shuinai Zhang, UMass Amherst
Thursday, February 16, 2017
3:45 p.m.
LGRT 1033
Title:
X-ray Spectroscopy of Galaxy Feedback Processes
Abstract:
AGN or starbursts play an important role in regulating galaxy evolution. However, underlying processes of such galaxy feedback remain very uncertain. I will show how X-ray spectroscopy of nearby galaxies can provide new insights into these processes. The central region of M31 is currently quiescent in both AGN and star formation, but shows strong indications for recent AGN activity in X-ray spectra obtained from XMM-Newton grating instruments. We find that these spectra can be well described by an AGN-relic model of diffuse hot gas, which we have developed, suggesting that M31 is a bright AGN about 0.4 Myrs ago. Applying this X-ray spectroscopic method to many galaxies, one could, in principle, investigate the recurrence history or frequency of AGN. By contrast, M82 is the prototype of superwinds driven by nuclear starbursts. The grating spectrum of the diffuse X-ray emission from this galaxy shows strong evidence for the contribution from charge exchange (CX) at interfaces between hot and cool gases of the superwind. Modelling this contribution enables us to estimate the interface area, which is about one order of magnitude greater than the simple geometric cross section of the superwind and must be greatly enhanced by turbulent mixing. These sample studies demonstrate the power of the X-ray spectroscopy, which will be greatly improved by upcoming X-ray missions, in our understanding of the physical processes in galaxy feedback.
Shy Genel, Center for Computational Astrophysics
Thursday, February 2, 2017
3:45 p.m.
LGRT 1033
Title:
Lagrangian analysis of mesh-based cosmological simulations
Abstract:
Galaxies form hierarchically, hence the material making up z=0 galaxies may be spread over many Megaparsecs of the IGM and in numerous progenitor galaxies at cosmic epochs z>0. Cosmological simulations allow following the time evolution of the individual 'mass elements' that make up galaxies. I will discuss techniques for performing so-called 'Lagrangian analysis' in mesh-based hydro codes using tracer particles, and several unique applications of such an approach. Among them are studies of the thermal histories of gas accretion, the baryon cycle, the angular momentum acquisition of galaxies, and the origin of stellar IMF variations.
Geoffrey Clayton, Louisiana State University
Thursday, January 26, 2017
3:45 p.m.
LGRT 1033
Title:
Dust Formation in Core-Collapse Supernovae
Abstract:
Recent detections of large amounts of dust in high redshift galaxies suggest that core collapse supernovae (CCSNe) may play an important role in the dust budget of the universe. In these high-z galaxies, with ages less than 1 Gyr, there has not been enough time for low-mass AGB stars to form, so much of the dust may come from high-mass stars in SN explosions. For the past decade, we have been following numerous, nearby CCSNe with Gemini, HST, Spitzer, Herschel, ALMA, and soon, JWST to look for indications of dust formation, which appear within the first few years of explosion. In particular, I will discuss the recent discovery of a large amount of cold dust associated with SN 1987A. I will discuss these results and their implications for SNe as major dust contributors in the universe.
Robert Fisher, Physics Department, UMass Dartmouth, Harvard-Smithsonian CfA Institute for Theory and Computation ('16 - '17)
Thursday, December 8, 2016
3:45 p.m.
LGRT 1033
Title:
The Fate of Exploding White Dwarfs
Abstract:
Type Ia supernovae play an important role as standardizable candles for cosmology, providing one of the most important probes into the nature of dark energy. Yet, the nature of the stellar progenitors which give rise to Type Ia supernovae remains elusive. For decades, the leading model explaining Type Ia supernovae properties consisted of a white dwarf accreting to near the Chandrasekhar mass, in the single-degenerate channel. More recently, a variety of lines of evidence point instead towards merging binary white dwarfs, in the double-degenerate channel, as the progenitors of Type Ia supernovae. In this talk, I will focus upon recent advances at the interface between observation and theory which will help crack the Type Ia progenitor problem. In particular, I will present recent multidimensional numerical simulations of both the double-degenerate and single-degenerate channels which I have undertaken with my students and collaborators. I will discuss how these models make clearly-defined predictions for current and planned late-time observations of nearby Type Ia supernovae, which will definitively establish the nature of their stellar progenitors.
Karin Oberg, CfA
Thursday, December 1, 2016
3:45 p.m.
LGRT 1033
Title:
Chemistry during planet formation
Abstract:
Exo-planets are common, and they span a large range of compositions. The origins of this compositional diversity are largely unconstrained. Among planets that are Earth-like, a second question is how often such planets form hospitable to life. A fraction of exo-planets are observed to be ‘physically habitable’, i.e. of the right temperature and bulk composition to sustain a water-based prebiotic chemistry. This does not automatically imply, however, that they are rich in the building blocks of life, in organic molecules of different sizes and kinds, i.e. that they are chemically habitable. In this talk I will argue that characterizing the chemistry of protoplanetary disks, the formation sites of planets, is key to address both the origins of planetary bulk compositions and the likelihood of finding organic matter on planets. The most direct path to constrain the chemistry in disks is to directly observe it. In the age of ALMA it is for the first time possible to image the chemistry of planet formation, to determine locations of disk snowlines, and to map the distributions of different organic molecules. Recent ALMA highlights include constraints on CO snowline locations, the discovery of spectacular chemical ring systems, and first detections of more complex organic molecules. Observations can only provide chemical snapshots, however, and even ALMA is blind to the majority of the chemistry that shapes planet formation. To interpret observations and address the full chemical complexity in disks requires models and laboratory experiments, and their contribution to our current state of knowledge will be highlighted throughout the talk.
Caitlin Casey, University of Texas Austin
Thursday, November 17, 2016
3:45 p.m.
LGRT 1033
Title:
The Ubiquity of Coeval Starbursts in Massive Galaxy Cluster Progenitors
Abstract:
The Universe’s largest galaxy clusters likely built the majority of their massive >10^11 M⊙ galaxies in simultaneous, short-lived bursts of activity well before virialization. The most challenging observational hurdle in identifying such pre-virialized “protoclusters” is their very large volumes, ~10^4 comoving Mpc^3 at z > 2, subtending areas ~half a degree on the sky. Thus the contrast afforded by an overabundance of very rare galaxies in comparison to the background can more easily distinguish overdense structures from the surrounding, normal density field. I will present five 2 < z < 3 proto-clusters from the literature which are found to contain up to 12 dusty starbursts or luminous AGN galaxies each, a phenomenon that is unlikely to occur by chance even in overdense environments. These are contrasted with three higher-redshift (4 < z < 5.5) dusty star-forming galaxy (DSFG) groups, whose evolutionary fate is less clear. Measurements of DSFGs’ gas depletion times suggest that they are indeed short-lived on ~100 Myr timescales, and accordingly the probability of finding a structure containing more than 8 such systems is 0.2%, unless their ‘triggering’ is correlated on very large spatial scales, ~10 Mpc across. The volume density of DSFG-rich protoclusters is found to be comparable to all >10^15 M⊙ galaxy clusters in the nearby Universe, a factor of five larger than expected in some simulations. Some tension yet exists between measurements and simulations. However, improved observations of protoclusters over large regions of sky will certainly shed more light on the assembly of galaxy clusters, thus fundamental parameters governing cosmology, and also the role of environment in shaping the formation and evolution of galaxies.

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