Colloquia Archives

Lorenzo Sorbo, UMass Amherst Physics Department
Thursday, September 8, 2016
3:45 p.m.
LGRT 1033
Title:
“Inflation, the polarized CMB, and the Universe in a mirror”
Abstract:
While the WMAP and Planck experiments have provided the ultimate measurement of the temperature fluctuations of the Cosmic Microwave Background for a large set of scales, an improved measurement of the polarization of the CMB is still the goal of several experiments. The polarization of the CMB contains precious information about the gravitational waves that should have been produced during primordial inflation, which occurred during the first 10ˆ(-30) or seconds of life of the Universe. In a large class of models, the inflaton - the agent responsible for inflation - is not invariant under parity, the mathematical operation equivalent to looking at the Universe in a mirror. After an introduction to inflation and the CMB, I will discuss how parity violation might be detectable in the polarization of the CMB. Then I will discuss other unusual signatures that might appear in the CMB in this class of models. Finally, I will mention how this class of models might even lead to gravitational waves that might directly detected by advanced LIGO in the next few years.
John Johnson, CfA
Monday, August 29, 2016
3:45 p.m.
LGRT 1033
Title:
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Abstract:
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Juna Kollmeier (Carnegie Observatory)
Thursday, December 10, 2015
3:45 p.m.
LGRT 1033
Title:
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Jo Bovy (University of Toronto)
Thursday, December 3, 2015
3:45 p.m.
LGRT 1033
Title:
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Nicolas Bouche (IRAP)
Thursday, November 19, 2015
3:45 p.m.
LGRT 1033
Title:
The 3D View of Galaxy Formation and Gas Flows
Abstract:
TBA
Daniela Calzetti (University of Massachusetts Amherst)
Thursday, November 12, 2015
3:45 p.m.
LGRT 1033
Title:
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Daniela Calzetti
Thursday, November 12, 2015
3:45 p.m.
LGRT 1033
Title:
Star Formation Across Space
Abstract:
Fundamental questions remain unanswered in understanding galactic-scale star formation, despite many decades of investigation and progress. These include: how do stars cluster in galaxies, and how do these structures evolve in time? Do we actually have a `clustered' and `diffuse' mode of star formation? When structures remain bound (star clusters), how do their populations evolve? How are they related to the galactic-scale star formation? Is the stellar Initial Mass Function universal? How are popular star formation rate indicators affected by the recent star formation history of a galaxy? How are these effects impacting our understanding of the scaling laws of star formation with the gas reservoir? The answers to these questions inform our theories for the evolution of galaxies through cosmic times. Many of these questions are being addressed by recent projects that combine UV and high-angular resolution with the Hubble Space Telescope, and which I will describe together with the results they have obtained so far.
Cara Battersby (CfA)
Thursday, November 5, 2015
3:45 p.m.
LGRT 1033
Title:
The Milky Way Laboratory for Star Formation
Abstract:
Our home Galaxy, the Milky Way, is our closest laboratory for understanding physical processes throughout the Universe. Submillimeter observations of the cool, dense gas and dust in our Milky Way provide insights on universal processes including how stars form in both 'regular' and 'extreme' environments and how gas is organized on galactic scales. On a tour through our Milky Way Laboratory, I will discuss 1) how we can use long, skinny molecular clouds, potential "Bones of the Milky Way," to trace our Galaxy's spiral structure, 2) how large surveys of our Galaxy have revealed that star clusters continue to grow even as they are forming, and 3) how observing the Central Molecular Zone (the inner few hundred parsecs of our Galaxy) can help us learn more about the conversion of gas into stars during the peak of cosmic star formation (z~2).
Francesca Civano (Yale University)
Thursday, October 29, 2015
3:45 p.m.
LGRT 1033
Title:
X-rays from the COSMOS Survey
Abstract:
Observations indicate that supermassive black holes (SMBHs) ordinarily dwell at the centers of local galaxies. Scaling relations have been identified between SMBHs and many large scale properties of the host galaxies that point to a joint cosmic evolution. Is this evolution regulated by the galaxy or by the black hole? Is star-formation triggering black hole activity or vice versa? For more than 30 years, X-ray surveys have provided a unique and powerful tool to find and study accreting SMBHs in the distant Universe. In the past decade alone, dozens of surveys in the 0.5-10 keV band with XMM-Newton and Chandra have covered a wide range in area and X-ray flux, corresponding to a similarly wide range of volume covered in the Universe. The advent of the NuSTAR telescope, with its 3-80 keV energy response, allows us to now complement the "soft" surveys, providing the ability to characterize the whole population, including highly obscured sources. In this talk, I will focus on the results I obtained using the COSMOS field X-ray surveys. I will highlight the importance of X-ray studies to compile a (quasi-) complete and unbiased sample of active SMBHs, suitable to study the mutual relation between the supermassive black hole and its host galaxy across the Universe. I will briefly discuss about the future and the idea of achieving fainter X-ray fluxes over large areas on legacy fields to match with upcoming deep optical and near-infrared data (JWST, HyperSuprimeCam, WFIRST).
Jeyhan Kartaltepe (RIT)
Thursday, October 22, 2015
3:45 p.m.
LGRT 1033
Title:
TBA
Abstract:
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Rosalba Perna (Stony Brook)
Thursday, October 15, 2015
3:45 p.m.
LGRT 1033
Title:
Highly magnetic neutron stars: bewildering astrophysical laborat
Abstract:
Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters (SGRs) are young neutron stars characterized by high X-ray quiescent luminosities, outbursts, and, in the case of SGRs, sporadic giant flares. They are believed to be magnetars, that is neutron stars powered by ultra-strong magnetic fields. However, the diversity of their behaviours, and, especially, the observation of magnetar-like bursts from 'low-field' neutron stars, has been a theoretical puzzle. In the first part of the talk, I will discuss results of long-term MHD simulations which, by following the evolution of magnetic stresses within the neutron star crust, have allowed to relate the observed magnetar phenomenology to the physical properties of the neutron stars, and in particular to their age and magnetic field strength and topology. The dichotomy of 'high-B' field pulsars versus magnetars is naturally explained, and occasional outbursts from old, low B-field neutron stars are predicted. In the second part of the talk, I will discuss how observations of highly magnetized neutron stars can be handy tools in the cosmological domain, and in particular as a way to set constraints on the hypothetical particle 'axion'.
Dan McIntosh (UMKC)
Thursday, October 8, 2015
3:45 p.m.
LGRT 1033
Title:
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Abstract:
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Fred C. Adams (University of Michigan)
Thursday, October 1, 2015
3:45 p.m.
LGRT 1033
Title:
Effects of Clusters on their Constituent Solar Systems and Possi
Abstract:
Most stars -- and most solar systems -- form within groups and clusters. One objective of this work is to explore how these star forming environments affect the solar systems forming within them via three channels: dynamical interactions, elevated radiation fields, and increased particle fluxes. The discussion starts with the dynamical simulations, which are used to study how cluster evolution depends on system size and initial conditions. Multiple realizations of equivalent cases are used to build up a statistical description of these systems, e.g., distributions of closest approaches and radial locations. These results provide a framework from which to assess the effects of clusters on solar system formation. Distributions of radial positions are used in conjunction with UV luminosity distributions to estimate the radiation exposure of circumstellar disks. Photoevaporation models determine the efficacy of radiation in removing disk gas and compromising planet formation. The distributions of closest approaches are used in conjunction with scattering cross sections to determine probabilities for solar system disruption. Finally, we determine the distributions of radioactive nuclei that are provided to circumstellar disks, where they enhance ionization and heating. This work provides a quantitative determination of the effects of clusters on forming solar systems. In the second part of the talk, these results are used to place constraints on the possible birth environments for our own solar system.
Lorenzo Sironi (Harvard University)
Thursday, September 24, 2015
3:45 p.m.
LGRT 1033
Title:
The Journey of High-Energy Photons in Blazar Jets
Abstract:
We investigate the origin and the fate of high-energy photons in blazar jets, by means of analytical theory and first-principles particle-in-cell (PIC) kinetic simulations. In magnetically-dominated jets, magnetic reconnection is often invoked as a mechanism to transfer the jet magnetic energy to the emitting particles, thus powering the observed non-thermal emission. With 2D and 3D PIC simulations, we show that magnetic reconnection in blazar jets satisfies all the basic conditions for the emission: extended non-thermal particle distributions (with power-law slope between -2 and -1), efficient dissipation and rough equipartition between particles and magnetic field in the emitting region. TeV photons emitted by the highest energy electrons accelerated by reconnection will interact in the intergalactic medium (IGM) with the extragalactic background light, producing a dilute beam of ultra-relativistic pairs. It is a matter of recent debate whether the energy of the pair beam is lost due to inverse Compton scattering off the CMB -- resulting in ~10-100 GeV photons -- or heats the IGM via collective plasma instabilities. The astrophysical stakes are very high because of the large amount of energy and extensive cosmic volume involved in this process. We study the relaxation of blazar-induced beams in the IGM, by means of 2D and 3D PIC simulations. We find that at most 10% of the beam energy is deposited into the IGM plasma, so that at least 90% of the beam energy will be ultimately re-processed in the multi-GeV band.
Darby Dyar (Mt. Holyoke College)
Thursday, September 17, 2015
3:45 p.m.
LGRT 1033
Title:
Laser-Induced Breakdown Spectroscopy on Mars: The Long Road towa
Abstract:
Laser-induced breakdown spectroscopy (LIBS) is being used almost daily on Mars by the ChemCam instrument on the Curiosity rover. The technique uses UV, VIS, and VNIR atomic emission spectra of surface rocks and soils to quantify elemental abundances. Because only 67 standards were run on the flight instrument before launch, calibration efforts are still ongoing. LIBS is different from other types of spectroscopy because atoms interact in the plasma, such that the ratio of intensity or area of an emission line to the abundance of the element producing that line is not a direct correlation. Thus LIBS quantitative analysis is complicated by chemical matrix effects related to abundances of neutral and ionized species in the plasma, collisional interactions within plasma, laser-to-sample coupling efficiency, and self-absorption. Atmospheric composition and pressure also influence plasma intensity. This talk will detail our group's contributions to calibrating Chemcam data, both through dramatic expansion of the laboratory calibration database and development of advanced machine learning tools for their analysis. We find that many previously-unappreciated steps in spectral processing, such as baseline/continuum removal, wavelength calibration, and normalization, have dramatic effects on prediction accuracy; many of these new insights have broader applications to other types of spectroscopy as well.
Anna Sajina (Tufts University)
Thursday, September 10, 2015
3:45 p.m.
LGRT 1033
Title:
Modelling dusty galaxies
Abstract:
I will present current work on comparing GADGET+Sunrise hydrodynamic simulations with observations of IR-luminous galaxies at z~0.3-3. I will especially focus on the relative roles of stars and AGN in heating the dust, the role of merger stage on the emergent SED, and the most likely initial gas fractions of the merger progenitors. In all cases, I will compare the results of the simulations with earlier, more direct, measurements. I will address the successes and limitations of the current generation of hydrodynamic simulations.
Jason Tumlinson (STScl)
Tuesday, September 8, 2015
3:45 p.m.
LGRT 1033
Title:
The Circumgalactic Medium: A New Window on Galactic Fueling, Que
Abstract:
Why do some galaxies quench while others continue to form stars? Where is all the normal matter that galaxies should have, but don't? What happens to all the heavy elements that stars produce? The gas flows that feed galaxies and return their enriched products back to their environments are arguably the most important and least understood processes driving galaxy evolution. I will survey our group's results from Hubble's Cosmic Origins Spectrograph on the diffuse "Circumgalactic Medium" surrounding galaxies that reveal it to be a massive and richly structured medium with important roles as the mediator of galaxy accretion and feedback and a potential answer to some of these open questions about galaxies.
Meredith Hughes (Wesleyan University)
Thursday, April 23, 2015
3:45 p.m.
LGRT 1033
Title:
Circumstellar disks and planet formation with ALMA
Abstract:
Circumstellar disks provide the raw material and initial conditions for planet formation. Millimeter-wavelength interferometry is a powerful tool for studying gas and dust in planet-forming regions, and it is undergoing an immense leap in sophistication with the advent of the ALMA interferometer. I will discuss some ways in which we are using millimeter-wavelength interferometry to study the process of planet formation in circumstellar disks, with particular emphasis on the kinematics of turbulence in protoplanetary disks and the surprising presence of gas in debris disks around main-sequence stars.
Tracy Webb (McGill University)
Thursday, April 16, 2015
3:45 p.m.
LGRT 1033
Title:
The Growth of the Most Massive Galaxies in the Highest Density R
Abstract:
The most massive galaxies in the local universe reside at the centres of galaxy clusters. Often called Brightest Cluster Galaxies (BCGs), they exhibit, as a class, highly uniform properties and are distinct from the general galaxy population. This suggests formation processes which are themselves distinct from those which dominate in massive galaxies outside of cluster cores. The mass growth of BCGs is likely linked to the overall physics of hierarchical structure formation on galaxy cluster scales, including the fundamental processes of gas cooling, star formation, energy feedback and galaxy mergers, at the centers of giant dark matter halos. In this talk I will present new results from the largest study of high-redshift BCGs conducted to date, drawn from the SpARCS optical/NIR cluster survey. Using archival infrared data we show the star formation rate within BCGs increases to z~2, and can add as much mass to the BCG population as the previous standard model of growth by dry mergers. At low redshifts, and in X-ray/SZ selected clusters, the rare examples of star forming BCGs appear to be fed by large-scale cooling flows. However, the first of the SpARCS systems we have studied in detail, SpARCS1049, has revealed a very different phenomenon - a train-wreck of a galaxy merger at the center of the cluster. This is the first example of such a process in high-redshift cluster cores and may represent a new phase of BCG evolution, previously unaccounted for.
Pieter van Dokkum (Yale University)
Thursday, April 9, 2015
3:45 p.m.
LGRT 1033
Title:
A history of the dense cores of massive galaxies over the past
Abstract:
Owing to several large surveys with new instruments on HST we are now able to measure the basic properties of galaxies over most of cosmic history. This talk will highlight one such survey, 3D-HST, which has obtained spectra and images of many thousands of faint galaxies. I will also discuss the next step, that is, how to reconstruct the histories of different kinds of galaxies from this wealth of data. This reconstruction has now been done for galaxies like the Milky Way, enabling us to look at "baby pictures" of galaxies like our own.
Katja Poppenhaeger (CfA)
Thursday, April 2, 2015
3:45 p.m.
LGRT 1033
Title:
A high-energy view of exoplanets and their host stars
Abstract:
Many exoplanets orbit their host stars at close distances, with orbital periods of only a few days. For such systems, the magnetic activity of the planet-hosting star determines the high-energy environment of its exoplanet and is an important factor in understanding the exoplanetary evolution. X-ray and UV observations allow us to determine the high-energy input into the exoplanetary atmosphere, which is the main driver of planetary evaporation. X-ray emission is also directly connected to the stellar rotation; without external input of angular momentum, the star spins down over time due to magnetic braking. However, if there is tidal interaction between a star and its Hot Jupiter, the spin-down of the host star may not follow the usual age/rotation/activity relations. I will discuss recent X-ray and UV observations which provide insights into both the evaporation of close-in exoplanets and a possibly altered age-activity relationship for host stars of such planets.
Rob Simcoe (MIT)
Thursday, March 26, 2015
3:45 p.m.
LGRT 1033
Title:
Heavy Element Enrichment in Early Intergalactic and Circumgalact
Abstract:
Infrared photometric surveys are discovering numerous quasars at z > 6.5, enabling absorption investigations of neutral Hydrogen and its associated heavy elements at the tail end of the reionization epoch. I will describe the status of my group's ongoing IR and optical spectroscopic surveys targeting metal pollution in the first few Gyr. Beginning with a systematic study of absorption candidates for "cold flows" at z~3, I will move on to describe a 100-sightline survey of circumgalactic MgII pollution with the Magellan/FIRE spectrometer, extending prior optical measurements (restricted to z<2) out to z~6.5. I will also describe our latest constraints on the CIV mass density and intergalactic carbon enrichment at z = 4.5-6.5. Finally, I will outline our first attempt at measuring actual chemical abundances in the z > 7 universe, and discuss their significance for reionization and the formation of the first stars.
Beth Willman (Haverford)
Thursday, March 12, 2015
3:45 p.m.
LGRT 1033
Title:
Triumphs and tribulations of near-field cosmology with wide-fiel
Abstract:
Over the last decade, wide-field surveys have revolutionized our view of the Milky Way’s stellar halo and dwarf galaxy population. Much of this observational progress has been motivated by a series of apparent “crises” for our cosmological model: the missing satellites problem, too big to fail, and the apparent planar distribution of dwarf satellite galaxies. These challenges have effectively functioned as flashlights pointing us to interesting galaxy formation physics. I will highlight related observational progress in our understanding of galaxy formation using near-field observations. I will then focus on the limiting impacts of observational bias and ways that current and future surveys will be used to tackle these biases. In particular, I will present new predictions for the number of Milky Way dwarf galaxies expected to be discovered in DES and LSST, RR Lyrae stars as a tool to discover dwarf galaxies in previously unexplored territory, and the use of M giant stars to map the Milky Way’s halo beyond its virial radius.
Elizabeth Blanton (Boston University)
Thursday, March 5, 2015
3:45 p.m.
LGRT 1033
Title:
Extragalactic Jets as Probes of Clusters of Galaxies
Abstract:
I will present multi-wavelength (X-ray, optical, infrared, and radio) observations of clusters of galaxies, including in-depth study of nearby objects and a survey of distant systems. Cooling of the hot intracluster medium in cluster centers can feed the supermassive black holes in the cores of the dominant cluster galaxies leading to AGN outbursts. This AGN feedback can reheat the gas, stopping cooling and large amounts of star formation. Most relaxed, cool core clusters host powerful AGN in their central galaxies and these AGN can significantly affect the distribution of e.g., temperature and abundance on cluster scales. AGN heating can come in the form of shocks, buoyantly rising bubbles that have been inflated by radio jets and lobes, and sound wave propogation. Sloshing of the cluster gas, related to minor, off-center interactions with galaxy sub-clusters or groups also affects the distribution of temperature and abundance on large scales. This sloshing gas can interact with the AGN's radio-emitting jets and lobes causing them to bend. This bending is also found in AGN jets and lobes embedded in clusters undergoing major, head-on cluster, cluster mergers. Since this bending is a signature of interaction within clusters, bent, double-lobed AGN observed in the radio can be used as beacons for clusters of galaxies at high redshifts. I will describe our large sample of high-redshift, bent-double radio sources that were observed in the infrared with Spitzer and that have yielded approximately 200 new, distant clusters of galaxies with z > 0.7. These clusters will serve as important laboratories for studying galaxy evolution and cosmology.
Andrey Kravtsov (University of Chicago)
Thursday, February 26, 2015
3:45 p.m.
LGRT 1033
Title:
Order out of chaos: formation of galaxies in hierarchical univer
Abstract:
Galaxy formation is a complex, hierarchical, highly non-linear process, which involves gravitational collapse of dark matter and baryons, supersonic, highly compressible and turbulent flows of gas, star formation, stellar feedback, as well as heating, cooling, and chemical processes. All of these processes appear to be critically important in shaping properties of galaxies. At the same time, despite the apparent complexity of these processes and the ways they interact, observed properties of galaxies exhibit a number of striking regularities, such as tight correlations between galaxy sizes, masses, luminosities, and internal velocities and surprisingly tight correlations between properties of stars and gas in galaxies and the mass and extent of their parent halos dominated by dark matter. Existence of such correlations indicates that powerful processes operate to bring order out of chaos. In this talk I will discuss some insights based on the recent work aimed to understand this aspect of galaxy formation, focusing on some specific issues in how mass of galaxies and their host dark matter halos is assembled and in how thermodynamics processes of diffuse gas, star formation, and stellar feedback conspire to produce galaxies quite close to observed systems.
Jill Naiman (CfA)
Thursday, February 19, 2015
3:45 p.m.
LGRT 1033
Title:
Gas Retention and Accumulation in Dwarf Galaxies: Implications
Abstract:
The effective supply and retention of gas in shallow gravitational potentials is a problem with implications in a diverse set of astrophysical systems. In particular, the magnitude of gas flows into mature dwarf galaxies can have large impacts on the star formation histories in these systems. In this talk, computational techniques will be used to show how such such weakly bound gravitational structures might be able to accumulate gas effectively. The implications for star formation in dwarf galaxies after their incorporation into a larger host halo will be presented.
Blakesley Burkhart (CfA)
Thursday, February 12, 2015
3:45 p.m.
LGRT 1033
Title:
New Frontiers of Magnetized Turbulence in the Multiphase Interst
Abstract:
The current paradigm of the ISM is that it is a multiphase turbulent environment, with turbulence affecting many important processes. For the ISM this includes star formation, cosmic ray acceleration, and the evolution of structure in the diffuse ISM. This makes it important to study interstellar turbulence using the strengths of numerical studies combined with observational studies. I shall discuss progress that has been made in the development of new techniques for comparing observational data with numerical MHD simulations in the star forming molecular medium and in neutral gas as traced by 21 cm emission.
Brice Menard (JHU)
Thursday, February 5, 2015
3:45 p.m.
LGRT 1033
Title:
De-projecting astronomical surveys
Abstract:
Observations of the sky are inherently a 2-dimensional measurement of flux density on the sphere of the sky. For astrophysical studies, however, one usually needs the knowledge of 3d positions, for example to convert an angle into a physical scale or a brightness into a luminosity. In the context of extragalactic surveys, distance or redshift information is usually done with "photometric redshifts", which rely on strong assumptions and often lead to problematic estimates. In this talk I will how it is possible to instead use clustering measurements and infer redshifts for any type of extragalactic sources. I will show applications of this "clustering-redshift" technique to various datasets at UV, optical, IR and radio wavelengths, and show a number of surprises.
Anne Jaskot (Smith/UMass/Five College Astronomy Department)
Thursday, January 29, 2015
3:45 p.m.
LGRT 1033
Title:
HI, Stars, and Ionizing Photons: Gas Cycles and LyC Escape in Lo
Abstract:
Neutral hydrogen gas plays a key role in galaxy evolution, by both providing the raw material for star formation and responding to the resulting stellar feedback. Two samples of low-redshift starburst galaxies, ALFALFA H-alpha and the Green Peas, give new insights into the relationship between HI gas, star formation, and UV radiation. Using 565 starburst and non-starburst galaxies from the HI-selected ALFALFA H-alpha survey, I will analyze the role of the HI gas supply in driving high levels of star formation and discuss factors that may promote efficient HI to H2 conversion. With both ground-based and HST observations of the Green Pea galaxies, some of the most extreme low-mass starbursts in the nearby universe, I will focus on the propagation of ionizing radiation through the neutral gas. In particular, the Green Peas may be leaking ionizing radiation into the intergalactic medium and are therefore potential analogs of the galaxies responsible for cosmic reionization.
Josh Shiode, John N. Bahcall Public Policy Fellow (American Astronomical Society)
Thursday, January 22, 2015
3:45 p.m.
LGRT 1033
Title:
There's Government in Your Science
Abstract:
The majority of basic science research in the United States — including that in the astronomical sciences — is funded by the federal government. This is both good and bad. Good because there are a lot of resources available, though basic research funding is but a small fraction of the total federal budget. Bad because individual scientific projects, and the careers of the scientists involved, can be affected by political winds they would otherwise never feel. In this talk, I’ll try to convey a sense of those winds. I’ll focus on the process of policymaking and long-term trends relevant to the scientific enterprise, and we’ll explore how individual scientists and science advocates can play a role in the political and policymaking process.

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