Physics from observations of merging galaxy clusters

Maxim Markevitch

NASA GSFC

Tuesday, January 25, 2011

Abstract

Mergers of galaxy clusters -- some of the most energetic events in the Universe -- produce disturbances in hot intracluster medium, such as shocks and cold fronts, that can be used as tools to study the physics of galaxy clusters. X-ray observations of shock fronts provide information on the shock Mach number and velocity, and for well-observed shocks, constrain the microphysical properties of the intracluster plasma. Cold fronts may constrain viscosity and the structure and strength of the cluster magnetic fields. Combined with radio data, these observations also shed light on the production of ultrarelativistic particles that are known to coexist with the cluster thermal plasma. While cold fronts are commonly seen in merging and relaxed clusters, only a few unambiguous shock fronts have been seen in X-rays so far. This talk will summarize the current X-ray observations of cluster mergers, as well as some recent radio data and high-resolution hydrodynamic simulations.

Circumstellar Disk Composition and Evolution

Alycia Weinberger

Carnegie

Tuesday, February 8, 2011

Abstract

This talk will discuss circumstellar disks as the birthplaces of planetary systems. Observations of the composition of dust over the transition from protoplanetary to debris disks teach us about the evolution of disks, the timescales for planet formation within them, and the processes that determine planetary compositions. In particular, I will show visual and�near-infrared multicolor imaging and mid-infrared spectroscopy of disks and discuss evidence for carbon rich chemistries in some, as well as evidence that planets may be modifying their environments.

High Resolution Measurements of the Sunyaev-Zel'dovich Effect

Tony Mroczkowski

UPenn

Tuesday, February 15, 2011

Abstract

Redshift independent observations of the Sunyaev-Zel'dovich effect (SZE) have long been sought as a probe of cosmology, and are finally detected new galaxy clusters. These measurements offer a low resolution window on the universe, but are in units of integrated SZE flux, a proxy expected to scale well with cluster total mass because it tracks thermal energy. I motivate a new way to access a cluster's mass directly from SZE observations based on the virial theorem and a few simplifying assumptions. Moving beyond this simple picture, I then discuss recent high angular resolution (9") SZE observations with MUSTANG, a 90-GHz bolometric receiver on the 100 meter Green Bank Telescope, that have taken a step toward using the SZE as a probe for cluster astrophysics. MUSTANG has now imaged several massive clusters of galaxies in some of the highest-resolution SZE imaging to date, revealing complex pressure substructure within the hot intra-cluster gas in merging clusters. Here I focus on two merging, intermediate redshift clusters: MACS J0744.8+3927 and MACS J0717.5+3745. In MACS J0744.8+3927, the MUSTANG observation revealed shock-heated gas previously undetected in X-ray (or any) observations, while preliminary observations of MACS J0717.5+3745 confirm the suspected shock found in X-ray and radio observations. Finally, I discuss prospects for MUSTANG2, the proposed successor to MUSTANG with over an order of magnitude higher sensitivity.

Bio

I did my thesis at Columbia University with Amber Miller, working to build and analyze data from the Sunyaev-Zel'dovich Array (SZA, now part of CARMA). Combining data from the SZA with Chandra surface brightness data, I was able to derive cluster temperatures reasonably consistent with those found in X-ray spectral analyses of the same clusters. As a postdoc at U Penn, I worked with Mark Devlin on instrumentation for the polarization upgrade to the Balloon-borne Large Aperture Submm Telescope (BLAST-Pol). Now an Einstein fellow (still at Penn), I am working on analysis of some of the highest resolution and sensitivity SZ observations to date, made with the 100-m Green Bank Telescope (GBT) using the MUSTANG 90-GHz bolometric receiver. I am also co-I on MUSTANG2, the proposed successor to MUSTANG designed to provide 30x higher sensitivity, which if built will offer an order of magnitude faster mapping of continuum emission at 90 GHz than ALMA will.

X-ray Emissions from Thunderstorms and Lightning

Joseph R. Dwyer

Florida Institute of Technology

Tuesday, March 1, 2011

Abstract

Until very recently, lightning was thought to be an entirely conventional discharge, involving only low-energy (a few eV) electrons. This picture changed completely with the discovery of intense x-ray and gamma-ray emission from natural cloud-to-ground lightning, rocket-triggered lightning and thunderstorms. Indeed, the gamma-rays generated by thunderstorms can reach 100 MeV and are so intense that bright bursts of these gamma-rays are observed from space, 600 km above the storms, as Terrestrial Gamma-ray Flashes (TGFs). In addition, it has been found recently that thunderstorms launch beams of electrons and positrons into the inner magnetosphere, where they are observed by spacecraft thousands of kilometers away. These energetic emissions cannot be produced by conventional discharges in air, and so the presence of x-rays and gamma-rays strongly implies that relativistic runaway electrons, accelerated in air by strong electric fields, play a role in thunderstorm and lightning processes. In this talk, I will give an overview of the x-ray and gamma-ray observations of thunderstorms and lightning, along with new results on terrestrial gamma-ray flashes. Finally, the physics of relativistic runaway breakdown will be presented, including some very recent theoretical advances.

Bio

Dr. Joseph Dwyer is a professor of Physics and Space Sciences at the Florida Institute of Technology in Melbourne, Florida. He received is PhD in physics from the University of Chicago in 1994 and worked as a research scientist at Columbia University and the University of Maryland before joining the faculty at Florida Tech in 2000. After moving to Florida, Dr. Dwyer became interested in lightning physics and his research now centers on understanding the high-energy radiation produced by thunderstorms and lightning.

Evidence for Universal Gamma-Ray Burst Pulse Properties

Jon Hakkila

College of Charleston

Tuesday, March 8, 2011

Abstract

The most luminous sources in the universe, gamma-ray bursts (GRBs), also appear to have among the simplest emission characteristics. The bulk of GRB prompt emission can be de-convolved into distinct pulses having simple, correlated behaviors; these behaviors include pulse duration, peak luminosity, lag, spectral hardness, and shape. Because these behaviors have not always been recognized and because pulses are faint and often overlap, many correlated properties attributed to GRB bulk emission are in fact pulse characteristics that have been smeared out when their properties have been indiscriminately combined. And yet, remarkably, correlated pulse properties are observed in both the Long and Short GRB classes, are significantly more pronounced than observed cosmological effects caused by time dilation and the inverse square law, and appear to indicate an energy relaxation process. This talk summarizes the correlated characteristics of GRB pulses, along with the repercussions of the pulse paradigm on theoretical GRB models.

Bio

Jon Hakkila received M.S. and Ph.D. degrees in Astronomy from New Mexico State University. He served as a faculty member at Minnesota State University at Mankato from 1986 to 2000 before taking his current position as department chair and professor of physics and astronomy at the College of Charleston. His current research interests are in gamma-ray bursts, galactic structure, and astrostatistics/ astroinformatics. He was a member of the Burst And Transient Source Experiment Science team on the Compton Gamma-Ray Observatory, contributed to the development of the National Virtual Observatory, is a member of the Large Synoptic Survey Telescope Informatics and Statistics collaboration, and serves on the Stellar Oscillations Network Group (SONG) steering committee.

Dark Matter and Baryons in Dwarf Spheroidal Galaxies

Rosemary Wyse

JHU

Tuesday, March 15, 2011

Abstract

The nature of dark matter is one of the outstanding questions of astrophysics. The internal motions of member stars reveal that the lowest luminosity galaxies in the Local Group are the most dark-matter dominated. New large datasets allow one to go further, and determine systematic properties of their dark-matter halos. In addition, the chemical abundances of the stars constrain star formation, gas flows and `feedback'. I will summarize recent results, emphasizing the critical role of dwarf spheroidal galaxies in understanding both dark matter and baryonic processes that shape galaxy evolution.

Bio

July 1993 - present : Full Professor, The Johns Hopkins University.
July 1990 - June 1993 : Associate Professor, The Johns Hopkins University.
Jan 1988 - June 1990: Assistant Professor, The Johns Hopkins University.
Aug 1986-Jan 1988 and Aug-Dec 1985 : UC President's Fellow, UC Berkeley.
January-July 1986 : Postdoctoral Research Fellow, Academic Affairs, Space Telescope Science Institute.
September 1983-July 1985 : Parisot Postdoctoral Fellow at UC Berkeley.
September 1982-August 1983 : Lindemann Fellow of the English Speaking Union of the Commonwealth. Held at Princeton University and University of California, Berkeley. Education
1978-1982 : University of Cambridge, Institute of Astronomy.
Degree : Ph.D. (Supervisor : Bernard Jones); awarded April 1983.
1977-1978 : University of Cambridge, Department of Applied Mathematics.
Degree : Part Three of the Mathematics Tripos. Passed with Distinction.
1974-1977 : University of London, Queen Mary College, Physics Department.
Degree : B.Sc (First Class Honors) in Physics with Astrophysics.

Prizes & Honors
2010: Elected to President, Aspen Center for Physics
2006: Elected to Trustee, Aspen Center for Physics
2003: Elected to General Membership, Aspen Center for Physics
2002: Visiting Fellow, New College, Oxford (Hilary term)
2002: Keeley Visiting Fellow, Wadham College, Oxford (Trinity term)
2002: NOVA visitor, Leiden University and Groningen University, The Netherlands.
1990-1992 : Fellowship from the Alfred P. Sloan Foundation.
1989 : Teaching Fellowship from the Eli Lilly Foundation.
1986 : Annie Jump Cannon Award from the American Association of University Women.
1981-1982 : Amelia Earhart Fellowship from ZONTA International.
1978-1981 : Bachelor Scholarship at Emmanuel College, Cambridge.

Planet Forming Disks around Young Stars

David Wilner

Institution

Tuesday, March 22, 2011

Abstract

The circumstellar disks that arise naturally from the star formation process are the sites where planets are born. Observations at millimeter wavelengths play a key role in probing these disks by providing direct access to the cool dust and gas that trace the bulk of the disk mass, with no contrast problem from starlight. I will discuss our recent observations of disks from the Submillimeter Array on Mauna Kea, Hawaii, designed to provide insight into disk physical conditions and planet forming potential. In particular, I will describe high resolution (0.25 arcsec = 35 AU) 870 micron observations of dust continuum emission from young disks where we have used 2D radiative transfer models to fit simultaneously the resolved submillimeter data and the broadband spectral energy distributions obtained from ground- and space-based telescopes to characterize disk structure and the likelihood of future (and even past) planet formation. I will also touch on the incredible advances expected with next generation facilities, especially the international Atacama Large Millimeter Array, now under construction.

Bio

David Wilner is a tenured Astrophysicist at the Smithsonian Astrophysical Observatory, a Lecturer on Astronomy at Harvard University, and in 2010 became the Associate Director of the Radio and Geoastronomy Division of the Harvard-Smithsonian Center for Astrophysics. He studies the origins of stars and planets, mainly using interferometric techniques at radio wavelengths to obtain high angular resolution. He received his A.B. in Physics from Princeton University, his Ph.D. in Astronomy from the University of California, Berkeley, and held a Hubble Postdoctoral Fellowship at the Harvard-Smithsonian Center for Astrophysics prior to joining the staff. His recent research has concentrated on resolving the structure of disks around young stars, to probe the physics of disk accretion and the planet formation process.

Evolution or Selection: The X-ray Properties of Moderate-redshift Optically-selected Clusters of Galaxies

Amalia Hicks

Michigan State

Tuesday, March 29, 2011

Abstract

By virtue of their size, galaxy clusters can be used to place important constraints on cosmological parameters. In particular, charting the evolution of the cluster mass function provides us with vital information on the progression of large-scale structure formation over time. The masses of clusters, however, are often inferred from observables such as gas temperature or X-ray luminosity, which can be influenced by non-gravitational processes that affect cluster baryons, such as energy injection (heating) and radiative cooling. In addition, many high-redshift cluster surveys select samples based on baryon observables such as gas density. Recent correlations between temperature, luminosity, and total cluster mass indicate significant discrepancies between observations and the theoretical expectations of self-similarity. Therefore understanding changes in cluster properties with redshift is of crucial importance to surveys that intend to use the evolution of the cluster population as a proxy for cosmic evolution, and ultimately for the determination of cosmological parameters. The results of our X-ray investigation of 13 high-redshift (0.6 < z < 1.1) optically-selected clusters suggest that the central entropy of these objects has been elevated by processes such as pre-heating, mergers, and episodic AGN outbursts, and that their ratio of gas mass to total gravitating mass is systematically lower than that found in lower-redshift X-ray selected clusters. To determine whether these effects are primarily associated with selection or evolution, we have designed a comparison sample of 10 moderate-z (0.2 < z < 0.6) optically-selected clusters, all of which have recently been observed by Chandra, Suzaku, or XMM. Here we present our final results from these observations, and discuss their relevance to cluster surveys which rely on the assumption of constant gas mass fraction to detect clusters and/or determine their masses.

Bio

My first degree was actually a B.A. in english literature and creative writing. �I then went back for a B.S. in Physics at the University of Wisconsin-Milwaukee, finished my masters in physics at MIT under Claude Canizaresin 2001, received a Ph.D. in Astrophysics from CU Boulder in 2005, completed a postdoc with Craig Sarazin at the University of Virginia, and am currently doing a second postdoc with Megan Donahue at Michigan State. �I have also collaborated closely with Richard Mushotzky over the last 7 or 8 years.

Coevolution of Black Holes and Galaxies: Recent Developments

Luis Ho

Carnegie

Tuesday, Apr 5, 2011

Abstract

I will review observational progress in defining and refining the various empirical scaling relations between black hole masses and host galaxy properties. I will emphasize ways in which the intrinsic scatter can be quantified, and present evidence that the scatter correlates with physical properties. I will discuss how to extend the scaling relations to active galaxies, and summarize preliminary efforts to probe the evolution of these scaling relations with redshift. I will present new measurements of the cold ISM content in AGN host galaxies, and constraints they place on currently popular models of AGN feedback. Lastly, I will discuss a new class of low-mass black holes in bulgeless and dwarf galaxies that serve as local analogs of seed supermassive black holes.

The Co-Evolution of Black Holes and Galaxies in Clusters

Paul Martini

Ohio State

Tuesday, April 12, 2011

Abstract

The evolution of AGN in clusters of galaxies traces the growth of some of the first and most massive black holes in the Universe. Observations have found that the fraction of luminous AGN in clusters is consistently about an order of magnitude below the field fraction from z=1 to the present. These results constrain the majority of cluster black hole growth to earlier times than field galaxies. We have now begun to measure star formation rates and stellar masses for galaxies in some of these same clusters. Intriguingly, the rate of galaxy growth, as traced by the fraction of galaxies with massive star formation, appears to have evolved in a similar manner to the AGN fraction. That is, the fraction of massively star forming galaxies in clusters is also suppressed by approximately an order of magnitude from z=1 to the present. This suggests that there is also black hole and galaxy co-evolution in clusters, even though both star formation and black hole growth are approximately an order of magnitude less common.

Bio

Paul Martini received his Ph.D. in 2000 from The Ohio State University. He then was a Carnegie Fellow at the Observatories of the Carnegie Institution of Washington and a Clay Fellow at the Harvard-Smithsonian Center for Astrophysics. He joined the faculty of The Ohio State University in 2005 and is an Assistant Professor. His research interests include the evolution of supermassive black holes, particularly as active galactic nuclei (AGN), astronomical instrumentation, and the formation and evolution of galaxies.

Ejecta blocks as tracers of the formation and evolution of asteroid regoliths

Dan Durda

SWRI

Tuesday, May 3, 2011

Abstract

Regoliths on small bodies represent valuable natural laboratories for evaluating various models of impact cratering processes since they may present crater structures or ejecta features that either do not form or are hidden on higher-gravity bodies like the Moon. Quantifying the extent to which impact processes generate and redistribute regoliths on small body surfaces is pivotal to the issue of how to relate meteoritical samples to their asteroidal parent bodies and a better understanding of the processes at work in these unique environments is crucial for designing technologies and techniques for future robotic and human exploration, resource utilization, and impact hazard mitigation. Ejecta blocks represent the coarsest fraction of small body regoliths and are important, readily-visible 'tracer particles' for crater ejecta blanket units that may be linked back to specific source craters, thus yielding valuable information on physical properties and constraining various aspects of impact cratering in low-gravity environments. These blocks, launched from the surface of a small, rapidly-rotating, and highly-elongated and irregularly-shaped body, are subjected to a complex dynamical process. Dynamical models of reaccretion of impact ejecta on asteroids thus provide important and necessary tools for a detailed investigation of the distribution and morphology of blocks and finer regolith across their surfaces.

Bio

Daniel D. Durda is a Principal Scientist in the Department of Space Studies of the Southwest Research Institute's Boulder CO office. He has more than twenty years of experience researching the collisional and dynamical evolution of main-belt and near-Earth asteroids, Vulcanoids, Kuiper belt comets, and interplanetary dust. He is an active pilot, with time logged in over a dozen types of aircraft including the F/A-18 Hornet and the F-104 Starfighter. He serves as a flight astronomer for the SWUIS-A airborne astronomical imaging system flown aboard NASA and military high-performance, high-altitude aircraft and has spent over 84 minutes of time in zero-gravity conducting experiments on NASA's KC-135 Reduced Gravity Research Aircraft. He is the program coordinator for the Planetary Society's Gene Shoemaker Near-Earth Object Grant Program. Dr. Durda has co-authored a book and published several articles popularizing planetary science and human exploration of space. His space art has appeared in many magazines and books and has been internationally exhibited. Dr. Durda is an experienced cave diver and holds multiple scuba and cave diving certifications, including Full Cave and Cave Recovery Specialist.

The Past, Present, and (Near) Future of Supernova Cosmology

Robert P. Kirshner

CfA

Tuesday, May 10, 2011

Abstract

Supernova observations provided the evidence that we live in an accelerating universe.� Today, Type Ia supernovae provide one of� several powerful paths to knowledge of dark energy.� As the best developed tool, the uncertainties for supernova cosmology are more limited by systematic errors than by sample size.� Bigger samples do not guarantee better results.� I will describe two methods for improving the supernova constraints on dark energy -- by using supernova spectra as luminosity indicators and by measuring the supernova light curves in the rest frame infrared.� The infrared light curves seem especially promising: SN Ia are better standard candles at H-band (1.6 micron) than in the optical and the pernicious effects of dust are reduced. In the (near) future, HST observations of SN Ia discovered with PanSTARRS could lead to significant improvement in our knowledge of dark energy.� In the (more distant) future, NIR observations with JWST and with WFIRST will provide accurate and precise knowledge of the history of cosmic expansion.

Gamma-rays and X-rays from Jets in Blazars

Alan Marscher

BU

Tuesday, May 17, 2011

Abstract

X-ray and gamma-ray flares are a common occurrence in blazars, active galactic nuclei with relativistic plasma jets that are pointing almost directly at us. This talk will present the results of a comprehensive multi-waveband monitoring program of blazars with gamma-ray flares observed by Fermi and X-ray flares observed with RXTE and Swift. The relative timing of the flares at different wavebands and the emergence of bright radio knots moving down the jet at apparent superluminal speeds allow us to locate where in the jet the high-energy emission arises. This information provides strong clues for determining the physics behind the gamma-ray mission, the acceleration and focusing of the jets, and acceleration of electrons to very high energies.

Bio

Alan Marscher (BS Cornell U., MS & PhD U. Virginia) is Professor of Astronomy at Boston University, where he has taught since 1981. He served as chair of the department from 1987-1997. A specialist in quasars, high energy astrophysics, and multi-waveband astronomy, he conducts research at observatories throughout the world as well as in space. His research group's website is at http://www.bu.edu/blazars. Marscher composes folk/pop songs, some with scientific and philosophical themes, which he sings to liven up his classes and to summarize the day's lesson. Recordings & lyrics are available at http://www.soundclick.com/cosmosii.

The Hot Galacic Halo -- what does it look like?

Dan McCammon

Wisconsin

Tuesday, May 24, 2011

Abstract

Since 1956 it has been supposed that the Galaxy should have a ubiquitous hot halo. Observations of O VI and soft X-rays provide much evidence that there is hot gas at high latitudes, but we still lack a convincing overall picture of its distribution and temperature. Steve Snowden and Kip Kuntz used 1/4-keV ROSAT data to determine a very clumpy distribution of halo gas at T = 1.0 x 10⁶ K. The derived halo emission measures are quite high, but due to significant interstellar absorption most of the observed 1/4-keV radiation comes from a much more local source that is some combination of 1.0x10⁶ K gas within 100 pc or so of the Sun and charge exchange on heavy solar wind ions in interplanetary space. In the 0.5-1 keV X-ray band, where characteristic temperatures are 2-3 x 10⁶ K and absorption lengths in the disk about 1 kpc, most of the sky appears quite uniform and there is little variation with Galactic latitude. Most people assume that the primary contributor is a uniform hot halo, and various fits usually give scale heights of a few kpc and temperatures 2-3 x 10⁶ K. However, none of these models can reproduce global observations in any detail, it is difficult to reproduce the observed intensity profiles near the Galactic plane, and the actual location of sources of the 0.5-1 keV X-ray background is still very much in doubt. I will discuss some preliminary shadowing results that appear to allow only a quite faint halo in the outer Galaxy, and the much brighter emission toward the inner parts, where the picture is complicated by extensive features such as the soft X-ray bulge, Loop I, the North Polar Spur, and Fermi bubbles.

X-ray Spectroscopy of Hot Plasma in and around Galaxies

Daniel Wang

UMass

Tuesday, May 31, 2011

Abstract

Much of the baryon matter in the present universe is believed in the form of diffuse hot plasma in and around galaxies. However, little is so far known about the actual properties of this plasma. I will review recent spectroscopic studies of the plasma, based chiefly on X-ray grating data from Chandra and XMM-Newton. The X-ray absorption line spectroscopy has led to the first characterization of the spatial, thermal, chemical, and kinetic properties of the plasma in our Galaxy. The plasma is concentrated around the Galactic bulge and disk on scales of a few kpc. The column density of the plasma on larger scales appears to be at least an order magnitude smaller, indicating that it may not account for the bulk of the missing baryon matter predicted for the Galactic halo according to the standard cosmology. Similar results have also been obtained for other nearby galaxies. Furthermore, a substantial fraction of diffuse soft X-ray emission observed in nearby galaxies cannot arise from optically-thin thermal plasma and may originate in charge exchange at the interface with neutral gas, as indicated by the Strong forbidden lines of the Kalpha triplets of He-like ions. In any case, the bulk of the mass, energy, and metals from the galactic feedback is likely gone with outflows in both stellar spheroids and nuclear starburst galaxies. These outflows can have profound effects on the galactic ecosystem and evolution.

Going to the ends of the earth to glimpse the beginnings of time:

CMB Polarimetry with the BICEP Telescope at the South Pole

Brian Keating

UCSD

Tuesday, June 7, 2011

Abstract

The Background Imaging of Cosmic Extragalactic Polarization (BICEP) experiment is the first polarimeter developed to measure the inflationary B-mode polarization of the CMB. During three seasons of observing at the South Pole, Antarctica beginning in 2006, BICEP mapped 2% of the sky chosen to be clean of polarized foreground emission, with sub-degree resolution. In this colloquium I will present initial results derived from a subset of the data acquired during the first two years of data and discuss the unique design features of BICEP which led to the first meaningful limits on the tensor-to-scalar ratio to come from B-mode polarization. Recently, Xia, Li & Zhang (2009) have claimed a detection of parity-violating "cosmic birefringence" effects using publicly available BICEP data. I will discuss polarimetric fidelity in the light of systematic errors and how such effects are particularly pernicious for probes of cosmic parity violation. I will conclude with a discussion demonstrating how BICEP, and its successor "BICEP2" will inform future measurements of the inflationary gravitational wave background and cosmic birefringence.

The Center of the Galaxy and its Activities

Farhad Yusef-Zadeh

Northwestern

Tuesday, June 14, 2011

Abstract

The processes occurring in the nuclear disk of our own Galaxy are interesting because this region can potentially provide a template for the study of more distant galactic nuclei. There are several sources of activity that lie in this unique region of the Galaxy. One is the underluminous supermassive black hole (Sgr A*) at the dynamical center of the Galaxy. Sgr A* exhibits flares which occur a few times a day, arising from the inner ten Schwarzschild radii of a 4 million solar mass black hole. The other is the nuclear stellar cluster which consists of a population of evolved stars as well as a young population of stars in a disk orbiting within 0.5 pc Sgr A*. On a larger scale, another source of high energy activity is evidenced by the distribution of 6.4 keV line emission from neutral iron tracing dense, cold giant molecular clouds in the inner 200pc of the Galaxy. In this talk, I will describe what we have learned from studying these three types of activities, namely, flares, massive star formation in a steep gravitational potential of a massive black hole and X-ray emitting molecular clouds.