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Astrophysics Seminars From 2009 - 2010


Fall 2009

October 7, 2009

 "Cosmological Constraints from Gravitational
Lens Time Delays"

Dan Coe, Caltech/JPL

 Future large ensembles of time delay lenses have the potential to provide interesting cosmological constraints complementary to those of other methods. Current constraints from 10-16 time delay lenses already yield the Hubble constant (h) in agreement with and to roughly the same level of precision (10%) as the HST Key Project which analyzed 40 Cepheids. Future surveys (Pan-STARRS, LSST, JDEM / IDECS, SKA, OMEGA) will yield hundreds or even thousands of lenses with well-measured time delays. We find in a flat universe with constant w including a Planck prior, LSST time delay measurements for ~4,000 lenses should constrain h to 0.007 (~1%), Omega_de to ~0.005, and w to ~0.026 (all 1-sigma precisions). Similar constraints could be obtained by a dedicated space-based gravitational lens observatory (OMEGA) which would obtain precise time delay and mass model measurements for ~100 lenses with spectroscopic redshifts. Constraints for a general cosmology are presented as well. We compare these to the "optimistic Stage IV" constraints expected from weak lensing, supernovae, baryon acoustic oscillations, and cluster counts, as calculated by the Dark Energy Task Force. As with any method, there are systematics we must learn to control, and we discuss these issues.

October 21, 2009

 "The evolution of AGN activity across cosmic time"

James Aird, UCSD-CASS Postdoctoral Scholar

 Determining the distribution and evolution of accretion activity in Active Galactic Nuclei (AGN) throughout the history of the Universe, traced by the luminosity function, is essential to constrain models of supermassive black-hole formation and growth, the triggering and fuelling of AGN and their co-evolution with galaxies. I will present new results on the evolution of the X-ray luminosity function (XLF) of AGN utilising data from the Chandra Deep Fields and the AEGIS survey, using new methods to account for uncertainties in photometric redshifts, selection effects and incompleteness. I will also present an alternative method of determining the XLF at high redshifts (z~2-3) using a rest-frame UV color pre-selection approach, and discuss the implications of these results for the accretion history of the Universe.

October 27, 2009

 "Faint Lyman alpha Emitters, Damped Lyman Alpha Absorbers, and their Relation to Other High Redshift Sources"

Michael Rauch, Carnegie Observatories

 I will talk about some recent attempts at measuring very faint Lyman alpha emission from the general intergalactic medium at redshirt 3. I will describe how a recently detected population of faint emitters may help us to understand the nature of other classes of high redshift objects, in particular damped Lyman alpha absorption systems, and what they may tell us about the properties of dwarf galaxies at high redshift.

November 4, 2009

 "Using Numerical Simulations to Study the Formation and Evolution of Galaxies"

TJ Cox, Carnegie Obervatories

 The past decade has produced an amazingly robust picture for the universe we live in. This picture predicts that structure forms hierarchically, i.e., small objects collapse at early times and grow via mergers and gravity. The prevailing idea for the formation of galaxies is that the morphology and structure that we observe is a direct byproduct of this hierarchical merger history; however, a detailed mapping between specific merger histories, and the wide variety of galaxy types, is still uncertain. By using a comprehensive set of state-of-the-art numerical simulations, we show how this process is being studied, and what some of the common scenarios might be. For example, we show that a single disk-disk merger, such as that which will occur in 5 Gyr between our own Milky Way and our nearest neighbor Andromeda, is a plausible mechanism to form many elliptical galaxies provided that dissipation is involved. We also show were this picture fails, and outline how current and future work will address these shortcomings and yield testable predictions of the model.

November 10, 2009

 "The Distribution of Thermal Pressures in the Diffuse Interstellar Medium of our Galaxy"

Edward Jenkins, Princeton University

 Over recent decades, there has been an increasing awareness of the importance of turbulence in governing the physical states and kinematics of the interstellar medium, a picture that advances us beyond the earlier simple concept of quiescent, dense clouds that are confined by the external pressure of a lower density warm medium. There are various ways to observe the strength and character of interstellar turbulence; one of them is to sense the distribution of thermal pressures, even though such pressures represent a small fraction of the total pressure in the medium. An effective way to examine this distribution is to measure the strengths of absorption features from interstellar neutral carbon atoms that are seen in the ultraviolet spectra of hot stars. The ground state of C I is split into three fine-structure levels; the relative populations of the upper two levels are governed by collisions with hydrogen atoms and thus reveal the local thermal pressures. An interpretation of high-resolution UV spectra in the HST archive for over 100 stars indicates that the mass-weighted pressure distribution function follows approximately a log-normal distribution centered on a value of p/k = 2600 cm-3 K, with an rms dispersion in log(p/k) = 0.16. Accompanying almost all of the gas is about a 0.1% fraction that seems to have an anomalously high pressure of p/k > 105 cm-3 K.

November 18, 2009

 "Blowing Up Massive Stars -- Core-Collapse Supernovae Explosion Mechanisms"

Christian Ott, Caltech

 Despite many decades of concerted theoretical effort and numerical modeling, the details of the core-collapse supernova explosion mechanism are still under debate. Indications are strong that the supernova mechanism is intrinsically multi-dimensional and involves (a combination of) postbounce energy deposition by neutrinos, convective instability, the standing-accretion-shock instability (SASI), protoneutron star core oscillations, rotation, and magnetohydrodynamic effects. I review the current status of core-collapse supernova modeling and introduce the ensemble of candidate explosion mechanisms that is emerging from recent multi-dimensional simulations. I go on to discuss gravitational-wave (GW) emission processes in core-collapse supernovae and present new results on the supernova gravitational-wave signature that were obtained with 2D and 3D general relativistic and Newtonian simulations. I demonstrate how GWs observed by current and future GW detectors can be used to constrain the core-collapse supernova explosion mechanism.

December 2, 2009

 "The Relation between Gas and Star Formation in Nearby Galaxies on sub-kpc Scales - Where is the Interstellar Medium Good at Forming Stars"

Frank Bigiel, UC Berkeley

 I will present results on the relation between gas and star formation in nearby galaxies using new, high-resolution radio data to trace the atomic and molecular gas ("The HI Nearby Galaxy Survey" and the "HERA CO Line Extragalactic Survey"). These results include a detailed pixel-by-pixel study of the star formation (Schmidt/Kennicutt) law on sub-kpc resolution, using THINGS HI, HERACLES CO, Spitzer IR and GALEX UV data. These measurements are made at sub-kiloparsec resolution in a systematic way across the entire star forming disks of a sample of ~20 nearby galaxies, including spirals and dwarfs. I will show that a Schmidt law with power law index N~1 relates star formation and molecular gas surface density in the star forming disks and that the star formation efficiency and the molecular gas fraction are both strong functions of radius and thus environment in a galaxy. We furthermore use these measurements to test proposed drivers of star and cloud formation, such as e.g. metallicity, ISM pressure, large-scale instabilities and spiral density waves, all quantities that have been predicted to affect the conversion of HI into H2 or the ability of gas to form stars. I will summarize these tests and give our currently best answer(s) to the basic question "Where is the ISM good at forming stars?".

December 16, 2009

 "Hierarchy, cooling thresholds and feedback in galaxy formation"

Martin Stringer, Durham University, UK

 Recent analysis of the Milky Ways satellite galaxies reveals that these objects share a common central mass density, even though their luminosities range over five orders of magnitude. This observation can be understood in the context of galaxy formation theory by quantifying the factors which restrict the central mass density to a small range. One limit is set by the maximum mass that can collapse into a given region by the hierarchical growth of structure in the standard cold dark matter cosmology. Another limit comes from the natural thresholds which exist for gas to be able to cool and form a galaxy. The wide range of luminosities in these satellites reflect the effects of supernova feedback on the fraction of cooled baryons which are retained.

Winter 2010

January 13, 2010

 "Catastrophic Collisions in the Terrestrial Planet Zone around Intermediate Mass Stars"

Carl Melis, UCSD-CASS Postdoctoral Scholar

 We have completed an extensive search for stars hosting terrestrial planet zone dust by cross-correlating the Tycho-2 and IRAS catalogs. Near-infrared to far-infrared excess emission has been discovered towards a ~10 Myr old, A-type member of the Upper-Centaurus-Lupus association. The hot dust component (750 K) in combination with the high fractional infrared luminosity (0.4%) suggest a recent catastrophic collision between rocky bodies in this intermediate mass stars inner planetary system. Synthesis of all published incidences of intermediate mass stars with evidence for terrestrial planet zone dust suggests that catastrophic collisions analogous to the Moon-forming event in our Solar System occur around intermediate mass stars when the star is 10-30 Myr old.

January 20, 2010

 "The ISM in Star-forming Galaxies at z~2-3: Surprising New Insights"

Alice Shapley, UCLA

 There has been a veritable explosion of surveys of z>1.5 galaxies within the few years. Much of this work has focused on the global properties of distant galaxy populations, rather than the detailed physical properties of individual objects. Indeed, given the faint apparent magnitudes and small angular sizes of typical star-forming galaxies in the distant universe, such detailed studies are challenging. However, aided by gravitational lensing and a wide array of multi-wavelength observations, we present some new results that provide insight into the detailed physical conditions in HII regions of star-forming galaxies at z~2-3, the nature of dust extinction in such systems, and the structure of their large-scale outflows -- all crucial ingredients in models of early star formation and its resulting feedback.

January 27, 2010

 "Baryon Oscillation Spectroscopic Surveys (BOSS and BigBOSS)"

David Schlegel, LBNL

 The Baryon Oscillation Spectroscopic Survey (BOSS) is a Stage III dark energy experiment which began on the Sloan Telescope in September 2009. I will describe the current status of baryon acoustic oscillation (BAO) results as standard rulers for dark energy, and the design goals of the BOSS experiment. For the five years from 2009-2014, we will map 1.5 million galaxies at z<0.7. A simultaneous survey of 160,000 QSOs will map the hydrogen gas in absorption at redshifts 2.3 < z < 3.BOSS will provide the definitive measurement of the low redshift (z<0.7) BAO scale, and it will pioneer a powerful new method of measuring BAO at high redshift. BigBOSS is a proposed Stage IV experiment that will extend this map to 50 million galaxies over 24,000 deg^2. I will describe this survey and its technical status.

February 3, 2010

 "Abundance of dwarf galaxies: is there a problem for the standard cosmological model"

Anatoly Klypin, NMSU

 I will discuss current status of theoretical predictions for properties and statistics of galaxies. I will show results from recent simulation named Bolshoi which in a large cosmological volume resolves the dark matter halos from small dwarfs of size of Magellanic Clouds to Clusters of galaxies comparable to the Coma Cluster. We find that a simple and reasonable corrections for baryonic content of dark matter halos produce remarkably accurate catalogs of "galaxies", which reproduce the luminosity function, the Tully-Fisher relation of observed galaxies, and the correlation function. However, there is a substantial disagreement in the abundance of dwarf galaxies.

February 17, 2010

 "The detection of z > 2 type IIn supernovae: Direct test for an evolving stellar initial mass function"

Jeff Cooke, Caltech

 Type IIn supernovae (SNe IIn) result from the deaths of massive stars (>40 M_solar) and comprise some of the brightest supernovae events ever recorded. Moreover, SNe IIn are the brightest SN-type in the rest-frame ultraviolet, making them ideal targets for high-redshift detection in deep optical surveys. The spectra of SNe IIn exhibit extremely luminous narrow emission lines as a result of the interaction between the supernova ejecta and cool circumstellar material shed in pre-explosion outbursts. The emission lines remain bright for years are used to confirm and study high-redshift photometric candidates at late times. I will introduce our method to detect high-redshift SNe IIn in archival CFHTLS images and present the sample of spectroscopically confirmed z ~ 2 discoveries to date. I will conclude by discussing the implications of the existing dataset on the evolution of the stellar initial mass function and the capabilities of imminent and future projects such as Hyper-SuprimeCam, the Large Synoptic Survey Telescope, and the Thirty Meter Telescope to detect and study large numbers of SNe IIn to z ~ 6.

February 19, 2010

 "Spatially resolved dynamics of z~2 forming galaxies"

Reinhard Genzel, Max-Planck Institute, Garching, Germany

 I report on two major programs of studying the kinematics, star formation and cold gas properties of z~2 massive star forming galaxies (z2SFGs) with spatially resolved spectroscopy. With the adaptive optics assisted, integral field spectrometer SINFONI on the ESO VLT we have studied about 90 z2SFGs and find compelling evidence for large, turbulent rotating disk galaxies in ~50% of the larger objects that we spatially resolve well. It appears plausible that these z2SFGs may be driven by continuous, rapid accretion of gas from their dark matter halos, and that their evolution is strongly influenced by internal, secular evolution. In a new program on the IRAM Plateau de Bure millimeter interferometer we have also detected for the first time CO 3-2 line emission in a sample of these z2SFGs (as well as in matched z~1.2 counterparts). We find that the z~1-2 SFGs are gas rich and that their star formation properties are fully compatible with a standard Kennicutt-Schmidt star formation relation. I will discuss the impact of these new observations on our understanding of galaxy evolution in the early Universe.

February 24, 2010

 "Neutrinos from stellar collapse"

Cecilia Lunardini, ASU

 I review the diffuse flux of neutrinos from core collapse supernovae, with emphasis on the motivation for their study. I also discuss the diffuse flux of electron antineutrinos from stellar collapses with direct black hole formation (failed supernovae). This flux is more energetic than that from successful supernovae, and therefore it might contribute substantially to the total diffuse flux above realistic detection thresholds. The total flux might be considerably higher than previously thought, and approach the sensitivity of SuperKamiokande. For more conservative values of the parameters, the flux from failed supernovae dominates for antineutrino energies above 30-45 MeV, with potential to give an observable spectral distortion at Megaton detectors.

Spring 2010

March 18, 2010

 "The Formation of Massive Stars"

Mark Krumholz, UC Santa Cruz

 The similarity between the mass and spatial distributions of pre-stellar gas cores in star-forming clouds and young stars in clusters provides strong circumstantial evidence that these gas cores are the direct progenitors of individual stars. I describe a physical model for the evolution of massive cores into stars, starting with the initial phases of collapse and fragmentation, through disk formation and evolution, and finally to the point where stellar feedback begins to dominate. This model shows that a direct mapping from cores to stars is the natural physical outcome of massive core evolution, and thereby allows us to explain many of the properties of young star clusters as direct imprints of their gas-phase progenitors.

March 31, 2010

 "Cosmic Clues for Dark Energy"

Katrin Heitmann, LANL

 Little more than ten years have passed since the astonishing discovery of a mysterious agent driving an accelerated expansion of the Universe. Since confirmed by different probes, this discovery has been hailed as the harbinger of a revolution in fundamental physics and cosmology. Why is there so much excitement? Simply because no fundamental understanding exists. Cosmic acceleration demands completely new physics. It challenges basic notions of quantum theory, general relativity, and the fundamental makeup of matter. In this talk I will discuss the current observational status of cosmic acceleration and future plans and challenges to shed light on the mystery. I will focus specifically on probes based on the large scale distribution of matter and illuminate the challenges from a theorist perspective.

April 7, 2010

 "Revisiting the Saturnian satellites following the recent Cassini-Huygens spacecraft observations: Resolving an ancient mystery"

Asoka Mendis, UCSD ECE Emeritus Professor

 A recent report appearing in Physics Today pertaining to the large brightness asymmetry between the leading and trialing faces of the synchronous Saturnian satellite, Iapetus, discusses an early model proposed by Prof. Ian Axford and me to explain this phenomenon, and states that the “data collected since 2004 by the Cassini-Huygens spacecraft offer the most compelling evidence yet for Mendis and Axfords view.” I will use this opportunity to revisit the question of the observed brightness variations, with orbital phase, not only of this outermost large Saturnian satellite Iapetus but also of the large inner ones (Tethys, Dione, Rhea, and Titan). While Titan (which has a significant atmosphere) shows no brightness variation with orbital phase, the remaining satellites show small but definite brightness variations with orbital phase, which, curiously, are in a sense opposite to that of Iapetus. In other words, while the leading face of Iapetus is much darker than its trailing face, the leading faces of the other satellites (with the exception of Titan) are somewhat brighter. Here I will explain the latter observation too by invoking the impact on the trialing faces of these inner satellites (which lie inside the planets magnetosphere) by a class of negatively charged dust grains in the E-ring, moving with super-Kepler speeds.

April 21, 2010

 "Locating the planetesimals belts in multiple-planet systems"

Amaya Moro-Martin, Spanish National Research Council (& visiting researcher at Princeton)

 In addition to the Sun, six other stars are known to harbor multiple planets and debris disks: HD 69830, HD 38529, HD 128311, HD 202206, HD 82943 and HR 8799. We set constraints on the location of the dust-producing planetesimals around the latter four systems. We use a radiative transfer model to analyze the spectral energy distributions of the dust disks (including two new Spitzer IRS spectra), and a dynamical model to assess the long-term stability of the planetesimals orbits. As members of a small group of stars that show evidence of harboring a multiple planets and planetesimals, their study can help us learn about the diversity of planetary systems.

April 28, 2010

 "The Progenitors of Core-Collapse Supernovae"

Doug Leonard, SDSU

 Theory holds that a star born with an initial mass between about 8 and 140 times the mass of the Sun will end its life through the catastrophic gravitational collapse of its iron core to a neutron star or black hole. This core collapse process is thought to usually be accompanied by the ejection of the stars envelope as a supernova, although direct collapse to a black hole is also considered a possibility. This established theory is now being tested observationally, with nearly three dozen core-collapse supernovae having had the properties of their progenitor stars directly measured through the examination of high-resolution images taken prior to the explosion. In this talk, I will summarize what we have learned from these studies by reviewing the progenitor characteristics inferred for each of the major core-collapse supernova types: II-Plateau, II-Linear, IIb, IIn, and Ib/c. Brief discussion of a few individual events will also be given, including SN 2005gl, a type IIn supernova that has been demonstrated to have had an extremely luminous -- and thus very massive -- progenitor that exploded shortly after a violent, luminous blue variable-like eruption phase, contrary to standard theoretical predictions.

May 5, 2010

 "The evolution of red galaxies in clusters over half of cosmic time"

Greg Rudnick, University of Kansas

 Tracking the growth of stellar mass in galaxies is a fundamental characterization of the galaxy population. Recent observations have shown that the total mass in L>L* red galaxies has increased by a factor of ~2 at z<1, although at different rates as a function of galaxy mass. Despite the advance made by these studies of the whole galaxy population, until recently it has not been clear if the growth of the red sequence depended on environment. Galaxy clusters are a useful probe of this as they sample the most extreme environments. However, progress toward answering this question has been hampered by a lack of deep multi-band imaging of a large sample of clusters that can be well linked to those in the local universe. I will show how the luminosity function (LF) of red-sequence galaxies in clusters and the field has evolved over 50% of cosmic time, highlighting the rapid buildup of the faint cluster galaxy population. Finally I will address how the total mass on the red sequence evolves in clusters and will use this to constrain the mechanisms of how red galaxies can be added to clusters. From this analysis it appears likely that some fraction of the light in recently added cluster red sequence galaxies is currently in the in the form of intracluster stars.

May 12, 2010

 "Star Formation, Stellar Mass, and Metals in Galaxies Since z=0.7"

John Moustakas, UCSD-CASS Postdoctoral Scholar

 The latest generation of galaxy redshift surveys like the Sloan Digital Sky Survey (SDSS) have quantified the multivariate distributions of galaxy properties in unprecedented detail. I highlight some of the insights we have gained into the relationships between galaxy morphology, color, optical luminosity, and stellar mass. The remarkably tight correlation between stellar mass and gas-phase metallicity is an especially powerful metric of galaxy evolution because both properties are related to the integrated star formation histories of galaxies. I will present new constraints on the evolution of the mass-metallicity relation for star-forming galaxies to z=0.7 using observations from the AGN and Galaxy Evolution Survey (AGES). We find that the mean metallicity of massive star-forming galaxies has increased by only ~25% over the last six billion years, indicating that the bulk of their metals were synthesized at z>1. I conclude with some preliminary work suggesting that a stellar initial mass function (IMF) that is more top-heavy than a Salpeter IMF is required to simultaneously match the present-day stellar mass density in galaxies with the buildup of stellar mass implied by the star formation history of the Universe.

May 19, 2010

 "Astrophysical Measurement of the Equation of State of Neutron Star Matter"

Feryal Ozel, University of Arizona

 Neutron stars are the densest objects in the universe and may contain hyperon-dominated matter, condensed mesons, or even deconfined or strange quark matter. Because of their low temperatures and high chemical potentials, the physical conditions in their interiors differ greatly from the dense conditions of the early universe or those achieved at hadron colliders. This region of the QCD phase diagram can best be probed through astrophysical observations that measure the masses and radii of neutron stars. I will discuss how we can break degeneracies in the measurements of neutron star properties by combining recent developments in our understanding of their atmospheres with observations of multiple spectroscopic phenomena from X-ray binaries. I will present unique measurements of the masses and radii of three neutron stars in low-mass X-ray binaries and show the equations of state of neutron star matter that are compatible with these observations. These measurement constrain, for the first time, the pressure of cold matter above nuclear saturation density and offer tantalizing evidence for new degrees of freedom at ultrahigh densities.

May 26, 2010

 "Constraining Non-Gaussianity and Lensing using the CMB"

Joseph Smidt , UC Irvine

 New methods have been developed to constrain lensing and primordial non-Gaussianity using power spectra from the skewness and kurtosis of CMB anisotropies. The advantage of this approach is the scale dependence of each statistic may be probed. We discuss recent constraints placed on primordial non-Gaussianity by analysing the bispectrum and trispectrum of the CMB using these estimators. Furthermore, we discuss how the lensed power spectrum, C_l^{phiphi}, may be measured directly from the trispectrum of the CMB and present preliminary results.

June 28, 2010

 "Early Galaxy Formation and the Chemical Enrichment of Intergalactic Matter"

Robert Simcoe/MIT, 12:00-1:00PM - SERF 329

 Heavy elements have been detected in tenuous intergalactic gas at essentially all observed redshifts. This suggests that nascent galaxies expelled some fraction of their interstellar material starting very early in the history of the universe. The physical process that distributed these metals may also govern many aspects of the galaxy formation process itself. I will review the status of intergalactic chemical abundance measurements and describe new efforts to push these techniques beyond redshift 6 using infrared spectroscopy. At these epochs, we may begin to witness nucleosynthetic byproducts from the generation of galaxies that reionized the universe.

June 28, 2010

 "Debris Disks in the Era of Terrestrial Planet Formation"

Christine Chen/MIT, 4:00-5:00PM - SERF 329

 The terrestrial planets in our Solar System are believed to have formed via a multi-stage process that included the conglomeration of sub-micron sized interstellar dust particles into kilometer-sized parent bodies, the growth of pluto-size planetary embryos from collisions of kilometer-sized parent bodies, and the accretion of the patina from giant collisions between planetary embryos. The Spitzer Space Telescope has enabled photometric surveys of mid- and far-infrared excess around stars in nearby young associations that constrain models for the formation of pluto-sized objects, and spectroscopic characterization of the circumstellar material that suggests that giant collisions may be occuring in some systems. In this talk, I will outline the properties of young debris disks in which terrestrial planets may be forming, compare MIPS photometric observations of stars in the 5-20 Myr old Scorpius-Centaurus OB Association to the evolution predicted by self-stirred disk models, and discuss possible diagnostics for giant collisions in young Solar Systems. I will also describe outstanding questions about solar system evolution that can not be addressed with Spitzer and will require future ground- and space-based instruments.