University of Minnesota
University of Minnesota

Astrophysics Colloquia

University of Minnesota

Minnesota Institute for Astrophysics

Astrophysics Colloquium

Spring 2012 - Coordinated by Liliya Williams
Held in Physics 210 at 3:35pm on Fridays
(unless otherwise noted)

January 20 Dr. Jean-Claude Passy (University of Victoria, Canada)
Title: Modeling Catastrophic Interactions in Binary Stars With Multi-Physics Simulations
Abstract: Most of the stars in the Universe are binaries. If both components of the system are close enough they will interact strongly, in particular during the giant phases of the primary. In some cases, a common envelope will start during which the secondary star spirals inside the envelope of the primary, exchanging momentum and energy with the gas. This particular interaction is an essential ingredient for many parts of astrophysics, including the shaping of planetary nebulae, the formation of Type Ia supernovae progenitors and the evolution of planetary systems. In this talk, I will show how we use different numerical techniques to gain a better understanding of the common envelope phase, how well these models compare to observations and what are the implications for the astrophysical phenomena mentioned above.
Host: P. Woodward
January 27 Dr. Michael Boylan-Kolchin (UC Irvine)
Title: Dark Matter and Dwarf Galaxies
Abstract: A firm prediction of the cold dark matter model of cosmological structure formation is that the Milky Way should host a huge population of self-bound dark matter subhalos. The vast majority of these subhalos are likely devoid of stars, but a tiny minority are expected to host the luminous dwarf galaxies observed around the Milky Way. These are the most dark matter-dominated galaxies presently known, and are therefore excellent laboratories for testing theories about galaxy formation and the properties of dark matter. I will show that the Milky Way's satellite galaxies appear to have substantially less dark matter than is predicted by numerical simulations. I will also review plausible explanations of this discrepancy, which include the possibility that galaxy formation is markedly different in low-mass dark matter halos than in larger systems, that baryonic processes wreak havoc on dark matter subhalos, or that the nature of dark matter differs from the canonical picture of a massive, weakly interacting particle.
Host: L. Williams
February 3 Dr. Falk Herwig (University of Victoria, Canada)
Title: Hydrodynamic combustion in stellar evolution and the origin of the elements
Abstract: Simulations of the origin of the elements provide comprehensive data sets that are compared to observations of stars in our own galaxy as well as extra-galactic sources. Such simulation data sets, for example to be used to interpret the data obtained from pre-solar grains, are now available from a wide range of sources from an internally consistent simulation approach. However, the underlying global modelling techniques defy those phases in stellar evolution that are subject to significant non-spherically symmetric and dynamic events, such as H-C12 combustion in convectively unstable shells. I present the latest multi-dimensional simulation results that aim to determine the properties of such extreme stellar environments that are encountered, for example in shell burning around WD-like strutters or stars of very low metal content.
Host: P. Woodward
February 10 Sally Brummel and Jim Rock (ExploraDome, Bell Museum)
Title: Indigenous Star Lessons from Turtle Island
Abstract: Did you know that throughout human history, cultures around the world have been using "cutting edge technology" to explain and explore scientific phenomena? Using the advanced technology of the Bell Museum's ExploraDome program, Dakota science educator Jim Rock and Bell Museum Planetarium Coordinator Sally Brummel will take us on a comprehensive journey of the night sky as the indigenous people of Minnesota see and understand it.
Host: L. Rudnick
February 17 Dr. Alberto Bolatto (University of Maryland)
Title: The Large Scale Picture of Star Formation in Galaxies: Bright Gas, Dark Gas, and Galaxy Evolution
Abstract:The importance of understanding baryonic processes in galaxies to properly interpret large scale cosmological simulations has been known for some time. Only recently, however, has there been compelling evidence showing that "normal" galaxies evolve mostly not through major mergers (although those produce some truly spectacular examples) but via a combination of cold mode accretion and star formation. In this colloquium I will review the evidence for passive evolution of galaxies and discuss some of our efforts to better understand the relation between gas and star formation activity in galaxies at present times, and throughout cosmic history.
Host: E. Skillman
February 24 Dr. John Cannon (Macalester College)
Title:SHIELD: The Survey of HI in Extremely Low-mass Dwarfs
Abstract: SHIELD is an ongoing multi-wavelength investigation of the properties of 12 extremely low-mass galaxies. These systems populate the very faint end of the HI mass function derived by the ALFALFA blind HI survey. This talk will provide the motivation for SHIELD and will present preliminary results from ongoing observing campaigns with the EVLA, HST, Spitzer, and WIYN. Macalester College students have played important roles in all aspects of the SHIELD program.
Host: L. Williams
March 2 Dr. Laura Trouille (CIERA Postdoctoral Fellow, Northwestern University)
Title: The Monster's Fiery Breath: Supermassive Black Holes at the Centers of Distant Galaxies
Abstract: Most, if not all, galaxies, including our own Milky Way, are host to a supermassive black hole (SMBH) at their center. Since the discovery of a tight empirical correlation between SMBH mass and host galaxy bulge mass, mounting observational evidence and advances in cosmological simulations suggest a link between galaxy evolution and central black hole growth. However, advances in this field are stymied by disagreement over accuracy of selection methods for accreting SMBHs and applicability of these selection methods to more distant galaxies. In this talk, I will discuss our resolution of the long-standing question with regards to the nature of BPT-composites and our new method for identifying distant active galactic nuclei (AGN). I will also present our recent results using the rare population of post-starburst galaxies as a probe to understanding the role of mergers and AGN in galaxy evolution.
Host: L. Fortson
March 9 Dr. Kristen McQuinn (University of Minnesota)
Title: The Starburst Mode of Star Formation in Dwarf Galaxies
Abstract: A significant number of dwarf galaxies in the local extragalactic environment show signs of a starburst mode of star formation. Understanding the starburst phenomenon is a key component to understanding the evolution of dwarf galaxies. Using HST optical observations of resolved stellar populations in 20 nearby starburst dwarf galaxies, the defining temporal and spatial characteristics of the burst events can be measured by reconstructing the star formation histories (SFHs) of the sample. Contrary to the short 5–10 Myr timescales often assumed for starbursts, the SFHs show elevated rates of SF sustained over a few hundred Myr. Further, the spatial distribution of young (<250 Myr) blue helium burning stars show that the spatial distribution of the starbursts lie on a continuum from highly centrally concentrated to more distributed. These longer lasting starbursts that are not solely co-located with the central regions of highest gas density create a new paradigm for the starburst mode of star formation in low mass galaxies. These results suggest that starbursts have a global triggering mechanism, are self-regulating, and have a larger impact on the evolutionary state of the host galaxy than previously thought.
March 16 No Colloquium — University Closed
March 23 Dr. Paul Ricker (University of Illinois, Urbana-Champaign)
Title: Magnetizing the Intracluster Medium with AGN Feedback
Abstract: Galaxy clusters have been shown using Faraday rotation of background radio sources to possess clusterwide magnetic fields of order 1–10 μG. These fields can have a variety of important effects on diffusive transport processes in the intracluster medium, and they help to accelerate nonthermal electrons that produce observable diffuse radio emission. However, while plasma mechanisms operating during structure formation readily produce very weak seed fields, how μG fields come to exist throughout cluster-size volumes is not clear. Magnetized feedback by the active galaxies known to exist at the centers of many clusters has been suggested as a possible field amplification and distribution mechanism. I will address this model using magnetohydrodynamic simulations of AGN feedback in clusters.
Host: T.W. Jones and L. Rudnick
March 30 Dr. Paul Smith (University of Arizona)
Title: Keeping up with the High-Energy Universe
Abstract: In 2008, NASA launched the Fermi Gamma-ray Space Telescope, providing a major advance in technology to study high-energy phenomena. The Large Area Telescope (LAT) aboard Fermi samples the entire sky every two orbits. The sensitivity of this instrument enables the gamma-ray flux at 0.1–300 GeV of the brightest sources in the sky to be monitored on daily time scales. Fermi has confirmed the findings of previous experiments that the blazar class of radio-loud AGNs dominates the extragalactic high-energy sky. The unprecedented ability of the LAT to systematically monitor the flux of a sizable sample of blazars on relevant time scales presents an exciting opportunity to study the extreme physics within the regions of the relativistic jet that produce the bulk of the continuum emission observed over essentially the entire electromagnetic spectrum. The Fermi project has funded an impressive array of supporting programs ranging from observations at TeV energies with ground-based Cherenkov telescopes to VLBI radio monitoring to place the Fermi/LAT measurements of blazars within the context of the overall source variability. I will describe a large program at Steward Observatory that supports Fermi by providing comprehensive optical monitoring of the polarization, flux, and spectra of gamma-ray- bright blazars and summarize the results of the major multi-wavelength efforts anchored by Fermi during the first four years of its projected 10-year mission.
Host: T.J. Jones
April 6 Dr. Roberta Paladini (Caltech)
Title: Tracing Dust Evolution in HII regions with Spitzer and Herschel
Abstract: The depletion of dust grains in the ionized gas is predicted by models and observed both in HII regions and in the diffuse ionized gas. However, the mechanism responsible for this depletion has yet to be clearly identified. After a brief review of the possible depletion scenarios, I will present the results of the analysis of a sample of Galactic evolved HII regions performed by combining IRAC 8 micron, MIPS 24 micron, PACS 70/160 micron and SPIRE 250, 350, 500 micron data. The analysis shows that dust inside HII regions is depleted mostly due to radiation pressure driven drift, as recently proposed by Draine (2011). In addition, we also find that radiation pressure induces a selection of the size of the grains, with the surviving grains being either small Big Grains (a ~ 0.01 micron) or large PAHs (a ~ 0.005 micron). In the last part of my talk, I will discuss implications of studies of HII regions for Anomalous Microwave Emission (AME), a recently discovered emission mechanism commonly attributed to spinning dust.
Host: C. Scarlata
April 13 No Colloquium — Kaufmanis Lecture on Apr 11
April 20 Dr. Joel Kastner (Rochester Institute of Technology)
Title: The Chandra X-ray Survey of Planetary Nebulae: Probing Binarity, Magnetic Fields, and Wind Collisions
Abstract: The community of planetary nebula (PN) astronomers is undertaking the Chandra Planetary Nebulae Survey (ChanPLaNS), the first systematic X-ray survey of PNe in the solar neighborhood. I present early results from the first phase of ChanPlaNS, which combines new (Cycle 12 Large Program) observations of 20 PNe within ~1.5 kpc of Earth with Chandra archival data for all (14) other PNe within ~1.5 kpc that have been observed to date. These data have yielded more than two dozen detections of either point-like or diffuse X-ray emission (or both) within the sample objects. Among other things, the early ChanPlaNS results provide new constraints on the frequency of appearance and X-ray spectral properties of X-ray-emitting PN central stars, and on the physical conditions and evolutionary timescales characterizing energetic wind shocks within PNe. I describe how these and future ChanPlaNS X-ray imaging spectroscopy results should inform and refine models describing PN shaping mechanisms and, in particular, provide insight into the potential pivotal role of central star binarity in determining PN structure and evolution.
Host: R. Humphreys
April 27 Dr. Mike Kelley (University of Maryland)
Title: The Latest from the EPOXI Mission to Comet Hartley 2
Abstract: Comets are time capsules of the ices and dust that formed the giant planets, and, possibly, were an important source of water and organic material for the terrestrial planets. Comets, however, are not wholly pristine. Understanding how comets work, that is, what drives their activity, is a crucial step towards determining how their current properties relate to their initial conditions. After the successful flyby of comet 9P/Tempel 1, the Deep Impact spacecraft next targeted comet 103P/Hartley 2, a so-called "hyperactive" comet. The activities of most comets are consistent with surfaces that are over 90% inert, i.e., activity is restricted to a few localized sources. In contrast, the gas production rates of the hyperactive comets suggest activity over 100% of their surfaces. Rather than just being the fifth comet nucleus to be imaged up close by a spacecraft, comet Hartley 2 has also become the archetype of the hyperactive comet class. I will review the EPOXI misson, and discuss results from both the spacecraft and ground-based observatories. The activity of most comets is primarily driven by the sublimation of water ice, but the activity of Comet Hartley 2 is instead driven by carbon dioxide.
Host: C. Woodward
May 4 Dr. Steven Bamford (University of Nottingham, England)
Title: The Detailed Dependence of Galaxy Morphology on Environment
Abstract: I will present the results of recent work studying the dependence of specific morphological features on various measures of galaxy environment. These studies examine not only the presence of key morphological features, but also quantify their characteristics, e.g. the number and winding tightness of spiral arms, using large, statistically powerful samples. I will show how we are able to quantify the prevalence and properties of particular galaxy types, some of which are rare, but which are all important for understanding the evolution of the galaxy population. This work is based on data from Galaxy Zoo 2, an online citizen science project which has involved many thousands of volunteers in the task of visually classifying over 250,000 galaxies from the Sloan Digital Sky Survey. Our results depend on the combination of large sample and robust morphological measurement that can currently only be obtained by utilising citizen science.
Host: L. Fortson