University of Minnesota
University of Minnesota

Astrophysics Colloquia

University of Minnesota

Minnesota Institute for Astrophysics

Astrophysics Colloquium

Fall 2011 - Coordinated by Evan Skillman
Held in Physics 210 at 3:35pm on Fridays
(unless otherwise noted)

September 16 No Colloquium — Fall Picnic
September 23 Dr. Franscesco Haardt, U Insubria
Title: The Cosmic UV/X-Ray Background
Abstract: The reionization of the all-pervading intergalactic medium (IGM) is a landmark event in the history of cosmological structure formation. Studies of Gunn-Peterson absorption in the spectra of distant quasars show that hydrogen is highly photoionized out to redshift z > 6, while polarization data from the Wilkinson Microwave Anisotropy Probe (WMAP) constrain the redshift of a sudden reionization event to be significantly higher, z = 11 to 14. It is generally thought that the IGM is kept ionized by the integrated UV emission from active nuclei and star-forming galaxies, but the relative contributions of these sources as a function of epoch are poorly known. In this talk I review the theories behind UV/X-ray background models, and describe new solutions of the radiative transfer of ionizing background radiation in a clumpy, expanding medium. I present improved synthesis models of the UV/X-ray cosmic background spectrum and its evolution, discussing several astrophysical consequences and possible tests.
Host: C. Scarlata
September 30 Dr. Thomas Nelson, U Minnesota, Physics
Title: V407 Cyg 2010: Exploring the anatomy of a gamma-ray emitting nova
Abstract: Classical nova events in symbiotic stars, although rare, offer a unique opportunity to probe the interaction between ejecta and a dense environment in stellar explosions. The 2010 outburst of the V407 Cyg offered such an opportunity. This nova caught the attention of a wide cross-section of the astronomy community when it was detected as a GeV gamma-ray transient with Fermi. I'll present an overview of the outburst across the electromagnetic spectrum, and discuss the rich physics that can be explored with this multiwavelength dataset, including evidence of an asymmetric blastwave, the possible launching of a jet, and the presence of a massive white dwarf at the heart of the system.
Host: L. Fortson
October 7 No Colloquium — Inflation Workshop
Host: FTPI
October 14 Dr. Massimo Marengo, Iowa State University
Title: Blowing a Standard Candle: the Disappearing Mass of δ Cephei
Abstract: Cepheid stars hold the keys of the cosmological distance scale. Thanks to their period-luminosity relation (Leavitt law), they are the first rung in the ladder we use to measure the size and age of the universe. They are also the benchmark for intermediate-mass stellar evolution models. Despite their importance, there are still outstanding puzzles in the theoretical understanding of Cepheids. Namely, the mass predicted by evolutionary models is significantly larger than the mass estimated by pulsation theory, or directly measured in binary systems. A possible solution is that these stars lose mass while they are in the Cepheid phase.

I will present the first direct observation that one Cepheid star, the class prototype δ Cephei, is currently losing mass. These observations are based on data obtained with the Spitzer Space Telescope in the infrared, and with the Extended Very Large Array in the radio. We found that δ Cephei is associated to a vast circumstellar structure, reminiscent of a termination shock. This structure is created as the wind from the star interacts with the local interstellar medium. We measure a velocity of the wind of ~ 36 km/s, and a mass loss rate in the range 10-7–10-6 M/yr. I will discuss the importance of this discovery in the context of the Cepheid mass discrepancy, and the cosmological distance scale.
Host: R. Humphreys
October 21
October 28 Dr. Eli Dwek, NASA GSFC
Title: The Origin of Dust in the High-Redshift Universe
Host: R. Gehrz
November 4 Dr. Amy Barger, U Wisconsin-Madison
Title: The History of Star Formation and AGN Activity from Deep X-ray Observations
Abstract: The Chandra X-ray satellite detects X-rays emitted during accretion onto supermassive black holes, even when they are obscured. I will describe how Chandra surveys have changed our thinking on when and how supermassive black holes formed. I will also present new results on the history of star formation obtained from these data.
Host: C. Scarlata
November 11 Dr. Kyle Willett, U Minnesota, Physics
Title: OH Masers from M31 Out to the Peak of Cosmic Star Formation
Abstract: Since their discovery at the beginning of the era of radio astronomy, masers have been used as unique probes of their astrophysical environments. I will be presenting results from multi-wavelength observations of extragalactic OH masers. Spitzer infrared observations of ULIRGs have demonstrated the ability to identify a megamaser host based on the global properties of the host galaxy, and form the testbed for the first detailed tests of OH pumping models. I will also present results from a radio survey for new OH megamasers out to z=1.5. Finally, I will show results of a search for OH masers in the Andromeda Galaxy (M31), and discuss the prospects of using masers to measure M31's proper motion.
Host: L. Fortson
November 18 Dr. Mario Mateo, U Michigan
Title: Collecting Fossils in the Galactic Halo
Abstract: Over the past few decades, our view of the Galactic Halo—the largest, most massive component of the Galaxy—has been revolutionized by our understanding of its smallest components, the dwarf galaxy satellites of the Milky Way. These small galaxies were once considered to be insignificant debris in an otherwise simple halo that formed from a monolithic collapse early in the Galaxy's history. A new paradigm has emerged in which it is recognized that these dwarfs are important fossils that allow us to probe the evolution of our Galaxy, and, by extension, about galaxy formation in general. But these galaxies are also fascinating in their own right: they are the darkest, smallest, closest galaxies known, placing fundamental constraints on the nature of Dark Matter. They exhibit a wide range of stellar populations, especially bewildering given their tiny luminosities and in the strong contrast to the seeming uniformity of classical halo populations. They exhibit a number of striking scaling relations that clearly reveal some sort of common history or formation environment. I will provide an overview of these enigmatic galaxies while highlighting studies that I am involved with—past, present and future—that aim at increasing our knowledge of these amazing systems and filling in their fossil record.
Host: E. Skillman
November 25 No Colloquium — Thanksgiving Holiday
December 2 Canceled
December 9 Dr. Julianne Dalcanton, U Washington
Title: Galaxies Viewed as Collections of Individual Stars
Abstract: In extragalactic astronomy, we routinely observe galaxies in broad-band filters, and then interpret the resulting spectral energy distribution to constrain the galaxies' masses, star formation rates, ages, and metallicities. The fidelity of this interpretation relies on having a detailed understanding of the stellar populations within the galaxy, and on accurately characterizing the luminosities and colors of the billions of stars which contribute to a galaxy's light. In this talk I will discuss several large programs which use the Hubble Space Telescope to resolve millions of the most luminous stars in nearby galaxies. I will highlight what we are learning about the star formation history of galaxies, the evolution of stars, and the structure of the dusty interstellar medium.
Host: E. Skillman