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Northwestern University


Weekly Astrophysics Seminars 2009-2010

Seminars are held at 4:00 PM on Tuesdays in Room F160
on the first floor of the Technological Institute (2145 Sheridan Road) unless otherwise noted



Winter Quarter 2010

  Date   Visitor / SeminarHost
  Jan. 12  

Ben Koester
    University of Chicago
     The Beginning of the End: The Galaxy Cluster Red Sequence from 0.1 < z < 1.0

The arrival of galaxies on the red-sequence (RS) marks the beginning of the end in galaxy evolution, and is thus a cornerstone of many galaxy formation models and cluster cosmology programs. In this seminar, I present recent results that demonstrate the utility of the RS as a cluster mass proxy in the maxBCG-SDSS cluster sample (0.1 < z < 0.3), a catalog which has already been used to place constraints at the few percent level on cosmological parameters. I then turn to galaxy formation, and describe new measurements of the RS in the maxBCG cluster catalog and in Hubble Space Telescope imaging of massive clusters at z ~ 1, which preliminarily indicate that RS galaxies in optically-selected clusters quench their star-formation significantly later than those in X-ray selected clusters. I conclude with a plan to address significant statistical and systematic uncertainties in these and other RS studies by combining HST, SDSS, and RCS2 data sets to make the first comprehensive, statistically robust measurement of the cluster RS at 0.1 < z < 1.0. The result will have a direct impact on models of galaxy formation and future cluster cosmology programs.

Mel Ulmer
  Jan. 20
  *Wed
    12:00pm

Eugene Chiang
    UC Berkeley
     Planet Formation in Transitional and Debris Disks

Myriad processes operate in circumstellar disks to give birth to planets. New insights are afforded by transitional T Tauri disks and debris disks. Transitional disks resemble classical T Tauri disks but have central AU-sized clearings that are swept clean of dust. They age into debris disks in which the dust is entirely optically thin. The decline in optical depth almost certainly reflects the accumulation of micron-sized grains into planets. In connection with these systems we offer progress reports on several problems in disk physics, planet formation, and planet-disk interaction: (1) How do T Tauri disks accrete and dissipate? (2) As dust grains sediment to disk midplanes, what maximum densities can they achieve? Are dust concentrations large enough for planetesimals to form by gravitational instability? (3) What physics underlies the surface brightness profiles of debris disks? In particular, how can we use the observed shapes of debris disks to infer the properties of attendant planets, such as the newly imaged extrasolar planet orbiting Fomalhaut?

Fred Rasio
  Jan. 26

Qizhou Zhang
    Harvard/Smithsonian CfA
     How to Make Massive Stars

Massive stars dominate the appearance and the evolution of galaxies. Despite their prominence, their formation is not well understood. Since most massive stars are found in clusters, their formation is linked to the fragmentation of dense molecular clouds. Due to a lack of observational constraints, opposing theoretical ideas have argued the relevance of cloud core mass function to stellar initial mass function. In addition, once protostars reach 8 Msun, the radiation pressure may halt mass infall and prohibit stars from further growth. In this talk, I will review recent observational results that help discriminate different theoretical ideas. I will present studies of massive infrared dark clouds that reveal fragmentation towards the beginning phase of a cluster formation. I will then present studies of accretion flows surrounding high mass protostellar objects. Finally, I will present measurements of dust polarization in high mass protostellar objects, which suggest that magnetic fields play an important role during the collapse of dense cores.

Farhad Zadeh
  Feb. 2

Bülent Kiziltan
    UC Santa Cruz
     New Constraints on Neutron Stars: A Compact Key to Unlock Galactic Mysteries

It's been more than 40 years since the discovery of the first pulsar. Since then, these objects have been used as unique cosmic yardsticks by which we continue to test a variety of exotic physics and astronomical phenomena. While we have reached a remarkable precision in pulsar observations, many aspects of our theoretical understanding regarding these objects are still in their infancy. In this talk, I will describe alternative multidisciplinary approaches which we can use to get new insights into a wide range of neutron star properties including details of their evolution, ages, magnetic fields, initial spin periods and masses. I will then discuss how we can use these new constraints to get a variety of interesting science from millisecond pulsars on a broader scale.

Vicky Kalogera
  Feb. 9

Charles Gammie
    University of Illinois, Urbana-Champaign
    The Black Hole in the Galactic Center

The black hole candidate in the galactic center, Sgr A*, is among the best understood such objects in the universe: its mass and distance are known with comparative precision, its spectrum and variability are well characterized from the radio to the X-ray, and its event horizon is large enough to be resolved by Very Long Baseline Interferometry at millimeter wavelengths. With appropriate modeling, these data may probe the strong gravitational field close to the event horizon. I will describe new models of the luminous accreting plasma that include many of the physical effects that are known to be important, including black hole spin, and show how the data and models can be used to constrain the spin and inclination of the black hole.

Vicky Kalogera
  Feb. 23

C. Barth Netterfield
    University of Toronto
    The Balloon-borne, Large Aperature Sub-mm Telescope

The Balloon-borne, Large Aperture Sub-mm Telescope (BLAST) operates at an altitude of 35km, above ~99.5% of the atmosphere. BLAST observes in three bands, between 250 and 500 microns, where the atmosphere from the ground is nearly opaque, and near the peak of a 10K thermal blackbody - the characteristic temperature of cold pre-star-forming clouds of gas and dust in the Milky Way, and other galaxies. BLAST has probed these clouds, providing a census of pre-stellar cores, constrained dust properties of nearby galaxies, and, by observations of red-shifted star-forming galaxies, measured the history and distribution of star formation in the Universe.

Giles Novak
  March 2

Jay Strader
    Harvard/Smithsonian CfA
    Globular Clusters and Their Galaxies

Globular clusters are fossils of star formation at all epochs. I will discuss how new photometric and spectroscopic surveys of globular cluster systems are providing important constraints on the formation of early-type galaxies, stellar halos, and the distribution of dark matter in the local universe. In addition, observations of individual clusters in nearby galaxies are turning up unexpected wrinkles, with potentially far-reaching implications for stellar populations.

Loren Hoffman


Past Astrophysics Seminars