Department of Astronomy Colloquia Schedule for Fall 2008
University of Texas at Austin

Sept. 2nd

No talk scheduled.

Sept. 9th

Rennan Barkana Caltech, and Tel Aviv University
"21-cm Cosmology"

Gravitational The earliest generations of galaxies are thought to have heated
and reionized the universe. The most promising way to study cosmic reionization
is to detect emission in the redshifted 21-cm line of atomic hydrogen. I will explore
the ability of measurements of the 21-cm power spectrum to enable the simultaneous
reconstruction of the reionization history and the properties of the ionizing sources.
I will show that our ignorance about the properties of the galaxies affects the accuracy
of the reconstruction, but the expected accuracy is still rather hight. I will also present
a detailed analysis of the 21-cm PDF (i.e., histogram of pixel values) and show that
it may offer an independent observational method for reconstructing the reionization
history. Finally, I will consider the early stages of hydrogen or helium reionization,
and show that even at quite high redshifts, a significant fraction of the ionized volume
resided in bubbles containing multiple sources.

Sept. 16th

Rachel Mandelbaum The Institute for Advanced Study, Princeton, NJ
"Constraining Galaxy Formation Scenarios and Density Profiles
with Galaxy-Galaxy Weak Lensing"

Gravitational lensing is a convenient tool for observing the total matter content
of the universe, including the dark matter. Galaxy-galaxy lensing thus allows a measure
of the matter content of galaxies and their environs, which may be compared against
optical tracers of galaxy contents to learn about the way galaxies form and evolve, and
about the nature of the dark matter halos in which they reside. I will present several useful
observational constraints from galaxy-galaxy weak lensing: (1) on the relation between
stellar mass, luminosity, and dark matter halo mass for early and late type galaxies;
(2) on the lensing profiles of field versus satellite galaxies; and (3) on the dark matter
halos and local environments of optical narrow-line and of radio-loud AGN. As I will show,
these data exist at sufficiently high S/N to make meaningful comparisons against
pre-existing N-body simulations and semi-analytic models of galaxy or AGN formation,
and future prospects for constraints from upcoming surveys are even more promising.

Sept. 23rd

Gregory B. Taylor University of New Mexico
"The Evolution of Radio Galaxies from Parsec to Megaparsec Scales"

Depending on where (and when) you look in our Universe, radio emission from active
galaxies is either rare (~ 1 in 10 for field galaxies) or commonplace (almost 100% in
the centers of dense clusters of galaxies at low redshifts). I will disculss what triggers
the radio emission, how the growth of the radio source is influenced by its environment,
and how it in turn can have a profound effect on the host galaxy. It is becoming
well established that some form of feedback mechanism regulates the fuel supplied
to the central dominant AGN in galaxy clusters. Details of the accretion and the energy
transport are still to be understood and are important for the study of not just clusters
, but galaxy evolution as well. By studying the radio emission of the central dominant
AGN over a broad range of physical scales we can read the history of energy input
to the cluster. I will describe some of the radio observations from parsec to
mega-parsec scales. Low frequency observations are of particular interest to explore
these problems, and I will spend some time describing the Long Wavelength Array

Sept. 30th

David S. P. Dearborn Lawrence Livermore National Laboratory (visit: 09/25-10/01)
"The Use of Nuclear Explosives to Disrupt or Divert Asteroids"

Nuclear explosives are a mature technology with well-characterized effects. Proposed
utilization for reducing the threat of an asteroid impact include a standoff burst to
ablate surface material and nudge the body to a safer orbit. This can be done without
risk of fragmentation when there are decades of warning, and the required speed
change is small (~1 cm/s). For shorter lead times, (a keyhole passage scenario like Apophis)
a direct sub-surface burst can fragment the body. With only 1000 days lead time,
shattering an NEO reduces the material impacting the Earth to about 1/100,000th
of the original body, a huge mitigation factor. To better understand these possibilities,
we have used a multidimensional radiation/hydrodynamics code to simulate sub-surface
and above surface bursts on an inhomogeneous, 1 km diameter body with an average
density of 2 g/cc. The body, or fragments (up to 750,000) are then tracked along

Oct. 7th

Special Event: Antoinette DeVaucouleurs Medalist Prize-Winner in Astronomy (visit: Oct. 6-10)

Christopher F. McKee
University of California/Berkeley
"Massive Star Formation: Theory and Observational Predictions"

Stars more than about 10 times the mass of the Sun are responsible for creating
most of the heavy elements in the universe, for governing the evolution of galaxies, and
most likely for reionizing the universe a few hundred million years after the Big Bang.
The formation of these stars can be understood as an extension of the theory of
low-mass star formation, but two major problems must be overcome: First, for massive
stars, the outward force due to radiation pressure exceeds the inward force due to gravity;
how can gas accrete onto the star in that case? Circumstellar disks, outflow cavities, and
radiative Rayleigh-Taylor instabilities all contribute to the solution of this problem.
Second, why does a gas cloud form a single massive star instead of fragmenting into
hundreds of low-mass stars? This problem can be solved by thermal feedback from
the first stars to form in the cloud. These conclusions are validated by the first 3D
radiation-hydrodynamic simulations of high-mass star formation. Observational
predictions include (1) Massive stars should form in cores with surface densities
of order 1 g cm^2; (2) the stellar initial mass function (IMF) should follow the mass
function of cores in the host molecular cloud, scaled down by a factor of a few;
(3) massive, turbulent disks detectable by ALMA and the EVLA should occur
around massive protostars; and (4) star formation in regions of low surface density
should have an IMF that is truncated at high mass, and as a result, the star

Oct. 14th

Donald E. Winget University of Texas at Austin
"The Physics of Crystallization in a Dense Coulomb Plasma
from Globular Cluster White Dwarf Stars"

White dwarf stars are cosmic laboratories for studying the equation of state of matter at high densities
and temperatures. They have a long history of pushing the envelope of our understanding of matter.
Nearly eighty years ago we began to come to grips with the physical nature of the structure of
white dwarf stars; the solution of the puzzle of their masses and radii in terms of an electron
degenerate equation of state was a tour de force for the quantum theory. The 1960’s saw major
advances in our theoretical understanding of the ions: a Coulomb liquid phase and ultimately ion
crystallization were predicted. I will outline the exploration of this theoretical landscape against
the backdrop of recent astronomical observations. Significant puzzles have arisen from spectroscopic
and photometric studies of white dwarf stars in the galactic disk, even with the large samples
produced by the SDSS. Asteroseismology of white dwarf stars has led to direct measurement
of stellar evolution and promises constraints on particle physics including our understanding of
electroweak theory at low energies and the properties of axions and super-symmetric particles
and gave us the first empirical probe of the extent of ion crystallization. A new frontier is opening up
from Hubble Space Telescope observations of large populations of coeval white dwarf stars in
globular clusters. Interpreting these observations challenges our understanding of the initial final
mass relation, nucleosynthesis, and basic internal white dwarf compositional structure, as well as
providing important additional confirmation of ion crystallization theory. Through the study of
white dwarf stars in the globular cluster, NGC 6397 I present our empirical evidence that
crystallization is a first-order phase transition – releasing latent heat. Most exciting is the new
tool these stars provide for measuring the Gamma of crystallization in dense Coulomb plasmas.
I will discuss the future of these important tools for cosmochronology, particle physics, and
the equation of state of matter.

Oct. 21st

Casey Papovich Texas A&M University
"Witnessing Galaxy Formation at High Redshifts"

The combination of NASA's Great Observatories provides a unique, panchromatic view of
distant galaxies. I will discuss how observations of cosmologically distant galaxies with
these facilities have improved our understanding of galaxy evolution. In particular, I will
discuss recent results from the Spitzer Space Telescope, which measures the infrared
emission from obscured star formation and AGN. I will focus on recent results from several
deep surveys to study the assembly of massive galaxies at high redshifts, which are based
on the properties of their infrared emission. I will also discuss a program to study gravitationally
lensed distant galaxies with Spitzer and other telescopes, which will provide a detailed view
of very distant star-forming galaxies. I will summarize the current constraints on the assembly
histories of distant galaxies, and discuss challenges that remain to understand galaxy evolution.

Oct. 28th

R. Michael Rich University of California, Los Angeles
"The Galactic Bulge: New Surveys and New Results"

The central bulge of the galaxy is the nearest example of an old, metal rich, spheroid
population. I will report on detailed abundances of its stars, and results of a new radial
velocity survey. The bulge shows some fo the dynamical characteristics of a pseudobulge,
yet also has a vertical abundance gradient and is older than 10 Gyr. Chemical signatures
are consistent with formation of the bulge on a timescale of < 1Gyr. The abundance
gradient and rapid formation timescale pose challenges if the bulge's structure

Nov. 4th

James P. Lloyd Cornell University
"Precision Radial Velocities in the Near Infrared with TEDI"

The TEDI (TripleSpec - Exoplanet Discovery Instrument) is a dedicated instrument
for precision near-infrared radial velocities combining the moderate resolution TripleSpec
spectrometer with an externally dispersed interferometer. With the goal of achieving meters
per second radial velocity precision, TEDI will enable searches for planetary companions
to the lowest mass stars. Heretofore, such planet searches have been limited almost entirely
to the optical band and to stars that are bright in this band. Consequently, knowledge about
planetary companions to the populous but visibly-faint low mass stars is limited. Current radial
velocity searches for planets around early M dwarfs with visible wavelength spectrometers
have already yielded remarkable discoveries. The extension of this capability to longer
wavelengths opens up new opportunities in numerous mid-late M dwarfs and brown dwarfs.
TEDI has been commissioned on the Palomar 200" telescope in December 2007, and is
currently in a science verification phase.

Nov. 11th -
Nov. 14th

Special Event: Conference on Galaxy Evolution: Emerging Insights and Future Challenges
(no colloquium scheduled on Nov. 11, to avoid conflict with this event.)

Nov. 12th

Special Physics and Astronomy Colloquium: 4:15 P.M. RLM 4.102 (John Wheeler Lecture Hall)

Andrew Hamilton University of Colorado at Boulder
"Black Holes, Schrodinger Cats, and Particle Accelerators"

What really happens inside black holes? As first pointed out by Poisson & Israel (1990),
the classical empty (Kerr-Newman) solutions for black holes, complete with their analytic
continuations through wormholes and white holes to new universe, are subject to the mass
inflation instability. The instability has profound consequences for the interior structure of
black holes. If the instability is suppressed by large dissipation, then the typical result is
the creation of a huge amount of entropy inside the black hole, orders of magnitude more
than the Bekenstein-Hawking entropy. If the second law of thermodyamics is to be saved,
then locality must break down inside black holes, so that entropy does not accumulate inside
black holes. In effect, what happens inside the horizon of a black hole must constitute an
alternate quantum reality for each person that travels inside it. Alternatively, if dissipation is
more modest, then mass inflation will occur. Mass inflation is caused by relativistic counter-
streaming between ingoing and outgoing streams. The result is a particle accelerator of
extraordinary power: the black hole accelerates ingoing and outgoing streams through each
other to center-of-mass energies that classically far surpass the Planck energy, easily reaching
conditions as extreme as those in the Big Bang. Like the Big Bang, the conditions are not
only energetic but of low entropy. What does Nature do with this remarkable beast?

Nov. 18th

Special Event: Beatrice M. Tinsley Visiting Professor in Astronomy (visit: Nov. 3 - Dec. 4)

Geoffrey A. Blake California Institute of Technology (Caltech)  
"The Search for Infant Solar Systems"

As conduits of mass and angular momentum, circumstellar accretion disks serve as the critical
link between astrophysics and (exo)planetary science. With new spectrometers on board the
Spitzer Space Telescope and the Keck/Very Large Telescopes in Hawaii/Chile, we have used
high resolution spectroscopy to discover systems in which planet(esimal) formation is likely ongoing.
In a few cases follow up imaging with aperture synthesis arrays has directly confirmed the disk
structure inferred from spectro-photometry. This talk will present the means by which such
discoveries are made as well as a number of recent results, and speculate on future
directions in the field.

Nov. 25th

No talk scheduled.

Dec. 2nd

Laurent Loinard
Instituto de Astronomia - UNAM, Morelia, Mexico
"The Space Distribution of Nearby Star-formation"

Using the excellent astrometry capabilities of the Very Long Baseline Array, we have measured
the trigonometric parallax (and the proper motions) of a number of young stars in several nearby
star-forming regions with a precision better than a few percents. This represents an improvement
by one to two orders of magnitude over previous determinations. In particular, we have found that
the distance to the core of Ophiuchus is 120 ± 4.5 pc, and we have confirmed that the mean
distance to Taurus is 140 pc. Our data also allow us to determine the 3D structure of the regions
under study. In the case of Taurus, we have shown that the total extent of the complex along the
line of sight is about 30 pc, the eastern portion being on the far side and the region around L1495
on the near side. Finally, the proper motion measurements, combined with existing radial velocities,
allow us to describe the internal kinematics of the regions under study. A full spatio-kinematical
description of star- forming regions, therefore, becomes possible.

I will end my talk by mentioning preliminary results in other regions (particularly Serpens and
Cepheus), as well as longer-terms prospects.

Last Modified: December 2008