Dept. of Astronomy Colloquia Schedule for Spring 2007
University of Texas at Austin

Jan. 16th

None Scheduled.

Jan. 25th

Rebecca A. Bernstein   University of Michigan, Ann Arbor  
Detailed Chemical Abundances of Extragalactic Globular Clusters
(On the Formation of Galaxies and Spectrographs)

My collaborators and I are developing a method for measuring the detailed
abundances of extragalactic globular clusters from high resolution spectroscopy
of their integrated light. To do so, we are using a "training set" of clusters in the
Milky Way and LMC. In these spatially resolved GCs, we can compare our
integrated light analysis with standard analysis of individual stars to test our results.
Our work to date shows that we can measure abundances of a wide range of key
elements with similar accuracy to that obtained from individual stars. Our goal is
to apply these techniques to GCs in galaxies within 4 Mpcs to constrain their
formation histories as the Milky Way's formation history has been studied. In this
talk, I will also describe the "formation" of the MIKE spectrograph, which I designed
and built for the Magellan telescopes and which we are using in this research.

Jan. 30th

Kepler Oliveira   Universidade Federal do Rio Grande do Sul, Brazil  
White Dwarf Mass Distribution

In all papers to date the mass distribution for hydrogen (DAs) and helium (DBs)
atmosphere white dwarf stars has been very similar, centered around 0.6 Msun.
But the samples have been limited because of the faintness of the stars.
The largest published sample is 318 DAs and 48 DBs. With the Sloan Digital
Sky Survey Data Release 4, we studied a sample of 7167 DAs and 507 DBs
and obtain distinct mean masses for the two populations,
<M(DA)>=0.593\pm 0.016M(Sun) and <M(DB)>=0.683\pm 0.008M(Sun).
We detect around 20% stars with M>0.8M(Sun) and the highest mass white
dwarf stars known, up to 1.33 M(Sun).

Feb. 6th

John Kormendy   University of Texas at Austin
Structure and Formation of Elliptical Galaxies

New surface photometry of all known elliptical galaxies in the Virgo cluster
is combined with published photometry to derive composite brightness profiles
over large radius ranges. They give enough radial leverage to show that Sersic
functions describe nearly all ellipticals remarkably well. Therefore we can confidently
identify central departures from these profiles that are diagnostic of galaxy formation:
    Two kinds of departures are seen. Bright ellipticals (M_B < -20) have cuspy cores --
"missing light" at small radii. Cores are well known and are naturally scoured by
binary black holes produced in dissipationless ("dry") galaxy mergers. Faint ellipticals
do not have cores; instead, they have extra light at the center above the inward
extrapolation of the outer Sersic profile. The properties of the extra light resemble those
of compact central components predicted in numerical simulations of mergers that
contain gas. We suggest that faint ellipticals were produced by dissipational ("wet")
mergers including central starbursts. Wet and dry mergers, respectively, explain how
the observed dichotomy between faint-disky-coreless ellipticals and bright-boxy-core
ellipticals was formed.
    Why wet and dry mergers formed the dichotomy became clearer at the 2006 Potsdam
Thinkshop on Black Holes and Galaxy Formation. This was a watershed in establishing
the importance of energy feedback from active galactic nuclei (AGNs) into the galaxy
formation process. The essential point is that hot, x-ray-emitting gas is required to store
AGN energy and make it available to solve a variety of problems in galaxy formation.
Remarkably, faint-disky-coreless ellipticals do not contain hot gas halos, while
bright-boxy-core ellipticals do contain such halos. We suggest that AGN feedback in
giant but not faint ellipticals explains why the E-E dichotomy arose.
    Finally, we verify that there is a dichotomy between elliptical and spheroidal galaxies.
Their properties are consistent with our understanding of their different formation processes:
mergers for ellipticals, and conversion of late-type galaxies into spheroidals by environmental
effects and by energy feedback from supernovae.

Feb. 13th

Kurtis Williams   University of Texas at Austin  
Focusing Gravitational Lenses by Focusing on their Environments

Strong gravitational lenses, where a single galaxy lenses a background source, have long
been touted as an independent and "simple" means of measuring cosmological quantities,
such as the value of the Hubble Parameter and the density of dark energy in the Universe.
Yet all too often, these supposedly straightforward measurements have yielded values that
contrast markedly with generally accepted values. If lenses tend to lie in complex environments,
such as groups of galaxies, these cosmological measurements (which generally ignore the
environments) will be biased. My collaborators and I have undertaken a large photometric and
spectroscopic survey of the environments at and along the line-of-sight toward 69 strong
gravitational lenses in order to determine what fraction of lenses lie in complex environments.
In addition, our data are able to provide a large sample of galaxy groups out to z~1, useful
for studying group evolution over a long time baseline. Further, we are finding evidence that
we can use the lenses themselves as probes of the dynamical state and shape of group
dark matter halos.

Feb. 20th

Tiziana Di Matteo   Carnegie Mellon University  
The Formation and Evolution of a Cosmological
Population of Black Holes and Galaxies

There is a growing observational evidence for a close connection between
the formation and evolution of galaxies and of their central supermassive
black holes. Motivated by this connection, we investigate the coupled formation
and evolution of black holes and galaxies using state-of-the-art cosmological
hydrodynamic simulations (a data set which includes the largest simulation yet run)
of structure formation in the Lambda-Cold Dark Matter model. Along with the
gravitational evolution of dark matter, gas dynamics, cooling and star formation,
the simulation follows black hole growth and associated feedback self-consistently.
I will discuss black hole growth in the centers of galaxies and their impact on
galaxy formation. I will show how we can use our model to investigate the global
history of black hole mass assembly in galaxies from the high redshift Universe

to today and the formation and fate of the first quasars and the properties of their hosts.

Feb. 27th

Michelle Creech-Eakman   New Mexico Institute of Mining and Technology
Magdalena Ridge Observatory Interferometer -
A Fully Optimized Aperture Synthesis Array

I will present a brief overview of the Magdalena Ridge Observatory Interferometer (MROI),
being built at an altitude of 10,500 feet just outside of Socorro, NM. The planned
architecture of the system will include 10 1.4-m class alt-alt telescopes, the most
modern detectors operating over the 0.6 - 2.4 micron range, with relocatable telescopes
capable of resolving sources with sizes in the range of 30 - 0.3 milliarcseconds.
I will present a brief overview of the technique of optical interferometry and then
demonstrate some of the science which has been done already with optical interferometers.
I will introduce our key science mission for the MROI and demonstrate how the design
for this array will provide unprecedented, model independent images for a wide variety
of stellar and some extragalactic sources. The MROI is being built in collaboration by
two principal institutions: New Mexico Tech and the University of Cambridge. We are
following an aggressive schedule and anticipate first light on the first baseline in 2009.

Mar. 6th

James E. Lawler   University of Wisconsin, Madison
Laboratory Astrophysics with Old and New Fourier Transform

The first part of this talk will be a description of our laser and Fourier transform spectrometer (FTS)
experiments used to measure accurate, absolute atomic transition probabilities. These laboratory
results are being systematically applied by UT-Austin astronomers (Sneden et al.) in studies
of metal-poor Galactic halo stars. The increasingly well-defined r-process abundance patterns
of some Halo stars provides a strong constraint on nucleosynthesis models.

    In the second part of this talk I will describe our Mark 1 Spatial Heterodyne Spectrometer (SHS).
This instrument is a new type of broad-band, high resolution FTS. The Mark 1 SHS will have
substantial laboratory applications, and may influence the design of spectrometers for next
generation orbiting and large (~30 m) ground-based telescopes.

Mar. 20th

Jonathan C. Tan   University of Florida  
Star Formation Near and Far

Star formation is a fundamental process that, as the final stage of cosmic structure formation,
dictates how galaxies form and evolve. At the same time it sets the environment for the birth
of planetary systems.
    We must understand star formation to know our own origins. Unfortunately we are a long way
from a complete theory of star formation. In this talk I describe a number of projects through
which we attempt to understand how stars from in a range of galactic environments. We start
with massive star and star cluster formation in our local Galactic neighborhood, exemplified
by the Orion Nebula Cluster. We look for differences between star cluster formation in dwarf
irregular and spiral galaxies. Continuing to the environs of supermassive black holes, we find
star formation is a natural and important part of their accretion process. Finally we apply
the knowledge we have gained about local star formation to make a theoretical prediction for
how it occurs in the high redshift, metal-free universe.

Apr. 3rd

John Peoples 
Fermi National Accelerator Laboratory (Fermilab)
The Dark Energy Survey

The need to understand the accelerating expansion of the universe has become a
critical problem for cosmology. It certainly requires something new: dark energy.
In order to begin to pin down its nature, more precise measurements of its properties
must be made; the key property of dark energy that determines the expansion history of
the Universe is its equation of state parameter, w. The Dark Energy Survey Collaboration
plans to measure w with an optical-near infrared survey of 5000 sq. deg of the South
Galactic Cap to ~24th magnitude in the SDSS griz filters. We propose to build a
state-of-the-art, wide-field CCD imager, DECam, and mount it on the Blanco 4-m telescope
at Cerro Tololo Inter-American Observatory. We also plan to build and deploy a powerful
data management system to process and distribute the data to the Collaboration and
the public. The project will be carried out with support of Fermilab, NCSA, NOAO and
partner universities from the U.K, U.S., Spain and Brazil with funding from DOE, FINEP,
MEC, PPARC, NSF, and the participating institutions.

Apr. 10th

Steven Kawaler 
Iowa State University
Numerology and Asteroseismology

The physics of stellar pulsation provides a limited set of rules for oscillation frequencies
that a star can show. Frequently these rules produce simple numerical relationships
between the observed frequencies that we can exploit as a "short cut" for determining
global properties and some structural details. But sometimes, pulsating stars show stark
systematics that theory does not readily anticipate. What (if anything) can we learn from
these systematics? How devious can Nature be in producting spurious apparent systematics
that send us down interesting, but ultimately fruitless, paths of analysis?
    To approach these questions, I will show examples of predicted and verified
asteroseismic tools, unanticipated but still valuable relationships, and a blind alley or two -
all taken from pulsating stars in their end stages of life.

Apr. 17th

David S. De Young 
National Optical Astronomy Observatory (NOAO)
AGN Outflows and Galaxy Cluster Evolution

Energetic outflows from active galactic nuclei have been a topic of astrophysical interest
for several decades, and many basic features of these outflows are still poorly understood.
In particular, the highly collimated bipolar jets emanating from the cores of AGN lack a
clear definition of their most basic parameters. More recently such outflows have been
suggested as a mechanism for solving the "overcooling" problems encountered by current
CDM cosmological models. In addition, AGN outflows have been hypothesized as sources
of energy for reheating the intra-cluster medium in rich clusters and thus as a solution for
the "cooling flow" problem in the ICM. This talk addresses these three issues via an
examination of the processes that can couple the energy of bipolar AGN outflows to the
ambient medium, with emphasis on the effects of AGN in galaxy clusters. These outflows
can provide new constraints on the "central engine" driving the outflow, and in addition
the evolution of such jets can lead to some energy transfer to the surrounding gas.
However, it is less clear that the outflow-environment coupling is efficient enough to
suppress star formation and overcome cooling flows in the time required.

Apr. 24th

Scott M. Ransom 
National Radio Astronomy Observatory (NRAO)  
A Millisecond Pulsar (and Basic Physics) Bonanza with the GBT

In the past 5 years, the Green Bank Telescope (GBT) has discovered at least 60 new
radio pulsars in globular clusters, effectively doubling the number known. The vast
majority of these new systems are millisecond pulsars and about half of them are
members of binaries. The rich cluster Terzan 5 alone now contains 33 known millisecond
pulsars, by far the most of any globular cluster. Many of the pulsars are truly unique
and/or exotic objects that could only have been produced in dense cluster cores after
stellar interactions. Some of the stranger systems include the fastest known spinning
neutron star (PSR J1748-2446ad at 716 Hz), 9 highly eccentric binary systems, at least
5 eclipsing systems, and 2 millisecond pulsars which seemingly have main-sequence-like
stellar companions. Several of these pulsars constrain the equation of state of matter at
supra-nuclear densities, while others will eventually provide masses of spun-up neutron stars
and interesting tests of gravitational theories. In addition, the pulsars will allow us to probe
a wide variety of other astrophysics such as eclipse mechanisms, cluster dynamics,
and the structure of the interstellar medium.

Last Modified: June 2007