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Fall 2007

Sept 14
 Dr. Mark Brodwin (NOAO)
Galaxy Clusters at 0 < z < 2 in the IRAC Shallow Cluster Survey
Using a probabilistic photometric redshift-based approach combining Spitzer mid-IR and NOAO optical data, we have discovered 335 new galaxy clusters and groups in the NDWFS Bootes field, over 100 of which are at 1 < z < 2. To date we have spectroscopically confirmed 10 of these high redshift clusters, spanning 1.06 < z < 1.41. Deep follow-up Spitzer and HST data, currently being obtained for 18 of the z > 1 clusters, allows detailed studies of the mass assembly and star formation histories in massive galaxies at this key epoch. In this regime massive cluster galaxies have the mean colors predicted by simple passive evolution models with high (z > 3) formation redshifts. They also show strong evidence of morphological evolution, with the fraction of mergers and disturbed morphologies increasing with lookback time. Finally we have measured strong clustering in the galaxy cluster population at 0.25 < z < 1.5, from which we infer that the cluster sample has an average mass of nearly 10^14 Msun.
Host: Bob Abraham
Sept 21
 Prof. Adam Frank (Rochester)
New Perspectives on Protostellar Outflows in 3 Acts
The role of outflows in global star formation processes has become hotly debated even as fundamental questions about the nature of these outflows continues to receive attention. In this talk I discuss both problems and new approaches to their resolution. Astrophysical outflows have always been a subject at the forefront of the numerical technologies and in the first act of the talk I introduce AstroBEAR, a new Adaptive Mesh Refinement MHD tool developed at Rochester for the study of star formation outflow issues. The question of "feedback", the mechanisms by which protostellar outflows can drive turbulence in either clouds or clusters, is then addressed via AstroBEAR simulations. In these studies we seek to understand the detailed mechanisms by which outflows can return energy to their environment and, perhaps, drive turbulent motions. Finally we turn to the MHD processes involved with the outflows themselves and focus on a new tool, High Energy Density Laboratory experiments. I will present results from a campaign of experiments carried out at Imperial Collage in London which directly address issues of magnetically dominated radiative outflows and jets.
Host: Chris Matzner
Sept 28
 Prof. Joe Mohr (Illinois)
The Evolution of the Intracluster Medium in Massive Galaxy Clusters from z=1.3 to z=0.2
Galaxy Clusters are important as laboratories for the study of galaxy populations and the intracluster medium over cosmic timescales. Developing an improved understanding of the physics driving the structure evolution of clusters has many benefits, because galaxy clusters are powerful tools for the study of cosmology. In particular, if we can develop a sufficiently detailed understanding of cluster structural evolution, we can use these systems to address the mysteries surrounding the cosmic acceleration. One sensitive technique for studying evolution of the intracluster medium is through X-ray imaging spectroscopy from the Chandra Observatory. We have used Chandra archival observations of a large sample of clusters to explore redshift evolution of the intracluster medium, testing consistency of the observations with simple no-evolution and self-similar evolution models as well as constraining the variation of the cluster gas fraction with redshift. Our analysis indicates that there is a clear and measurable signature of physics beyond simple gravitational collapse at work in the redshift evolution of the structure and mass of the intracluster medium.
Host: Howard Yee
Oct 5 Dr. Nitya Kallivayalil (MIT)
The Motions of the Magellanic Clouds About the Milky Way: New Insights into an Enigmatic System
It is expected from current models of hierarchical structure formation that the interaction between the Large and Small Magellanic Clouds (LMC & SMC) and the Milky Way (MW) will have played an important role in the dynamical evolution of the MW's outer parts. A major uncertainty in decoding this evolution is the space velocity of the Clouds and thus their orbital motion. While the radial velocities of the Clouds have been measured to high precision, the velocity transverse to the line of sight (the proper motion) has been harder to constrain. I will present a project that has allowed us to determine the systemic proper motions of the Clouds to better than 5% accuracy for the LMC, and 15% for the SMC, using two epochs of HST/ACS (Advanced Camera for Surveys) data and just a ~2 year baseline. This measurement has the required accuracy to constrain models of the Galactic dark halo as well as formation mechanisms for the Magellanic Stream: the implications of which are unexpected in light of our previous understanding of the MW-LMC-SMC system.
Host: Ray Jayawardhana
Oct 12
 Prof. Art Wolfe (UCSD)
Star Formation in Neutral Gas Reservoirs at High Redshift
Neutral Gas at z ~ 3 covers 0.333 of the sky, yet star forming galaxies in the same redshift interval cover less than 0.001 of the sky. I describe results of a Hubble Ultra Deep Field search for in situ star formation in the gas. The results demonstrate that application of the Kennicutt-Schmidt relation at high redshift leads to overestimates of star formation rates by as much as two orders of magnitude. I then describe how investigations of gas cooling rates leads to siginficant evidence for bimodality. There is evidence for a `high cool' population in which isolated star forming bulges heat the surrounding quiescent gas and a `low cool' population that is either heated by background radiation alone or by a low level of in situ star formation. The connection with bimodality in nearby galaxies is briefly described.
Host: Chris Matzner
Oct 19
 Prof. Mary Putman (Michigan)
Fuel for Galaxy Disks
Galaxies show a range of stellar metallicities and ages that indicate they accrete star formation fuel over the majority of their lifetimes; however, the methods this process actually occurs remain highly uncertain. I will discuss the observational evidence for different modes of accretion through neutral hydrogen observations of halo gas and comparison to simulations. The focus will be on Local Group galaxies where stronger limitations can be put on the properties and total amount of halo gas. Time permitting, star formation in halo gas outside of galaxy disks will also be discussed.
Host: Yanqin Wu
Oct 26
 Dr. Mike Muno (Caltech)
Massive Stars and Magnetars
When a massive star (at least eight times the mass of the sun) exhausts its nuclear fuel at the end of its life, its core collapses, producing a supernova explosion and leaving behind a "compact object," a neutron star or a black hole. Currently, there are large gaps in our knowledge of this process, in part because basic observational data is missing. First, it is unclear which massive stars form neutron stars, and which form black holes. Second, it has recently been appreciated that young neutron stars can have a wide variety of appearances, including radio pulsars, magnetars, and compact central objects in supernova remnants. The relationships between these objects is uncertain, as are the properties of the progenitors that lead to their birth. I will I will describe my efforts to address these questions by using observations with Chandra and XMM-Newton. I will discuss the discovery of a highly-magnetized neutron star (a magnetar) in the massive young star cluster Westerlund 1. Its presence in this coeval cluster implies that very massive (>40 Msun) stars sometimes produce neutron stars. I will also describe a search for new magnetars in archival Chandra and XMM observations that I have used to place limits on the birth rates of magnetars. These results provide observational insight into the role of magnetic fields in the collapse of a massive star.
Host: Dae-Sik Moon
Nov 2
 Prof. Howard Yee (Toronto)
The Red-Sequence Cluster Survey
The Red-Sequence Cluster Survey (RCS) is a 1000-square-degree multi-color imaging survey carried out using the MegaCam on the 3.6m CFHT. The survey provides images in z', i, r, and g over 22 patches on the sky, optimized for the search of galaxy clusters with 0.15 < z < 1.0 using the red-sequence method. Designed to create a well-characterized, large sample of clusters, the survey has the main goals of constraining cosmological parameters and studying galaxy cluster evolution. I will present some recent results using RCS-1, including a determination of the cosmological parameters Omega_m and sigma_8, the evolution of the cluster galaxy LF and star formation history for clusters up to z~0.9, the discovery of a high-redshift supercluster, and a study of X-ray properties of RCS clusters. I will also show some initial results from an extension of the RCS method to using Spitzer IRAC images to search for galaxy clusters in the "cluster desert" redshift range of 1 < z < 2.
Nov 9 Dr. Vincent Geers (Leiden-->Toronto)
PAHs in Disks around Young Solar-Type Stars: New Results from Spitzer and VLT
In the past decade, the topic of formation of low-mass stars, comparable to our own Sun, has become one of the most rapidly developing fields in modern astrophysics. The arrival of highly sensitive ground- and space-based telescopes at mid-infrared wavelengths has enabled us for the first time to study the faint solar-type young stars in nearby molecular clouds during their early formation phases. I will present a summary of my thesis study on Polycyclic Aromatic Hydrocarbons (PAHs), a family of large molecules, or small dust grains, which I carried out partly within the context of the "Cores to Disks" Spitzer Legacy program and the Dutch VISIR GTO program.I'll present our Spitzer and VLT mid-infrared spectroscopy and imaging surveys as well as comparisons with 3D radiative transfer models, and summarize our main results on the presence and location of PAH emission toward embedded young stellar objects and circumstellar disks around young solar-type stars.
Host: Kevin Bundy
Nov 16
 Prof. Andy Gould (Ohio State)
Recent Developments in Gravitational Microlensing
Over the past 15 years, gravitational microlensing has radically diversified from a method narrowly focused on finding dark matter to a very general astronomical technique. Microlensing has now detected 7 planets, including several that are inaccessible by other search methods. It has resolved the surfaces of distant stars, served as a magnifying glass to take spectra of extremely faint objects, and revealed a number of surprising phenomena. I give a sweeping look at this remarkable technique, including its successes and future challenges.
Peter Martin
Inaugural Karl W. Kamper Memorial Lecture
Host: 4pm ** Note special time **
Nov 23
 Prof. Laura Parker (McMaster)
Mapping Dark Matter with Weak Lensing
Dark matter remains one of the most interesting and mysterious topics in modern astrophysics. We can hope to understand more about its nature by measuring the total amount and distribution of dark matter in the universe. A powerful probe of dark matter is weak gravitational lensing, which uses small distortions in the shapes of background galaxies to map the mass along the line-of-sight. This allows us to study the mass distribution directly, without visible tracers or equilibrium assumptions that other techniques require. In this talk I will introduce the theory of weak lensing and then highlight some of the well publicised results of the past year. I will then focus some attention on recent results from the CFHT Legacy Survey before concluding with a look to what we can expect from future facilities and surveys.
Host: Ray Carlberg
Nov 30 Prof. Simon White and Prof. Rocky Kolb
Toronto Dark Energy Smackdown - a Debate on the Future of Astronomy
2pm, MP102 ** Note special location **
Host: Bob Abraham
Dec 7
 Dr. Carles Badenes (Princeton)
The Persistence of Memory: Insights on Type Ia Supernova explosions and progenitors from the X-ray Spectra of Young Supernova Remnants
Recent advances in our understanding of the X-ray spectra of Type Ia Supernova Remnants (SNRs) allow us to use the excellent observations from Chandra and XMM-Newton to learn more about these intriguing explosions and their mysterious progenitors. I will discuss several examples, focusing on the SNRs Tycho and 0509-67.5. This last object, located in the LMC, is specially interesting given the recent results on the spectroscopy of its light echo. Given the mounting evidence that dim and bright Type Ia SNe might come from different kinds of progenitors, these SNR studies open new possibilities to investigate the local environments where the progenitor populations arise.
Host: Chris Matzner
Last modified: 21 Oct 2008, 14:06:27

Dae-Sik Moon (moon@astro) / Yanqin Wu (wu@astro) / Ray Jayawardhana (rayjay@astro) / Marten van Kerkwijk (mhvk@astro)

 Colloquia are Fridays, at 2 PM, in Cody Hall. They are followed by refreshments in the lounge, AB 201.

Instructions for speakers

Schedule with abstracts

Schedule without abstracts

Student seminars

CITA seminars

Physics colloquia

Past colloquia: 2003A, 2003B, 2004A, 2004B, 2005A, 2005B, 2006A, 2006B, 2007A, 2007B, 2008A, 2008B, 2009A, 2009B, 2010A, 2010B, 2011A

Current colloquia: 2011B