Sept 12

"On the Origin of Cosmic Magnetic Fields"
Swadesh Mahajan, University of Texas at Austin, Institute for Fusion Studies

The origin of "vorticity" in a fluid, or of its structural equivalent the "magnetic field" in a plasma, has been a long-standing quest common to many different disciplines of physics. For an ideal fluid in thermodynamic equilibrium, a fundamental topological constraint forbids the creation (destruction) of generalized vorticity, a combination of the electromagnetic and the fluid parts.

Thus to find the origin of magnetic fields that are universally found at all scale lengths, one has to invent various mechanisms that will violate the forbidding ideal constraint. After giving a review of the "nonideal" mechanisms suggested in the literature, we will show that magnetic fields can be generated within the framework of purely ideal fluid dynamics provided space-time distortions through special/general relativity are included in the dynamics.

The mechanism will be elaborated and exaaminrd in the context of the cosmic magnetic fields.

Sept 19

"The Formation and Evolution of Massive Black Holes in Cosmological Simulations"
Jillian Bellovary, University of Michigan

Massive black holes (MBHs) are inextricably connected to the formation of massive galaxies, but their formation, evolution, and specific effects on their hosts are not clearly understood. Cosmological simulations of galaxy formation, including prescriptions for MBH formation, mergers, accretion, and feedback, are a unique way to shed light on this issue. I adopt a novel approach to forming seed black holes in galaxy halos which is dictated directly by the physics of primordial, zero-metallicity gas and motivated by physical models of massive black hole formation. Our simulations explain why massive black holes are found in some bulgeless and dwarf galaxies, but we also predict that their occurrence becomes rarer and rarer in low-mass systems. I also predict a population of "wandering" MBHs in the halos of massive galaxies, which are the remnant cores of tidally stripped satellite galaxies. These objects may be observed as off-nuclear ultraluminous X-ray sources if the cores retain a gas reservoir and are perturbed in some way, or if a nearby star is tidally disrupted.

Oct 3

"The Millennium Run Observatory"
Roderik Overzier, University of Texas at Austin

I will present an exciting new project specifically designed to narrow the divide between theory and observations of galaxy evolution. Key to this project is a new addition to the suite of "Millennium" cosmological simulations that allows us to produce artificial yet highly realistic multi-wavelength data in the observer's frame using arbitrary cosmologies, population synthesis models, or resolution. I will try to convince you of the power of this true "virtual observatory", and discuss its practical purposes for a wide range of subjects in cosmology including clustering, galaxy luminosity/mass functions, and structures of galaxies in deep extragalactic surveys (e.g. CANDELS, UDF), as well as studies of galaxy clusters. I will also demonstrate various online services that will be made available to the community later this year.

Oct 17

"Lyman-Alpha in Three Dimensions"
Anze Slosar, Brookhaven National Laboratory (TCC Visiting Speaker)

The Lyman-alpha forest is a series of absorption features in the spectra of distant quasars, blue-ward of the Lyman-alpha emission line. These features arise as the light from the quasar is absorbed by the intervening neutral hydrogen. This gives one-dimensional information about the fluctuations in the neutral hydrogen density along the line of sight to the quasar. When spectra of many quasars are combined, it allows one to build a three-dimensional image of the fluctuations in the neutral hydrogen density and thus infer the corresponding fluctuations in the matter density. This makes the Lyman-alpha forest a unique probe of the distant Universe, opening a novel window on understanding dark energy, dark matter, neutrino properties and inflation. Using the 14,000 quasars from the first year data, the BOSS experiment has detected, for the first time, three-dimensional correlations in the Lyman-alpha forest fluctuations to cosmological distances. The signal has thhe expected amplitude and redshift-space distortions and we find no evidence for overwhelming instrumental or astrophysical contamination. The BOSS experiment was projected to measure the distance to the redshift of z ~ 2.5 with a better than 2% precision through detection of baryonic acoustic signature in the flux correlations. The present results give these forecasts new credibility.

Oct 24

"From OWLS to FiBy: Numerical Simulations of Galaxy Formation and Evolution"
Claudio Dalla Vecchia, Max Planck Institute for Extraterrestrial Physics (TCC Visiting Speaker)

I will summarise my contribution to the OverWhelmingly Large Simulation (OWLS) project, highlighting the improvements on the numerical implementation of baryon's physics. In particular, I will emphasise the modelling of SN feedback. I will present some preliminary results of the First Billion Years (FiBY) simulation project on the formation of the first proto-galaxies in the Universe.

Nov 14

"The Plasma Physics and Cosmological Implications of TeV Blazars"
Philip Chang, University of Wisconsin - Milwaukee (TCC Visiting Speaker)

The universe is teeming with very high energy gamma ray sources (> 100 GeV), but it is generally thought that their impact on the universe is minor at best. On energetic grounds, this assumption seems well-founded because the energy density in TeV photons is 0.2% of that of ionizing photons from quasars. However, as I hope to show in this talk, this is not the case. Rather, the greater efficiency by which TeV photons can be converted to heating in the intergalactic medium (IGM) allows TeV blazars dominate the heating of the IGM at low redshift. I will discuss the nature of this conversion via beam instabilities. I will then discuss how the resultant heating from these TeV sources makes dramatic differences in the formation of structure in the universe. In particular, I will discuss how it gives rise to the inverted temperature-density profile of the IGM, the bimodality of galaxy clusters, and the paucity of dwarf galaxies in galactic halos and voids.

Nov 21

"Toward an Understanding of the Large Scale Structure of the Universe with Galaxy Surveys"
Masatoshi Shoji, University of Texas at Austin (PhD Defense Presentation)

A galaxy survey directly observes a distribution of structures in the sky using galaxies as tracers of the underlying density distribution, and it yields constraints on cosmological models when compared to a physical theory of structure formation based on a given cosmological model. Among many cosmological and astronomical phenomena to be understood from a galaxy survey, the nature of the observed accelerated expansion of the universe is the most profound problem in modern physics.

Motivated by various planned and ongoing galaxy surveys, including our own Hobby-Eberly Telescope Dark Energy eXperiment (HETDEX), we show the way to fully exploit the data from a galaxy survey. We improve a model of structure formation to include the effect of the free-streaming of massive neutrinos at a mildly non-linear regime. Future galaxy surveys are intended to reach the level of accuracy where the effect of massive neutrinos on the observed power spectrum is no longer negligible. Proper understanding of these effects gives a way to measure the absolute masses of neutrinos: one of the most fundamental particles, which, by itself, will be a major development in the field of particle physics.

Yet, most of the space (~80%) observed by galaxy surveys is occupied by voids. An ellipticity probability distribution function of voids offers yet another way of probing cosmology. Especially, a distribution of ellipticities in redshift space provides a unique way to measure the growth rate of the structure of the universe apart from other cosmological parameters.

Dec 5

"Hydrogen and Helium Reionization"
Adam Lidz, University of Pennsylvania (TCC Presentation)

One of the primary goals of observational cosmology at present is to detect, and elucidate the nature of, the Epoch of Reionization. Current observations suggest that star-forming galaxies reionized hydrogen sometime before z > ~6. These sources should simultaneously singly ionize helium, but are unlikely to doubly ionize it. Helium may be doubly-ionized only later on, perhaps near z ~ 3, by bright quasars. I will discuss efforts to theoretically model the Epochs of Hydrogen and Helium Reionization and focus on a few observational implications. First, I will describe how it might be possible to map out large scale structure during hydrogen reionization using rotational emission lines from the CO molecule in high redshift star-forming galaxies. I will explain how this might complement future measurements of redshifted 21 cm emission from the intergalactic medium. Next, I will discuss the impact of helium reionization on the statistics of the hydrogen Lyman-alpha forest.