List of Past CTC Theory Lunches : 01-Sep-2011 to 31-Dec-2011

Date:   Monday 19-Sep-2011
Speaker:   Cole Miller (UMD)

Date:   Monday 26-Sep-2011
Speaker:   Umberto Cannella (UMD)
Title:  Testing gravity with gravitational waves: a field-theoretical perspective

So far experiments of relativistic gravity have probed dynamical regimes only up to order "(v/c)^5" in the post-Newtonian expansion, which corresponds to the very first term of the radiative sector in General Relativity. In contrast, by means of gravitational-wave astronomy, one aims at testing gravity up to (v/c)^(12)! It is then relevant to envisage testing frameworks which are appropriate to this strong-field/radiative regime. Using a field theory approach, gravitational interactions are described by Feynman diagrams in which classical gravitons interact with matter sources and among themselves. Tagging the self-interaction vertices of gravitons with parameters it is possible, for example, to translate the measure of the period decay of Hulse-Taylor pulsar into a constraint on the three-graviton vertex at the 0.1% level. With future observations of gravitational waves, higher order graviton vertices can in principle be constrained through a Fisher matrix analysis.

Date:   Monday 03-Oct-2011
Speaker:   Laura Blecha (CfA/Harvard University)
Title:  Signatures of Supermassive Black Hole Mergers: Before and After

Because central supermassive black holes (SMBHs) are ubiquitous in local galaxies, SMBH pairs are expected to form in major galaxy mergers. Until recently, however, few observational constraints existed on the population of SMBH pairs. Even less is known empirically about their subsequent evolution to a possible merger and gravitational-wave recoil kick. I will review the remarkable recent progress in finding candidate SMBH pairs, as well as recent identifications of candidate recoiling SMBHs. I will then describe our efforts to interpret these findings and make predictions for future observations using hydrodynamic simulations of galaxy mergers that include SMBHs. Specifically, I will focus on 1) kpc-scale SMBH pairs and 2) recoiling SMBHs. Because many candidate SMBH pairs have been identified as double-peaked narrow-line (NL) AGN, we have made a first attempt to model the NL region in galaxy merger simulations. In general, we find that double-peaked NL AGN occur as a short-lived but generic phase of gaseous major mergers, and that they originate from a varied combination of SMBH motion and gas kinematics. I will discuss the implications of our findings for follow-up confirmation of candidate SMBH pairs. Additionally, I will summarize the results of our large study of recoiling SMBHs in galaxy merger simulations. I will describe the dynamics of recoil in gas-rich versus gas-poor mergers, as well as the possible effects of recoil on star formation and the black hole - bulge relations. Recoiling SMBHs may be observed as kinematically- or spatially-offset AGN with lifetimes of up to tens of Myr; I will conclude by describing efforts to model a candidate recoiling SMBH that displays both types of offset.

Date:   Monday 10-Oct-2011
Speaker:   Rob Olling (UMD)
Title:  How to Weigh a Stellar Disk

After decades of study, we still don't really know how heavy stellar disks are. Are the "maximal," "submaximal," or how can we find out anyway. And why would we want to find out? The methods can be broadly divided into methods based on stellar population synthesis models on the one hand and on dynamical analysis on the other. I will discuss a method from the latter category: stellar velocity dispersions. After several decades, this method is now slowly reaching maturity from the observational perspective. I will illustrate the current theoretically shortcomings and possible fixes.

Date:   Monday 17-Oct-2011
Speaker:   Dale Kocevski (UC Davis)
Title:  What the CANDELS Survey is Telling Us About AGN Host Galaxies at z~2

Current AGN feedback scenarios suggest that supermassive black holes play an integral role in the evolution of their host galaxies by regulating their growth and eventually suppressing their star formation activity. In order to test these models and to better understand the coevolution of galaxies and their central black holes, it is vital to determine the key properties of galaxies undergoing active black hole growth, such as their star formation histories and morphologies. While this has been done extensively at z~1, few studies have thus far extended this work to z~2, the era when nuclear activity and star formation activity in the Universe are at their peak. This is now possible for a large sample of galaxies due to the CANDELS survey, which is providing high-resolution HST/WFC3 imaging beyond the Balmer break for thousands of galaxies at z~2. With an allotment of 902 orbits, CANDELS is the largest project undertaken in the history of Hubble. I will discuss the project's primary science drivers, the design of the survey, and what early CANDELS results are revealing about the morphologies, colors and triggering mechanisms of galaxies hosting AGN at high redshift.

Date:   Monday 24-Oct-2011
Speaker:   KwangHo Park (UMD)
Title:  Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback : Growth Rate and Duty Cycle

We present simulations of radiation-regulated accretion onto black holes from galactic scales. We find that the thermal pressure of the ionized sphere forming around the black hole is the dominant mechanism regulating the accretion rate, producing periodic and short-lived luminosity burst. We also focus on the effects of radiation pressure and gas angular momentum on the black hole growth rate and duty cycle. Our simulations focus on intermediate-mass black hole, but we derive general scaling relationships that are solutions of the classic Bondi problem when radiation feedback is considered. We found two distinct modes of oscillations with very different duty cycles, governed by different depletion processes of the gas inside the ionized bubble.

Date:   Monday 31-Oct-2011
Speaker:   TBD (Any Volunteers?)

Date:   Monday 07-Nov-2011
Speaker:   Xiaoyue Guan (University of Virginia)
Title:  Simulations of Tilted BH Accretion Disks

There are many situations in astrophysics in which accretion disks are believed to be non-planer. In this talk I will review the physics of warped disks and present new results from numerical studies of an accretion disk misaligned with the black hole spin. I will also discuss the computational challenges of resolving a global black hole accretion disk.

Date:   Monday 14-Nov-2011
Speaker:   Roman Shcherbakov (UMD)
Title:  Features of ultra-close tidal disruptions of stars by black holes

We present numerical relativity results of tidal disruptions of white dwarfs from ultra-close encounters with a spinning, intermediate mass black hole. These encounters require a full general relativistic treatment of gravity. We show that the disruption process and prompt accretion of the debris strongly depend on the magnitude and orientation of the black hole spin. However, the late-time accretion onto the black hole follows the same decay, $\dot M \propto t^{-5/3}$, estimated from Newtonian gravity disruption studies. We compute the spectrum of the disk formed from the fallback material using a slim disk model. The disk spectrum peaks in the soft X-rays and sustains luminosities above the Eddington limit for 1-3 yrs after the disruption. For arbitrary black hole spin orientations, the disrupted material is scattered away from the orbital plane by relativistic frame dragging, which often leads to obscuration of the inner fallback disk by the outflowing debris. Ultra-close disruption near IMBH lead to extreme accretion rates over the timescales of minutes, which may give rise to a collimated outflow (jet) and prompt burst of radiation from it. The energetics and timing of the events resemble those of gamma-ray bursts.

Date:   Monday 21-Nov-2011
Speaker:   Massimo Ricotti (UMD)
Title:  The Formation History of Globular Clusters and their Impact on Reionization

I will present arguments in support of the idea that the formation of proto-globular clusters may have been a dominant mode of star formation in the early universe, and the dominant source of ultraviolet radiation at redshifts 6-10, responsible for the reionization of the intergalactic medium.

Date:   Monday 28-Nov-2011
Speaker:   Jillian Bellovary (University of Michigan)
Title:  The Formation and Evolution of Massive Black Holes in Cosmological Simulations

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.

Date:   Monday 05-Dec-2011
Speaker:   Snezana Prodan (CITA, University of Toronto)
Title:  The Curious Case of 4U 1820-30: On the Resonant Trapping and Tidal Dissipation Rate of the White Dwarf Secondary

It has been suggested that the 170 day period in the light curve of the low mass X-ray binary 4U 1820-30 arises from the presence of a third body with a large inclination to the binary orbit. We show that this long period motion arises if the system is librating around the stable fixed point in a Kozai resonance. We demonstrate that mass transfer drives the system toward this fixed point, and calculate, both analytically and via numerical integrations, that the period of libration is of order 170 days when the mutual inclination is near the Kozai critical value. The non-zero eccentricity of the binary, combined with tidal dissipation, implies that the rate of change of the binary period would be slower than, or even of opposite sign to, that implied by standard mass transfer models. If the 170 day period results from libration, then, contrary to appearances, the orbital period of the inner binary is increasing with time; in that case, (e/0.009)^2Q/k_2 > 2.5 x 10^9, where k_2=0.01 is the tidal Love number and $e=0.009$ is the fiducial eccentricity of the inner binary. It appears unlikely that the observed negative period derivative results from the smaller than expected (but positive) value of period derivative combined with the previously suggested acceleration of the system in the gravitational field of the host globular cluster NGC 6624. The discrepancy between the observed and expected period derivative requires further investigation.

Date:   Monday 12-Dec-2011
Speaker:   Daniel Jontof-Hutter (UMD)
Title:  Dynamics in Dusty Rings: Magnetic Field Effects on Charged Grains

The optically thin, dusty ring systems around the outer planets are formed and replenished by impact debris from large ring particles or satellites. As dust grains pick up electric charges due to interactions with the plasma environment and sunlight, the motions of grains ranging from micron-scale down to ions, are determined by gravity and electromagnetism.

Following launch at the local Kepler orbital speed, positively-charged dust grains for which EM and gravity are roughly comparable, are immediately unstable to either escape at high speed or collide with the planet in the equatorial plane. Negatively-charged grains remain radially stable. Some positive and negative grains are unstable to vertical perturbations, which cause dust grains to spiral up magnetic field lines to collide with the planet at high latitude. The boundaries between stable and unstable orbits depend only on the charge-to-mass ratio of a dust grain and its radial launch position. For the idealized configuration of a rotating planetary magnetic field aligned and centered with the rotation axis of the planet, a reasonable approximation at Jupiter, but especially applicable at Saturn, we conduct numerical simulations to locate these boundaries, and derive analytical expressions to explain these data, improving upon prior models to provide a complete description of the azimuthal, radial and vertical motions of highly-charged dust grains.

We then test the robustness of our analytical solutions in the idealized case described above with the higher order magnetic field terms of Jupiter's magnetic field. These cause destabilizing Lorentz resonances, which we find numerically and analytically for all charge-to-mass ratios. The resonances provide an escape mechanism for negatively-charged dust grains trapped in the Io plasma torus, the likely source of the high speed dust streams escaping from the Jupiter system.

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