List of Past CTC Theory Lunches : 01-Jan-2012 to 01-Jun-2012

Date:   Monday 30-Jan-2012
Speaker:   Demos Kazanas (Goddard)
Title:  Echoes of the Kerr Metric: A Clue on its Spin

The ISCO of accretion disks around black holes with spin parameter a > 0.94 is inside the ergosphere. It is shown by direct computation of a large number of photon orbits, that if radiation is emitted in flares close to ISCO, a significant number of photons (~15%) will reach a faraway observer will after one additional orbit around the BH. As such they will produce a secondary peak in the ACF of the light curve at lag DT ~ 14 M. In the frequency domain this will produce a QPO with frequency nu ~ 1.4 (10 MSolar/M) kHz, where M is the BH mass. Flares outside the ergosphere do not yield a similar signal if produced randomly over the entire ISCO circumference. A search for these signatures in the light curves of galactic black holes and neutron stars will be discussed.

Date:   Monday 06-Feb-2012
Speaker:   Shigeki Inoue (Tohoku University / Mullard Space Science Laboratory (UCL))
Title:  Nature of a clump-origin bulge

Recently, both high-redshift observations and numerical simulations have revealed that infant disk galaxies are expected to be highly gas-rich. Giant gas clouds collapse due to dynamical instability and form giant stellar "clumps" in the premature galactic disk. The giant clumps fall into the galactic centers and finally form a single bulge (clump-origin bulge). By the way, galactic bulges have been discussed to be classified into classical bulges and pseudobulges. Classical bulges are remnants of hierarchical galaxy mergers, whereas pseudobulges are thought to form through secular evolution caused by non-axisymmetric structures such as bars and spirals in a galactic disk. The formation scenario of clump-origin bulges mentioned above is neither galactic mergers nor secular evolution. Therefore, the clump-origin bulges may open-up the third category of the bulge classification. Using N-body/SPH simulations, I will discuss the nature of the clump-origin bulges and find that the bulges are pseudobulge-like structure in dynamical properties, but old and metal-rich like classical bulges. I will also discuss thick disk formation and density structure of a dark matter halo in clumpy galaxies from the simulation results.

Date:   Monday 13-Feb-2012
Speaker:   Julie Comerford (UT Austin)
Title:  Dual Supermassive Black Holes as New Observational Tracers of Galaxy Evolution

In recent years dual supermassive black holes (SMBHs), which are kpc-scale separation SMBHs in merger-remnant galaxies, have emerged as a new testing ground for theories of galaxy evolution. Dual SMBHs are direct observational tracers of galaxy mergers and SMBH mass growth during mergers, and, because they are the smallest-separation SMBH pairs that are resolvable, provide the tightest observational constraints on SMBH mergers. However, the full potential of dual SMBHs for studies of these topics has not yet been realized, due to the small number of known dual SMBHs. I will describe the first systematic search for dual SMBHs. This search employs a combination of large spectroscopic surveys of galaxies, longslit spectroscopy, imaging, and X-ray and radio observations to identify and characterize dual SMBHs that power active galactic nuclei. I will present the first batch of dual SMBHs that have been discovered with these techniques. This systematic survey will produce the first large catalog of dual SMBHs, which will enable measurements of the galaxy merger rate, SMBH growth via gas accretion during mergers, and the SMBH merger rate of interest to future gravitational-wave experiments.

Date:   Monday 20-Feb-2012
Speaker:   Javier Garcia (UMD)
Title:  Time-dependent Photoionization Modeling

Photoionization modeling codes have been developed over the last three decades, achieving powerful predictions based on a high degree of complexity in terms of the physical processes considered in the simulations. Nevertheless, the temporal dependency of the equations is usually neglected or included through very simplistic approximations. Here we present the efforts on the development of a self-consistent photoionization model where the energy, ionization balance and radiative transfer equations are considered in their full time-dependent form. With these models we are able to predict the departures from the steady-state of a photoionized gas given the time variability of the radiation source. These results are particularly applicable to active galactic nuclei outflows where the energy source, the optical/UV/X-ray continuum, is highly variable.

Date:   Monday 27-Feb-2012
Speaker:   Athena Stacy (Goddard)
Title:  New Insights into Primordial Star Formation

The formation of the first stars, also known as Population III (Pop III), marked a pivotal point in the universe's evolution from relative smoothness and homogeneity to its current highly structured state. I will summarize key aspects of Pop III star formation as determined by recent studies. This recent work utilized three-dimensional cosmological simulations to follow the evolution of gas and DM from z~100 until the first minihalo forms, which subsequently hosts Pop III star formation. A disk forms around the initial Pop III star and fragments to form secondary stars with a range of masses (1 - 50 M_sol). This is markedly different from the previous paradigm of one single, massive star forming per minihalo. I will next discuss the effect of radiative feedback on protostellar growth and disk fragmentation. This feedback will not prevent the formation of secondary stars within the disk, but will reduce the final mass reached by the largest Pop III star. I will finally comment on how the multiplicity of Pop III stars will affect the formation of so-called Pop III 'dark stars,' primordial stars that could have become supermassive (10^5 M_sol) and extremely long-lived through the effects of DM annihilation.

Date:   Monday 05-Mar-2012
Speaker:   Kari Helgason (UMD)
Title:  The nature of the sources producing the near-IR Background Fluctuations. Do we need high-z galaxies?

Significant anisotropies have been measured in the Cosmic Infrared Background (CIB) after removing foreground sources down to faint levels. The signal, which reflects the distribution of sources that are too faint to be observed directly, has been interpreted as being cosmological in nature but this notion is subject to debate. Several upcoming measurements are likely to bring this picture into focus in the near future. I will give an overview of our current knowledge and discuss the theoretical implications of current and forthcoming measurements. To evaluate the signal expected from known galaxy populations, we reconstruct the CIB-fluctuations arising from local and intermediate redshift galaxies which lie beyond the detection threshold of current NIR observatories. We use over 230 luminosity functions in the literature obtained in a variety of surveys probing rest-frame UV, optical and near-IR to predict the projected fluctuations arising anywhere in the redshift cone. Our empirical-based calculations suggest that yet unknown populations are required to explain the bulk of the measured signal seen by Spitzer/IRAC and AKARI/IRC.

Date:   Monday 12-Mar-2012
Speaker:   Rodrigo Nemmen (Goddard)
Title:  Probing the weak end of black hole activity in the centers of nearby galaxies

Low-luminosity AGNs represent the bulk of the AGN population in present-day galaxies and they trace underfed supermassive black holes. In this talk, I will show how high-resolution, multiwavelength observations of weak AGNs in nearby galaxies can be used to probe the properties of the accretion flow, the relativistic jet and the disk/jet connection. In particular, I will describe how the radio-to-X-rays SEDs can be used to constrain important parameters of the black hole engines such as mass accretion and outflow rates. I will discuss how Fermi gamma-ray observations can advance our understanding of the physics of weak AGNs.

Date:   Monday 26-Mar-2012
Speaker:   Ashley Zauderer (CfA)
Title:  Observational Signatures of Tidal Disruption Events: Are Jets and Radio Emission Ubiquitous?

I will present a brief overview of theoretical predictions for observational signatures of the tidal disruption of stars by supermassive black holes. One year ago, the Swift satellite observed a source, J1644+57, which arguably is the most clear-cut case for a tidal disruption event. This event is unique because it deviates from the simple predictions: strong radio emission was observed starting at early times (with the EVLA, the SMA and CARMA), providing evidence for a collimated, mildly-relativistic jet. I will outline the utility of radio observations to monitor the expansion and energy scale of the relativistic outflow and to probe the parsec-scale environment around a SMBH that was previously dormant. Radio observations up to ~200 days reveal a significant change in the evolution starting at ~1 month, with a brightening at all frequencies that requires an increase in energy by an order of magnitude, as well as a change in the density profile. I will conclude with a summary of interpretations for this event questions and open theoretical questions.

Date:   Monday 02-Apr-2012
Speaker:   Alexander (Sasha) Tchekhovskoy (Princeton)
Title:  Getting the Most out of a Black Hole

Recent advances in computer simulations of black hole accretion systems provide us with unprecedented insights into the physics of jet production in active galactic nuclei (AGN) and other black hole accretion systems. For the first time, time-dependent 3D general relativistic numerical simulations allow us to determine the maximum efficiency with which radiatively inefficient accretion onto black holes can produce energy in the form of relativistic jets and winds. I will present the dependence of this maximum efficiency on black hole spin and accretion disk thickness and discuss the implications for radio loud/quiet dichotomy of AGN and recent observations of high jet efficiency in a number of AGN.

Date:   Monday 09-Apr-2012
Speaker:   Drew Clausen (Penn State)
Title:  Optical Emission Lines From Tidally Disrupted White Dwarfs and Other Evolved Stars

When a black hole tidally disrupts a star, accretion of the debris will produce a luminous flare and reveal the presence of a dormant black hole. The accretion flare can also photoionize a portion of the debris that is not bound to the black hole. I will present models of the emission line spectrum produced in the debris released when a white dwarf is tidally disrupted by an intermediate-mass black hole and discuss the possibility of using the emission lines to identify such events and constrain the properties of the black hole. I will also compare our models with observations of white dwarf tidal disruption candidates in globular clusters associated with NGC 4472 and NGC 1399. The bright [O III] line observed in each system is consistent with our models, but there are some drawbacks to interpreting these sources as tidally disrupted white dwarfs. On the other hand, models of the emission line spectrum produced when a horizontal branch star is disrupted by a ~100 solar mass black hole are in good agreement with the source in the NGC 1399 globular cluster.

Date:   Monday 16-Apr-2012
Speaker:   Owen Parry (UMD)
Title:  Simulating the First Galaxies

The earliest star formation in the Universe occurs in "minihalos" with masses >10^5 M_sun, where the dominant coolant is molecular Hydrogen. For the subset of these halos that never grow large enough for atomic cooling to become efficient (~10^8 M_sun), star formation effectively ends after their local environment is reionized. The simulated galaxies hosted by such halos have been shown to share many properties with Local Group dwarf spheroidals, raising the possibility that a significant fraction of the Milky Way's satellites are pre-reionization "fossils". In this talk I will describe numerical simulations that follow the formation of the first galaxies, using a code that includes H_2 cooling and radiative transfer. I will discuss some of the first results from these simulations and outline some specific issues we aim to study, including: i) The relative importance of radiative, mechanical and chemical feedback mechanisms in regulating star formation at high redshift. ii) Distribution of metals by Pop III stars and the transition to Pop II. iii) Development of a multiphase ISM in the first galaxies and its effect on early star formation.

Date:   Monday 23-Apr-2012
Speaker:   Mike Koss (University of Hawaii)
Title:  Understanding Dual AGN Activation in the Nearby Universe

One of the most important problems in modern astronomy is what process triggers the growth of supermassive black holes. If galaxy mergers trigger AGN, then there should be time when AGN can be detected in both of the merging galaxies as a dual AGN. However, AGN surveys of quasars and double peaked emission line sources find dual AGN are exceedingly rare, by some estimates only ~0.1% of AGN. I present a study of the fraction of dual AGN in a sample of nearby X-ray selected AGN with high quality data from Gemini and Chandra with a less biased sample selected in the ultra hard X-rays from SWIFT. Additionally, I will present initial results of a study of the link between molecular gas, star formation, and AGN fueling of SWIFT AGN using the new instrument SCUBA2 (

Date:   Monday 30-Apr-2012
Speaker:   John ZuHone (Goddard)
Title:  The Physics of Gas Sloshing in the Cores of Galaxy Clusters

Many X-ray observations of relaxed galaxy clusters reveal the presence of sharp, spiral-shaped discontinuities in the surface brightness of the X-ray emitting gas. Spectral analysis of these features shows that the colder gas is on the brighter side, hence they have been dubbed "cold fronts." These features arise naturally in simulations from the cool-core gas "sloshing" in the gravitational potential. Their sharpness and stability has important implications for the microphysics of the ICM. The sloshing motions may have other effects, such as contributing to the heating of the cluster core and the acceleration of relativistic particles. I will present simulations of gas sloshing in clusters, explain their formation and evolution, and discuss the implications for the physics of the ICM, including fresh results on heat conduction and radio mini-halos.

Date:   Monday 07-May-2012
Speaker:   Megan DeCesar (UMD)
Title:  Probing Gamma-ray Pulsar Emission with the Fermi Large Area Telescope

The high-quality Fermi LAT observations of gamma-ray pulsars have opened a new window to understanding the generation mechanisms of high-energy emission from these systems. The high statistics allow for careful modeling of the light curve features as well as for phase-resolved spectral modeling. We model the LAT light curves of four bright LAT pulsars using simulated high-energy light curves. The model light curves and phase-dependent radii of curvature are generated using geometrical representations of the outer gap and slot gap/two-pole caustic emission models, within the context of both the vacuum retarded dipole and force-free magnetosphere models. These simulated light curves are compared with observed LAT light curves via maximum likelihood using a Markov Chain Monte Carlo method to explore the phase space of fitted parameters such as magnetic inclination, viewing angle, maximum emission radius and gap width. We find that the observed light curves can be fit within both the vacuum dipole and force-free fields, but the force-free magnetosphere produces phase lags between the gamma-ray and radio peaks larger than those observed. We have also used the measured phase-dependent spectral cutoff energies to estimate the accelerating parallel electric field dependence on emission radius for each pulsar, under the assumptions that the high-energy emission is dominated by curvature radiation and the geometry (radius of emission and minimum radius of curvature of the magnetic field lines) is determined by the best fitting light curves for each model.

Date:   Monday 14-May-2012
Speaker:   Abderahmen Zoghbi (UMD)
Title:  Decoding the echoes from black holes

Studies of physical processes around black holes in active nuclei has recently gained a great boost with the discovery of light reverberation on scales of a few gravitational radii. The initial detections were made in sources with bright so-called soft excess, where the delay was between the hard continuum and the 'reflected' soft excess. The ambiguous nature of the soft excess has, however, limited a full understanding of the phenomena. More recently, we discovered light echoes in the iron K band in the bright AGN NGC 4151, the first to be seen. The object not just shows delays between the iron line and the continuum, but also shows a delay between different parts of the line emitted at different radii in the accretion disk. I will discuss how spectroscopy and variability in AGN are used to probe the inner regions of active galaxies.

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