List of Past CTC Theory Lunches : 01-Jan-2014 to 01-Jun-2014

Date:   Monday 03-Feb-2014
Speaker:   Wen-Fai Fong (Harvard/CFA)
Title:  The Origin of Short-duration Gamma-ray Bursts

While long-duration gamma-ray bursts (GRBs, duration > 2 sec) are linked to the catastrophic deaths of massive stars, until recently, the progenitors of short-duration GRBs (duration < 2 sec) were uncertain. Over the past two decades, theoretical predictions pointed to a compact object merger progenitor, which are the premier candidates for gravitational wave signals detected by Advanced LIGO. In this talk, I present the most robust observational evidence to date from their host galaxies and afterglows that short GRBs originate from the mergers of compact objects. I also present preliminary evidence that some short GRBs are collimated in narrow jets, which has significant implications on the merger event rate. Finally, I discuss necessary next steps to take prior to the first gravitational wave detections.

Date:   Monday 17-Feb-2014
Speaker:   Rob Olling (UMD)
Title:  Supernovae at Kepler Cadence

A number of supernovae (SNe) have been discovered by the Kepler mission while monitoring a few hundred small galaxies. Two of the five SNe are clearly white-dwarf (WD) supernovae (type Ia), one mostly likely a Ia. One is clearly a core-collapse (massive star) supernovae of type IIp, while the last one is as of yet untyped. Supernovae are typed by the absence (type I) or presence (type II) of hydrogen [silicon and helium] lines in their spectra. The Ia subset results from the collapse of a white dwarf, while all other types (Ib, Ic, Ib/c and II...) arise from the collapse of massive stars.

In this talk I will focus on the early lightcurves (LCs) of type Ia SNe, within about 10 days after explosion or first light. The Kepler satellite takes observations about twice per hour and we have about two years of data. No supernova has ever been observed so frequently.

It is not clear how exactly the white dwarfs actually explode. Among the many explanations put forward, two are most popular: 1) the single degenerate model where the WDs gradually acquire extra mass from a regular star (main-sequence or giant) and 2) the double degenerate case where the explosion is initiated through a collision/merger between two white dwarfs. When the explosion runs into the companion, a shock results that can be brighter than the emission from the blastwave itself (in the optical), the exact brightness depending on the size of the companion and observing geometry. Our two Kepler SNe indicate that the companion is smaller than, about, a 3 M_SUN main-sequence star, possibly a white dwarf. These data lend support to the double-degenerate precursor model for type Ia supernovae.

Date:   Friday 24-Feb-2014
Speaker:   Anne Lohfink (UMD)
Title:   A discussion about X-ray observations of AGN

Date:   Monday 28-Feb-2014
Speaker:   Nader Haghighipour (IFA)
Title:  A New Model for the Formation of Terrestrial Planets and the Origin of Mars

The formation of the terrestrial planets of our solar system is one of the most outstanding problems of planetary astronomy. Traditional models of terrestrial planet formation have been successful to produce planets in the range of Venus and Earth. However, they have failed to produce Mars and Mercury analogs. Only when Jupiter and Saturn are assumed to have formed in their current positions and in highly eccentric orbits, do simulations succeed to produce planets with masses comparable to that of Mars. However, the intense interactions of these planets with the protoplanetary disk, combined with the short time of the formation of Mars, as suggested by cosmochemical data, point to a new approach to understanding the formation of the inner solar system. In this approach terrestrial planets are formed in a small and rather compact region of the protoplanetary disk and are scattered into their current orbits through interactions with one another and with planetary embryos. The appearance of such small-compact regions can be a natural consequence of the growth of solid objects in a circumstellar disk with a non- uniform surface density. I will present a new model of terrestrial planet formation capable of forming Mars based on this scenario, and will address the effects of giant planets on the evolution of the protoplanetary disk and the formation of the inner solar system. I will also show how this model can account for the water contents of terrestrial planets, the formation of the asteroid belt, and explain the formation of Mars as a stranded embryo within the first 10 Myr.

Date:   Tuesday 04-Mar-2014
Speaker:   Victoria Grinberg (MIT)
Title:  A holistic view of a black hole binary: bringing together spectral, timing and polarization analysis of Cygnus X-1

I will present recent analyses of the representative high mass X-ray binary Cygnus X-1, with a focus on X-ray source states that are essential for interpretation of data at all wavelength, from radio over IR to the gamma-rays. In particular, I will introduce a recently developed novel approach that uses data from all sky monitors such as RXTE-ASM, MAXI, Swift-BAT, and Fermi-GBM to define states and state transitions on a timescales of a few hours over a period of more than 17 years and enables us to conduct state-resolved polarization analysis. I will then combine spectral and model-independent X-ray timing analysis of over 1900 RXTE orbits over 14 years and investigate the evolution of Fourier-dependent timing parameters over all spectral states in unprecedented detail. This spectro-timing behavior, particularly the correlation between the shape of the power and time lag spectra in all hard and intermediate states, is crucial for constraining physical models for accretion and ejection processes in compact objects on all mass scales.

Date:   Monday 10-Mar-2014
Speaker:   Kate Whitaker (GSFC)
Title:  The Quenching of Star Formation in Massive Galaxies

Nearby galaxies exhibit a bimodal color distribution, where actively star-forming galaxies have blue colors and quiescent galaxies have red colors. It is generally thought that red galaxies arise from blue galaxies when star formation is quenched. However, the origin of this color bimodality remains unknown. Furthermore, it is not well understood how actively star-forming galaxies quench and migrate to form the well-defined color-mass relation, known as the “red sequence”. In this talk, direct evidence is presented that the massive end of the red sequence is most-rapidly building up when the universe was only 3 billion years old, with an influx of young recently quenched galaxies that are almost non-existent over the past 8 billion years. Presenting results from the NEWFIRM Medium-Band Survey and the 3D-HST Survey, I will discuss the properties of these massive galaxies in the context of current galaxy formation and evolution theories.

Date:   Monday 24-Mar-2014
Speaker:   Laura Blecha (UMD)
Title:  Discussion on Active BHs in Unexpected Places

The co-evolution of massive black holes and their host galaxies is a standard paradigm of galaxy formation, but new results have challenged how "standard" their evolutionary histories may be. Substantial scatter in the empirical BH-bulge correlations at the low- and high-mass ends, BHs found in dwarf galaxies, and even the possibility of off-nuclear, recoiling AGN suggest that a more nuanced understanding is required. I plan to lead an informal discussion on what we might learn from these "unusual" BHs about the growth history of BHs and galaxies.

Date:   Monday 31-Mar-2014
Speaker:   Gregory Snyder (STSCI)
Title:  Observing the Origins of Galaxy Structure in Simulations

Abstract: It remains debated whether galaxy mergers, gas flows, or internal processes shape the population of galaxies we see today. I will describe work to disentangle these issues using mock data derived from hydrodynamical simulations of galaxy formation. Primarily, I will describe how we study galaxy morphology by building "mock observatories" around cosmological simulations. With these, we address how the common galaxy shapes emerged and how they relate to cessation of star formation in the universe. Early results suggest realistic populations of galaxy disks and bulges are a natural consequence of galaxy formation models with black hole and star formation feedback set to reproduce global quantities. Such simulations make readily falsifiable predictions that can be pressured by future surveys, and I will describe how we use these mock data to create diagnostics of galaxy dynamics.

Date:   Monday 07-Apr-2014
Speaker:   Jane Rigby (NASA Goddard)
Title:  Galaxy Evolution in High Definition, Via Gravitational Lensing

In hundreds of known cases, "gravitational lenses" have deflected, distorted, and amplified images of galaxies or quasars behind them. As such, gravitational lensing is a way to "cheat" at studying how galaxies evolve: lensing can magnify galaxies by factors of 10--100 times, transforming them from objects we can barely detect to bright objects we can study in detail. I'll summarize new results from a comprehensive program, using imaging from Hubble and Spitzer, and high-quality spectroscopy from Keck, Magellan, and Hubble, to study how galaxies formed stars at redshifts of 1--3, the epoch when most of the Universe's stars were formed. These results give insight into the process by which galaxies form elements and stars.

Date:   Monday 14-Apr-2014
Speaker:   Kai Noeske (STSCI)
Title:  The Galaxy Star Formation-Stellar Mass relation ("Galaxy Main Sequence"): Insights into what we do and don't know about galaxy-wide star formation

The Galaxy Main Sequence is a tight relation between star-forming galaxies' star formation rates and their stellar masses. It persists to at least $z\sim 2$ and possibly to the highest observable redshifts. This relation indicates that star formation in galaxies over the past 10 billion years, and likely beyond, was predominantly regular and gradually declining over the past 10 billion years, and that star-forming galaxies of similar stellar and halo masses at a given redshift experienced similar star formation histories. The Galaxy Main Sequence provides us with an observational reference star formation history for normal star-forming galaxies, and can identify galaxies that are outliers in either a starburst or suppressed star formation mode.

The slope and redshift evolution of this relation, and various independent evidence, points to a delayed onset of efficient star formation in less massive galaxies. The amount of delay, and possibly the fraction of affected galaxies, increase towards lower galaxy masses. It has not been successfully reproduced in most galaxy models. Non-standard star formation prescriptions, based on small-scale ISM physics, appear promising, but are computationally challenging.

Importantly, delayed efficient star formation appears to smoothly depend on galaxy mass, and may not be limited to dwarf galaxies, but may persist to L* galaxies and possibly beyond. It is being discussed as the origin of several problems in low- and high mass galaxy evolution.

Date:   Monday 21-Apr-2014
Speaker:   Anna Pancoast (UCSB)
Title:  XXI century reverberation mapping: inferring black hole mass, geometry and dynamics of the broad line region in active galaxies

Constraining the inner regions of active galactic nuclei (AGN) is difficult due to the very small scales involved. By using reverberation mapping, we can substitute time resolution for spatial resolution and begin to probe AGN on the scales of the broad line region. However traditional reverberation mapping analysis uses only a fraction of the available information. We have been developing advanced analysis methods that give us new insights into the properties of AGN, including the absolute black hole mass and the geometry and dynamics of the broad line region. Applying this technique to five AGNs from the Lick AGN Monitoring Project 2008 sample, we find that the BLR geometry is consistent with a thick disk and the dynamics are a combination of elliptical orbits and inflow.

Date:   Monday 05-May-2014
Speaker:   Yuexing Li (PSU)
Title:  Bridging the gap between theory and observations of galaxies across cosmic time

A major recent milestone in observational cosmology is the detection of a large number of galaxies and quasars across cosmic time through multi-wavelength surveys. In order to interpret the wealth of data and to understand the origin and destination of these objects, a comprehensive model which fully accounts for the formation, evolution and multi-band properties of structures is imperative. However, despite the strong observational push, theoretical modeling in this field has lagged behind. Here, I report some progresses my group made recently in bridging this gap. We have developed a physical model for cosmological simulations which for the first time self-consistently reproduces both observed cosmic histories of star formation and black hole accretion, and a state-of-the-art radiative transfer code which offers direct comparisons between simulations and observations of galaxies over a wide range of wavelengths. By combining multi-scale cosmological simulations and multi-wavelength radiative transfer, I will present new results of the formation and evolution of galaxies and quasars at different redshifts, and the underlying physical processes that determine their properties. Furthermore, I’ll discuss the detectability of the first galaxies with the next generation instruments such as JWST and ALMA.

Date:   Thursday 08-May-2014
Speaker:   Roger Deane (UCT)
Title:  A triple supermassive black hole system at a cosmologically significant distance

Multiple supermassive black hole (SMBH) systems have long been predicted to play an important role in galaxy evolution and dominate the stochastic gravitational wave background spectrum in the nHz-microHz frequency range. However, observational constraints on these impacts are limited since very few low separation binary (or multiple) SMBH systems are known. I will present our recent discovery of a triple SMBH system at z~0.4. This includes a ~140 parsec (26 milli-arcsecond) separation inner binary, which is roughly an order of magnitude lower separation than known binary/dual SMBH systems at comparable redshifts (with direct imaging confirmation). This discovery suggests that close-pair binary SMBHs are more common than previous observations suggest and that their radio-jet morphology may be an efficient way to find similar systems.

Date:   Monday 12-May-2014
Speaker:   Sweta Shah (Radboud University, The Netherlands)
Title:  Gravitational wave astrophysics of compact Galactic binaries.

Millions of short period binaries in our Galaxy will be observed by the future space-based Gravitational wave (GW) detector, eLISA. It is expected that thousands of these binaries will be individually resolved for which we can measure their GW parameters. From GW observations we expect to measure their GW frequencies to nano-Hertz accuracies. However most other parameters greatly vary in their accuracies owing to signal strengths and the relative geometry of the detector with the source. I will address to what extent will combining the GW observations (for systems mostly made of white-dwarfs) with their potential electro-magnetic (EM) data can improve our knowledge of the binary parameters further. I will also address the issue of measuring tidal deviations in detached short-period binaries using both GW and EM observations.

Date:   Monday 30-June-2014
Speaker:   Brenamir Sesar (Caltech)

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