List of Past CTC Theory Lunches : 01-Sep-2014 to 31-Dec-2014

Date:   Monday 15-Sept-2014
Speaker:   Sanchayeeta Borthakur (JHU)
Title:  A Local Clue to the Reionization of the Universe

Identifying the population of galaxies that were responsible for the reionization of the universe is a long standing quest in astronomy. We present the first viable low-z candidate that has an escape fraction of ionizing flux of 21% – the largest ever detected in the low-z universe. This detection confirms the existence of gaps in the neutral gas cover enveloping the starburst. Such gaps are believed to be an integral property of galaxies at the epoch of reionization. The candidate is a massive yet highly compact starburst and the gaps are most likely created by the unusually strong winds and intense ionizing radiation produced by this extreme starburst.

Another interesting aspect of our study is that it also validates the indirect technique of using the residual flux in saturated interstellar absorption lines for identifying such leaky galaxies. Since direct detection of ionizing flux is impossible at the epoch of reionization, this is a highly valuable technique for the next generation of studies.

Date:   Monday 29-Sep-2014
Speaker:   Shy Genel (CfA, Harvard)
Title:  The Illustris simulation: large-scale hydrodynamical modelling of galaxy formation

I will present results from the Illustris simulation, a large cosmological hydrodynamical simulation that follows a (100Mpc)^3 volume down to z=0, resolving <~kpc scales. It is run using the Arepo moving-mesh code, including also stars and black holes, and various feedback processes. I will present a broad range of observables at z=0->5, including galaxy mass, morphology, size, angular momentum content, and star-formation activity, compare them to observations, and discuss how they are set or affected by feedback processes. I will also present the evolution of scaling relations as well as how individual galaxies evolve with time with respect to these relations.

Date:   Monday 06-Oct-2014
Speaker:   Ken Chen (UCSC)
Title:  The Cosmic Dawn - Physics of the First Luminous Objects

One of the frontiers in modern cosmology is understanding the end of the cosmic dark ages, when the first stars, supernovae, and galaxies transformed the simple early Universe into a state of ever-increasing complexity. I will talk about the possible physics behind the formation of these first luminous objects by presenting the results from our simulations. I will also discuss the possible observational signatures of the cosmic dawn that will be the prime targets for the future telescopes such as the James Webb Space Telescope (JWST).

Date:   Monday 13-Oct-2014
Speaker:   Shea Garrison-Kimmel (UCI)
Title:  Lessons in Near-Field Cosmology from Simulating the Local Volume

Studies of the Milky Way (MW) and Andromeda (M31) galaxies, along with their associated satellites and nearby dwarf galaxies, have proven immensely useful for constraining the cosmology of the Universe, particularly on small scales. I will present a number of simulations, many of which are a part of the ELVIS Suite, cosmological zoom-in simulations of Local Group-like volumes of MW/M31 pairs. Using these, and other simulations, I will highlight existing tensions within the LCDM paradigm, as well as illustrate how simulations can provide links between near-field and deep-field observations.

Date:   Monday 20-Oct-2014
Speaker:   Tanmoy Laskar (CfA, Harvard)
Title:  The diversity and versatility of Gamma-ray Bursts

Gamma-ray bursts (GRBs) are the most energetic explosions since the Big Bang, thus providing a unique opportunity to study the most extreme conditions in the Universe. At the same time, the large luminosities resulting from the interaction of the ejecta with their environment also makes these events premier probes of star-formation in galaxies out to the highest redshifts. I will highlight the versatility of GRBs through three related studies: i) using GRBs to measure the first galaxy mass-metallicity relation at 36 through afterglow observations and modelling, and iii) probing GRB shock physics through the first multi-wavelength detection and characterisation of the reverse shock. I will conclude with a discussion of future directions in the study of the GRB progenitors and their use as probes in the ALMA and JWST era.

Date:   Monday 27-Oct-2014
Speaker:   Chien-Ting Chen (Dartmouth)
Title:  AGN accretion, obscuration and star formation in luminous galaxies

Abstract: I will present studies of connections between star formation (SF) and supermassive black hole (SMBH) accretion in galaxies, using large galaxy and AGN samples from the wide-field Boötes survey. We find a linear relationship between galactic SF rate and the average SMBH accretion rate, consistent with a strong physical connection between these processes despite the fact that SMBH and galaxy growth rates in individual AGN host galaxies are not directly correlated due to the short variability timescale of AGN relative to SF. I will also present evidence for a link between nuclear obscuration and host galaxy SF in powerful quasars. These results support a scenario in which galaxy and SMBH grow from a common gas reservoir that can obscure the central SMBH during the luminous quasar phase.

Date:   Monday 03-Nov-2014
Speaker:   Simin Mahmoodifar (UMD)
Title:  "Ultra-Dense Matter Physics and Neutron Star Seismology"

Neutron stars are among the most compact objects in the universe and provide a unique laboratory for the study of cold ultra-dense matter. Their core may contain exotic forms of matter that are predicted to exist at supranuclear densities, such as, for example, hyperons, strange quark matter, and color superconducting quarks. We still don't know what the nature of cold ultra-dense matter is, and whether or not any of the exotic states of matter actually exists inside neutron stars. Asteroseismology, the measurement of the characteristic frequencies of the normal modes of oscillation of stars, can provide a powerful probe of their interiors. The frequencies, amplitudes and damping timescales of the global oscillation modes of neutron stars, such as the Rossby waves (r-modes) and gravity modes (g-modes), which have the Coriolis force and buoyancy as their respective restoring forces, depend on their interior structure and composition. In this talk I will explain how the physics of ultra-dense matter and the interior composition of neutron stars can be constrained with their X-ray observations, especially with the detection of their global oscillation modes.

Date:   Monday 10-Nov-2014
Speaker:   Mahmuda Afrin Badhan (UMD)
Title:  Constraining the Structure of Exoplanet Atmospheres using the NEMESIS Retrieval Algorithm

In order to understand the origins and evolution of extreme planets in nearby solar systems such as "hot Jupiters" and "super-Earths," it is crucial to constrain and understand the physical properties governing their atmospheres. In recent years, studies on several key exoplanet candidates have employed atmospheric retrieval algorithms, where a model spectra is calculated via a radiative transfer model for the planet of interest, using best estimates for the atmospheric parameters. Upon generating this "forward model," either a Markov-Chain Monte Carlo (MCMC) and/or Bayesian numerical schemes, such as Optimal Estimation, are employed to update the parameter values through a series of successive iterations till a final spectral fit (or a series of spectral fits) to the observed spectra are obtained. The best-fit model parameters "retrieved" by solving this type of "inverse problem" are used to make conclusions about the atmospheric composition. In my work, I employ one of such retrieval tool, the NEMESIS Optimal Estimation algorithm, originally created for Cassini CIRS and presently being developed for exoplanet. A thorough exploration of the parameter space is necessary for exoplanet retrievals, where solutions usually are degenerate. My goal is to develop this tool such that it is able to robustly quantify plausible solutions with uncertainty estimates on atmospheric variables. I have completed a) reproducing existing work in literature of the planets GJ1214b and HD189733b using the latest version of NEMESIS and b) used these to identify and diagnose any changes in retrievals due to i) updates in the NEMESIS algorithm and ii) updates in line list databases since these results were published. Line databases pertaining to exoplanet atmospheres are continually being improved. Since calculation of the forward model utilizes correlated distribution of absorption coefficients (k-distribution) amongst atmospheric layers, updates to these line parameters could impact the retrievals quite significantly. Thus I compile a list of the latest line database and seek to utilize this in creating up-to-date k-tables for future retrieval work. Additionally, for ill-constrained problems (such as exoplanet retrievals), OE-based retrieval techniques often fails to meet all convergence criterions simultaneously, resulting in oscillating retrievals that do not complete. Thus I have implemented an alternate set of criterions to mitigate this issue, in addition to developing an MCMC framework for the NEMESIS platform. Today, I will be presenting an overview of my work on NEMESIS thus far.

Date:   Monday 17-Nov-2014
Speaker:   Michael Kesden (UT Dallas)
Title:  Effective potentials and morphological transitions for binary black-hole spin precession

Generic binary black holes have spins that are misaligned with their orbital angular momentum. When the binary separation between the black holes is large compared to their gravitational radii, the timescale on which the spins precess is much shorter than the radiation-reaction time on which the orbital angular momentum decreases due to gravitational-wave emission. We use conservation of the total angular momentum and the projected effective spin on the precession time to derive an effective potential for BBH spin precession. This effective potential allows us to solve the orbit-averaged spin-precession equations analytically for arbitrary mass ratios and spins. These solutions are quasiperiodic initial relative orientations. We classify black-hole spin precession into three distinct morphologies between which the black holes can transition during their inspiral. Our new solutions constitute fundamental progress in our understanding of black-hole spin precession and also have important applications to astrophysical black holes. We derive a precession-averaged evolution equation that can be numerically integrated on the radiation-reaction time, allowing us to statistically track black-hole spins from formation to merger. This will greatly help us predict the signatures of black-hole formation in the gravitational waves emitted near merger and the distributions of final spins and gravitational recoils.

Date:   Monday 15-Dec-2014
Speaker:   Antonino Cucchiara (GSFC)
Title:  Gamma-ray Bursts: the Ancient Keys of Cosmic Explorations

A new frontier of cosmic exploration is approaching, bringing the z=6-12 redshift range rapidly at reach. While the HUDF exploration has been a cornerstone effort in our understanding of the early Star-formation and galaxy assembly up to 500Myr after the Big Bang, it comes with a large expenditure of observing time and with natural observational limitations. Moreover, the faintest galaxies, which likely play a fundamental role in the first billion years, are still missing. Long Gamma-ray Bursts (GRB), originating from the explosion of the most massive stars (PopII/III), can be detected up to z~20 and provide a direct approach for studying the cosmic star-formation: they, in fact, are detected independently from the nature of their host galaxies and enable the investigation of the environment in which these primordial objects live in and the way in which re-ionization occurred. In particular, I will focus on the metals, dust and neutral hydrogen content of the GRB hosts using rapid afterglow spectroscopic observations, which provide a much better "handle" on these properties respect, e.g., quasars studies or galaxy surveys. Finally, I will briefly describe the important role and future synergies between GRB afterglow/host studies and the high-redshift Universe exploration, in particular in relation with current and new generation of NIR spectrographs mounted on ground based facilities (AST, DCT to be ready for the year 2018, the beginning of the JWST era.

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