List of Past CTC Theory Lunches : 01-Sep-2016 to 31-Dec-2016

Date:   Monday 19-Sep-2016
Speaker:   Josh Wiener (U. of Wisconson, Madison)
Title:  "Cosmic Rays in Galaxies -- Drivers of Winds?"

Cosmic rays are often cited as an appealing driver of galactic winds. Cosmic rays(CRs) have some advantages as a wind-driving mechanism, such as long cooling times which allow the CRs to keep their energy for long periods of time. In this talk I will summarize the physics of CR transport, and how CRs interact with the ambient gas in a galaxy. I present numerical simulations of isolated galaxies, which reveal how the characteristics galactic winds depend strongly on how the CR physics are modeled.

Date:   Monday 03-Oct-2016
Speaker:   Clement Bonnerot (U. of Leiden)
Title:  "Stream evolution in tidal disruption events"

Tidal disruption events occur when a star gets scattered on a trajectory that takes it so close to a supermassive black hole that it is torn apart by strong tidal forces. Such events represent unique probes of otherwise quiescent black holes, which constitute the majority of black holes in the local universe. However, this great potential is hampered by the lack of a clear picture of how exactly these events take place. One particular uncertainty concerns the circularization process during which the stellar debris dissipate their large orbital energy to form an accretion disc around the black hole. Hydrodynamical simulations point towards shocks driven by debris self-intersections as an efficient dissipation mechanism. In my talk, I will present a recently developed semi-analytical model that treats the circularization process, accounting for the impact of both shocks and magnetic stresses. This model proves that the net effect of magnetic stresses is to strengthen shocks, thus accelerating circularization. It also allows to predict the form of the lightcurve associated to shock luminosity and demonstrates that the thermal energy excess imparted by shocks is most likely to cause the rapid formation of a thick structure. To conclude the talk, I will present latest advances on ongoing projects and mention future research directions.

Date:   Monday 10-Oct-2016
Speaker:   Dr. Dipanjan Mukherjee (Australian National University)
Title:  "Impact of relativistic jets on host galaxies"

Relativistic jets from AGNs are an important driver of feedback in galaxies. Interaction of such jets with an inhomogeneous ISM significantly affect the morphology and the evolution of the host galaxy. I will present the results of our recent 3D relativistic hydrodynamic simulations, performed on scales of several kpc, of jets interacting with a turbulent ISM of the host galaxy. The jet intially couples strongly with the ISM, driving fast moving lateral outflows of dense gas, which though fast, do not escape the galaxy’s potential. I will discuss the differences in results due to variations in ISM densities and jet power, especially the effect of low power jets. From our simulations we have also computed expected luminosities in optical line emissions, Xrays and radio wavelengths, which can be compared with existing and future high resolution observations.

Date:   Monday 24-Oct-2016
Speaker:   Dr. Dong Zhang (University of Virginia)
Title:  "On the Theory of Galactic Winds"

Galactic winds are ubiquitous in most rapidly star-forming galaxies. They are crucial to the process of galaxy formation and evolution, regulating star formation, shaping the stellar mass function and the mass-metallicity relation, and enriching the intergalactic medium with metals. Although important, the physics of galactic winds is still unclear. Many theoretical mechanisms have been proposed. Winds may be driven by the heating of the interstellar medium by overlapping supernovae explosions, the radiation pressure by continuum absorption and scattering of starlight on dust grains, cosmic rays, and so on. The comparison between theory and observation is still incomplete. The growing observations of emission and absorption of cold molecular, cool atomic, and ionized gas in galactic outflows in a large number of galaxies have not been well explained by any models over a vast range of galaxy parameters. A full understanding of these issues requires both better theoretical explorations and comparisons with new and existing observations. I will be taking about the theoretical models of both supernova-driven and radiation-pressure-driven galactic winds, and compared these models with observations. I will also be talking about our recent radiative hydrodynamical numerical simulations on momentum coupling between gas and radiation field in starbursts and rapidly star-forming galaxies.

Date:   Monday 14-Nov-2016
Speaker:   Cecilia Chirenti (UFABC, Sao Paulo, Brazil)
Title:  "Gravitational waves from F-modes excited by the inspiral of highly eccentric neutron star binaries"

Abstract: As gravitational wave instrumentation becomes more sensitive, it is interesting to speculate about subtle effects that could be analyzed using upcoming generations of detectors. One such effect that has great potential for revealing the properties of very dense matter is oscillations of neutron stars. These have been found in numerical simulations of the hypermassive remnants of double neutron star mergers, but here we sketch out some ideas for the production, gravitational-wave detection, and analysis of neutron star oscillations that are produced by a succession of close passages in highly eccentric double neutron star orbits. These events will be rare (perhaps as many as tens per year could be detected using the planned Einstein Telescope), but they would have unique diagnostic power for the analysis of cold, catalyzed, dense matter. The full analysis of such events will require significant further work in nuclear physics and general relativistic nonlinear mode coupling, and thus we also discuss some further directions that will need to be pursued.

Date:   Monday 28-Nov-2016
Speaker:   Neven Caplar (ETH Zurich)
Title:  “Growth, co-evolution and variability of AGN“

Big surveys enable us to study both global and local properties of AGN population in a statistical manner. In the first part of the talk I will discuss global properties of AGN population and how physical parameters of AGN, such as mass and luminosity, evolve with cosmic time. I will present a simple model which connects evolution of galaxy mass function and quasar luminosity function. This model makes powerful predictions for the quasar duty cycle, black hole-galaxy co-evolution and black hole-sigma relation. In the second part of my talk I will concentrate on the local accretion properties of AGN as inferred from their optical variability. I will show results of the analysis of the largest fully re-calibrated single-band dataset collected with Palomar Transient Factory Survey. I will again focus on the connection of variability with the physical parameters of AGN. At the end, I will discuss how AGN spin influences variability and the growth of black holes in Universe.

Date:   Monday 05-Dec-2016
Speaker:   Gongjie Li (CFA, Harvard University)
Title:  "Dynamical Origin of Planetary Systems Under Observational Constraints"

The unexpected diversity of observed extrasolar planetary systems has posed new challenges confronting our classical understanding of planetary formation. A lot of these challenges can be addressed by a deeper understanding of the dynamics in planetary systems, which will also allow us to construct more accurate planetary formation theories consistent with observations. To this end, I will first use my results on hierarchical three-body interactions to explain the observed spin-orbit misalignments. Then, I will present our statistical approach for removing selection bias from the detected circumbinary planetary systems and infer the origin of such systems.

Date:   Monday 12-Dec-2016
Speaker:   Vivienne Baldassare (University of Michigan)
Title:  "Properties of active galactic nuclei in dwarf galaxies"

In the last few years, the number of known active galactic nuclei in dwarf galaxies has increased by an order of magnitude, from just a handful of examples to a collective sample of several hundred dwarf galaxies with AGN signatures. I will discuss work I have done to characterize black hole masses and accretion properties of active galactic nuclei in dwarf galaxies. In particular, I'll discuss my discovery of a 50,000 M_sun BH in the nucleus of RGG 118 - the smallest BH yet reported in a galaxy nucleus (Baldassare et al. 2015). I will also discuss my work using multi-epoch optical spectroscopy to study the nature of broad H-alpha emission in dwarf galaxies. A characteristic signature of dense gas orbiting a BH, broad emission can also be produced by transient stellar processes. I showed that broad H-alpha in star-forming dwarf galaxies fades over a baseline of several years, and is likely produced by a Type II SN as opposed to an AGN. However, broad emission in dwarf galaxies with AGN/composite narrow lines is persistent and consistent across observations, suggesting an AGN origin (Baldassare et al. 2016). Finally, I'll discuss my analysis X-ray and UV observations of dwarf galaxies with broad and narrow-line AGN signatures. All had nuclear X-ray emission significantly higher than expected from X-ray binaries, providing strong confirmation that these are bona-fide AGN (Baldassare et al. submitted).

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