List of Past CTC Theory Lunches : 01-Sep-2022 to 31-Dec-2022

Date:   Wednesday 07-Sept-2022
Speaker:   Matt Orr (Rutgers)
Title:  "Connecting Star Formation and Stellar Feedback with Local Conditions in Galaxies from Cosmic Noon to the Milky Way"

Understanding how the highly turbulent gas in galaxies forms stellar populations is critical to understanding the evolution of gaseous galactic ecosystems, a key focus of the 2020 decadal survey. In this seminar, I will present recent results from the FIRE simulations, among the first simulations to resolve a multiphase ISM and include meaningful feedback physics (e.g., supernovae and stellar winds). I will demonstrate how the FIRE simulations are ideal tools for studying the relationship between star formation and turbulence in galaxies. This talk will explore the emergence of scaling relations like the spatially resolved Kennicutt-Schmidt relation (relating the local gas reservoir to the rate of star formation) and connections between gas turbulence (velocity dispersions/line widths) and star formation rates. I will show how highly efficient local star formation translates to low global star formation efficiency, with the addition of stellar feedback. Additionally, I will lay out conceptual models I have developed to interpret observations and these simulations, from time-delayed feedback driving cyclic star formation episodes, to the importance of clustered supernovae in blowing superbubbles in the ISM of galaxies and as drivers of galactic superwinds. These models give us insight as to where and why stellar feedback injects momentum into the ISM versus the near-CGM, and has strong implications for the formation of galactic disks near z ~ 1.

Date:   Wednesday 14-Sept-2022
Speaker:   Adam Stevens (UWA)
Title:  "Towards a convergent theory of galaxy evolution across environments"

After decades of hype and advancement, galaxy evolution remains one of the forefront research fields of astrophysics. The ultimate challenge lies in developing a theoretical framework that not only covers the vast breadth of astrophysical processes that affect galaxies’ formation and evolution, but also predicts and explains the enormous and ever-growing wealth of multi-wavelength observational data we possess. In this talk, I will discuss how we test the outcome of modern galaxy formation models and cosmological simulations against low-redshift galaxy surveys. In particular, I will highlight research into the influence of galaxy environment on the acquisition and expulsion of galaxies’ gas. With simulation suites like IllustrisTNG, and radio surveys like VERTICO and WALLABY, we are riding a new frontier of insight into the gas flows of galaxies in dense environments.

Date:   Wednesday 21-Sept-2022
Speaker:   Libby Tolman (CCA)
Title:  "Electric field screening in pair discharges and generation of pulsar radio emission"

Pulsar radio emission may be generated in pair discharges which fill the pulsar magnetosphere with plasma as an accelerating electric field is screened by freshly created pairs. In this talk we present a simplified analytic theory for the screening of the electric field in these pair discharges and use it to estimate total radio luminosity and spectrum. The discharge has three stages. First, the electric field is screened for the first time and starts to oscillate. Next, a nonlinear phase occurs. In this phase, the amplitude of the electric field experiences strong damping because the field dramatically changes the momenta of newly created pairs. This strong damping ceases, and the system enters a final linear phase, when the electric field can no longer dramatically change pair momenta. Applied to pulsars, this theory may explain several aspects of radio emission, including the observed luminosity, 10^{28} erg s^{-1}, and the observed spectrum, ω^{-1.4+-1.0}.

Date:   Wednesday 28-Sept-2022
Speaker:   Daniel Thorngren (JHU)
Title:  "Formation and Mass Loss of Exo-Saturns"

The interiors of Exo-Saturns are among the most diverse of any class of planet. Objects with the same mass and insolation can range in radius by more than a factor of two! Even among the cooler planets, the metallicity can run essentially the whole range from 0 to 100% (roughly). Furthermore, hot Saturns rarely have surface gravities less than 3 m/s2 even though structure models of such planets are readily produced. For this talk, I will discuss our recent progress in understanding these objects. In particular, I will argue that hot Saturns of sufficiently low gravity can experience cataclysmic runaway mass loss under the right conditions, and explain why this effect has not been well-discussed in the past. I will also show my results for the compositions of the cooler Saturns, which show a clear jump between the metal rich and poor objects, which is a potential signature of runaway gas accretion during formation.

Date:   Wednesday 05-Oct-2022
Speaker:   Kavitha Arur (Georgia Institute of Technology)
Title:  "Using the bispectrum to understand rapid X-ray variability from black hole binaries"

Quasi-periodic oscillations (QPOs) observed from X-ray binaries are a signature of rapid X-ray variability and a powerful diagnostic tool to study the regions closest to the central compact object. However, the physical origin of these QPOs is still under debate. One way to break degeneracies between the different models that have been proposed is to go beyond the simple power spectrum, and use higher order time series analysis techniques. One such technique is the bispectrum, and the closely related bicoherence; which is a measure of the strength of phase coupling among triplets of frequencies. In this talk, I will summarize the results obtained from applying such analysis to X-ray binaries and how these can be used to probe the geometry of the region close to black holes.

Date:   Wednesday 12-Oct-2022
Speaker:   Rostom Mbarek (UMD)
Title:  "Title: Particle Acceleration in Active Galactic Nuclei: From the Large Structures of Jets to the Kinetic Scale of Plasma Turbulence"

The origin of Ultra-High-Energy Cosmic Rays (UHECRs) and the highest-energy astrophysical neutrinos remains as one of the most prominent unresolved questions in astrophysics. We can shed light on such phenomena employing a thorough bottom-up approach to understand the spectra of UHECRs, neutrinos, and eventually x/gamma-rays from Active Galactic Nucleus (AGN) jets. In this respect, I will initially discuss an original theory of particle acceleration in AGN jets, i.e., the espresso mechanism, that we back by propagating protons and heavier elements in relativistic 3D MHD simulations of AGN jets accounting self-consistently for i) particle injection, ii) particle acceleration, iii) spectra of UHECRs, iv) effects of losses on UHECRs, and v) the resulting neutrino spectral features. Moving from the global scale of jets to the kinetic scales of the plasma, I will also present the first steps in understanding asymmetric reconnection in the relativistic regime both analytically and using Particle-in-Cell (PIC) simulations. Considering the turbulent nature of AGNs, asymmetric reconnection can potentially be the main driver of nonthermal lepton acceleration, and thus nonthermal radiation, important to modeling UHECR losses and neutrino production. If we have time, I will touch upon the interplay of these two regimes and potential ways of combining them.

Date:   Wednesday 19-Oct-2022
Speaker:   Paul Bennet (STScI)
Title:  "The proper motions of globular clusters in the Large Magellanic Cloud"

We have obtained the proper motions for a comprehensive sample of 32 globular clusters (GCs) around the Large Magellanic Cloud (LMC) using a combination of Gaia eDR3 and Hubble Space Telescope data. Combining these proper motions with literature values for line-of-sight velocity and distance provides full 6D position velocity data for 28 of these GCs, almost doubling the size of previous samples. This expanded data set allows new insights into the LMC and the kinematics of its GC population. I will present the results of this ongoing work, where in contrast to previous proper motion work we find no evidence of kinematically distinct populations within the LMC’s GCs. We also explore the velocity dispersion and anisotropy of the GCs and the implications for the LMC. This research has provided a strong test case for the benefits of combining HST and Gaia data to increase the proper motion accuracies for faint stars beyond those provided by Gaia alone.

Date:   Wednesday 26-Oct-2022
Speaker:   Oren Slone (NYU)
Title:  "Observational Signals of Self-Interacting Dark Matter in Astrophysical Sub-Structure"

Dark matter self interactions can leave distinctive signatures on the properties of satellite galaxies around Milky Way-like hosts. By analyzing a number of Milky Way dwarf galaxies, we were able to place new constraints on models of self-interacting dark matter which interact via a Yukawa potential. The results push the theory into a parameter space with a very specific prediction: self-interactions within satellite galaxies can be either very large (so large that new dynamical effects become important), or very small (so small that such models are usually thought of as collisionless), but not intermediate. Specifically, if self-interactions are large, some dwarfs of the Milky Way must be undergoing a process of gravothermal collapse, and this process has a number of distinct observational predictions which can be searched for in current and upcoming data.

Date:   Wednesday 02-Nov-2022
Speaker:   Aaron Yung (Goddard)
Title:  "The future of Semi-Analytic Forecasts -- Interpretation of JWST galaxies and predictions for wide-field survey telescopes"

The work series "Semi-analytic forecasts for JWST" presented a wide range of predictions for the photometric and physical properties of high-redshift galaxies and AGN, as well as their ionizing photon production rate and their influence on the Universe's reionization history. These predictions are made with physically-motivated models that include many physical processes that are known to have leading effects on shaping galaxy formation, and are essential to the interpretation of the intrinsic properties and physical origins of observed galaxies. In turn, JWST observations will deliver new constraints on the physical models and help refine our understanding of galaxy formation in the earliest episode of cosmic history. The physical modeling pipeline developed for this work can easily be adapted to serve other survey instruments. I will showcase preliminary results for future deep-wide survey predictions for NASA's Roman Space Telescope, as well as ESA's Euclid and the Canadian mission concept CASTOR. All data products are publicly available; I will give a brief demonstration on how they have been used in the planning of high-z Webb galaxy surveys and how to gain access to these publicly available mock catalogs.

Date:   Wednesday 09-Nov-2022
Speaker:   Ashley Bransgrove (Columbia)
Title:  "Magnetic Activity of Neutron Stars and Black Holes"

Pulsars are rotating magnetic neutron stars which produce powerful beams of coherent radio emission. They are also prolific sources of broadband high-energy emission. In this talk I will describe recent advances in modelling pulsar radio emission using first-principles kinetic plasma simulations. I will discuss the radio emission anomaly of the Vela pulsar at the time of its rotational glitch, and how it could be used to probe the physics of neutron star interiors. I will also discuss the behavior of magnetic fields on black hole event horizons as it relates to the no-hair theorem of general relativity.

Date:   Wednesday 16-Nov-2022
Speaker:   Odelia Teboul (HUJI)
Title:   "TBD"

Date:   Wednesday 30-Nov-2022
Speaker:   Aleksandra Hamanowicz
Title:  "Two talks. 1. METAL-Z – measuring dust depletion in the low metallicity environments. 2: Dramatic Evolution in the Dusty ISM Across the Local Group"

Abstract 1: A critical yet poorly understood aspect of the baryon cycle in galaxies is the depletion of metals from the gas to the dust phase via dust formation, and vice versa, the return of heavy elements from the dust to the gas phase via dust destruction. Understanding dust depletion is especially crucial in the low metallicity regime (5-10 % solar), corresponding to the typical metallicity of Damped Lyman-alpha (DLA) systems, which are used to track the chemical enrichment of the universe. Corrections for dust depletion in abundance measurements in DLAs rely on locally-derived calibrations of depletions based on abundance ratios. Yet, these calibrations do not exist at metallicities lower than that of the SMC. In this talk, I will present results from the METAL-Z Hubble program (GO-15880), which obtained COS spectra toward UV-bright O/early B stars in local dwarf galaxies IC1613 (0.15 Z_sun) and Sextans-A (0.08 Z_sun), with the goal to measure dust depletions in these low metallicity environments. We combine the gas-phase abundances derived from METAL-Z with photospheric abundance measurements in young OBA stars from the literature to derive interstellar depletions of iron and sulfur, two key elements for understanding the chemical enrichment of the universe. Combined with depletion measurements available in the Milky Way, LMC (0.5 Z_sun), and SMC (0.2 Z_sun), the METAL-Z measurements allow us to constrain dust depletions as a function of metallicity down to 0.08 Z_sun. Abstract 2: I will present the evolution of the dust-to-gas ratio with density for four well-resolved galaxies in the local group - the LMC, SMC, M31, and M33. We explore this using new Herschel maps of these galaxies, which restore extended dust emission that had been missed by previous Herschel reductions. Combining this sensitivity to diffuse dust emission with excellent angular - and hence physical - resolution, allows us to probe the dust-to-gas ratio across 2.5 orders of magnitude in ISM surface density. We find very strong evolution in the dust-to-gas with density, by as much as a factor of 22 within a single galaxy. This is strong evidence for very high levels of dust grain growth in the denser regions of the ISM. We find M31 and M33 have very similar dust-to-gas evolution profiles, despite their large differences in mass, metallicity, and star formation rate; conversely, we find M33 and the LMC to have very different dust-to-gas evolution profiles, despite their close similarity in terms of mass, metallicity, and star formation rate. Our dust-to-gas ratios address previous disagreement between UV- and FIR-based dust-to-gas estimates for the Magellanic Clouds, removing the disagreement for the LMC, and considerably reducing it for the SMC - with our new dust-to-gas measurements being factors of >2 greater than the previous far-infrared estimates. This indicates how issues with angular sensitivity & data calibration can lead to dust-to-gas ratios being underestimated, especially at lower metallicities, with implications for the vast majority of low-metallicity galaxies for which available far-infrared data is poorly- or un-resolved.

Date:   Wednesday 07-Dec-2022
Speaker:   Eric Moseley (Princeton)
Title:   "Modeling dust using PIC methods in RAMSES"

Date:   Wednesday 14-Dec-2022
Speaker:   Yakov Faerman (University of Washington)
Title:  "Modeling the (Cool) Circumgalactic Medium"

Modeling the (Cool) Circumgalactic Medium

This page was automatically generated on: 22-Aug-2023.