Fall 2022 Schedule

Fall 2022 Schedule:

September 15th:

Aritra Ghosh (Graduate Student, Yale University)

Title: Estimating Galaxy Morphological Parameters for ~8 Million Galaxies in the Hyper Suprime-Cam Wide Survey using Bayesian Machine Learning

Abstract: In this talk, I will introduce a novel machine learning framework for estimating the Bayesian posteriors of morphological parameters for arbitrarily large numbers of galaxies. The Galaxy Morphology Posterior Estimation Network (GaMPEN) estimates values and uncertainties for a galaxy's bulge-to-total light ratio, effective radius, and flux. GaMPEN also contains a Spatial Transformer Network (STN) that automatically crops input galaxy frames to an optimal size before determining their morphology. The STN will be crucial in applying GaMPEN to new survey data with no radius measurements. GaMPEN is the first machine learning framework for determining posterior distributions of morphological parameters and is one of the first applications of an STN to astronomy.

Using GaMPEN, we have determined the full Bayesian posteriors for the morphological parameters of ~ 8 Million galaxies in the Hyper Suprime-Cam (HSC) Wide Survey with z < 0.75 and m < 23. Using a novel technique of first training on simulations and then transfer-learning on real data, we have been able to train GaMPEN with < 1% of our dataset. By analyzing a sub-sample using light-profile fitting, we have shown that the posterior distributions predicted by GaMPEN are accurate and well-calibrated for all three parameters with < 5% deviation. This is one of the largest morphological catalogs of galaxy parameters currently available and is allowing us to use morphology to probe galaxy evolution for lower mass galaxies with extremely high statistical significance.

September 22nd:

Thales Gutcke (NASA Hubble Fellow and Lyman Spitzer, Jr. Postdoctoral Fellow, Princeton)

Title: LYRA: The smallest survivors of Reionization

Abstract: The dividing line between galaxies that are quenched by reionization (“relics”) and galaxies that survive reionization (i.e. continue forming stars) is commonly discussed in terms of a halo mass threshold. I study this threshold using five extremely high resolution (M_target = 4 M⊙) cosmological zoom-in simulations of dwarf galaxies within the halo mass range 1-4 x 10^9 M_sun. The employed LYRA simulation model features resolved interstellar medium physics and individual, resolved supernova explosions. In our results, we discover an interesting intermediate population of dwarf galaxies close to the threshold mass but which are neither full reionization relics nor full reionization survivors. These galaxies initially quench at the time of reionization but merely remain quiescent for ~500 Myr. At z~5 they recommence star formation in a synchronous way, and remain star-forming until the present day. These results demonstrate that the halo mass at z = 0 is not a good indicator of survival close to the threshold. While the star formation histories are diverse, I show that they are directly related to the ability of a given halo to retain and cool gas. Whereas the latter is most strongly dependent on the mass (or virial temperature) of the host halo at the time of reionization, it also depends on its growth history, the UV background (and its decrease at late times) and the amount of metals retained within the halo.

September 29th:

Fengwu Sun (Graduate Student, University of Arizona)

Title: JWST/NIRCam Wide-Field Slitless Spectroscopy: In-flight Performance and First Sample of z>6 [OIII]+Halpha Line Emitters

Abstract: In this talk, I will convince you that the Wide-Field Slitless Spectroscopic (WFSS) mode of JWST/NIRCam will become a game changer for the observational extragalactic astronomy. The in-flight spectral sensitivity of NIRCam WFSS is 20-40% higher than the pre-launch prediction, and it started to unveil the rest-frame optical emission lines (e.g., [OIII] 5007 and Halpha) of z>6 galaxies even with the shallow (10~20-min integration) commissioning data taken in April 2022. In two of our recent works, we discovered four z>6 [OIII]+Ha emitters using the flux-calibration data of NIRCam WFSS. I will show the first [OIII]/Hb-[NII]/Ha BPT diagram of galaxies in the Epoch of Reionization, and a nice mass-metallicity relation that suggests rapid metal enrichment with galaxies at z>6. We also obtain the first direct measurement of [OIII]5007 and Halpha luminosity functions at z>6, both are under-predicted by certain previous cosmological simulation by a factor of ~10. Our studies suggest an enhanced ionizing photon production rate in the early Universe, and the ubiquity of strong Hα and [O III] line emitters in the Epoch of Reionization, both will be further uncovered in the era of JWST.

October 6th:

Yubo Su (Lyman Spitzer Jr. Postdoctoral Fellow, Princeton)

Title: Tilted Planets: Exciting Exoplanetary Obliquities via Spin-Orbit Resonances

Abstract: The obliquity of a planet, the tilt between its spin and orbital axes, reflects the evolutionary history of the planet. In our own Solar System, the 98 degree obliquity of Uranus is hypothesized to be the result of giant impacts during its formation, and the 4 and 26 degree obliquities of Jupiter and Saturn are thought to be the result of spin-orbit resonances due to the gravitational influence of Uranus and Neptune respectively. While there have been few direct constraints on the obliquities of sub-stellar-mass objects beyond our Solar System, there are prospects for better constraints on exoplanetary obliquities in the coming years. Such measurements are important for informing the surface conditions and potential habitability of exoplanets. In this talk, I will describe some mechanisms by which exoplanetary obliquities can be excited due to the rich dynamics resulting from the interaction between tides and spin-orbit resonances. I will use these results to assess the prospects of substantially oblique exoplanets in a few important systems of interest. I will also discuss my results on the spin evolution in dynamically-formed binary black hole mergers, which appear similar in many ways to the planetary dynamics.

October 13th:

Yinhao Wu (Graduate Student, Leicester University)

Title: Using Planet Migration and Dust Drift to Weigh Protoplanetary Discs

Abstract: ALMA has spatially resolved dozens of protoplanetary discs, discovering widespread signatures of young massive planets. To translate these observations into quantitative measures of the system, such as disc and planet masses, one must perform expensive numerical simulations. To simplify these, a steady state approach is commonly used: the planet position is kept fixed, and a constant source of dust is introduced at the outer edge of the computational domain. Here we take the famous system -- HD163296 as an example to argue against this approach by demonstrating how planet and dust dynamics can constrain "good" steady-state models.

October 20th:

Fall break

October 27th:

Lizhong Zhang (Graduate Student, University of California, Santa Barbara)

Title: Simulating the Dynamics of Super-Eddington Accretion onto a Neutron Star

Abstract: The physics of a magnetically confined, radiation pressure supported column of plasma plays a defining role in understanding the observations of accretion-powered X-ray pulsars, including pulsating ultraluminous X-ray sources (ULXs). Near the neutron star accretor, the accretion flow is constrained by the strong magnetic field to fall along the magnetic field lines. At a sufficiently high accretion rate, the inflow is shocked above the stellar surface and forms a columnar structure below, radiating most of accretion power via sideways emission in a so-called ‘fan-beam’ pattern. The misalignment of the anisotropic radiation emission with respect to the neutron star spin axis results in the observed pulsations. We perform radiative relativistic MHD simulations to study the nonlinear dynamics of the accretion column. The column structure is extremely dynamical and exhibits kHz quasi-periodic oscillations. The existence of the photon bubble instability is identified in simulated accretion columns but proved to be not responsible for triggering the oscillatory behaviors. Instead, the oscillations originate from the inability of the system to resupply heat to locally balance the sideways cooling. The column structure is very sensitive to the shock geometry, which directly determines the cooling efficiency. The time-averaged column structures from the simulations can be approximately reproduced by a 1D stationary model provided one corrects for the actual 2D shape of the time-averaged column. I will also discuss how reduction of scattering opacity by the magnetic field can alter the column structure and variability.

November 3rd:

Oliver Zier (Graduate Student, Max Planck Institute for Astrophysics, Garching, Germany)

Title: How to beat the noise: Accurate simulations of accretion disks on a moving mesh

Abstract: Rotationally supported, cold, gaseous disks are ubiquitous in astrophysics and appear in a diverse set of systems, such as protoplanetary disks, accretion disks around black holes, or large spiral galaxies. By using a cold, two-dimensional, gaseous, keplerian disk I show in this talk that traditional Lagrangian methods such as SPH as well as codes using a static cartesian grid fail to evolve the disk accurately for several orbits. The moving-mesh method as implemented in the AREPO code performs better but the so-called mesh noise on the grid scale finally destabilizes the disk. To increase the local resolution I present a novel implementation of the shearing-box approximation in AREPO. The implementation offers manifest translational invariance across the shearing-box boundaries and offers continuous local adaptivity. But again the unstructured mesh leads to grid noise. I show that this can be rectified by high-order integrations of the flux over mesh boundaries. The higher-order integration also removes the noise in global simulations which leads the disk from the beginning of my talk to be stable. As first applications of the new shearing-box implementation, I present parameter studies for the magnetorotational instability and gravitational instability.

November 10th:

Chang-Goo Kim (Post-Doctoral Associate Research Scholar, Princeton)

Title: Introducing TIGRESS-NCR: current status of numerical modeling of the star-forming ISM

Abstract: The importance of star formation “feedback” to the energetics of the interstellar medium (ISM) has been appreciated throughout the modern history of astronomy. Star formation is inefficient in gas consumption because feedback efficiently maintains the pressure support against gravity, which is otherwise rapidly lost via cooling and turbulence dissipation. At the same time, collective actions of feedback drive galactic-scale outflows, controlling the baryonic cycle in galaxy halos. In this talk, I will introduce the TIGRESS framework and its non-equilibrium cooling and radiation (NCR) extension. We solve magneto-hydrodynamics equations in a local shearing box representing a patch of galactic disks to take advantage of limited outer dimensions (~kpc) to achieve uniformly high resolution (~pc). The TIGRESS-NCR framework synthesizes our current best knowledge on governing physics of the star-forming ISM, including supernova and UV radiation feedback as well as photochemical reactions associated with UV (and cosmic rays) to set radiative heating rates and abundances for major coolants self-consistently. I will present the first results from a suite of simulations using the TIGRESS-NCR framework and explain the co-regulation of SFRs and the ISM. Specifically, I will delineate the self-regulation of SFRs in the context of pressure-regulated, feedback-modulated star formation theory and ISM phase structure and energetics with detailed breakdowns into energy source/sink from different processes and in different phases.

November 17th:

Tsun Hin Navin Tsung (Graduate Student, University of California, Santa Barbara)

Title: Simulating cosmic ray streaming at the Meso-scale

Abstract: Cosmic ray (CR) protons with energy between 1-300GeV, which dominate the CR population, are believed to be self-confined, i.e. the CRs are scattered by the waves they generate. The end result is that they travel along the magnetic field lines at the local Alfven speed. CRs undergoing this kind of transport are said to be streaming, and in this talk, I will present 'meso-scale' MHD+CR simulations exploring the drastic implications of streaming CRs on gas dynamics in galaxies. First, CR streaming gives rise to a unique feature called 'bottlenecks', where a globally smooth CR pressure profile is shaped into a step-like function in the presence of density bumps, redistributing the energy and momentum transfer from the CRs to the gas. Second, it can drive acoustic waves unstable and give rise to a 'staircase' structure in the CR pressure profile, which can change the mass outflow rate. Third, it alters thermal instability, both in the linear and nonlinear stages. For instance, in the linear state it leads to propagating entropy modes, leading to a new timescale t_alfven that should be considered in addition to t_cool and t_ff for the onset of thermal instability. In the nonlinear stage, as the galactic atmosphere thins out due to thermal instability, CR heating, which is a weaker function of density than radiative cooling, can dominate the energy budget, causing the atmosphere to be overheated and can drive a wind.

November 24th:

Thanksgiving Day

December 1st:

Ore Gottlieb (Rothschild Fellow, CIERA Postdoctoral Fellow, Northwest University)

Title: Multi-messenger Collapsars

Abstract: The collapsar model suggests that when the core of a massive star exhausts its fuel, it collapses to a black hole which launches a pair of relativistic jets. Collapsars offer unique opportunities to study a wide range of cutting-edge astrophysical phenomena: heavy element nucleosynthesis, physics of relativistic jets, birth of black holes, variety of cosmic fireworks and gravitational wave sources. I will present state-of-the-art collapsar simulations that, for the first time, follow jets from a newly formed black hole to outside the star, and offer a new paradigm of wobbly jets that naturally explain quiescent times in gamma-ray bursts (GRBs). Our numerical results also suggest that newborn black holes that launch GRB jets inevitably have small spins and high kick velocities. The interaction of jets with the star leads to the formation of a hot bubble - cocoon that collimates the jets and powers a variety of electromagnetic transients that can be detected by high cadence optical surveys. I will end with the discovery of the first class of non-inspiral gravitational wave sources, powered by collapsar cocoons, that is powerful and abundant enough such that its detection by LIGO/Virgo/KAGRA can be imminent. The gravitational waves will be accompanied by detectable bright supernova and cocoon electromagnetic emission, making jetted stellar explosions promising multi-messenger sources, enabling targeted searches for gravitational waves based on the electromagnetic counterpart detection.

December 8th:

Sihao Cheng (Postdoc Member at the Institute for Advanced Study)

Title: How to quantify textures and random fields in astrophysics?

Abstract: Extracting information from stochastic fields is a ubiquitous task in science. However, from cosmology to biology, it tends to be done either through a power spectrum analysis, which is often too limited, or the use of convolutional neural networks, which require large training sets and lack interpretability. I will present a new powerful tool called the “scattering transform”, which stands nicely between the two extremes. I will use various examples in astrophysics and beyond to demonstrate its power, interpretability, and its advantage over traditional statistics.