The goal of the Thunch seminar is to encourage the scientific interaction of the Princeton Astrophysics community. In addition, it should give everybody (students and postdocs in particular) a chance to present their research to a broad audience. Potential visiting speakers may contact the Thunch Czars to request a talk.

The talks are held on Thursdays at 12:15 pm, currently in Grand Central. They are rather informal, so leave time for questions throughout. Please try to keep your talk to about 50 minutes.

Most importantly, attendees are supposed to eat lunch throughout the talk!  Participants bring their own lunch; to honor tradition, you may wish to pick up a hoagie from Hoagie Haven.   

Please send the title and abstract of your talk a week prior to the Thunch Czars, Matt Sampson and Rachel Wang.

A few rules apply:

  • Priority goes to those who have not given a talk recently, and to postdocs and graduate students.
  • Visiting speakers must be explicitly invited by our department
  • There is, unfortunately, no money in Thunch to pay for travel, so visitors need another source of funding.

You now know everything about Thunch! So, go grab an empty slot for your chance to give a Thunch Talk this semester, and plan to join the group to learn about the fascinating research being done right next door!


Spring 2023 Upcoming Speakers:


Date Name and Institution Title Abstract
February 16th Chia-Yu Hu (University of Florida)

Co-evolution of the interstellar dust and chemistry in low-metallicity dwarf galaxies

The cycle of gas in and around galaxies plays a fundamental role in galaxy formation. Recent hydrodynamical simulations have managed to reach (sub-)parsec-scale resolutions to follow the small-scale physics in the interstellar medium (ISM). As our knowledge of the gas cycle comes from multi-wavelength observations, detailed modeling of the ISM chemistry is of fundamental importance. However, interstellar dust, the major catalyst of ISM chemistry, is often regarded as a non-evolving species. In this talk, I will present our recent progress on resolved hydrodynamical simulations of low-metallicity dwarf galaxies coupled with chemistry and dust evolution. Our resolution (1 M_sun per gas particle, spatial resolution ~0.2 pc) allows us to resolve not only the individual supernova blastwaves but also the detailed structure of the star-forming clouds. I will demonstrate how dust evolution helps explain the observed CO luminosity and why the observationally derived dust-to-gas ratio might have been underestimated at low metallicity. Finally, I will also discuss dust-enriched galactic outflows as a possible origin of the intergalactic dust.

February 23rd Dan Foreman-Mackey (CCA) An intro to JAX for astronomical computation In this informal presentation, I will demonstrate how you can use JAX as a computational framework in your Python code, and discuss the reasons why you might want to do such a thing. Using my Gaussian Process library “tinygp” as a case study, I will present some of the strengths and weaknesses of JAX for common applications in astronomical data analysis. There will be some live coding and plenty of time for discussion. 
March 2nd Lyla Jung (ANU) Search for magnetized circumgalactic clouds using Faraday rotation High-velocity clouds (HVCs) around the Milky Way provide direct evidence of ongoing gas accretion that could fuel the future star formation activity of the Galaxy. Theories and numerical simulations predict that the magnetic field significantly affects the efficiency of the mixing at the cloud-halo interface, which could in the long run affect the survival of the clouds. There have been active searches for magnetized HVCs using polarization observations. In this presentation, I will discuss possible obstacles in detecting magnetized HVCs using Faraday rotation and how future polarization surveys will improve the situation. This presentation will provide insight on how to find magnetized HVCs as the first step towards understanding the role of magnetic fields on the evolution of gas clouds around the Galaxy.
March 16th Vicente Valenzuela-Villaseca
(Princeton University)
Experiments on high-energy-density rotating plasmas: towards laboratory modelling of accretion disks and jets in laser and pulsed-power facilities

Rotating plasma disks orbiting a central object, like a black hole, are ubiquitous in the universe. However, questions regarding their dynamical evolution, such as the mechanisms of angular momentum transport and the role of magnetic fields in seeding instabilities, turbulence and launching jets, remain outstanding.

In this talk I will give an overview of a new generation of laboratory experiments fielded in high-energy-density facilities (such as the MAGPIE pulsed-power generator at Imperial College London and the OMEGA laser at the University of Rochester), designed to probe plasma physics relevant to accretion disks and jet launching regions.

In these experiments, a differentially rotating plasma column is driven and sustained by the collision of multiple inflowing plasma jets. The free-boundary design allows the plasma to expand axially, forming supersonic rotating jets which remain collimated as they propagate through the vacuum chamber. The rotating plasma flow is high magnetic Reynolds number (ranging from 10 to 103) and has a quasi-Keplerian stratification. I will discuss the potential of  these experiments to study the magneto-rotational instability, the Omega-effect, and the overall effect of magnetic fields in high-Rm rotating plasmas on laboratory scales.

March 30th Roohi Dalal (Princeton)

Cosmology from Cosmic Shear Power Spectra with Hyper Suprime-Cam Year 3 Data

The Hyper Suprime-Cam (HSC) survey is the deepest Stage III weak lensing experiment, going to ~26 mag in five filters with exquisite seeing. The depth and image quality achieved by HSC allow us to probe cosmology using weak lensing up to z~1.5. I will discuss our constraints on cosmological parameters using weak lensing cosmic shear power spectra measured from the Year 3 shape catalog of HSC. The shape catalog, prepared by Li et al. 2022, covers 416 square degrees of the northern sky, with a mean i-band seeing of 0.59 arcsec and an effective galaxy number density of 19.9 arcmin^-2. With an i-band magnitude limit of 24.5 mag, and four tomographic redshift bins spanning 0.3<z<1.5, we obtain a high-significance measurement of the cosmic shear power spectra in the multipole range 300<\ell<1800. I will describe our cosmological analysis, including the steps we take to prevent confirmation bias as well as our modeling of various sources of systematic uncertainties. I will then share our new constraint on the S8 parameter, and show that this constraint is robust to uncertainties in our modeling and analysis choices. Finally, I will discuss the implication of our S8 constraint in light of the apparent 2-3 sigma tension between constraints on S8 from weak lensing experiments and those from the CMB.
April 6th David Velasco (Princeton) Spectral-Difference Method for Astrophysical Fluid Dynamics A key aspect in computational astrophysical fluid dynamics is the need to deliver higher and higher accuracy for longer and longer time integration. Modern architectures such as massively parallel computers or graphical
processing units (GPU) can be leveraged to increase the spatial and time resolution of simulations. We present a new numerical scheme which combines the Spectral Difference (SD) method up to arbitrary high order with a-posteriori limiting. It delivers very accurate solutions in smooth regions of the flow, while capturing sharp discontinuities without spurious oscillations.
April 13th Zili Shen (Yale) Ultra-Diffuse Galaxies lacking dark matter: from properties to formation The formation of ultra-diffuse galaxies (UDGs) is not well-understood. UDGs have large sizes (several kpc) for their dwarf-galaxy-like stellar mass, and have a wide range in dark matter content and other observed properties. In particular, NGC1052-DF2 and NGC1052-DF4 are two puzzling UDGs: they have low velocity dispersions indicating little to no dark matter content, as well as overly luminous and large globular clusters compared to the Milky Way population. Recently, we used the Keck Cosmic Web Imager (KCWI) to measure the velocity dispersion of the diffuse stellar light in NGC1052-DF4. High-resolution spectroscopy confirmed that the stellar kinematics are consistent with the globular cluster velocity dispersion, and rules out a normal NFW dark matter halo. We proposed that the two galaxies lacking dark matter formed jointly in the aftermath of a single bullet-cluster-like collision of dwarf progenitors. This is the first formation theory that can explain all the abnormal properties of both galaxies. I will also introduce the ongoing Dragonfly Ultrawide Survey. This survey covers the entire SDSS footprint in g and r band, and I will show examples of initial data. We will conduct a systematic search for large, nearby UDGs with the Ultrawide Survey.
April 20th Sabrina Appel (Rutgers) How the Gas Dynamics Set the Star Formation Rate of Molecular Clouds

Gravity, turbulence, magnetic fields, and stellar feedback all play important roles in star formation and in setting the star formation rate (SFR). The impact of these physical processes is reflected in the distribution and dynamics of the gas of these star forming regions. In this talk, I present our work using a suite of 3D magnetohydrodynamical (MHD) simulations to study  the shape and evolution of the density distribution and how the compression and expansion rates of the gas are linked to the gas density distribution. We find that supersonic turbulence produces an approximate equilibrium between compressing and expanding gas near the mean density and that protostellar jets produce rapidly expanding and compressing gas at low densities.  We also find that the net gas mass flux becomes constant at a density above the transition density (the transition between the lognormal and power-law portions of the PDF) but below the sink formation threshold.  At this density, the net gas mass flux matches the mean SFR, suggesting that the gas dynamics at this density sets the overall star formation rate of the simulation.

May 4th  Ewine van Dishoeck Leiden
(Leiden Observatory, the Netherlands)
Protostars and protoplanetary disks with JWST:
 first from the JWST JOYS and MINDS GTO programs
This talk will present the first results of the MIRI-MRS spectra of a
number of protostars, T Tauri and brown dwarf disks resulting from the
JOYS and MINDS GTO programs. Rich ice spectra are seen in the cold
outer envelopes of protostars hinting at the presence of complex
molecules in ices. In young and mature disks, many lines from gaseous
CO, H2O, CO2, C2H2 and HCN are found, but also surprising new
molecules are detected. Together they point to a rich chemistry that
is linked to the physical structure of the inner regions of these
disks and thereby provides a unique diagnostic of it.

As background information: the 55 hr JWST Observations of Young
protoStars (JOYS) GTO program (PI: van Dishoeck) uses MIRI to
investigate the physical and chemical properties of two dozen
protostars and their immediate environment. JOYS studies the physical
characteristics of embedded disks, accretion signatures onto the
protostars, feedback from the young protostars on their environment
through primordial jets and outflows, as well as the chemical gas and
ice constituents of the protostellar envelopes to feed the disks.

The goal of the 120 hr MIRI Mid INfrared Disk Survey (MINDS) GTO
program (Th. Henning, I. Kamp co-PIs) is to use JWST to (1)
investigate the chemical inventory of the terrestrial planet forming
zone, (2) to follow the gas evolution into the disk dispersal stage,
and to (3) study the structure of protoplanetary and debris disks in
the thermal mid-IR. The program builds a bridge between the chemical
inventory of planet-forming disks and the properties of exoplanets.
In total, about 50 targets (Herbig Ae stars, T Tauri stars, brown
dwarfs and young debris disks) will be observed.
May 11th Charlotte Ward (Princeton University) Probing the extremes of the supermassive black hole population with the Zwicky Transient Facility and LSST Large time-domain surveys such as the Zwicky Transient Facility are providing new insight into the early-universe seeding and accretion/merger-driven growth of supermassive black holes (SMBHs) via transient phenomena from important SMBH sub-populations. These rare sub-populations include: ‘hidden’ low mass AGN in dwarf galaxies, analogs to early MBH seeds; SMBHs ejected from galactic nuclei during mergers; ‘state-changing’ AGN with recently triggered episodes of accretion; and ‘disk-emitting’ broad-line AGN which reveal the geometries of AGN accretion disks. I will present a multi-wavelength and spectroscopic analysis of a population of variable disk-emitting AGN discovered in ZTF and discuss the relationship between AGN accretion disk properties, optical variability levels and radio jet features. I will also discuss recent work on the optically variable dwarf galaxy AGN discovered in the deep HSC-SSP transient survey, including the effort to constrain the fraction of ‘wandering’ IMBHs located outside of their dwarf galaxy nuclei. Finally, I will explain how multi-resolution image analysis techniques, which combine time-resolved ground and space-based survey data, are enabling us to better extract the spectra, positions and morphologies of transients and their host galaxies for an even wider range of applications, with an eye to the future with LSST and Roman.

Fall 2022:

September 15th: Aritra Ghosh (Graduate Student, Yale University)
September 22nd: Thales Gutcke (NASA Hubble Fellow and Lyman Spitzer, Jr. Postdoctoral Fellow, Princeton)
September 29th: Fengwu Sun (Graduate Student, University of Arizona)
October 6th: Yubo Su (Lyman Spitzer Jr. Postdoctoral Fellow, Princeton)
October 13th: Yinhao Wu (Graduate Student, Leicester University)
October 20th: Fall break
October 27th: Lizhong Zhang (Graduate Student, University of California, Santa Barbara)
November 3rd: Oliver Zier (Graduate Student, Max Planck Institute for Astrophysics, Garching, Germany)
November 10th: Chang-Goo Kim (Post-Doctoral Associate Research Scholar, Princeton)
November 17th: Tsun Hin Navin Tsung (Graduate Student, University of California, Santa Barbara)
December 1st: Ore Gottlieb (Rothschild Fellow, CIERA Postdoctoral Fellow, Northwest University)
December 8th: Sihao Cheng (Postdoc Member at the Institute for Advanced Study)

Spring 2022:

February 3rd: Benjamin Crinquand (Post-Doctoral Associate Research Scholar, Princeton)
February 10th: Matthew Coleman (Post-Doctoral Associate Research Scholar, Princeton)
February 17th: Riddhi Bandyopadhyay (Post-Doctoral Associate Research Scholar, Princeton)
February 24th: Alex Gagliano (Pre-Doctoral Fellow, CCA Flatiron)
March 10th: Igor Andreoni (Postdoctoral Fellow, Joint Space-Science Institute)
March 17th: Sihao Cheng (Postdoctoral Fellow, Johns Hopkins University)
March 24th: Keith Hawkins (Assistant Professor, University of Texas at Austin)
March 31th: Frank van den Bosch (Professor of Theoretical Astrophysics, Yale)
April 7th: Mor Rozner (Graduate Student, Technion - Israel Institute of Technology)

April 14th: Sam Yee (Graduate Student, Princeton)

Click here for the latest schedule information


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