2021 Flatiron Institute Center for Computational Astrophysics Pre-Doctoral Program

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Purpose

The Center for Computational Astrophysics (CCA) at the Flatiron Institute is a vibrant research center in the heart of New York City with the mission of creating new computational frameworks that allow scientists to analyze big astronomical datasets and to understand complex, multi-scale physics in a cosmological context.

The CCA Pre-Doctoral Program will enable graduate student researchers from institutions around the world to participate in the CCA mission by collaborating with CCA scientists for a period of 5 months on site. With this opportunity, the selected group of researchers will be able to participate in the many events at the CCA and interact with CCA scientists working on a variety of topics in computational astrophysics (including both numerical simulations and sophisticated analyses of observational data), thereby deepening and broadening their skill sets.

Application

CCA Pre-Doctoral Program participants will be employed for up to 5 months at the CCA as Research Analysts. More information about this paid position is available on the application page, which can be accessed by clicking ‘Apply Now‘.

Research Analysts will collaborate with one or more CCA scientists on a project of mutual interest. Potential applicants can find a list of projects proposed by possible CCA mentors by going to the mentors tab above. Alternatively, applicants may propose a project related to the interests of one or more of the CCA mentors listed. Before applying, applicants must contact one or more potential mentors to discuss the project of interest in detail and specify the selected mentor(s) in the research proposal.

Deadline

Applications for the Research Analyst position should be submitted here by June 1. Applicants will be notified about the status of their applications by June 22.

Supporting material for the application includes the following:

  • CV and publication list
  • Description of previous research experience (not to exceed two pages)
  • Research proposal of not more than 2 pages outlining planned work at Flatiron
  • Two (2) letters of recommendation submitted confidentially by the letter writers to ccapredoc@flatironinstitute.org. One letter must be from the applicant’s PhD supervisor and must explicitly approve the applicant’s possible participation in the Pre-Doctoral Program
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Important Dates
  • Application Deadline
  • Applicant notification
  • Fellowship start date(s)
  • or
Contact Info
  • Please send inquiries
    about the program to ccainfo@flatironinstitute.org

Position Description 

TITLE: Research Analyst REPORTS TO: Associate Research Scientists and/or Group Leaders
DEPARTMENT: CCA FLSA STATUS: Exempt
LOCATION: NY office (162)

DATE: March 1, 2021

EMPLOYMENT CLASSIFICATION: Fixed-term

POSITION SUMMARY

The Center for Computational Astrophysics (CCA) at the Flatiron Institute seeks temporary full-time Research Analysts as part of its Pre-Doctoral Program. The aim of this program is to provide graduate students from institutions worldwide the opportunity to be employed at the CCA for 5 months for the purpose of working on a research project with one or more CCA staff mentors. The program is open to individuals who are currently pursuing a Ph.D. in a relevant field.

Before applying, candidates for this position must contact one or more potential mentors. For further details about the program and a full list of mentors, please visit: https://www.simonsfoundation.org/grant/flatiron-institute-center-for-computational-astrophysics-pre-doctoral-program-2021/

MINIMUM QUALIFICATIONS

Education
• Currently enrolled in a Ph.D. program in a field relevant to the proposed research

Experience
• 1-2 years of advanced course work in a relevant field
• Demonstrated understanding of basic research skills

Related Skills & Other Requirements
• Knowledge of software engineering practices for working in groups, including software development life cycles, coding standards, code review and version control systems (e.g., Git)
• Expertise in algorithms and data structures and/or in computational methods
• Technical and scientific curiosity
• Professional communication skills

REQUIRED APPLICATION MATERIALS

• CV and publication list
• Description of previous research experience (not to exceed two pages)
• Research proposal of not more than 2 pages outlining planned work at Flatiron
• Two (2) letters of recommendation submitted confidentially by the letter writers to ccapredoc@flatironinstitute.org. One letter must be from the applicant’s Ph.D. supervisor and must explicitly approve the applicant’s possible participation in the Pre-Doctoral Program

Deadline
• All applications must be submitted no later than June 1, 2021

THE SIMONS FOUNDATION’S DIVERSITY COMMITMENT

Many of the greatest ideas and discoveries come from a diverse mix of minds, backgrounds and experiences, and we are committed to cultivating an inclusive work environment. The Simons Foundation provides equal opportunities to all employees and applicants for employment without regard to race, religion, color, age, sex, national origin, sexual orientation, gender identity, genetic disposition, neurodiversity, disability, veteran status, or any other protected category under federal, state and local law.

Important Dates
  • Application Deadline
  • Applicant notification
  • Fellowship start date(s)
  • or
Contact Info
  • Please send inquiries
    about the program to ccainfo@flatironinstitute.org

Phil Armitage
 
Projects or project ideas are welcomed in any of the science areas covered by the Planet Formation group, including studies of the formation and evolution of protoplanetary disks, planet formation physics, migration, and planetary dynamics. Collaborative projects that would involve members of the Machine Learning or Compact Objects groups are also encouraged.
 
 
Megan Bedell
 
I am broadly interested in spectroscopy of Sun-like stars, stellar variability, and extreme precision radial velocity (EPRV) data analysis, and I’d welcome student projects connected to any of those topics. Some potential projects could include work on:
 
 • Applying the “wobble” data-driven method to extract precise RVs across large RV archives
 • Developing open-source tools to simulate stellar activity in the spectrum (with Rodrigo Luger, Christina Hedges)
 • Optimizing target selection for EPRV surveys
Please email me to discuss further if you are interested in any of the above ideas, or if you have another suggestion!
 
 
Matteo Cantiello & Adam Jermyn
 
More info at: https://www.stellarphysics.org/projects
 
Stars in AGN Disks
 
Stars are likely formed in, or captured by, the disks of active galactic nuclei (AGN). The disk conditions profoundly change the star’s evolution, with AGN stars accreting large amounts of mass and becoming massive / very massive. This project could involve either modeling the accretion stream with radiation hydrodynamics software instruments like Athena, modeling the long-term stellar evolution in the MESA software instrument, or studying the interplay of stellar dynamics, AGN disk models, and evolution, tying together output from a variety of tools with semi-analytic models. Some questions that need to be answered are:
 
 1. Does AGN star chemical pollution explain observed metallicities in AGN disks?
 2. How does AGN star evolution change with redshift and/or AGN metallicity?
 3. Does accretion during the AGN phase of the Milky Way explain the lack of Red Giants in the Galactic Center?
 4. Does AGN star evolution impact the growth of SMBHs?
 
Ref: https://ui.adsabs.harvard.edu/abs/2020arXiv200903936C/abstract
 
Modeling Stellar Ingestions with MESA
 
Planets and stars can be engulfed when, e.g., their host / companion star ascends the giant branch. Using a 1D code (MESA) it is possible to account for the energy deposited during the spiral in and determine the evolution of the primary star. Some questions that need to be answered:
 
 1. Is it possible to use this simple approach to reproduce observed Luminous Red Novae? (e.g. V1309Sco and AT 2018bwo?)
 2. Can the light-curve produced using this 1D approach be used to determine basic properties of the binary system (M1,R1,q,a)? And if so, in what regimes?
 3. Is it possible to use a large grid of such 1D models to help the identification of these transients as observed by e.g. the VRO/LSST?
 
Modeling Stellar Convection
 
Stellar modeling is limited by our understanding of turbulence. We are looking for a student interested in studying stellar convection, either near the star’s surface where it can cause measurable brightness fluctuations or in deeper regions where it plays a crucial role in distributing angular momentum and setting stellar rotation rates. These projects likely involve a combination of hydrodynamic simulations with the Athena or Dedalus software instruments, as well as semi-analytic work developing prescriptions for 1D stellar evolution instruments like MESA.
 
Apogee Binaries
 
Binary stars represent a powerful probe of stellar evolution. Data from the Apogee radial velocity survey have been used to identify a large population of binary main-sequence and red giant stars. This project would involve studying the chemical abundances, eccentricities, periods, and rotation periods of the Apogee binaries, and leveraging semi-analytic models as well as 1D stellar evolutionary models to gain insight into how both single and binary stars evolve.
 
 
James Cho
 
My research interests mainly center on studying hydro and MHD dynamics using idealized simulations and analytics. I would be happy to work with a student interested in any of the following under the two general topics mentioned:
 • Weakly and/or strongly ionized planetary/stellar atmospheric dynamics (including possible coupling to radiative transfer)
 • Vortices or instability in protoplanetary/accretion disks– Convection and jets and spots in giant planet or stellar interiors– Hamiltonian fluid dynamics (symmetry analysis of quasi-linear/wave-meanflow equations)
 
 
Rachel Cochrane & Chris Hayward
 
We are interested in collaborating with pre-doc students on topics in galaxy evolution that bridge simulations and observations.
 
Several potential projects could stem from work we have been doing to predict observables using the Feedback In Realistic Environments 2 (FIRE-2) simulations (Hopkins et al. 2018). We have recently run radiative transfer modelling on a series of massive galaxies (Angles-Alcazar et al. 2017), spanning redshifts 1<z<5 (Cochrane+19). Our output comprises spatially resolved predictions for the UV-FIR emission on scales of 10s of parsec. Extensions to this work could include:
 • The implementation of an AGN heating source into the radiative transfer, to quantify the role of AGN in shaping observed FIR emission.
 • A study of the recoverability of star formation histories from observed spectra, using SED fitting codes such as Prospector (Johnson et al. 2020).
 • Detailed modelling of mock observables from JWST, and development of techniques to infer physical properties from observations.
 • Studies of resolution effects in radiative transfer – how robust are predictions of spectra and spatially resolved maps to the particle resolution of the simulation?
 
We are also interested in the connection between galaxies and their host halos. Work within the SMAUG and CAMELS collaborations (see links below) has generated thousands of numerical simulations with controlled variations of cosmological and astrophysical input parameters. Tools have also been developed to create mock emission line fluxes for these galaxies (Hirschmann+17). Possible applications of this work include studying the sensitivity of the luminosity functions, halo occupation distributions, and auto/cross-correlation functions of emission line-selected galaxies to the underlying parameter choice.
 
References:
Angles-Alcazar D. et al. 2017, MNRAS, 472, L109
Cochrane R. K., et al., 2019, MNRAS, 488, 1779
Hirschmann, M. et al. 2017, MNRAS, 742, 2468
Hopkins P. F., et al., 2018, MNRAS, 480, 800
Johnson et al. 2020 arXiv:2012.01426
 
https://camels.readthedocs.io/en/latest/index.html
https://www.simonsfoundation.org/flatiron/center-for-computational-astrophysics/galaxy-formation/smaug/
 
 
Emily Cunningham & Melissa Ness
 
Spectroscopic surveys of stars in the Milky Way are attempting to extract as many elemental abundances as possible from stellar spectra. However, many of these abundances are well known to be extremely correlated, and the number of abundances that are necessary to measure accurately remains in dispute. For example, Ness et al. (2019) found that iron abundances and age are sufficient to predict other elemental abundances to within current measurement precision. However, such predictive models always have some level of residuals; in this project, we propose to explore the potential information content of those residuals.
 
The student will first train a regression model to predict the abundances for a sample of APOGEE spectra, using only stellar parameters, [Fe/H], and [Mg/H]. The student will then explore the correlation structure of the residuals from this model to identify which elements are providing additional information. In addition, for stars with high residuals, the student will use kinematic data from the Gaia mission to investigate if the abundance signature, combined with the star’s orbital properties, could be a result of the star’s formation environment.
 
To complement the analysis of observations, the student will also explore abundance correlations for stellar populations in different environments in cosmological simulations. The student will look for differences in correlations between abundances both for disk stars born in different birth clusters as well as halo stars born in satellite galaxies of different masses.
 
 
John Forbes & Rachel Somerville
 
Intra-galactic profiles and correlations: when, how, and why?
 
A recent generation of IFU surveys have revealed a range of correlations, for instance the resolved star-formation main sequence, and average radial profiles of gas- and stellar-phase kinematics. At the same time, the kinematics and chemical fingerprints of stars in the Milky Way have become incredibly detailed. These extremely rich datasets compel theoretical interpretation somewhere between the simplest considerations and comparisons to expensive hydro simulations. The student would employ a state-of-the-art radially-resolved semi-analytic model to investigate, depending on their interests, the age-velocity dispersion correlation of MW stars, the radially and vertically resolved distribution of alpha/Fe abundances as a function of metallicity, the observed exponential profiles of stars, gas, and stellar velocity dispersions, the resolved star formation main sequence, or any other observational relation of interest. The flexibility and speed of the model allows large parameter studies, and tracing the progenitor galaxies back in time to understand the origin of these relations.
 
 
Dan Foreman-Mackey
 
I am broadly interested in the development of novel methods for data analysis in astronomy, with a focus on producing high-performance and user-friendly software implementations. I am interested in mentoring a student who would like to work on these topics within any subfield of astrophysics so please get in touch to discuss further, but here are two potential project ideas:
 
A systematic study of spectroscopic binaries observed by TESS (Co-mentor: Adrian Price-Whelan)
 
There are large catalogs of spectroscopic binary star systems detected using the radial velocity method but, in many cases, the precision with which we characterize these systems is limited because the orbits are not well sampled and there are covariances between the physical and orbital parameters. Many of these systems have also been observed by NASA’s TESS mission and in some cases eclipses and phase curves can be detected in the light curves. A joint analysis of these systems observed with both radial velocities and photometry yields extremely precise characterization of these stellar systems. In this project, we will perform a systematic analysis of all of these datasets and produce a catalog of benchmark stars that can be used to study the population of multiple star systems and place constraints on stellar evolutionary theory.
 
Inferring data-driven limb darkening laws from transiting multi-planet systems (co-mentor: Rodrigo Luger)
 
Limb darkening affects almost any photometric or spectroscopic measurement one can make of a star, whether it is resolved or not. Traditionally, exoplanet light curve analyses make use of simple two-, three-, or four-parameter laws that are motivated by–and whose coefficients are computed from–stellar radiative transfer models. These simple laws admit closed-form and/or computationally efficient expressions for the transit light curve model, making them an attractive option for inference problems. We have derived an efficient, closed-form expression for the transit light curve corresponding to a polynomial limb darkening law of arbitrary order. This model is flexible enough in principle to capture *any* functional form for the limb darkening profile. However, the true limb darkening profiles of main sequence stars likely live in a fairly low-dimensional space. In this pre-doctoral project, we will use this framework to model transit light curves of multiple-planet systems observed by Kepler and TESS, with the goal of deriving empirical limb darkening models as a function of various stellar properties.
 
 
Shy Genel
 
My interests are in modeling galaxy formation using cosmological simulations such as IllustrisTNG or CAMELS, with a particular focus on galaxy morphology and dynamics, galaxy growth through accretion and mergers, and mock observations of simulated galaxies. I welcome candidates’ ideas for specific areas of study and will be happy to develop a project vision together.
 
 
Sultan Hassan & Rachel Somerville
 
I am interested to work with a student on any of the following areas below.
 
Cosmic Reionization
 • Development of self-consistent AGN model.
 • Parameterizing key observables using symbolic regression.
 • Emulating Reionization Simulations using Deep Generative Models.
 • Constraining cosmology and astrophysics using key observables.
 • Lya emitters evolution and clustering.
 • Reconciling EDGES results with current reionization constraints.
 
Circumgalactic medium (CGM) and Damped Lyman Alpha Systems (DLAs)
 • DLAs kinematics in relation to the hosting properties.
 • Nature and evolution of metal-poor DLAs and their relation to the ultra-faint dwarf galaxies at high redshift.
 • Nature and evolution of dusty DLAs at high column densities.
 • DLAs as a function of cosmology and astrophysics using the CAMELS simulations.
 
 
Chris Hayward
 
I would be interested in mentoring a student on a project connected to any of my areas of research, so please email me if you have an ongoing project or project idea related to one or more of the following topics:
 • Radiative transfer, dust and predicting observables from simulations
 • Stellar feedback, turbulence and outflows
 • Galactic magnetic fields (with Ulrich Steinwandel)
 • Black hole accretion and AGN feedback (with Kung-Yi Su and Daniel Anglés-Alcázar)
 • Circumgalactic medium (CGM) and the physics of multiphase gas (with Drummond Fielding)
 • Infrared/submillimeter-selected galaxies
 
 
David W. Hogg (Flatiron/NYU), Sarah Pearson (NYU) & Melissa Ness (Columbia)
 
The Milky Way’s Galactic Bar in Gaia and APOGEE
 
There are many unknowns about the Milky Way’s Galactic bar: when was it formed, how was it formed, has it spun down throughout its lifetime, how long is it, and for how long will it persist? With insight from rich datasets such as Gaia and APOGEE, as well as access to sophisticated simulations of barred galaxies, we are entering a unique era in which we can answer these questions. The goal with this project is to gain a deeper understanding of stellar motions near the center of our Galaxy, and to determine the length of the Milky Way’s galactic bar. You will investigate the orbit structure of the Milky Way’s Galactic bar by integrating orbits of stars from the APOGEE survey cross-matched with Gaia. With 6D phase space information of stars in the Milky Way’s bar, we can answer whether the stars support specific bar potentials, and determine whether there is wiggle room in our interpretation of the length of the bar (e.g. what’s the radius of the last trapped bar orbit). By looking at all APOGEE star orbits, you will also determine what fraction of the APOGEE stars are on bar-type orbits as compared to expectations from theoretical predictions of barred N-body simulations and ultimately gain a deeper understanding of the nature of the Milky Way’s Galactic bar.
 
 
Yan-Fei Jiang
 
I am generally interested in radiation MHD simulations of various astrophysical systems and development of new numerical techniques. I have a few projects that can be appropriate for the pre-doctor program. One project is to study gas accretion onto supermassive black holes with super-Eddington accretion rates in the immediate radial range (a few thousand gravitational radii to the Bondi radius). The goal of this project is to understand how the black hole is fed with super-Eddington accretion rates based on multi-dimension radiation MHD simulations. Particularly, we will focus on the spatial scale that is too small for galactic simulations but too large for typical accretion disk simulations. This project will also be collaborated with people from the galaxy formation group. Another project is to analyze existing 3D radiation hydrodynamic simulations of massive star envelopes to understand realistic 3D envelope structures of massive stars for a wide range of mass and luminosity. We will also need to run more simulations to cover a larger parameter space in the HR diagram. I am also interested in the dynamics of cosmic ray driven outflow in galaxies. Particularly, I am actively thinking the properties of cosmic ray driven outflow with multi-phase gas created by thermal instability.
 
 
Natascha Manger & Phil Armitage
 
Turbulence properties in protoplanetary disks are still not fully understood, but current models and observations suggest that hydrodynamic instabilities can develop near the disk mid plane. Understanding which instabilities develop at different radii and how they interact with the disk magnetic field is crucial in finding a consistent model of early disk development and potential routes to planetesimal formation. The student would work with Natascha Manger and Phil Armitage on projects related, but not limited to, simulations of VSI turbulence at different disk radii (optionally with non-ideal MHD), or dusty protoplanetary disks and the onset of planetesimal formation.
 
 
Chiara Mingarelli
 
Supermassive black holes (SMBHs) in the million to 10 billion solar mass range form in galaxy mergers and live in the centers of galaxies with large and poorly constrained concentrations of gas and stars. When SMBHs merge, they create low frequency gravitational waves: ripples in the fabric of spacetime However, there are currently no observations of merging SMBHs— it is in fact possible that they stall at their final parsec (3.3 light years) of separation and never merge, called the final parsec problem. The only way to detect SMBHBs is by timing radio pulsars, which are excellent clocks. Their radio waves sweep across the Earth, like cosmic lighthouses, and GWs induce a delay or advance in the pulse arrival time. Thus, an array of precisely timed pulsars forms a galactic-scale GW detector, called a pulsar timing array. The cosmic merger history of SMBHs should generate a GW background (GWB), which may have already shown a hint of its existence in the NANOGrav 12.5 year data. If the signal is really from the GWB, a detection is imminent.
 
The amplitude and the shape of the GWB’s strain spectrum is affected by galaxy merger rates, SMBH mass estimates, and solutions to the final parsec problem. Together, we will develop methods to provide insights into all aspects of low frequency GW astronomy: from the amplitude of the GWB as a function of SMBHB merger physics to the likeliest SMBHB host galaxies. Our advanced physical models will implement cutting edge gas dynamics and binary eccentricity for the first time, and serve to forward model the GWB. When the GWB is detected, we will in turn have the necessary tools to constrain the underlying astrophysics and demographics of the SMBHB population, addressing long-standing questions in galaxy evolution and fundamental physics.
 
 
Chirag Modi, Shy Genel, & Rachel Somerville
 
Exploring new methods for predictions for the next generation of large scale structure galaxy surveys
 
A large number of new galaxy surveys coming on line in the next decade (DESI, Euclid, VRO, Nancy Grace Roman) will measure the spatial distribution of different types of galaxies over very large volumes. These observations have the potential to provide exciting new constraints on both cosmology and the physics of galaxy formation. However, these processes are complex and intertwined, and the ‘gold standard’ method to make detailed physics-based predictions of galaxy clustering, numerical hydrodynamic simulations, are far too expensive to simulate these large volumes or to explore the large space of both cosmological and astrophysical parameters. A new generation of Semi-analytic models (SAMs), that are designed and calibrated to “emulate” the results of a numerical hydrodynamical simulation at a much reduced computational cost, appear promising. Machine learning based methods (including Deep Learning) provide another promising avenue for creating realistic mock galaxy surveys at a low computational cost.
 
In this project, we will explore one or more new methods to address this problem. The focus of the project is flexible depending on the interests of the candidate, with the ultimate goal of making progress towards generating state-of-the-art full sky galaxy survey simulations that can be used for inference.
 
A few examples of techniques that could be explored are:
 1. Learning halo properties such as mass, position and velocity, and developing novel generating models to learn halo merger histories/mass accretion histories (which have traditionally been the computational and memory bottleneck for SAMs) in a hybrid fashion with approximate simulations. This has advantages such as fidelity to generate several merger trees with different features of interest and algorithms as well as applicability to existing large suites of simulations. Both of these can be fleshed out as projects in themselves.
 2. Develop code to extract high-dynamic range merger trees for outputs of ML-based simulations that evolve only density fields. These can be developed in conjunction with super-resolution techniques to scale to larger volumes.
 3. Develop probabilistic merger tree methods to learn galaxy data likelihoods and developing inference pipelines with differentiable forward models in a SAM framework.
 4. Develop ML based methods to emulate SAMs or hydro sims with a Halo Occupation Distribution (HOD) style occupancy framework.
 
 
Sasha Philippov
 
My main research area is relativistic plasma astrophysics. Interested students may work on subjects including (but not limited to) kinetic plasma simulations of pulsars, binary neutron star and black hole magnetospheres and astrophysical jets. These first-principles simulations are instrumental for understanding plasma production, particle acceleration and emission of nonthermal photons in the environments of compact objects. I’m also broadly interested in production of coherent radio emission in the universe, ranging from solar radio bursts to pulsar radio emission and the enigmatic fast radio bursts.
 
 
Adrian Price-Whelan
 
I am interested in a range of topics from stellar astrophysics to gravitational dynamics, and many places in between! Some themes and topics that I am working on or interested in working on with you:
 • Dynamical modeling of stellar kinematics in the Galactic disk, especially toward the Galactic anticenter, using data from the APOGEE survey and Gaia mission.
 • The large-scale (dark matter) mass distribution in the Galactic halo, using stellar kinematics and stellar streams.
 • Discovery of stellar streams and substructures in the Milky Way halo, using elemental abundances (APOGEE) and kinematics (Gaia).
 • Dynamical modeling of stellar streams, especially in the presence of time-dependent perturbations from the Magellanic Clouds and small-scale impacts from dark matter subhalos.
 • Binary star populations, especially using spectroscopic data from the APOGEE survey and photometric data from the TESS and Kepler missions.
 
Many projects related to these topics would be in collaboration with Dan Foreman-Mackey, David Hogg, Kathryn Johnston, and other members of the Dynamics and Astronomical Data groups.
 
 
Robyn Sanderson
 
Understanding the orbit structure of cosmologically accreted stellar halos
 
The stellar halos of galaxies like the Milky Way are made up of stars accreted from many smaller galaxies, spread into tidal streams by the gravitational field of the host galaxy. Stars in the same tidal stream are expected to have similar orbital invariants (constants of motion) since they come from the same progenitor globular cluster or dwarf galaxy, which occupies a smaller total phase space volume than the total available in the host. In principle, stars can be assigned to a particular stream by association with the nearest invariant-space cluster, which would be a powerful way to determine the accretion history of our Galaxy. However, in a real galaxy some or all of these quantities will only be approximately invariant, both due to time-evolution in the potential (which could be non-adiabatic in some cases) and due to departures from perfect symmetry in the potential (which breaks the assumption of separable equations of motion underlying the choice of a coordinate system). Both of these types of symmetries are expected to be broken to different degrees at different locations in the gravitational potential, which can lead to distortion of the cluster corresponding to a single stream. Additionally, the extent of each structure in invariant space varies in proportion to the mass of the progenitor, while filamentary infall can lead several progenitors to create streams on similar orbits and hence partially overlap in invariant space. Several aspects of a realistic galactic potential and cosmological background can thus impact the certainty with which one can assign stars to a particular tidal stream.
 
The student will work with simulated stellar halos from the FIRE suite, as well as simulated Gaia surveys of these systems, to understand how these different effects influence the structure of [approximately] invariant space in cosmologically accreted stellar halos, especially compared to the effect of current measurement uncertainties in calculating orbital invariants. Possible topics of investigation include the effect on constraints on the mass distribution from action-space clustering, how to evaluate the certainty with which stars can be assigned to streams, and whether distortions in invariant space can point to specific discrepancies between a model and the actual galactic gravitational field.
 
 
Stephanie Tonnesen
 
My research broadly spans galaxy evolution, and I would be happy to work with students on a range of projects, especially those related to:
 • galaxy quenching
 • outside-in (or inside-out) star formation
 • satellite galaxy evolution
 • environmental impacts on galaxy formation and evolution
 • multiphase gas mixing (in the CGM and ICM)
 
Some specific projects we could work on together are:
 
Do Satellites Stir the CGM?
 
The CircumGalactic Medium (CGM) is an important reservoir of gas that has accreted into a galaxy halo, or been ejected from a galaxy disk. Because this gas is very difficult to observe, the distribution of gas density, temperature and velocity is poorly constrained. In this project, we (the student, Drummond Fielding, and myself) would examine z=0 Milky-Way mass galaxies in cosmological simulations, and determine the effect of satellites on the CGM. Some questions to consider are: Do satellites drive turbulence in the CGM? Induce cooling? Effect the metallicity?
 
Ram Pressure Stripped Tails
 
Ram Pressure Stripping is a gas-removal mechanism that acts on satellite galaxies in which an interaction between the ISM and surrounding gas removes the ISM. Interestingly, recent papers on clouds mixing into a wind (using “cloud-crushing” simulations) have shown that some clouds can grow in mass through entrainment in the tail (e.g. Gronke & Oh 2018). In fact, we see some “dense” gas growth in stripped tails in some recent simulations (Tonnesen & Bryan 2021). However, we don’t know how much of this growth can be observed in, for example, HI. In this project we will make mock observations of “wind-tunnel” simulations to directly compare with observed stripped galaxies.
 
Ram Pressure Stripping in Fornax
 
Galaxies plunging into X-ray clusters can be stripped of their gas, leading to tail of dense, neutral gas. This project uses the just-released TNG50 high-resolution simulation to compare the predicted HI gas distribution against a new, very deep Meerkat observation of the Fornax cluster which shows intriguing evidence for low column density clouds (with Greg Bryan). The goal will be both to test the simulations against a new observable and to understand the complex astrophysics of cold gas clouds moving through a hot medium.
 
 
Yajie Yuan & Sasha Philippov
 
I am interested in a range of topics related to particle acceleration and high energy emission around black holes and neutron stars. Here are two possible projects, but I am also happy to work with you on any related topics.
 
Pair production in black hole magnetospheres and gamma-ray flares
 
Black holes can launch powerful jets through electromagnetic extraction of their rotational energy. But this requires sufficient plasma in the jet funnel to conduct the necessary current, which may be a problem especially for low luminosity active galactic nuclei. In this case, electrostatic gaps can form at the base of the jet to inject pair plasma. We will study the gap physics using state-of-the-art general relativistic particle-in-cell codes, focusing on how the gap dynamics and resulting high energy radiation depend on the black hole environment factors (e.g., radiation background, magnetic field, etc).
 
Radio polarization of millisecond pulsar with multipole magnetic field
 
NICER has observed a few millisecond pulsars where detailed geometry of the X-ray emitting hotspots on the neutron star is studied in order to obtain the neutron star mass and radius, constraining the nuclear equation of state. One example, PSR J0030+0451, is shown to possibly have significant multipolar magnetic fields at the stellar surface. We have done pioneering work using force-free simulations of the magnetosphere structure to show that the radio, X-ray and gamma-ray light curves can be modeled simultaneously with appropriate field configuration. A more stringent test is to compare radio polarization with observations. This, if works, can constrain the radio emission site. Similar approach can also be applied to other radio pulsars to help constrain the radio emission physics.

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