The Puzzling Properties of Galaxies in the First Billion Years

Supervisor: Prof. Pratika Dayal and Haowen Zhang

Galaxy formation in the first billion years mark a time of great upheaval in our cosmic history: the first sources of light in the Universe, these galaxies ended the ‘cosmic dark ages’ and produced the first photons that could break apart the hydrogen atoms suffusing all of space starting the process of ‘cosmic reionization’. At the forefront of astronomical research, the past few years have seen cutting-edge instruments such as the James Webb Space Telescope (JWST) and Euclid provide tantalising glimpses of such galaxies assembling in an infant Universe. Puzzlingly, these observations indicate an over-abundance of bright, dusty galaxies in addition to yielding unexpectedly numerous and massive black holes (up to a 100 million solar masses) within the first 600 million years, posing an enormous challenge for galaxy formation models. The project can focus on any of these puzzles and will be defined together with the SURF student.

Potential methodology for the project includes analytical and/or empirical modeling of galaxy formation, as well as the analysis of hydrodynamical simulations. From this project, the student can expect to learn the basics of galaxy formation in the early Universe, including the halo—galaxy connection, seeding mechanism(s) of supermassive black holes, star formation, feedback, and how to forward model observable galaxy properties based on galaxy assembly histories.

 

Instruments such as the JWST are allowing unprecedented glimpses of the earliest structures assembling in an infant Universe. However, these observations are yielding a number of puzzles such as an over-abundance of bright galaxies and supermassive black holes. The project will focus on shedding light on these key outstanding issues in the first billion years.

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When MHD Breaks Down: Probing the Turbulent Dynamo in Weakly Collisional Plasmas

Supervisors: Radhika Achikanath Chirakkara, Shantanu Basu

Astrophysical plasmas such as the hot intracluster medium of galaxy clusters, the solar wind, and the hot ionized interstellar medium are all weakly collisional. Magnetic fields are ubiquitous throughout the Universe, and the turbulent small-scale dynamo provides an efficient mechanism for amplifying initially weak seed magnetic fields to dynamically significant strengths, where they can influence gas dynamics. The turbulent small-scale dynamo has been studied primarily using collisional magnetohydrodynamic (MHD) simulations and these models are not directly applicable to weakly collisional plasmas. Only recently have particle-in-cell (PIC) simulations begun to explore the turbulent dynamo in the weakly-collisional regime where kinetic physics and pressure-anisotropy effects become important (see figure below).

This project aims to carry out a systematic comparison between the collisional and weakly collisional turbulent dynamo, using high-resolution MHD simulations for the former and hybrid PIC simulations for the latter. The work will include a detailed investigation of magnetic field structure and intermittency in the saturated state of the dynamo, and will assess how kinetic effects modify the classical MHD picture. The results will have broad implications for our understanding of magnetic-field evolution in weakly collisional astrophysical environments. Through this project, the student will gain experience in plasma physics and MHD theory, programming and high-performance computing, and the use of both MHD and PIC simulation tools. They will also learn statistical techniques for characterizing turbulent magnetic field amplification, with the possibility of publication. The project can be tailored to the student’s background and interests.

Plot: Achikanath Chirakkara et al 2025 

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Statistical Signatures in Polarimetric Light Curves from Black Hole Accretion

Supervisor: Dr. Rohan Dahale 

The supermassive black hole Sagittarius A* (Sgr A*) is a dynamic environment where the magnetized accretion flow produces variable high-energy flares observed in X-ray, infrared, and radio wavelengths. While recent Event Horizon Telescope (EHT) imaging has confirmed the ring-like structure of the black hole shadow, the physical nature of the time-variable emission remains unknown. Leading hypotheses attribute these flares to orbiting compact hotspots or flux tubes formed near the event horizon, yet simulations suggest that stochastic fluctuations within a turbulent disk can produce similar transient features. Distinguishing between discrete coherent structures and continuous turbulence requires a rigorous statistical study that connects disk properties to observables. 

This project addresses this challenge by forward-modeling the polarimetric signatures of turbulent accretion flows. The student will develop a Python-based pipeline to simulate synthetic observations of the Galactic Center. First, the student will generate 3D, time-evolving Gaussian Random Fields (GRFs) which capture the inhomogeneous and anisotropic nature of accretion turbulence, incorporating differential rotation (shear) consistent with Keplerian orbital dynamics (using a python library pynoisy). Second, these 3D scalar fields will be ray-traced to integrate emissivity along the geodesics in the curved spacetime of a Kerr black hole (using a deep learning based JAX – python library bhnerf and kgeo). 

By varying correlation length, anisotropy, magnetic field configurations (e.g., vertical vs. toroidal), the student will generate synthetic polarimetric light curves (Stokes I, Q, U). The analysis will focus on two key statistical diagnostics: (1) the slope of the power spectral density (PSD), which theoretically encodes the correlation length of the underlying turbulent eddies, and (2) the topological morphology of loops in the Stokes Q-U plane. Ultimately, this work aims to determine if purely stochastic turbulence can mimic the observed coherent hotspot signatures, thereby providing new constraints on the physics of black hole accretion. 

Plot: From Levis, et al. Nature Astronomy, 8, 765-773 (2024) 

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JWST MIRI observations of the Galactic Center 

Supervisors: Dr. Sebastiano von Fellenberg

This project aims at uncovering the dynamical and chemical properties of the luminous mid infrared regime of the Galactic Center. Specifically, the student will learn to calibrate JWST MIRI data, deal with instrumental artifacts such as fringing and the undersampling of the PSF, to extract the highest quality spectra of the Galactic Center to date. The student will study stellar sources such as IRS 16 stars, extract the dynamical information from the gas shells surrounding the stars and the measure the chemical composition. If time allows for it, the study can be extended to include gaseous sources like the ominous X7 source, which have never been studied in spectroscopy outside the near infrared K-band.

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The Feeding Rate Towards Central Black Holes Across the Bondi Radius 

Supervisors: Dr. Yanlong Shi

The accretion of black holes (BHs) involves gas inflow from large to small scales, with the Bondi radius as the boundary of the BH’s gravitational influence. Though the mass influx cross the Bondi radius can be easily estimated, the fraction that finally reached the central BH needs additional care, particularly for the super-Eddington regime which may dominate the growth of supermassive black holes.  

In this project, we explore this problem with zoom-in radiation hydrodynamical simulations that bridge the Bondi radius and the disk scale, treating the BH+disk system as a central engine of radiative and mechanical feedback. We will also explore the role of large-scale turbulence, angular momentum, and magnetic fields in regulating the BH accretion. This study may serve as a sub-grid model for cosmological simulation where detailed BH accretion dynamics are hardly resolved. The student should gain experience with the basics of computational astrophysics at the end of the project.

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