Kevin Seokwoo Hong

Research

Projected Density Fields using N-body Simulations

Brookhaven National Laboratory

Weak gravitational lensing data from galaxy surveys can be used to probe the underlying dark matter distribution and predict cosmological measurements. Traditionally, most weak lensing analyses utilize summary statistics (e.g. two-point correlation function / power spectrum) to simplify weak lensing maps for efficient and precise measurements. However, two-point statistics are not insufficient for extracting all available information that is contained in a cosmic map, such as non-Gaussianities. Field-level inference is a technique that parameterizes the entire cosmic map for more accurate cosmological measurements. In another Science Undergraduate Laboratory Internship (SULI) with Dr. Anže Slosar, I ran a suite of 100 dark-matter-only N-body simulations and studied the statistical variance in the projected density fields. The goal of this project is to understand how the 2D projection of the density field evolves over time so we can perform field-level inference on 2D weak lensing maps (for more accurate cosmological constraints). I have been comparing my simulation results to predictions from 2nd-order Lagrangian perturbation theory. The image shows the projected density fields of all 100 simulations over time, with the mean of the fields on the bottom row.

GIF of projected density field over time

High-Resolution Spectroscopy of Hot Jupiters

University of California, Los Angeles

With Prof. Michael Fitzgerald and Luke Finnerty, I have been studying high-resolution emission spectroscopy (HRES) using the Keck Planet Imager and Characterizer (KPIC). HRES is a technique of analyzing the atmospheres of exoplanets, particularly hot Jupiters (big and hot exoplanets). By observing a high-resolution spectrum emitted by the planet-star system, we can use Doppler shift to isolate the planet's spectrum, which is varying over time, from the stellar spectrum, which is relatively constant. This technique utilizes Bayesian inference to compare data with generated planet models. For my first project, I determined the minimum planet velocity shift and signal-to-noise (SNR) required for a significant hot Jupiter detection. I simulated a grid of planet spectra with varying velocity shift and SNR, and I ran each simulated spectra through the HRES pipeline to generate a detection. By plotting the detection strengths of each spectrum (see plot), we can estimate the range of planet velocity shift and SNR that is needed for some detection strength (say, a 6\sigma detection). These detection estimates are useful during observation planning and for reliability checks. I am now studying the atmosphere of a specific hot Jupiter called MASCARA-1 b. I am investigating a chemical equilibrium model that will produce more realistic constraints on chemical abundances in the atmosphere.

Weak Lensing Forecasting with LSST

SLAC National Accelerator Laboratory

The Vera C. Rubin Observatory will be conducting the Legacy Survey of Space and Time (LSST), which is an upcoming all-sky survey that will image millions of galaxies in the entire night sky over the next ten years. In particular, LSST will be measuring weak gravitational lensing, or the small distortion of background galaxies due to the deflection of light by the underlying (dark) matter distribution. The Rubin Observatory will be producing much more precise measurements of weak lensing that can be used to test cosmological models. As part of the Science Undergraduate Laboratory Internship (SULI) program with Dr. Agnès Ferté, I forecasted results from LSST weak lensing and galaxy clustering data. By using a Python package called CosmoSIS, I simulated LSST data and performed Bayesian inference using nested sampling to produce constraints on cosmological parameters, such as the Hubble constant H₀. I also tested the impact of different shapes for the prior distributions of parameters (see plot). As part of this internship, I attended my first conference at the Rubin Project and Community Workshop in Tucson, where I presented my first poster.

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