Donghyeon "Jeff" Khim

"Disco Ball: A Closer Look at an Unusual Ultra-Diffuse Galaxy"

We studied an unusual ultra-diffuse galaxy (UDG), nicknamed the Disco Ball, using Keck/KCWI spectroscopy to investigate its internal kinematics and dark matter distribution. Although the galaxy has largely stopped forming stars, we identified compact star-forming regions with strong emission lines, along with a population of globular clusters and a nuclear star cluster. We used these emission-line sources to measure galaxy rotation and found a rotational velocity of 39 ± 9 km/s at half the effective radius. The galaxy also shows stellar rotation consistent with the motions of these sources. We estimated its halo mass using globular cluster counts and compared it with dynamical masses derived from the internal kinematics. These measurements suggest that the Disco Ball has a substantial dark matter component despite its diffuse stellar distribution. Our results demonstrate that compact emission-line sources can be used to measure the internal kinematics of UDGs efficiently, even when their diffuse stellar light is too faint for conventional spectroscopy.

"Properties of Nuclear Star Clusters in Low Surface Brightness Galaxies"

Using the Systematically Measuring Ultra-Diffuse Galaxies and Sloan Digital Sky Survey catalogs and our own reprocessing of the Legacy Survey imaging, we investigate the properties of nuclear star clusters (NSCs) in galaxies having central surface brightnesses as low as 27 mag arcsec‑2. We find that galaxies with low central surface brightness are more likely to contain an NSC if they (1) have a higher stellar mass, (2) have a higher stellar-to-total mass ratio, (3) have a brighter central surface brightness, (4) have a larger axis ratio, or (5) lie in a denser environment.

Image copyright: DESI Legacy Imaging Surveys

"Star-Gas Misalignment in Galaxies I & II"

Recent integral field spectroscopy observations have found that about 11% of galaxies show star-gas misalignment. We used the Horizon-AGN cosmological simulation to study the properties and origins of star-gas kinematic misalignment in galaxies and compared our results with observations from the SAMI Galaxy Survey. We found that Horizon-AGN reproduces the observed distribution of misalignment angles and their relationships with galaxy morphology and gas fraction. By tracing the evolution of galaxies, we identified four main causes of misalignment: mergers (35%), interactions with nearby galaxies (23%), environmental interactions (21%), and secular evolution, including gas accretion (21%). We also found that misalignment is mainly caused by changes in the rotational axis of gas rather than stars. We then examined how long misalignment persists and found that its lifetime depends on galaxy kinematics, gas fraction, environment, and formation history. These findings helped us understand how the internal properties of galaxies and their interactions with the surrounding environment influence their kinematic evolution.

"Systematically Measuring Ultra-diffuse Galaxies (SMUDGes)"

We constructed the complete Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) catalog using optical images from the DESI Legacy Imaging Surveys, identifying 7,070 ultra-diffuse galaxy (UDG) candidates over approximately 20,000 square degrees of sky. We estimated distances for 1,529 galaxies and halo masses for 1,436 galaxies. We found that UDGs typically reside in halos with masses between 10¹⁰ and 10¹¹·⁵ solar masses and have 1.5-7 times fewer stars than other galaxies with similar halo masses. This stellar deficiency increases with galaxy size and halo mass but shows no clear dependence on large-scale environment. In subsequent studies, we visually classified the galaxies and identified mergers, ring galaxies, and tidally disturbed systems, leaving 6,553 viable undisturbed UDG candidates. We also combined optical, UV, and IR photometry for 966 galaxies to investigate their recent star formation histories. We found that star formation episodes typically last less than 1 Gyr in galaxies that are not quenched. These studies provide a large sample for investigating the formation, structure, and evolution of UDGs.

Image copyright: DESI Legacy Survey Viewer

"The MAGPI Survey - science goals, design, observing strategy and early results"

We present an overview of the Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey, a Large Program on the European Southern Observatory Very Large Telescope. MAGPI is designed to study the physical drivers of galaxy transformation at a lookback time of 3-4 Gyr, during which the dynamical, morphological, and chemical properties of galaxies are predicted to evolve significantly.

Image copyright: MAGPI, Foster et al. 2020

"Finding jellyfish galaxies in the Horizon-AGN simulation based on the Convolutional Neural Networks (CNN) deep-learning technique"

Collaborators: Kim HyeongHan, Hoseung Choi, Sangho Choi, Dasol Yoo, Sukyoung Yi

Image copyright: NASA

"Variable Stars in the regions of BX-Dra, M34 and NGC 6871"

Performed a multiple-frequency analysis and found new variable stars with a time-series BV CCD photometry. We used 155mm optical telescope at Bohyunsan Optical Astronomy Observatory (BOAO). We found 5 new variable stars in the region of BX-Dra, 25 variable stars in M34, and 54 variable stars in NGC 6871.