The Astrophysics and Cosmology Group is headed by Prof. Changbom Park and Research Prof. Juhan Kim and QUC Fellow Dr. Ena Choi. Postdocs in the group include Drs. Hyunsung Jun, Jaehyun Lee, Yonghwi Kim, Junsup Shim, Yongmin Yoon, Jaehong Park, Fuyu Dong, Anki Singh, Celine Gouin.

Prof. Park is carrying out the Korea Dark Energy Survey (KDES) program, which aims to uncover the nature of the dark energy component of the universe. As a member of Sloan Digital Sky Survey (SDSS) IV, he is analyzing the MaNGA and eBOSS survey data. He is also a member of the Dark Energy Spectroscopic In­strument (DESI) consortium, which is carrying out a deeper redshift survey. He is leading the Horizon Run 5 (HR5) collaboration. HR5 is a 1 Gpc-volume high resolution (l pkpc) gravity/hydrodynamics simulation made by an international consortium of Korea, France, and U.K. He has also created a collaboration called Lyman Al­pha Simulation and Observation Research (LASOR) with Purdue, Korea Astronomy and Space Science Institute (KASI), and Rutgers to observe the Lyman-a objects at high redshifts (2

Individual members of the Astrophysics and Cosmology Group were active in their research in 2020. Research prof. Jearch Prof. Juhan Kim has worked on the galaxy stellar mass functions using the HR5 data to bridge the gap between the cold dark matter model predictions and the extragalactic observations. He is now developing the next-generation cosmological simulation code im­plementing gravity and hydrodynamics. This code will be used for the cosmological and galaxy formation studies, and will be run on the next generation Korea Supercomputer, which will be installed and operated in 2023.

QUC Fellow Dr. Ena Choi investigated the formation and chemi­cal evolution history of the outer halo of giant elliptical galaxies by analyzing high-resolution hydrodynamic simulations. She com­pared the stellar metallicity distribution functions (MDF) of simulat­ed galaxies to the MDF of NGC 5128, the central giant elliptical in the Centaurus group. Even though the simulated galaxies have different merging histories and age distributions, all predicted MDFs are in good qualitative agreement with the observed ones. By tracking stellar particles in the simulations, she investigated the origin of the stars ending up in the outer halo field of simulated galaxies and found that most have an 'accreted' origin, formed in other small galaxies and later accreted in mergers.

Dr. Hyunsung Jun investigated the reason for finding a massive z= 1 .47 quasar at the outskirt of a galaxy protocluster, awaiting spectroscopic confirmation and a followup study from a larger survey data. He has been collecting optical-lR light curves of inter­esting active galaxies to uncover their unresolved geometry and to understand unusually varying light curves. Based on extensive spectrophotometric variability monitoring, he finds a meaningful portion of a sample of highly IR-variable active galaxies to show varying galaxy classifications over timescales of years, to be inter­preted comprehensively.

Dr. Yonghwi Kim has been working on a post-process of HR5+ which is the spin-off simulation of HR5, and generating halo and galaxy catalogs from these data as a first step for research using these cosmological simulations. He is investigating galactic evolu­tion in the Hubble sequence and its environmental effects using lo­cal and global cosmological simulations. Recently, he has started to prepare the next generation of cosmological hydrodynamic sim­ulations collaborating with researchers inside and outside of KIAS.

Dr. Jaehyun Lee has completed generating the galaxy and halo catalogues and the structure merger trees of HR5. He is now supporting the HR5 collaborators conducting a variety of studies using the catalogues as well as investigating the formation and evolution of proto-clusters in the early universe based on HR5. He has also examined the origin of molecular clouds and young stars found in the tails of ram pressure-stripped galaxies in cluster envi­ronments using radiation hydrodynamical simulations with a very high spatial resolution. This study shows that the gas content in a galactic disk is a primary parameter governing the tail properties of ram pressure-stripped galaxies.

Dr. Junsup Shim developed a new concept of defining cosmic voids as cluster-counterpart structures and showed that these clus­ter-counterpart voids can be successfully identified from galaxy density fields without using the initial information of the density field. He also has been working on developing a method to con­strain the nature of dark energy by utilizing the clustering of critical points of density fields as a standard ruler.

Dr. Yongmin Yoon analyzed the MaNGA integral field spectre­scopic survey data to study rotation curves of galaxies and their dependence on morphology and stellar mass, and the impact of galaxy mergers on stellar kinematics of early-type galaxies. He also investigated the warped nature of the fundamental plane of early-type galaxies and its implications for evolution histories of early-type galaxies.

Dr. Jaehong Park has been working on modelling Lyman-alpha emitters (LAEs) in HR5. This modelling is one of the main projects in LASOR (Lyman-Alpha Simulation and Observation Research) collaboration to compare the properties of predicted LAEs from simulation with those observed from ODIN survey. To this end, he has computed the rest-frame UV luminosity functions and Lyman-al­pha luminosity functions. Also, he finished a project to develop a methodology to conserve ionizing photons during the epoch of reionization for semi-numerical simulations based on the excur­sion-set approach.

Dr. Fuyu Dong has been worked on the Alock-Paczynski test using SDSS galaxy catalogues. After considering the non-linear evolu­tion effect, she is now able to make cosmological constraints using the shape of two-point correlation function. She has also studied the CMB lensing effect using DESI galaxy catalogues and got nice results.

Dr. Ankit Singh is currently working on the impact of the large­scale environment on active galactic nuclie (AGN). He is also studying the effect of AGN on star formation rate estimation in infrared wavelength using Monte-Carlo radiative transfer for gal­axies in Horizon-Run 5 simulation. He has also been working on how the large-scale filament environment affects the properties of the galaxies residing in it at high redshift.

Dr. Gouin Celine joined the group in November 2021. She is currently working on the impact of large-scale cosmic web envi­ronment on the properties and evolution of galaxy clusters in HR5. She is also probing the comic web connectivity of galaxy clusters with the HeCSomnibus sample, built from MMT/Hectospec obser­vations and SDSS data.