2019.1 ~ 2019.12

The Astrophysics and Cosmology Group is headed by Prof. Changbom Park and Research Profs. Juhan Kim and Ena Choi. Postdocs in the group include Drs. Stephen Appleby, Motonari Tonegawa, Hyunsung Jun, Jaehyun Lee, Yonghwi Kim, Junsup Shim, and Yongmin Yoon.

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. He is leading the Korean Scientist Group (KSG), which is participating in the Sloan Digital Sky Survey (SDSS) IV, whose survey programs are APOGEE-2, MaNGA, and eBOSS. He is also a member of the Dark Energy Spectroscopic Instrument (DESI) consortium, which plans to carry out a deeper redshift survey starting from 2020. He worked as the president of the Korean Astronomical Society from Jan. 2018 to Dec. 2019.

Individual members of the Astrophysics and Cosmology Group were active in their research in 2018. Research prof. Juhan Kim has been leading the simulation group to make a large cosmological hydrodynamic simulation called Horizon Run 5 (HR5) with Drs. Jihye Shin (KASI), Yonghwi Kim, and Jaehyun Lee. The HR5 project is being carried out by an international consortium (KIAS, KASI, KISTI, IAP, and the University of Hull) to study galaxy formation on the cosmological scale with the Ramses simulation code. Various astrophysical prescriptions on the formation of stars and super massive black holes (SMBHs), and on their feedback to the ambient medium are implemented in the simulation code. The output data are being analyzed for the study of galaxy formation and cosmology.

QUC research Prof. Ena Choi joined the group in September 2019. She has studied the AGN (active galactic nuclei)-host galaxy connection, using hydrodynamic simulations. She created mock images of simulated galaxies by post-processing the simulation with a radiative transfer code in order to fully study the predicted morphologies and stellar populations of both AGN hosts and normal galaxies with similar stellar masses. She has also worked as a group leader in the IQ (Isolated & Quenched) Collaboration, which aims to bridge the gap between simulations and observations of star-forming and quiescent galaxies to better characterize internal star formation quenching processes.

KIAS Scholar Prof. Christophe Pichon has worked on the connection between the 3D geometry and dynamics of the neighboring cosmic web and the properties of embedded galaxies impacting their morphology. He studies in particular how the evolution of the Hubble sequence is driven in part by the geometry of the cosmic web. To pin down cosmological parameters, he also uses the cosmic web, which is an ideal probe for understanding the nature of dark energy and obtaining an accurate description of the non-Gaussian statistical properties of cosmic fields that are either primordial or brought about by gravitational evolution. Finally, he makes use of novel extended kinetic theories capturing the secular evolution of galaxies on three different scales, from halos to disks to nuclear regions. Eventually he aims to connect this line of work to address key questions, such as the epoch of disc settling, radial migration, and deeper understanding of extreme-TDE and mass-ratio inspirals as sources for GW detectors.

Dr. Stephen Appleby has been extracting topological information from the large-scale distribution of galaxies, to place constraints on the growth and expansion histories of the Universe. He measured the genus from the SDSS spectroscopic galaxy survey data, as well as the higher redshift BOSS data, to constrain the matter density parameter and the equation of state of dark energy. He also constrained the shape of the matter power spectrum, finding consistency with early Universe measurements of the Cosmic Microwave Background. In parallel, he has been studying the Minkowski tensors of random fields, and was able to predict the ensemble expectation value of these statistics in the case of a field with an anisotropic signal due to redshift space distortion.

Dr. Motonari Tonegawa investigated the small-scale redshift space distortion effect of galaxy clustering using the Multiverse Simulation. By comparing the clustering signals from the simulations and those from the KIAS-VAGC spectroscopic galaxy sample, he constrained the matter density of the universe, the merger timescale (which controls the abundance of galaxies), and the galaxy velocity bias. He also found that the strength of the finger-of-god effect can be defined and this was useful for breaking the degeneracy of parameters as well as for performing a consistency check.

Dr Hyunsung Jun studied the feeding of supermassive black holes and the feedback from AGN. Using luminous, obscured AGN, he found that the accretion rates are close to the highest limit and that the feedback is effective through spatially-extended ionized outflows rather than radiation pressure to the immediate surroundings. He has continued monitoring observations using variability to estimate the size of the unresolved AGN structures.

Dr. Jaehyun Lee has been working on the cosmological hydrodynamical zoom-in simulation, HR5, using the RAMSES code. He updated the code, calibrated the physical parameters that govern galaxy growth in HR5, and developed pipelines for processing output data. The simulation was finished and he is investigating the formation of proto-clusters and galaxy evolution in various environments.

Dr. Yonghwi Kim has been working on generating galaxy catalogues and visualizing three-dimensional data in HR5 simulations and on investigating the correlations between star formation rate and local density in these galaxy catalogues. He has studied the secular evolution of gas and stars in disk galaxies using magnetohydrodynamic simulations. He has also investigated the physical properties of spiral galaxies in both observational and numerical samples based on spiral density wave theory.

Dr. Yongmin Yoon joined the group in September 2019. He has studied the impact on galaxy properties of violent changes in large-scale environments, such as cluster-cluster interactions. He has also been working on finding sub-populations of early-type galaxies that have smaller scatters in the Fundamental Plane.

Dr. Junsup Shim has been working on the identification of cosmic voids. By analyzing halo-antihalo pairs in a pair of cosmological simulations whose initial density fluctuations are sign inverted, it is possible to assign physical meanings to the cosmic voids identified by density and volume fraction thresholds. He has also been studying the clustering properties of the various critical points of the density field by measuring their two-point statistics with collaborators at IAP.