The Astrophysics and Cosmology group is headed by Prof. Changbom Park, Research Profs. Juhan Kim and Ho Seong Hwang, and KIAS scholar Prof. Christophe Pichon. There are currently eight research fellows: Drs. Stephen Appleby, Owain Snaith, Motonari Tonegawa, Hyunsung Jun, Yi Zheng, Jaehyun Lee, Sungryong Hong, and Christoph Saulder.

Prof. Park is carrying out the Korea Dark Energy Survey (KDES) project, which aims to uncover the nature of the dark energy component of the universe. Prof. Park is leading the Korean Scientist Group (KSG) participating in the Sloan Digital Sky Survey (SDSS) IV that started in July 2014. SDSS IV consists of three survey programs: APOGEE-2, MaNGA, and eBOSS. He is also a member of the Dark Energy Spectroscopic Instrument (DESI) consortium, which plans to construct a new wide- field multi-object spectrograph to carry out dark energy-related survey sciences. Individual members of the astrophysics group have been active in their research in 2017.

Prof. Pichon built estimators for dark energy and primordial non-Gaussianity using count-in-cells in the large deviation limits and validated them on the Horizon Run 4 simulation. He is leading a group of astronomers, the Spine collaboration, which quantifies the impact of the cosmic web on galactic assembly and morphology in simulations and data (COSMOS, VIPERS, GAMA). He estimated in particular the effect of intrinsic alignments of galaxies on weak lensing, the role of AGN feedback on cusps, and the importance of mergers in driving stellar mass growth. He also investigated the secular evolution of galactic discs and nuclear clusters near supermassive black holes.

Research Prof. Juhan Kim is making the next- generation GOTPM code, which adopts a new domain decomposition scheme and a new hierarchical network topology for large cosmological simulations. He is also simulating non-standard cosmological models, including clustering Quintessence, CPL dark energy, and non-general relativity, aiming to study the model- dependent Large-Scale Structure clustering. Along with Young Jun Park, a research assistant from Kyung Hee University, he is studying machine- learning applications to the galaxy formation model in the cosmic density field.

Research Prof. Ho Seong Hwang has worked on galaxy redshift surveys for cosmology and galaxy evolution studies. He studied the evolution of cosmic voids identified from observations (HectoMAP) and simulations (Horizon Run 4), and found that the physical properties of voids are consistent with the predictions of the standard Lambda cold dark matter model. He also studied the galaxy properties in a wide range of environments, including galaxy clusters and galactic satellite systems, to examine environmental effects on galaxy evolution. He worked on the analysis of spectral energy distributions of nearby star-forming galaxies using JCMT JINGLE survey data.

Dr. Hyunmi Song has now moved to KASI. Together with Dr. Sungwook Hong and Prof. Park, Dr. Song has continued to work on planting quasars in dark matter halos of a cosmological N-body simulation, Horizon Run 4. Dr. Hyunmi Song, Prof. Ho Seong Hwang, and Prof. Park have on the spatial and kinematic distributions of the Abell 2199 cluster galaxies and rotational motion of Abell 2107 using MMT/Hectospec redshift data.

Dr. Raphael Gobat has focused on quiescent galaxies at z~2. He has accomplished the first spectroscopic detection of stellar population gradients in high-redshift passive galaxies and has estimated the amount of residual gas in these galaxies. Both results point to inside-out gravitational quenching as the main mechanism for the cessation of star formation in massive galaxies. He left KIAS at the end of the year for a tenure-track position at the Pontifical Catholic University of Valparaiso in Chile.

Dr. Stephen Appleby has written codes to extract the Minkowski functionals and tensors from two- and three-dimensional density fields, and is currently applying them to the latest galaxy catalogs with the intention of constraining the expansion history of the universe and exploring the nature of dark energy. Along with Prof. Eric Linder (LBNL), he is also studying a class of scalar-tensor dark energy models which attempt to resolve the cosmological constant problem by breaking the coordinate invariance of de Sitter vacuum states.

Dr. Jihye Shin moved to KASI in November. She has studied the effects of large-scale density fluctuation modes on cosmological hydrodynamic simulations. She has been also preparing a large-volume high- resolution hydrodynamic simulation, Horizon Run 5, with Prof. Park, Prof. Kim, and Dr. Snaith. She published two papers on hydrodynamic simulations of the Galactic center, and on the probability distribution function of the cosmic density field.

Dr. Owain Snaith, along with Prof. Kim and Prof. Park, has worked on the effects of initial conditions and spatial resolution in cosmological simulations with RAMSES. He has published a paper on the outskirts of dark matter halos, and the link between the splashback and virial radii. He is studying the impact of changing the friends-of-friends linking length on galaxy group catalogues. He is working on the Horizon Run 5 simulation, along with Dr. J. Shin (now as KASI), and is focusing on tuning the subgrid physics to match observations.

Dr. Motonari Tonegawa is analyzing the Horizon Run 4 galaxy simulation data, focusing on the pairwise velocity probability distribution function as a function of pair separation, angle, and galaxy mass, to construct a model which describes the effects of redshift space distortion accurately within the framework of the streaming model. He has also measured intrinsic alignments of blue star-forming galaxies at the highest redshift so far (z~1.4), using the spectroscopic catalog of the FastSound survey.

Dr Hyunsung Jun joined the astrophysics group in June 2017. He is working on understanding the physics of active galactic nuclei (AGN) in terms of black hole growth, obscuration, and galaxy-black hole coevolution. He is also working on how AGN affects the host galaxy through ionized outflows, and has initiated long-term monitoring programs to utilize AGN variability in estimating the size of the unresolved substructures.

Dr. Yi Zheng, together with Prof. Park and Dr. Tonegawa, is working on modeling the small-scale galaxy redshift space correlation function with streaming model strategy. Meanwhile, he is working on improving the redshift space distortion (RSD) model of the galaxy power spectrum, through the TNS model strategy and velocity decomposition strategy. He has also helped his collaborators to conduct an accurate measurement of halo velocity bias in simulations, fulfilling a loophole in precision cosmology.

Dr. Jaehyun Lee joined the astrophysics group in August 2017. He has investigated the assembly history of galaxies in various environments using a semi-analytic model of galaxy formation and evolution. He has also looked into directional changes of galaxy spin vectors in dense environments using cosmological zoomed hydrodynamic simulations. He is currently working with Prof. Park and Prof. Kim on galaxy assignment in cosmological simulations with varying cosmological parameters by utilizing semi-analytic approaches.

Dr. Sungryong Hong joined the astrophysics group in October 2017. He is working with Prof. Park on investigating topological structures of the large- scale structures in the universe. Along with Dr. Ho Seong Hwang, he works to measure graph statistics from the Sloan Digital Sky Survey (SDSS) data and compare them with theoretical predictions from the KIAS mock galaxy catalogs.

Dr. Christoph Saulder arrived at KIAS in November. His primary focus of research is studying the global properties of early-type galaxies to improve fundamental plane distance estimates. Using this distance indicator, he wants to obtain peculiar motions from SDSS data to measure the growth factor β and to study large-scale structures by comparing observational data with expectations from the Horizon Run 4.