High-resolution galaxy formation simulations provide a unique opportunity to test our understandings of the interactions between galactic ingredients. At the same time, however, it is equally imperative to verify that astrophysical assumptions are accountable for any success in galaxy simulations, not artifacts of particular numerical implementations. While numerical experiments have become one of the most powerful tools in formulating theories of galaxy formation, it is this requirement of reproducibility that precludes theorists from drawing a definitive conclusion based on a single simulation technique.
The numerical community’s collective response to such a challenge is the AGORA High-resolution Galaxy Simulations Comparison Project that promotes a multi-platform approach to outstanding problems in galaxy formation. I have spearheaded an inter-institutional effort (more than 160 participants from over 60 institutions worldwide as of 2021) along with J. Primack and P. Madau to establish the project infrastructures. They include common cosmological initial conditions (i.e., 4 halo masses and 2 assembly histories by MUSIC), common isolated initial conditions (e.g., generated by MakeDisk), common astrophysics models (e.g., cooling and UV background by GRACKLE package), and common analysis platform (i.e., yt toolkit), all publicly available to the community. Using the components assembled, the ongoing collaborative efforts aim to raise the realism of numerical experiments collectively by comparing galaxy simulations across platforms, resulting in multiple publications in the next years.
●  Roca-Fabrega, S., Kim, J. -H. (corr. author), Hausammann, L., Nagamine, K., & 19 other co-authors for the AGORA Collaboration, “The AGORA High-resolution Galaxy Simulations Comparison Project. III: Cosmological Zoom-in Simulation of A Milky Way-mass Halo”,   ApJ submitted (2021)   [Project Website]
●  Shin, E. -J., Kim, J. -H. (corr. author), & Oh, B. K., “How Metals Are Transported In And Out Of A Galactic Disk: Dependence On The Hydrodynamic Schemes In Numerical Simulations”,   ApJ in press (2021)   [Local/high-resolution]
●  Roca-Fabrega, S., Kim, J. -H., Primack, J. R., & 11 other co-authors for the AGORA Collaboration, “The AGORA High-resolution Galaxy Simulations Comparison Project: Public Data Release”,   [astro-ph:2001.04354]   [Local/high-resolution]   [Project Website]   [Collection of Useful Links]
●  Kim, J. -H., Agertz, O., Teyssier, R., Butler, M. J., Ceverino, D., & 38 other co-authors for the AGORA Collaboration, “The AGORA High-resolution Galaxy Simulations Comparison Project. II: Isolated Disk Test”,   ApJ 833 (2016) 202   [astro-ph:1610.03066]   [Local/high-resolution]   [Project Website]
●  Kim, J. -H., Abel, T., Agertz, O., Bryan, G. L., Ceverino, D., & 41 other co-authors for the AGORA Collaboration, “The AGORA High-resolution Galaxy Simulations Comparison Project”,   ApJS 210 (2014) 14   [astro-ph:1308.2669]   [Local/high-resolution]   [Project Website]   [1-slide Introduction]   [UC-HiPACC AstroShort Article]   [UCSC Press Release] (EurekAlert)   [UC-HiPACC Press Release] (EurekAlert)
As computational resolution of modern cosmological simulations reach ever so close to resolving individual star-forming clumps in a galaxy, a need for "resolution-appropriate" physics for a galaxy-scale simulation has never been greater. To this end, we introduce a self-consistent numerical framework that includes explicit treatments of feedback from star-forming molecular clouds (SFMCs) and massive black holes (MBHs). In addition to the thermal supernovae feedback from SFMC particles, photoionizing radiation from both SFMCs and MBHs is tracked through full 3-dimensional ray tracing. A mechanical feedback channel from MBHs is also considered. Using our framework, we perform a state-of-the-art cosmological simulation of a quasar-host galaxy at z~7.5 for ~25 Myrs with all relevant galactic components such as dark matter, gas, SFMCs, and an embedded MBH seed of ~> 1e6 Ms. We find that feedback from SFMCs and an accreting MBH suppresses runaway star formation locally in the galactic core region. Newly included radiation feedback from SFMCs, combined with feedback from the MBH, helps the MBH grow faster by retaining gas that eventually accretes on to the MBH. Our experiment demonstrates that previously undiscussed types of interplay between gas, SFMCs, and a MBH may hold important clues about the growth and feedback of quasars and their host galaxies in the high-redshift Universe.
●  Kim, J. -H., Wise, J. H., Abel, T., Jo, Y., Primack, J. R., & Hopkins, P. F., “High-redshift Galaxy Formation with Self-consistently Modeled Stars and Massive Black Holes: Stellar Feedback and Quasar Growth”,   ApJ 887 (2019) 120   [astro-ph:1910.12888]   [Local/high-resolution]
●  Bryan, G. L. et al. including Kim, J. -H. for the ENZO Collaboration, “Enzo: An Adaptive Mesh Refinement Code for Astrophysics (Version 2.6)”,   JOSS 4(42) (2019) 1636
●  Kaehler, R., Abel, T., & Kim, J. -H., “Visualization of a High-resolution Galaxy Formation Simulation”,   SuperComputing 2011 Scientific Visualization Companion Proceedings pp.133 (2011), Seattle, WA, November 2011   [Local]   [Visualization by Ralf Kaehler]
●  Kim, J. -H., Wise, J. H., Alvarez, M. A., & Abel, T., “Galaxy Formation with Self-consistently Modeled Stars and Massive Black Holes. I: Feedback-regulated Star Formation and Black Hole Growth”,   ApJ 738 (2011) 54   [astro-ph:1106.4007]   [Local/high-resolution]   [Selected Plots and Movies]   [Astrobites Article]   [Visualization by Ralf Kaehler]
●  Kim, J. -H., “Galaxy Formation and Mergers with Stars and Massive Black Holes”,   Ph.D. Dissertation, Stanford Univ., May 2011   [Stanford Library]   [Google Books]   [Local]
The recent discovery of diffuse dwarf galaxies that are deficient in dark matter appears to challenge the current paradigm of structure formation in our Universe. We describe the numerical experiments to determine if the so-called dark matter deficient galaxies (DMDGs) could be produced when two gas-rich, dwarf-sized galaxies collide with a high relative velocity of ~300 km/s. Using idealized high-resolution simulations with both mesh-based and particle-based gravito-hydrodynamics codes, we find that DMDGs can form as high-velocity galaxy collisions separate dark matter from the warm disk gas which subsequently is compressed by shock and tidal interaction to form stars. Then using a large simulated universe IllustrisTNG, we discover a number of high-velocity galaxy collision events in which DMDGs are expected to form. However, we did not find evidence that these types of collisions actually produced DMDGs in the IllustrisTNG100-1 run. We argue that the resolution of the numerical experiment is critical to realize the "collision-induced" DMDG formation scenario. Our results demonstrate one of many routes in which galaxies could form with unconventional dark matter fractions.
●  Shin, E. -J., Jung, M., Kwon, G., Kim, J. -H. (corr. author), Lee, J., Jo, Y., Oh, B. K., “Dark Matter Deficient Galaxies Produced Via High-velocity Galaxy Collisions In High-resolution Numerical Simulations”,   ApJ 899 (2020) 25   [astro-ph:2007.09889]   [Local/high-resolution]   [More info at Eun-jin Shin's website]
●  Kim, J. -H., Wise, J. H., & Abel, T., “Galaxy Mergers with Adaptive Mesh Refinement: Star Formation and Hot Gas Outflow”,   ApJL 694 (2009) L123   [astro-ph:0902.3001]   [Local/high-resolution]   [Selected Plots and Movies]   [KIPAC Computing Article]   [Visualization by Ralf Kaehler]
●  Kim, J. -H., Wise, J. H., & Abel, T., “Galaxy Evolution on Adaptive Mesh Refinement”,   AIP Conference Proceedings Vol. 990 (2008) 429, First Stars III Conference, Santa Fe, NM, July 2007   [Local/high-resolution]   [Selected Plots and Movies]
Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, I have investigated the formation of star clusters and examine one of the formation hypothesis of present-day metal-poor globular clusters. We found that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ~>1e5−6 Msun collectively to high density, at which point it rapidly turns into stars before stellar feedback can stop star formation. The high dynamic range of the FIRE simulation is critical in realizing such dense star-forming clouds with a small dynamical timescale less than 3 Myr, shorter than most stellar feedback timescales. The simulation then allows us to trace how clusters could become virialized and tightly-bound to survive for up to ~420 Myr till the end of the simulation. Because the cluster’s tightly-bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.
●  Ma, X. et al. including Kim, J. -H., “Self-consistent Proto-Globular Cluster Formation in Cosmological Simulations of High-redshift Galaxies”,   MNRAS 493 (2020) 4315   [astro-ph:1906.11261]
●  Kim, J. -H., Ma, X., Grudic, M. Y., Hopkins, P. F., Hayward, C. C., & 5 other co-authors for the FIRE Collaboration, “Formation of Globular Cluster Candidates in Merging Proto-galaxies at High Redshift: A View from the FIRE Cosmological Simulations”,   MNRAS 474 (2018) 4232   [astro-ph:1704.02988]   [Local/high-resolution]
●  Wetzel, A., Hopkins, P. F., Kim, J. -H., Faucher-Giguere, C-A., Keres, D., & Quataert, E., “Reconciling Dwarf Galaxies with LCDM Cosmology: Simulating A Realistic Population of Satellites Around A Milky Way-Mass Galaxies”,   ApJL 827 (2016) L23   [astro-ph:1602.05957]   [Caltech News Article]
We have developed a pipeline to estimate baryonic properties of a galaxy inside a dark matter (DM) halo in DM-only simulations using a machine trained on high-resolution hydrodynamic simulations. As an example, we use the IllustrisTNG hydrodynamic simulation of a (75 Mpc/h)^3 volume to train our machine to predict e.g., stellar mass and star formation rate in a galaxy-sized halo based purely on its DM content. An extremely randomized tree (ERT) algorithm is used together with multiple novel improvements we introduce here such as a refined error function in machine training and two-stage learning. Aided by these improvements, our model demonstrates a significantly increased accuracy in predicting baryonic properties compared to prior attempts -- in other words, the machine better mimics IllustrisTNG's galaxy-halo correlation. By applying our machine to the MultiDark-Planck DM-only simulation of a large (1 Gpc/h)^3 volume, we then validate the pipeline that rapidly generates a galaxy catalogue from a DM halo catalogue using the correlations the machine found in IllustrisTNG. We also compare our galaxy catalogue with the ones produced by popular semi-analytic models (SAMs). Our so-called machine-assisted semi-simulation model (MSSM) is shown to be largely compatible with SAMs, and may become a promising method to transplant the baryon physics of galaxy-scale hydrodynamic calculations onto a larger-volume DM-only run.
●  Jo, Y., & Kim, J. -H. (corr. author), “Machine-assisted Semi-Simulation Model (MSSM): Estimating Galactic Baryonic Properties from Their Dark Matter Using A Machine Trained on Hydrodynamic Simulations”,   MNRAS 489 (2019) 3565   [astro-ph:1908.09844]   [Local/high-resolution]   [More info at Yongseok Jo's website]
I have developed a comprehensive description of stellar feedback in a galaxy simulation by combining the ultraviolet (UV) radiation from star clusters and the thermal energy of supernova explosions. In this method, which we call star-forming molecular cloud (SFMC) particles, we use a ray-tracing technique to follow the ultraviolet photons emitted by thousands of distinct particles on the fly. I have applied the realistic description of stellar feedback to a dwarf-sized galactic halo of 2.3e11 Msun with high spatial resolution. By tracing UV photons from thousands of SFMC particles, this feedback scheme has enabled us to study the escape of ionizing photons from an individual SFMC particle and from a galaxy, and to examine the evolving environment of star-forming clouds. I have discovered that the overall UV escape fraction is dominated by a small number of SFMCs with high escape fractions. In addition, the escape fraction from a SFMC particle on average rises from 0.27% at its birth to 2.1% at the end of its lifetime, 6 Myrs. This is because SFMCs drift away from the dense clumps in which they were born, and because the gas around the them is dispersed by stellar feedback.
Because we self-consistently locate the ionized gas, we can make mock observations of Hα emission, a star formation rate (SFR) tracer. With this, I have made a direct comparison between simulated and observed galaxies, and studied how stellar feedback manifests itself in the spatially-resolved star formation relation. I find that the correlation between SFR density (by mock Hα) and H2 density shows large scatter, especially at high resolutions of 75 pc that are comparable to the size of giant molecular clouds (GMCs). This is because an aperture of GMC size captures only particular stages of GMC evolution, and because Hα traces hot gas around star-forming regions, displaced from the H2 peaks themselves.
●  Butsky, I., Zrake, J., Kim, J. -H., Yang, H. -I., & Abel, T., “Ab Initio Simulations of A Supernova Driven Galactic Dynamo in An Isolated Disk Galaxy”,   ApJ 843 (2017) 113   [astro-ph:1610.08528]   [Local/high-resolution]
●  Pineda, J. L. et al. including Kim, J. -H., “Bridging the Gap: Observations and Theory of Star Formation Meet on Large and Small Scales - Study Report”,   Keck Institute for Space Studies Report, Pasadena, CA, November 2014   [Caltech Library]   [Local]
●  Kim, J. -H., & Lee, J., “How Does the Surface Density and Size of Disk Galaxies Measured in Hydrodynamic Simulations Correlate with the Halo Spin Parameter?”,   MNRAS 432 (2013) 1701   [astro-ph:1210.8321]   [Local/high-resolution]
●  Kim, J. -H., Krumholz, M. R., Wise, J. H., Turk, M. J., Goldbaum, N. J., & Abel, T., “Dwarf Galaxies with Ionizing Radiation Feedback. II: Spatially-resolved Star Formation Relation”,   ApJ 779 (2013) 8   [astro-ph:1210.6988]   [Local/high-resolution]
●  Kim, J. -H., Krumholz, M. R., Wise, J. H., Turk, M. J., Goldbaum, N. J., & Abel, T., “Dwarf Galaxies with Ionizing Radiation Feedback. I: Escape of Ionizing Photons”,   ApJ 775 (2013) 109   [astro-ph:1210.3361]   [Local/high-resolution]   [KIPAC Highlights Article]