Research

Ji-hoon Kim

ASTROPHYSICIST at University of California Santa Cruz

$Ongoing Research Topics Dwarf galaxies with ionizing radiation feedback: escape of ionizing photons & spatially-resolved star formation relation Collaborators: Mark R. Krumholz, John H. Wise, Matthew J. Turk, Nathan J. Goldbaum, Tom Abel We describe a new method for simulating ionizing radiation and supernova feedback in analogues of low-redshift galactic disks. In this method, which we call star-forming molecular cloud (SFMC) particles, we use a ray-tracing technique to solve the radiative transfer equation for ultraviolet photons emitted by thousands of distinct particles on the fly. Joined with high numerical resolution of 3.8 pc, the realistic description of stellar feedback helps to self-regulate star formation. By simulating a galactic disk in a halo of 2.3×1011 Msun, we find that average escape fraction from all radiating sources considered fluctuates between 0.08% and 5.9% during a ∼20 Myr period with a mean value of 1.1%. The flux of escaped photons is not strongly beamed, but manifests a large opening angle of more than 60◦ from the galactic pole. Further, we investigate the escape fraction per SFMC particle, fesc(i), and how it evolves as the particle ages. We discover that the average escape fraction fesc is dominated by a small number of SFMC particles with high fesc(i). On average, the escape fraction from a SFMC particle rises from 0.27% at its birth to 2.1% at the end of a particle lifetime, 6 Myrs. This is because SFMC particles drift away from the dense gas clumps in which they were born, and because the gas around the star-forming clumps is dispersed by ionizing radiation and supernova feedback. In addition, because we have self-consistently calculated the location of ionized gas, we for the first time are able to make spatially-resolved mock observations of star formation tracers, such as Hα emission. We can also observe how stellar feedback manifests itself in the correlation between ionized and molecular gas. We find that the correlation between SFR density (estimated from mock Hα emission) 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. -- 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 submitted (2012), astro-ph:1210.6988 [arXiv] [NASA-ADS] [Local/high-resolution] - New!! -- 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 submitted (2012), astro-ph:1210.3361 [arXiv] [NASA-ADS] [Local/high-resolution] [KIPAC Highlights Article] - New!! Galaxy formation with self-consistently modeled stars and Massive black holes: Self-regulated star formation & Bh growth Collaborators: John H. Wise, Marcelo A. Alvarez, Tom Abel There is mounting evidence for the coevolution of galaxies and their embedded massive black holes (MBHs) in a hierarchical structure formation paradigm. To tackle the nonlinear processes of galaxy - MBH interaction, we describe a self-consistent numerical framework which incorporates both galaxies and MBHs. The high- resolution adaptive mesh refinement (AMR) code Enzo is modified to model the formation and feedback of molecular clouds at their characteristic scale of 15.2 pc and the accretion of gas onto a MBH. Two major channels of MBH feedback, radiative feedback (X-ray photons followed through full 3D adaptive ray tracing) and mechanical feedback (bipolar jets resolved in high-resolution AMR), are employed. We investigate the coevolution of a 9.2×1011 Msun galactic halo and its 105 Msun embedded MBH at redshift 3 in a cosmological ΛCDM simulation. The MBH feedback heats the surrounding ISM up to 106 K through photoionization and Compton heating and locally suppresses star formation in the galactic inner core. The feedback considerably changes the stellar distribution there. This new channel of feedback from a slowly growing MBH is particularly interesting because it is only locally dominant, and does not require the heating of gas globally on the disk. The MBH also self-regulates its growth by keeping the surrounding ISM hot for an extended period of time. These first results demonstrate that our comprehensive numerical framework provides a powerful means in understanding the coevolution of galaxies and MBHs. Two merging disk galaxies of 2×1011 Msun each are also being investigated. -- Kim, J. -H., Wise, J. H, & Abel, T., “Galaxy Formation with Self-consistently Modeled Stars and Massive Black Holes. II: Mergers and Triggered Star Formation”, ApJ to be submitted (2013) -- 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 [Selected Images & Plots - Galaxy Formation with MBH] [arXiv] [NASA-ADS] [Local/high-resolution] [Astrobite Article] [viz by R. Kaehler] - New!! Galaxy MergerS with ADAPtive mesh refinement: Star Formation & Hot Gas OutFlow Collaborators: John H. Wise, Tom Abel In hierarchical structure formation, merging of galaxies is frequent and known to dramatically affect their properties. To comprehend these interactions high-resolution simulations are indispensable because of the nonlinear coupling between pc and Mpc scales. To this end, we present the first AMR simulation of two merging, low mass, initially gas-rich galaxies (M = 1.8×1010 Msun each), including star formation and feedback. With galaxies resolved by ~20 million total computational elements, we achieve unprecedented resolution of the multiphase interstellar medium, finding a widespread starburst in the merging galaxies via shock-induced star formation. The high dynamic range of AMR also allows us to follow the interplay between the galaxies and their embedding medium depicting how galactic outflows and a hot metal-rich halo form. These results demonstrate that AMR provides a powerful tool in understanding interacting galaxies. -- Kim, J. -H., Wise, J. H., & Abel, T., “Galaxy Mergers with Adaptive Mesh Refinement: Star Formation and Hot Gas Outflow”, ApJ 694 (2009) L123 [Selected Movies & Plots - Galaxy Mergers] [arXiv] [NASA-ADS] [Local/high-resolution] [KIPAC Computing Article] [Enlarged Picture] ISOLATED Galaxy Formation: supernova feedback & star formation relation Collaborators: John H. Wise, Tom Abel Galaxy formation is one of the most intriguing topics in astrophysics as it needs correct understandings of the large scale structure evolution and the small scale microphysics such as supernova explosion. Traditional galaxy formation simulations have been carried out by Smoothed Particle Hydrodynamics (SPH) techniques, but with ever-improving AMR technique we explore this amazing phenomenon down to 3.8 pc scale allowing ourselves to compare our simulated results with observed star forming galaxies at various redshifts. We employ Eulerian AMR code Enzo to incorporate all physics previously discussed to study galactic evolution, such as multiphase gas dynamics, radiative cooling, star formation, and Type II supernovae feedback, and follow the formation history of a dwarf-sized galaxy (M = 1.8×1010 Msun). [Selected Movies & Plots - Galaxy Formation] [NASA-ADS] [Local] [FirstStar III Poster] [Enlarged Picture] Realistic Galaxy Formation BASED ON a cosmological mass accretion history Collaborators: Marcelo A. Alvarez, Tom Abel Many studies of galaxy formation have been focusing on the case of gas embedded in an isolated dark matter halo, while the real galaxy formation is far from this idealized picture. In accordance with the cosmological mass accretion history of an individual halo we initialize the perturbed density field at a very high redshift in a large scale, and follow the realistic formation history of a dark matter halo and a galaxy in a cosmological context. Using a state-of-the-art AMR technique, we investigate numerous topics including the shock formation at the virial radius, the effect of baryonic back-reactions to the infalling gas, and the mass-metallicity relation. If any question, please send me an e-mail at: me@jihoonkim.org$ shapeimage_2_link_1