Jeong-Sun Hwang

Korea Institute for Advanced Study, Sŏul, Seoul, South Korea

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Publications (7)21.07 Total impact

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    Jeong-Sun Hwang · Changbom Park
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    ABSTRACT: We use $N$-body/smoothed particle hydrodynamics simulations of encounters between an early-type galaxy (ETG) and a late-type galaxy (LTG) to study the effects of hot halo gas on the evolution for a case with the mass ratio of the ETG to LTG of 2:1 and the closest approach distance of $\sim$100 kpc. We find that the dynamics of the cold disk gas in the tidal bridge and the amount of the newly formed stars depend strongly on the existence of a gas halo. In the run of interacting galaxies not having a hot gas halo, the gas and stars accreted into the ETG do not include newly formed stars. However, in the run using the ETG with a gas halo and the LTG without a gas halo, a shock forms along the disk gas tidal bridge and induces star formation near the closest approach. The shock front is parallel to a channel along which the cold gas flows toward the center of the ETG. As a result, the ETG can accrete star-forming cold gas and newly born stars at and near its center. When both galaxies have hot gas halos, a shock is formed between the two gas halos somewhat before the closest approach. The shock hinders the growth of the cold gas bridge to the ETG and also ionizes it. Only some of the disk stars transfer through the stellar bridge. We conclude that the hot halo gas can give significant hydrodynamic effects during distant encounters.
    Preview · Article · May 2015 · The Astrophysical Journal
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    Jeong-Sun Hwang · Changbom Park · Jun-Hwan Choi
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    ABSTRACT: We construct several Milky Way-like galaxy models containing a gas halo (as well as gaseous and stellar disks, a dark matter halo, and a stellar bulge) following either an isothermal or an NFW density profile with varying mass and initial spin. In addition, galactic winds associated with star formation are tested in some of the simulations. We evolve these isolated galaxy models using the GADGET-3 $N$-body/hydrodynamic simulation code, paying particular attention to the effects of the gas halo on the evolution. We find that the evolution of the models is strongly affected by the adopted gas halo component. The model without a gas halo shows an increasing star formation rate (SFR) at the beginning of the simulation for some hundreds of millions of years and then a continuously decreasing rate to the end of the run at 3 Gyr. On the other hand, the SFRs in the models with a gas halo emerge to be either relatively flat throughout the simulations or increasing over a gigayear and then decreasing to the end. The models with the more centrally concentrated NFW gas halo show overall higher SFRs than those with the isothermal gas halo of the equal mass. The gas accretion from the halo onto the disk also occurs more in the models with the NFW gas halo, however, this is shown to take place mostly in the inner part of the disk and not to contribute significantly to the star formation unless the gas halo has very high density at the central part. The rotation of a gas halo is found to make SFR lower in the model. The SFRs in the runs including galactic winds are found to be lower than the same runs but without winds. We conclude that the effects of a hot gaseous halo on the evolution of galaxies are generally too significant to be simply ignored, and expect that more hydrodynamical processes in galaxies could be understood through numerical simulations employing both gas disk and gas halo components.
    Preview · Article · Dec 2012 · Journal of the Korean Astronomical Society
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    ABSTRACT: We present smoothed particle hydrodynamic models of the interactions in the compact galaxy group, Stephan's Quintet. This work is extension of the earlier collisionless N-body simulations of Renaud et al. in which the large-scale stellar morphology of the group was modeled with a series of galaxy-galaxy interactions in the simulations. Including thermohydrodynamic effects in this work, we further investigate the dynamical interaction history and evolution of the intergalactic gas of Stephan's Quintet. The major features of the group, such as the extended tidal features and the group-wide shock, enabled us to constrain the models reasonably well, while trying to reproduce multiple features of the system. We found that reconstructing the two long tails extending from NGC 7319 toward NGC 7320c one after the other in two separate encounters is very difficult and unlikely, because the second encounter usually destroys or distorts the already-generated tidal structure. Our models suggest the two long tails may be formed simultaneously from a single encounter between NGC 7319 and 7320c, resulting in a thinner and denser inner tail than the outer one. The tails then also run parallel to each other as observed. The model results support the ideas that the group-wide shock detected in multi-wavelength observations between NGC 7319 and 7318b and the starburst region north of NGC 7318b are triggered by the high-speed collision between NGC 7318b and the intergalactic gas. Our models show that a gas bridge is formed by the high-speed collision and clouds in the bridge continue to interact for some tens of millions of years after the impact. This produces many small shocks in that region, resulting a much longer cooling time than that of a single impact shock.
    Preview · Article · Sep 2011 · Monthly Notices of the Royal Astronomical Society
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    Curtis Struck · Clare L. Dobbs · Jeong-Sun Hwang
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    ABSTRACT: We have discovered long-lived waves in two sets of numerical models of fast (marginally bound or unbound) flyby galaxy collisions, carried out independently with two different codes. In neither simulation set are the spirals the result of a collision-induced bar formation. Although there is variation in the appearance of the waves with time, they do not disappear and reform recurrently, as seen in other cases described in the literature. We also present an analytic theory that can account for the wave structure, not as propagating transients, nor as a fixed pattern propagating through the disc. While these waves propagate through the disc, they are mantained by the coherent oscillations initiated by the impulsive disturbance. Specifically, the analytic theory suggests that they are caustic waves in ensembles of stars pursuing correlated epicyclic orbits after the disturbance. This theory is an extension of that developed by Struck and collaborators for colliding ring galaxies. The models suggest that this type of wave may persist for a couple of Gyr, and galaxy interactions occur on comparable time-scales, so waves produced by the mechanism may be well represented in observed spirals. In particular, this mechanism can account for the tightly wound, and presumably long-lived, spirals seen in some nearby early-type galaxies. These spirals are also likely to be common in groups and clusters, where fast encounters between galaxies occur relatively frequently. However, as the spirals become tightly wound, and evolve to modest amplitudes, they may be difficult to resolve unless they are nearby. None the less, the effect may be one of several processes that result from galaxy harassment, and via wave-enhanced star formation, contributes to the Butcher–Oemler effect.
    Full-text · Article · Feb 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We use smoothed particle hydrodynamics (SPH) models to study the large-scale morphology and dynamical evolution of the intergalactic gas in Stephan's Quintet, and compare to multiwavelength observations. Specifically, we model the formation of the hot X-ray gas, the large-scale shock, and emission line gas as the result of NGC 7318b colliding with the group. We also reproduce the N-body model of Renaud and Appleton for the tidal structures in the group. Comment: To appear in the Proceedings of the Galaxy Wars: Stellar Populations and Star Formation in Interacting Galaxies Conference
    Preview · Article · Aug 2009
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    ABSTRACT: We present Spitzer infrared, Galaxy Evolution Explorer UV, and Sloan Digitized Sky Survey and Southeastern Association for Research in Astronomy optical images of the peculiar interacting galaxy pair Arp 285 (NGC 2856/4), and compare with a new numerical model of the interaction. We estimate the ages of clumps of star formation in these galaxies using population synthesis models, carefully considering the uncertainties on these ages. This system contains a striking example of "beads on a string": a series of star-formation complexes ~1 kpc apart. These "beads" are found in a tail-like feature that is perpendicular to the disk of NGC 2856, which implies that it was formed from material accreted from the companion NGC 2854. The extreme blueness of the optical/UV colors and redness of the mid-infrared colors implies very young stellar ages (~4-20 Myr) for these star-forming regions. Spectral decomposition of these "beads" shows excess emission above the modeled stellar continuum in the 3.6 μm and 4.5 μm bands, indicating either contributions from interstellar matter to these fluxes or a second older stellar population. These clumps have –12.0 < M B< –10.6, thus they are less luminous than most dwarf galaxies. Our model suggests that bridge material falling into the potential of the companion overshoots the companion. The gas then piles up at apogalacticon before falling back onto the companion, and star formation occurs in the pile-up. There was a time delay of ~500 Myr between the point of closest approach between the two galaxies and the initiation of star formation in this feature. A luminous (M B ~ –13.6) extended (FWHM ~ 1.3 kpc) "bright spot" is visible at the northwestern edge of the NGC 2856 disk, with an intermediate stellar population (400-1500 Myr). Our model suggests that this feature is part of a expanding ripple-like "arc" created by an off-center ring-galaxy-like collision between the two disks.
    Full-text · Article · Mar 2008 · The Astronomical Journal
  • Beverly Smith · Mark Giroux · Mark Hancock · Jeong-Sun Hwang · Curt Struck
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    ABSTRACT: In Taffy galaxies, a head-on collision between two gas-rich equal-mass galaxies has occurred, stripping out a large quantity of gas from the disks, and creating a radio continuum-bright bridge between the two galaxies. To better understand the physics of dust formation, excitation, and destruction, an examination of the dust properties in these unusual systems would be useful. We propose to use Spitzer to obtain mid-infrared images of two candidate Taffy systems, UGC 813/6 and Arp 261. We will use these images to search for dust associated with the bridge, and to test for PAH destruction or dust heating by shocks during the impact. We will compare the data for these two systems with archival Spitzer data for the first Taffy galaxy discovered, UGC 12914/5. We will also compare with results from a Spitzer imaging survey of nearby interacting galaxies that we have recently completed, and with numerical simulations of the collisions between the galaxies.
    No preview · Article · Mar 2008

Publication Stats

69 Citations
21.07 Total Impact Points


  • 2012-2015
    • Korea Institute for Advanced Study
      • School of Physics
      Sŏul, Seoul, South Korea
  • 2009-2011
    • Iowa State University
      • Department of Physics and Astronomy
      Ames, Iowa, United States
  • 2008
    • East Tennessee State University
      • Department of Physics and Astronomy
      Johnson City, Tennessee, United States