Gregory S.H. Paek (a SNU grad student) receives the best poster award from an International Workshop

Gregory S.H. Paek, a SNU student from the astronomy graduate program, receives the best poster award from the international workshop "Gravitational Wave Physics and Astronomy Workshop 2022 (GWPAW, link)" in December 9th, 2022. The workshop was held in Melbourne, Austrailia in December 5th to 9th, 2022, hosted by OzGrav (ARC Centre of Excellence for Gravitational Wave Discovery), the Australian Research Council Centre of Excellence for Gravitational Wave Discovery. Gregory S.H. Peak presented a poster about the detection of Kilonovas using 7DT project, which he has worked with Prof. Myungshin Im and Dr. Ji Hoon Kim (SNU astronomy program). He won "Winners of best posters at GWPAW Melbourne" with 1000 AUD price money. Below is the abstract of his presentation:  Identifying the electromagnetic counterparts of gravitational waves (GW), namely kilonovae, is a crucial step toward the success of GW multi-messenger astronomy. In this poster, we introduce the 7-Dimensional Telescope (7DT; PI: Prof. Myungshin Im), the biggest ground-based multi-telescope system designed to efficiently and rapidly search for these counterparts. Its unique feature is the use of 20-40 medium-band filters on each of its 20 independent telescopes, allowing for wide-field, low-resolution spectral imaging, which is a powerful tool in identifying and distinguishing kilonovae from other astronomical sources. We present the results of our experiments demonstrating how robust 7DT can detect and classify kilonovae. This research was supported by the Center for the Gravitational-Wave Universe granted by National Science Challenge Initiatives (NSCN). Congratulations, Gregory! 

2023-03-23

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Breakthrough Prize For Event Horizon Telescope Collaboration

  Prof. Sascha Trippe     The Event Horizon Telescope (EHT) Collaboration, including Prof. Sascha Trippe of Seoul National University, has been awarded the 2020 Breakthrough Prize in Fundamental Physics. The prize money of 3 million USD will be split equally among the collaboration members.   The EHT is a global network of radio observatories dedicated to the observation of supermassive black holes and active galactic nuclei. In April 2019, the EHT Collaboration released the first image of the shadow of the 6-billion solar mass black hole in the galaxy M 87, located at a distance of about 54 million light years. This groundbreaking achievement has now been recognized.   Announcement of the Breakthrough Prize: https://breakthroughprize.org/News/54    Information on the EHT: https://eventhorizontelescope.org/ 

2019-09-09

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Researchers discover that a giant smash of galaxy clusters forms bar structure in spiral galaxies

Researchers discover that a giant smash of galaxy clusters forms bar structure in spiral galaxies         Prof. Myunhshin Im (Advisor/Corresponding author) PhD stedent Yongmin Yoon (first author)     Researchers in Korea reported in 2019 June 24th issue of Nature Astronomy that a giant smash of two clusters of galaxies can form bar structure, an important structure inside many spiral galaxies. It has been known that galaxies, systems made of hundreds of billions of stars, can take many different shapes. The reason why galaxies take many different shapes has been one of the most important questions in Astronomy during the past 100 years. The most common kind of galaxies is spiral galaxies that have spiral arms, and about one third of spiral galaxies are known to have a structure called “bar”. Bar structure is an elongated structure that extends from the central region of a galaxy, and bar has been known to play a key role in regulating star formation and nuclear black hole growth. Furthermore, bar has been suggested as a structure responsible for forming another important galactic structure called “bulge”, a central concentration of stars in elliptical shape. Therefore, the formation mechanism of bar has been a central issue for understanding the galaxy formation and evolution.   Previous studies have suggested two key mechanisms for bar formation, one that is driven by the internal dynamical instability of galaxies (internal mechanism), and another due to interaction of a galaxy with other galaxies in the neighbor (environmental mechanism). However, the exact physical mechanism for the bar formation has been in debate.    In this new study, the research group led by Prof. Myungshin Im and Mr. Yongmin Yoon at Seoul National University identified a completely new mechanism that can form bars in spiral galaxies - interaction of two galaxy clusters. Galaxy clusters are the most massive gravitationally bound objects in the universe, that are made of hundreds to thousands of galaxies. When such massive monsters collide with each other, the colliding system exerts a rapid and strong change in the gravitational force on galaxies that belong to the clusters. This rapid change of the gravitational force can induce instability in spiral galaxy structure and causes bars to form. The researchers identified 105 galaxy clusters and 1377 spiral galaxies belonging to the clusters, and identified bar spiral galaxies among them. To their surprise, bar spiral galaxies are by 1.5 times more frequently found in interacting clusters than in non-interacting clusters. The researchers then examined if other mechanisms that follow the galaxy cluster interaction can be responsible for the bar formation, e.g. galaxy-galaxy interaction during the cluster interaction, but found no observational evidence that any derivative mechanisms are responsible for the enhancement of the bar fraction in the interacting clusters. Therefore, they concluded that the enhancement of the bar fraction in the interacting clusters is due to the cluster interaction itself.     The discovery is rather unexpected, and provides a fresh, new view on how galaxy shape are determined. When studying galaxy shapes, previous studies focused on internal physical parameters such as galaxy mass or static environmental parameters such as mass density of the environment. However, the new result suggests that the rapid and violent change of the large scale environment such as galaxy cluster interaction must be additionally considered seriously when understanding galaxy shapes. Myungshin Im notes that “This result provides a very new perspective to the study of galaxy shapes, and could galvanize this field.”. The bar structure may not be the only thing that has been induced by galaxy cluster interaction. Other physical properties of galaxies, such as how strongly they form stars, could be affected as well. “We are now looking into how galaxy cluster interaction changes many other properties of galaxies.”, Yongmin Yoon adds. Just like human society, violent change in the environment at large scale seems to change the lives of galaxies in the universe.    This research was supported by the National Research Foundation of Korea.        Examples of an interacting galaxy cluster, Abell 520 (top figure), a non-bar spiral galaxy M81 (bottom left), and a bar spiral galaxy NGC 1300 (bottom right). A violent interaction of the universe’s most massive structures, galaxy clusters (top), can induce bars in spiral galaxies as illustrated in the bottom. This is the first time such a mechanism has been observationally proven to be responsible for the formation of bars in spiral galaxies.   Figure credits Whole diagram: CEOU, Seoul National University Abell 520: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University) M81: NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: A. Zezas and J. Huchra (Harvard-Smithsonian Center for Astrophysics) NGC 1300: NASA, ESA, and The Hubble Heritage Team (STScI/AURA), P. Knezek (WIYN)     [Reference]  Yongmin Yoon, Myungshin Im, Gwang-Ho Lee, Seong-Kook Lee, and Gu Lim, Nature Astronomy, 10.1038/s41550-019-0799-7   [Main Author] Myugnshin Im(Seoul National University), Yongmin Yoon(Seoul National University)   * Contact : Myungshin Im myungshin.im@gmail.com                    Yongmin Yoon yymx2@astro.snu.ac.kr

2019-07-05

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THE VARIABILITY TIME SCALE - ACCRETION RATE RELATION OF ACTIVE GALACTIC NUCLEI

THE VARIABILITY TIME SCALE - ACCRETION RATE RELATION OF ACTIVE GALACTIC NUCLEI         PhD stedent Jongho Park (first author)  Prof. Sascha Trippe (Advisor/Corresponding author)     Active galactic nuclei (AGN) are the strongest sources of energy in the universe, being up to 10,000 times brighter than our home galaxy, the Milky Way. Their radiation is powered by accretion of interstellar gas into supermassive black holes located in the centers of galaxies; each of those black holes has a mass of a few million to a few billion times the mass of the sun. The brightness of AGN is highly variable and can change by more than one order of magnitude within few years. The amount of variability is stronger over longer time scales and shows no periodicities. In statistics terms, AGN are sources of "red noise": the power of variability is proportional to the sampling frequency (or inverse variability time scale) taken to a power beta, with beta being smaller than zero.   SNU PhD student Jongho Park and Prof. Sascha Trippe analyzed the radio light curves of 43 bright AGN. The light curves, which were provided by the University of Michigan Radio Astronomy Observatory (UMRAO), span about 30 years in time. Park and Trippe calculated the power law indices beta for each light curve and searched for correlations with the physical properties of the target AGN (like, e.g., black hole mass). Surprisingly, their analysis found a correlation between beta and the accretion rate, which is the amount of mass a black hole accretes per time: beta is proportional to the accretion rate taken to the power of 0.25. In other words: if a black hole accretes more mass per time, variations of its brightness take more time. This new relation is not yet understood, and investigations of its cause continue. REFERENCE: Park, J. & Trippe, S. 2017, ApJ, 834, 157 [http://iopscience.iop.org/article/10.3847/1538-4357/834/2/157/meta]       FIGURE CAPTION:  The variability time scale - accretion rate relation in logarithmic representation, showing the time scale parameter beta (corrected for Doppler boosting) as function of accretion disk luminosity (accretion rate times squared light speed). The parameter alpha, shown in the left panel, is the slope of the best-fit line.

2017-03-15

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Astronomy graduate student Geumsook Park, discovery of a Galactic supershell formed by the collision of a high-velocity cloud

stronomy graduate student Geumsook Park, discovery of  a Galactic supershell formed by the collision of a high-velocity cloud   highlighted on AAS Nova as one of the most interesting recent results being published in AAS journals         PhD stedent Geumsook Park (first author) Prof. Bon-Chul Koo (Advisor/Corresponding author)      Geumsook Park discovered the large neutral atomic hydrogen (HI) gas shell, called as HI supershell, which is likely to be formed by the collision of the compact High-Velocity cloud (HVC) in the outskirts of the Milky Way. The HVC-supershell system detected shows that supershells can be generated by the collision of HVCs with the disk, as well as multiple supernova explosions. This system also will be a good example to imply that some compact HVCs can survive their trip through the Galactic halo and inject energy and momentum into the Milky Way disk. This research was recently published in the Astrophysical Journal Letters, and highlighted in the AAS (American Astronomical Society) Nova. Also, other science magazines, such as New scientist and Sky and Telescope, introduced this study.             Figure. HI 21-cm image of HI supershell GS040.2+00.6-70. The color presents HI integrated intensity in gray scale, varying from black at the weakest intensity to white at the strongest. The overlaid red contours show the overall appearance of the HVC.     Reference: Park, G., Koo, B.-C., Kang, J.-h., et al. 2016, “A High-Velocity Cloud Impact Forming a Supershell in the Milky Way”, ApJL, 827, L27 http://iopscience.iop.org/article/10.3847/2041-8205/827/2/L27 (astroph: http://adsabs.harvard.edu/abs/2016arXiv160707699P)     □ AAS(American Astronomical Society) Nova: http://aasnova.org/2016/08/19/a-high-velocity-collision-with-our-galaxys-disk/   □ New Scientist: ​https://www.newscientist.com/article/2100157-small-gas-cloud-caught-blowing-a-huge-bubble-in-the-milky-way/​  □ Sky and Telescope: ​​http://www.skyandtelescope.com/astronomy-news/milky-way/cloud-punches-hole-milky-way-0508201623/

2016-08-24

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