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Kim, Yongjung,Im, Myungshin,Jeon, Yiseul,Kim, Minjin,Hyun, Minhee,Kim, Dohyeong,Kim, Jae-Woo,Taak, Yoon Chan,Yoon, Yongmin,Choi, Changsu,Hong, Jueun,Jun, Hyunsung David,Karouzos, Marios,Kim, Duho,Kim, American Astronomical Society 2018 The Astrophysical journal Vol.855 No.2
<P>To date, most of the luminous quasars known at z similar to 6 have been found to be in maximal accretion with the Eddington ratios, lambda(Edd) similar to 1, suggesting enhanced nuclear activities in the early universe. However, this may not be the whole picture of supermassive black hole (SMBH) growth, since previous studies have not reached faint quasars that are more likely to harbor SMBHs with low lambda(Edd). To gain a better understanding of the accretion activities in quasars in the early universe, we obtained a deep near-infrared (NIR) spectrum of a quasar, IMS J220417.92+011144.8 (hereafter IMS J2204+0112), one of the faintest quasars that has been identified at z similar to 6. From the redshifted C IV lambda 1549 emission line in the NIR spectrum, we find that IMS J2204+0112 harbors a SMBH with a solar mass of about a billion and lambda(Edd) similar to 0.1, but with a large uncertainty in both quantities (0.41 dex). IMS J2204+0112 has one of the lowest Eddington ratios among quasars at z similar to 6, but a common value among quasars at z similar to 2. Its low lambda(Edd) can be explained with two scenarios; the SMBH growth from a stellar-mass black hole through short-duration super-Eddington accretion events or from a massive black hole seed (similar to 10(5) M-circle dot) with Eddington-limited accretion. NIR spectra of more faint quasars are needed to better understand the accretion activities of SMBHs at z similar to 6.</P>
DISCOVERY OF A FAINT QUASAR AT <i>z</i> ∼ 6 AND IMPLICATIONS FOR COSMIC REIONIZATION
Kim, Yongjung,Im, Myungshin,Jeon, Yiseul,Kim, Minjin,Choi, Changsu,Hong, Jueun,Hyun, Minhee,Jun, Hyunsung David,Karouzos, Marios,Kim, Dohyeong,Kim, Duho,Kim, Jae-Woo,Kim, Ji Hoon,Lee, Seong-Kook,Pak, IOP Publishing 2015 ASTROPHYSICAL JOURNAL LETTERS - Vol.813 No.2
<P>Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars (M-1450 > -24 mag) at z greater than or similar to 6, but only a very small number of such quasars have been spectroscopically identified so far. Here, we report the discovery of a faint quasar IMS J220417.92+011144.8 at z similar to 6 in a 12.5 deg(2) region of the SA22 field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar shows a sharp break at similar to 8443 angstrom, with emission lines redshifted to z = 5.944 +/- 0.002 and rest-frame ultraviolet continuum magnitude M-1450 = -23.59 +/- 0.10 AB mag. The discovery of IMS J220417.92+011144.8 is consistent with the expected number of quasars at z similar to 6 estimated from quasar luminosity functions based on previous observations of spectroscopically identified low-luminosity quasars. This suggests that the number of M-1450 similar to 23 mag quasars at z similar to 6 may not be high enough to fully account for the reionization of the universe. In addition, our study demonstrates that faint quasars in the early universe can be identified effectively with a moderately wide and deep near-infrared survey such as the IMS.</P>
Are Quasars Growing Fast in the Early Universe?: The Lowest Eddington Ratio Quasar at z~6
Yongjung Kim,Myungshin Im,Yiseul Jeon,Minjin Kim,Minhee Hyun,Dohyeong Kim,Jae-Woo Kim,Yoon Chan Taak,Yongmin Yoon,Changsu Choi,Jueun Hong,Hyunsung David Jun,Marios Karouzos,Duho Kim,Ji Hoon Kim,Seong- 한국천문학회 2017 天文學會報 Vol.42 No.2
Discovery of a Faint Quasar at z ~ 6 and Implications for Cosmic Reionization
Yongjung Kim,Myungshin Im,Yiseul Jeon,Minjin Kim,Changsu Choi,Jueun Hong,Minhee Hyun,Hyunsung David Jun,Marios Karouzos,Dohyeong Kim,Duho Kim,Jae-Woo Kim,Ji Hoon Kim,Seong-Kook Lee,Soojong Pak,Won-Kee 한국천문학회 2016 天文學會報 Vol.41 No.1
Kim, Dohyeong,Im, Myungshin,Canalizo, Gabriela,Kim, Minjin,Kim, Ji Hoon,Woo, Jong-Hak,Taak, Yoon Chan,Kim, Jae-Woo,Lazarova, Mariana American Astronomical Society 2018 The Astrophysical journal Supplement series Vol.238 No.2
<P>We present medium-resolution spectra (R similar to 2000-4000) at 0.4-1.0 mu m and 0.7-2.5 mu m of 16 active galactic nuclei (AGNs) selected with red color in the near-infrared (NIR) of J - K > 2.0 mag at z similar to 0.3. We fit the H beta, H alpha, P beta, and P alpha lines from these spectra to obtain their luminosities and line widths. We derive the E(B - V) color excess values of the NIR-red AGNs using two methods, one based on the line-luminosity ratios and another based on the continuum slopes. The two E(B - V) values agree with each other at rms dispersion similar to 0.249. About half of the NIR-red AGNs have g' - K < 5 magnitude, and we find that these NIR-red, but blue in optical-NIR AGNs, have E(B - V) similar to 0, suggesting that a significant fraction of the NIR color-selected red AGNs are unobscured or only mildly obscured. After correcting for the dust extinction, we estimate the black hole (BH) masses and the bolometric luminosities of the NIR-red AGNs using the Paschen lines to calculate their Eddington ratios (lambda(Edd) ). The median Eddington ratios of nine NIR-red AGNs (log(lambda(Edd) ) similar or equal to -0.654 +/- 0.176) are only mildly higher than those of unobscured type 1 AGNs (log(lambda(Edd)) similar or equal to -0.961 +/- 0.008). Moreover, we find that the M-BH-sigma* relation for three NIR-red AGNs is consistent with that of unobscured type 1 AGNs at similar redshift. These results suggest that the NIR-red color selection alone is not effective at picking up dusty, intermediate-stage AGNs.</P>
Effect of LiCoO<sub>2</sub>-Coated Cathode on Performance of Molten Carbonate Fuel Cell
Kim, Dohyeong,Kim, Hyung Tae,Song, Shin Ae,Kim, Kiyoung,Lim, Sung Nam,Woo, Ju Young,Han, Haksoo The Korean Electrochemical Society 2022 Journal of electrochemical science and technology Vol.13 No.1
Molten carbonate fuel cells (MCFCs) are environmentally friendly, large-capacity power generation devices operated at approximately 650℃. If MCFCs are to be commercialized by improving their competitiveness, their cell life should be increased by operating them at lower temperatures. However, a decrease in the operating temperature causes a reduction in the cell performance because of the reduction in the electrochemical reaction rate. The cell performance can be improved by introducing a coating on the cathode of the cell. A coating with a high surface area expands the triple phase boundaries (TPBs) where the gas and electrolyte meet on the electrode surface. And the expansion of TPBs enhances the oxygen reduction reaction of the cathode. Therefore, the cell performance can be improved by increasing the reaction area, which can be achieved by coating nanosized LiCoO<sub>2</sub> particles on the cathode. However, although a coating improves the cell performance, a thick coating makes gas difficult to diffuse into the pore of the coating and thus reduces the cell performance. In addition, LiCoO<sub>2</sub>-coated cathode cell exhibits stable cell performance because the coating layer maintains a uniform thickness under MCFC operating conditions. Therefore, the performance and stability of MCFCs can be improved by applying a LiCoO<sub>2</sub> coating with an appropriate thickness on the cathode.
The Stress-Reducing Effects of Forest Healing Activities in Maladjusted Military Force Members
Jihye Kim,Changseob Sin,Jihye Kim,Dohyeong Kim,Yunsu Kim,Hyunchae Lee 인간식물환경학회 2020 인간식물환경학회지 Vol.23 No.3
Background and objective: In the Republic of Korea, military service is mandatory. Some of new recruits have the stress from the special environment, which could cause psychological maladjustment. The military forces have operated education programs such as green camp and healing camp. The study was conducted to investigate changes in psychological and physiological stress by conducting forest healing activities along with plant scent treatment for soldiers participating in a green camp. Methods: A total of 52 soldiers were participated including maladjustment soldiers and those recommended by their military units to protect the unfit soldiers who participated in forest healing activities in the green camp. The programs that were certified by the Korea Forest Service between 2014 and 2019, and were applied for stress reduction and relaxation training were classified into stress-coping programs and forest experience programs. Post-hoc surveys and cortisol measurements were carried out. Results: Green camp soldiers experience and differences in the level of stress responses were found to be very statistically significant between the treatment and control groups. The techniques for coping with stress were not significantly different in the control group, and the treatment group showed statistically significant results. In addition, the results of analyzing changes in the concentration of cortisol and measuring physiological stress were very statistically significant in forest healing activities at 4 p.m. Conclusion: Once green camp soldiers face stressful situations, forest healing programs using forest plant scents for green camp soldiers can have positive responses and forest healing activities can reduce psychological and physiological stress responses, improving maladjustment behaviors caused by stress and positively affecting the reduction of cortisol.