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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>
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 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
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
김두회 ( Duhoe Kim ),김용현 ( Yonghyun Kim ),김동화 ( Donghwa Kim ),신동규 ( Dongkyoo Shin ),신동일 ( Dongil Shin ) 한국정보처리학회 2017 한국정보처리학회 학술대회논문집 Vol.24 No.2
정보통신 기술의 발전으로 개인뿐만 아니라 경제, 행정, 국방 등 사회 전반에서 사이버 공간의 중요성이 대두되고 있다. 특히 국방부에서는 사이버 관련 공격들에 관한 피해를 평가하는 연구가 활발히 진행되고 있다. 본 논문에서는 사이버 전투 피해평가 프레임워크를 제안한다. 사이버 전투 피해평가 프레임워크는 아군이 사이버 공격에 의해 피해를 입은 뒤 지휘 통제실에게 아군의 피해를 알리고 장비의 손상도는 얼마인지 작전에 이상을 미치는 영향은 얼마인지 계산하여 제공한다. 본 프레임워크를 사용하면 현 상황을 아군의 사령부가 파악할 수 있게 되어 지휘 결심을 하는데 도움을 주어서 작전을 성공 시킬 수 있게 도와준다.
로그 분석을 이용한 공격 트리 생성과 실시간 공격 탐지 시스템
김두회 ( Duhoe Kim ),김용현 ( Yonghyun Kim ),김동화 ( Donghwa Kim ),신동규 ( Dongkyoo Shin ),신동일 ( Dongil Shin ) 한국정보처리학회 2017 한국정보처리학회 학술대회논문집 Vol.24 No.1
최근 국방부는 우리 군이 운영하는 내부망이 해킹에 의해 공격당한 사실을 발표했다. 국방부는 내부망은 외부와의 망 분리를 통해 안전하게 관리되고 있다고 밝혔지만, 이번 사건으로 군에서 사용하는 내부망에도 악성코드 공격이 가능한 취약점이 존재한다는 사실이 밝혀졌다. 본 논문에서는 국방부에서 사용되는 내부망과 일반적으로 사용하는 외부망에서도 보안을 강화할 수 있는 시스템을 개발했다. 본 시스템은 공격 등급을 적용한 트리를 생성하여 일정 수준이 되면 사용자에게 알리고 공격에 이르기 전에 시스템 단에서 차단이 가능하다. 본 논문에서 개발한 시스템을 사용하면 중요한 국가기밀자료나 지적 재산이 높은 자료를 가진 국가 및 기업들이 데이터를 보호 할 수 있고 시스템이 파괴당하는 것을 사전에 방지하여 물리적인 측면도 이점을 얻을 수 있다.
THE INFRARED MEDIUM-DEEP SURVEY. II. HOW TO TRIGGER RADIO AGNs? HINTS FROM THEIR ENVIRONMENTS
Karouzos, Marios,Im, Myungshin,Kim, Jae-Woo,Lee, Seong-Kook,Chapman, Scott,Jeon, Yiseul,Choi, Changsu,Hong, Jueun,Hyun, Minhee,Jun, Hyunsung David,Kim, Dohyeong,Kim, Yongjung,Kim, Ji Hoon,Kim, Duho,Pa IOP Publishing 2014 The Astrophysical journal Vol.797 No.1
<P>Activity at the centers of galaxies, during which the central supermassive black hole is accreting material, is nowadays accepted to be rather ubiquitous and most probably a phase of every galaxy's evolution. It has been suggested that galactic mergers and interactions may be the culprits behind the triggering of nuclear activity. We use near-infrared data from the new Infrared Medium-Deep Survey and the Deep eXtragalactic Survey of the VIMOS-SA22 field and radio data at 1.4 GHz from the FIRST survey and a deep Very Large Array survey to study the environments of radio active galactic nuclei (AGNs) over an area of similar to 25 deg(2) and down to a radio flux limit of 0.1 mJy and a J-band magnitude of 23 mag AB. Radio AGNs are predominantly found in environments similar to those of control galaxies at similar redshift, J-band magnitude, and (M-u - Mr) rest-frame color. However, a subpopulation of radio AGNs is found in environments up to 100 times denser than their control sources. We thus preclude merging as the dominant triggering mechanism of radio AGNs. By fitting the broadband spectral energy distribution of radio AGNs in the least and most dense environments, we find that those in the least dense environments show higher radio-loudness, higher star formation efficiencies, and higher accretion rates, typical of the so-called high-excitation radio AGNs. These differences tend to disappear at z > 1. We interpret our results in terms of a different triggering mechanism for these sources that is driven by mass loss through winds of young stars created during the observed ongoing star formation.</P>