star-galaxy separation은 전천후 측광 관측 데이터에서 좋은 galaxy (또는 star, quasar 등) catalogue를 만드는데 중요한 이슈이다.
기존에 주로 쓰이던 방법(& 이것의 문제점) 및 최근에 제안된 방법을 소개하고, 몇가지 데이터(SDSS, HSC, DECaLS)에 대해 진행된 star-galaxy separation을 담은 논문들의 일부를 소개한다.
The SDSS photometric pipeline performs a morphological star/galaxy separation. The quality of this separation is therefore intimately related to the seeing and sky brightness. Photo's classifications have been tested against those of the COMBO survey (which effectively uses low-resolution spectroscopy for star/galaxy separation) in a region of the sky that both surveys have scanned. While the comparison was carried out for DR1 data, the DR2/DR3 data are not significantly different.
The classificaiton_extendedness? parameter, which is based on the difference between the CModel and PSF magnitudes, is stored in the database and is a reasonable star/galaxy classifier. Using the star/galaxy separation from HST/ACS in COSMOS as the truth table, here we characterize the accuracy of our star/galaxy separation. The completeness is defined as a fraction of ACS stars properly classified as stars in HSC. The contamination is a fraction of ACS galaxies among objects classified as stars in HSC. Both the completeness and contamination rates are strong function of the seeing size and depth.
The mergers of galaxy clusters are the most energetic events in the universe after the Big Bang. With the increased availability of multi-object spectroscopy and X-ray data an ever increasing fraction of local clusters are recognised as exhibiting signs of recent or past merging events on various scales. Our goal is to probe how these mergers affect the evolution and content of their member galaxies. We specifically aim to answer the following questions: Is the quenching of star formation in merging clusters enhanced when compared with relaxed clusters? Is the quenching preceded by a (short lived) burst of star formation? We obtained optical spectroscopy of >400 galaxies in the field of the merging cluster Abell 520. We combine these observations with archival data to get a comprehensive picture of the state of star formation in the members of this merging cluster. Finally, we compare these observations with a control sample of 10 non-merging clusters at the same redshift from The Arizona Cluster Redshift Survey (ACReS). We split the member galaxies in passive, star forming or recently quenched depending on their spectra. The core of the merger shows a decreased fraction of star forming galaxies compared to clusters in the non-merging sample. This region, dominated by passive galaxies, is extended along the axis of the merger. We find evidence of rapid quenching of the galaxies during the core passage with no signs of a star burst on the time scales of the merger (~0.4Gyr). Additionally, we report the tentative discovery of an infalling group along the main filament feeding the merger, currently at ~2.5 Mpc from the merger centre. This group contains a high fraction of star forming galaxies as well as ~2/3 of all the recently quenched galaxies in our survey.
Shells are low surface brightness tidal debris that appear as interleaved caustics with large opening angles, often situated on both sides of the galaxy center. In this paper, we study the incidence and formation processes of shell galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We identify shells at redshift z=0 using stellar surface density maps, and we use stellar history catalogs to trace the birth, trajectory and progenitors of each individual star particle contributing to the tidal feature. Out of a sample of the 220 most massive galaxies in Illustris (M200crit > 6 × 1012 M⊙), 18%±3% of the galaxies exhibit shells. This fraction increases with increasing mass cut: higher mass galaxies are more likely to have stellar shells. Furthermore, the fraction of massive galaxies that exhibit shells decreases with increasing redshift. We find that shell galaxies observed at redshift z = 0 form preferentially through relatively major mergers ( 1:10 in stellar mass ratio). Progenitors are accreted on low angular momentum orbits, in a preferred time-window between ∼4 and 8 Gyrs ago. Our study indicates that, due to dynamical friction, more massive satellites are allowed to probe a wider range of impact parameters at accretion time, while small companions need almost purely radial infall trajectories in order to produce shells. We also find a number of special cases, as a consequence of the additional complexity introduced by the cosmological setting. These include galaxies with multiple shell- forming progenitors, satellite-of-satellites also forming shells, or satellites that fail to produce shells due to multiple major mergers happening in quick succession.
Aims: 1156 + 295 is a flat-spectrum quasar that is loud at both radio and γ-ray wavelengths. Previous observations of the source revealed a radio morphology on pc to kpc scales consistent with a helical jet model. We carried out VLBA observations at centimeter and millimeter wavelengths to study the structure of the innermost jet and understand the relation between the helical structure and astrophysical processes in the central engine.
Methods: The source 1156 + 295 was observed with the VLBA at 86, 43, and 15 GHz at four epochs from 10 May 2003 to 13 March 2005. The observations were carried out in a full polarization mode. The highest resolution of the observations is 0.08 mas (~0.5 pc) at 86 GHz.
Results: A core-jet structure with six jet components is identified in 1156 + 295. Three jet components are detected for the first time. The apparent transverse velocities of the six jet components derived from proper motion measurements range between 3.6 c and 11.6 c, suggesting that highly relativistic jet plasma moves in the direction close to the line of sight. The overall jet has an oscillating morphology with multiple curvatures on pc scales, which might be indicative of a helical pattern. Models of a helical jet are discussed in the context of both Kelvin-Helmholtz (K-H) instability and jet precession. The K-H instability model is in closer agreement with the observed data.
Conclusions: The overall radio structure on scales from sub-pc to kpc appears to be fitted with a hydrodynamic model in the fundamental helical mode of Kelvin-Helmholtz (K-H) instability. This helical mode with an initial characteristic wavelength of λ0 = 0.2 pc is excited at the base of the jet on the scale of 0.005 pc (or 103Rs, the typical size of the broad line region of a supermassive black hole of 4.3 × 108 M⊙). A precessing jet model can also fit the observed jet structure on scales of between 10 pc and 300 pc. However, with the precessing jet model, additional astrophysical processes may be required to explain the bendings of the inner jet structure (1 to 10 pc) and re-collimation of the large-scale jet outflow (>300 pc).
The deep connection between galaxies and their supermassive black holes is central to modern astrophysics and cosmology. The observed correlation between galaxy and black hole mass is usually attributed to the contribution of major mergers to both. We make use of a sample of galaxies whose disk-dominated morphologies indicate a major-merger-free history and show that such systems are capable of growing supermassive black holes at rates similar to quasars. Comparing black hole masses to conservative upper limits on bulge masses, we show that the black holes in the sample are typically larger than expected if processes creating bulges are also the primary driver of black hole growth. The same relation between black hole and total stellar mass of the galaxy is found for the merger-free sample as for a sample which has experienced substantial mergers, indicating that major mergers do not play a significant role in controlling the coevolution of galaxies and black holes. We suggest that more fundamental processes which contribute to galaxy assembly are also responsible for black hole growth.
1) 연구원 실적 관리 시스템
- 개인업적평가 저널 분류 기준 (이전 년도 논문 등급을 JCR 기준으로 분류)
- 연구관리시스템 (실적 검색 관련 팁)
2) 한국천문학회 수상 : 젊은 천문학자상, 우수논문상, 샛별상, 우수포스터상
3) 한국우주과학회 수상 : 두진 우주과학자상, 신진 우주과학자상, SELab상
4) 한국천문연구원 논문상 : 학생논문상 (SCI 기준 주저자 1~1.5편, 40점 이상)
5) 학생 연구원으로서 개인 실적 관리
6) 연구원 실적 평가 시스템의 문제점 (정량 평가의 문제점)