• 천문학회보
• /
• 제39권2호
• /
• pp.67.2-67.2
• /
• 2014

The presence of multiple populations is now well-established in most globular clusters in the Milky Way. In light of this progress, here we suggest a new model explaining the origin of the Sandage period-shift and the difference in mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In our models, the instability strip in the metal-poor group II clusters, such as M15, is populated by second generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the relatively metal-rich group I clusters like M3 are mostly produced by first generation stars (G1) without these enhancements. This population shift within the instability strip with metallicity can create the observed period-shift between the two groups, since both helium and CNO abundances play a role in increasing the period of RR Lyrae variables. The presence of more metal-rich clusters having Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most metal-rich clusters having RR Lyraes with longest periods (group III) can also be reproduced, as more helium-rich third and later generations of stars (G3) penetrate into the instability strip with further increase in metallicity. Therefore, although there are systems where the suggested population shift cannot be a viable explanation, for the most general cases, our models predict that the RR Lyraes are produced mostly by G1, G2, and G3, respectively, for the Oosterhoff groups I, II, and III.

• 천문학회지
• /
• 제37권5호
• /
• pp.315-322
• /
• 2004

Several arguments have been presented in the literature to support the connection between radio halos and cluster mergers. The spectral index distributions of the halos in A665 and A2163 provide a new strong confirmation of this connection, i.e. of the fact that the cluster merger plays an important role in the energy supply to the radio halos. Features of the spectral index (flattening and patches) are indication of a complex shape of the radiating electron spectrum, and are therefore in support of electron reacceleration models. Regions of flatter spectrum are found to be related to the recent merger. In the undisturbed cluster regions, instead, the spectrum steepens with the distance from the cluster center. The plot of the integrated spectral index of a sample of halos versus the cluster temperature indicates that clusters at higher temperature tend to host halos with flatter spectra. This correlation provides further evidence of the connection between radio emission and cluster mergers.

• 천문학회보
• /
• 제37권1호
• /
• pp.44.2-44.2
• /
• 2012

Cosmological shock waves result from supersonic flow motions induced by hierarchical clustering during the large-scale structure formation in the Universe. Suprathermal particles are known to be produced via plasma interactions at collisionless shocks in tenuous plasmas and they can be further accelerated to become cosmic rays (CRs) via diffusive shock acceleration (DSA). The presence of CR electrons has been inferred from observations of diffuse radio halos and relics in some merging galaxy clusters. We have calculated the emissions from CR electrons accelerated at weak planar shocks, using time-dependent DSA simulations that include energy losses via synchrotron emission and Inverse Compton scattering. The simulated nonthermal emission are used to model the synchrotron emission from several observed radio relics.

• 천문학회보
• /
• 제41권1호
• /
• pp.32.3-32.3
• /
• 2016

The presence of multiple stellar populations is now well established in most globular clusters in the Milky Way. Here we show that two populations of RR Lyrae stars and the double red clump observed in the Milky Way bulge are another manifestations of the same multiple population phenomenon observed in halo globular clusters. We will discuss the implications of this result on the stellar populations and formation of early-type galaxies.

• 천문학회보
• /
• 제44권2호
• /
• pp.76.3-76.3
• /
• 2019

Intracluster stars are believed to be gravitationally bound to a galaxy cluster, however, not to individual cluster galaxies. Their presence is observed as diffuse light typically in the central region extended from the brightest cluster galaxy. The diffuse light, often referred to as intracluster light (ICL), is difficult to quantify in distant high-redshift galaxy clusters because of the significant surface brightness dimming although ICL observations in high-redshift clusters provide powerful constraints on the origin of intracluster stars. In this poster, we present ICL study of the distant galaxy cluster SPT2106-5844 at z=1.132 with Hubble Space Telescope IR imaging data. With careful control of systematics, we successfully quantify the total amount of the ICL, measure the color profile, and obtain its two-dimensional distribution. Our measurement of the high abundance of the intracluster stars in this young cluster favors the ICL formation scenario, wherein production of intracluster stars are predominantly associated with the BCG formation.

• 천문학회보
• /
• 제45권1호
• /
• pp.29.4-29.4
• /
• 2020

Structure in the universe forms hierarchically with the small scales forming first and merging into larger scales. Galaxy clusters are at the pinnacle of the formation process. Peering far into the universe, we can observe galaxy clusters early in their evolution. SpARCSJ1049+56 is a galaxy cluster located at a redshift of 1.71. It has been shown to be rich in cluster galaxies, to have intense star formation, and to have a significant amount of molecular gas. Through careful control of systematics, we detected the weak-lensing signal from this distant galaxy cluster. I will present our HST infrared weak-lensing detection of the cluster with a focus on the method. Our lensing analysis found that the cluster is massive and is rare in a LambdaCDM universe. I will also present the Chandra X-ray discovery of cold gas coincident with the intense star formation and discuss the implications of the detection.

• 천문학회보
• /
• 제46권2호
• /
• pp.42.2-42.2
• /
• 2021

Intraclusterlight (ICL) is diffuse light from stars that are bound to the clusterpotential, not to individual member galaxies. Understanding the formationmechanism of ICL provides critical information on the assembly and evolution ofthe galaxy cluster. Although there exist several competing models, the dominantproduction mechanism is still in dispute. The ICL measurement between z=1 and 2strongly constrains the formation scenario of the ICL because the epoch is whenthe first mature clusters begin to appear. However, the number of high-redshiftICL studies is small mainly because of observational challenges. In this study, based on deep HST WFC3/IR data, we measured ICL of 10 galaxy clusters atredshift beyond unity, which nearly doubles the sample size in this redshiftregime. With careful handling of systematics including object masking, skyestimation, flatfielding, dwarf galaxy contamination, etc., we quantified thetotal amount of ICL, measured the color profile, and examined the transitionbetween BCG and ICL.

• 천문학회보
• /
• 제35권1호
• /
• pp.82-82
• /
• 2010

We measure the topology of the galaxy distribution using the Seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7), examining the dependence of galaxy clustering topology on galaxy properties. The observational results are used to test galaxy formation models. A volume-limited sample defined by Mr<-20.19 enables us to measure the genus curve with amplitude of G=378 at 6h-1Mpc smoothing scale, with 4.8% uncertainty including all systematics and cosmic variance. The clustering topology over the smoothing length interval from 6 to 10h-1Mpc reveals a mild scale-dependence for the shift and void abundance (A_V) parameters of the genus curve. We find strong bias in the topology of galaxy clustering with respect to the predicted topology of the matter distribution, which is also scale-dependent. The luminosity dependence of galaxy clustering topology discovered by Park et al. (2005) is confirmed: the distribution of relatively brighter galaxies shows a greater prevalence of isolated clusters and more percolated voids. We find that galaxy clustering topology depends also on morphology and color. Even though early (late)-type galaxies show topology similar to that of red (blue) galaxies, the morphology dependence of topology is not identical to the color dependence. In particular, the void abundance parameter A_V depends on morphology more strongly than on color. We test five galaxy assignment schemes applied to cosmological N-body simulations to generate mock galaxies: the Halo-Galaxy one-to-one Correspondence (HGC) model, the Halo Occupation Distribution (HOD) model, and three implementations of Semi-Analytic Models (SAMs). None of the models reproduces all aspects of the observed clustering topology; the deviations vary from one model to another but include statistically significant discrepancies in the abundance of isolated voids or isolated clusters and the amplitude and overall shift of the genus curve. SAM predictions of the topology color-dependence are usually correct in sign but incorrect in magnitude.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
• /
• 한국우주과학회 2008년도 한국우주과학회보 제17권2호
• /
• pp.29.4-30
• /
• 2008

We present elemental abundances of 12 red giants obtained with the BOAO 1.8m telescope and its fiber-fed echelle spectrograph. We perform the abundance analysis using the Kurucz model atmosphere and MOOG. Comparisons of our alpha- and neutron-capture elemental abundances and those in globular clusters and nearby dwarf galaxies will be presented.

• 천문학회보
• /
• 제37권2호
• /
• pp.232.1-232.1
• /
• 2012

With flourishing of new facilities, we are now set to expand the horizon of the human knowledge of the universe deep into the very beginning of the cosmic history of astronomical objects. In such an endeavor, extreme objects and cosmic events such as violent cosmic explosions, rapidly growing supermassive black holes, and proto-clusters of galaxies, are the objects to be investigated in order to unveil what was happening in the very early universe. We will discuss what we have learned and are expecting to discover from such extreme objects about the infant universe both currently and in future, highlighting some of our recent works.