• 천문학회지
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• 제37권5호
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• pp.387-392
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• 2004

X-ray observations of galaxy clusters have played an important role in cosmology, especially in determining the cosmological density parameter and the fluctuation amplitude. While they represent the bright side of the universe together with the other probes including the cosmic microwave background and the Type Ia supernovae, the resulting information clearly indicates that the universe is dominated by dark components. Even most of cosmic baryons turns out to be dark. In order to elucidate the nature of dark baryons, we propose a dedicated soft-X-ray mission, DIOS (Diffuse Intergalactic Oxygen Surveyor). Recent numerical simulations suggest that approximately 30 to 50 percent of total baryons at z = 0 take the form of the warm-hot intergalactic medium (WHIM) with $10^5K < T < 10^7K $which has evaded the direct detection so far. The unprecedented energy resolution (${\~} 2eV$) of the XSA (X-ray Spectrometer Array) on-board DIGS enables us to identify WHIM with gas temperature $T = 10^6 {\~} 10^7K$ and overdensity $\delta$ = 10 ${\~}$ 100 located at z < 0.3 through emission lines of OVII and OVIII. In addition, WHIMs surrounding nearby clusters are detectable with a typical exposure time of a day, and thus constitute realistic and promising targets for DIOS.

• 천문학회지
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• 제46권2호
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• pp.75-91
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• 2013

We estimate the power spectra of the cosmic microwave background radiation (CMB) temperature anisotropy in localized regions of the sky using the Wilkinson Microwave Anisotropy Probe (WMAP) 7-year data. We find that the north and south hat regions at high Galactic latitude ($|b|{\geq}30^{\circ}C$) show an anomaly in the power spectrum amplitude around the third peak, which is statistically significant up to 3. We try to explain the cause of the observed anomaly by analyzing the low Galactic latitude ($|b|$ < $30^{\circ}C$) regions where the galaxy contamination is expected to be stronger, and the regions weakly or strongly dominated byWMAP instrument noise. We also consider the possible effect of unresolved radio point sources. We find another but less statistically significant anomaly in the low Galactic latitude north and south regions whose behavior is opposite to the one at high latitude. Our analysis shows that the observed north-south anomaly at high latitude becomes weaker on regions with high number of observations (weak instrument noise), suggesting that the anomaly is significant at sky regions that are dominated by the WMAP instrument noise. We have checked that the observed north-south anomaly has weak dependences on the bin-width used in the power spectrum estimation, and on the Galactic latitude cut. We also discuss the possibility that the detected anomaly may hinge on the particular choice of the multipole bin around the third peak. We anticipate that the issue of whether or not the anomaly is intrinsic one or due to WMAP instrument noise will be resolved by the forthcoming Planck data.

• 천문학회보
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• 제40권1호
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• pp.59.3-60
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• 2015

We investigate the clustering properties of Lyman-break galaxies (LBGs) at z ~ 4. Using the hierarchical galaxy formation model GALFORM, we predict the angular correlation function (ACF) of LBGs and compare this with the measured ACF from combined survey fields consisting of the Hubble eXtreme Deep Field (XDF) and CANDELS. We find that the predicted ACF is in a good agreement with the measured ACFs. However, when we divide the model LBGs into bright and faint subset, the predicted ACFs are less consistent with observations. We quantify the dependence of clustering on luminosity and show that the fraction of satellite LBGs is important for determining the amplitude of ACF at small scales. We find that central LBGs predominantly reside in ${\sim}10^{11}h^{-1}M_{solar}$ haloes and satellites reside in haloes of mass ${\sim}10^{12}-10^{13}h^{-1}M_{solar}$. The model predicts fewer bright satellite LBGs than is inferred from the observation. LBGs in the tails of the redshift distribution contribute significant additional clustering signal, especially on small scales. This spurious clustering may affect the interpretation of the halo occupation distribution, including the minimum halo mass and abundance of satellite LBGs.

• 천문학회지
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• 제38권2호
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• pp.191-195
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• 2005

We used Subaru observations of A1689 (z = 0.183) to derive an accurate, model-independent mass profile for the entire cluster, r$\le$2Mpc/h, by combining magnification bias and distortion measurements. The projected mass profile steepens quickly with increasing radius, falling away to zero at r${\~}$1.0Mpc/h, well short of the anticipated virial radius. Our profile accurately matches onto the inner profile, r $\le$200kpc/ h, derived from deep HST / ACS images. The combined ACS and Subaru information is well fitted by an NFW profile with virial mass, $(1.93 \pm 0.20) {\times}10^{15} M_{\bigodot}$, and surprisingly high concentration, $C_{vir} = 13.7^{+1.4}_{-1.1}$, significantly larger than theoretically expected ($C_{vir} {\le}4$), corresponding to a relatively steep overall profile. These results are based on a reliable sample of background galaxies selected to be redder than the cluster E/SO sequence. By including the faint blue galaxy population a much smaller distortion signal is found, demonstrating that blue cluster members significantly dilute the true signal for r $\le$ 400kpc/ h. This contamination is likely to affect most weak lensing results to date.

• 천문학회보
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• 제38권2호
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• pp.59.1-59.1
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• 2013

Type Ia supernovae (SNe) are providing the most conclusive evidence for accelerating universe with dark energy in observational cosmology. In these investigations, look-back time evolution of SNe luminosity is regarded as negligible on the basic assumption. However, several recent works present some systematic differences among hosts which have different characteristics of stellar population. For more direct investigation, we are proceeding with our YONSEI (YOnsei Nearby Supernovae Evolution Investigation) project. Only early-type hosts in our catalogue were chosen in order to estimate the luminosity-weighted mean age and metallicity directly using Single Stellar Population (SSP) models and ignore the effect from the dust extinction. Observations using low-resolution spectrographs are still in progress at Las Campanas Observatory with 2.5m telescope and at McDonald Observatory with 2.7m telescope. We have thus far obtained spectra for 30 early-type hosts. After weak emission line correction, Lick/IDS absorption-line indices are measured and YEPS spectroscopic evolution model was applied to determine mean population ages and metallicities. Our preliminary results show that SNe Ia hosted in older galaxies seem to be brighter at 1.4 - 3 sigma levels, however, more observations and analyses are still needed to confirm this correlation.

• 천문학논총
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• 제30권2호
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• pp.237-240
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• 2015

Classical novae (CNe) are interacting binary systems whose outbursts are powered by a thermonuclear runaway in accreted material onto the surface of a white dwarf (WD). The secondary star in such systems fills its Roche lobe and material is transferred onto the WD primary star via an accretion disk. Recurrent novae (RNe) show many similarities to CNe, but have had more than one recorded outburst. RNe play an important role as one of the suspected progenitor systems of Type Ia supernovae, which are used as primary distance indicators in cosmology. Thus, it is important to investigate the nature of their central binary systems to determine the relation between the parameters of the central system and the outburst type, and finally ascertain the population of novae that might be available to give rise to the progenitors of Type Ia SNe. A low outburst amplitude is adopted as a criterion that may help distinguish RNe from CNe and was therefore used to select targets for observations from ground-based observatories including the Liverpool Telescope and the Southern African Large Telescope as well as the full-sky space-based archive of the Solar Mass Ejection Imager (SMEI). We found that at least four objects currently classified as CNe are possibly RNe candidates based on their quiescent spectra. We also searched the SMEI archive for additional outbursts of bright CNe that might otherwise have been missed but did not find a conclusive example.

• 천문학회보
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• 제43권2호
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• pp.40.4-41
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• 2018

The current 'standard model' of cosmology provides a minimal theoretical framework that can explain the gaussian, nearly scale-invariant density perturbations observed in the CMB to the late time clustering of galaxies. However accepting this framework, requires that we include within our cosmic inventory a vacuum energy that is ~122 orders of magnitude lower than Quantum Mechanical predictions, or alternatively a new scalar field (dark energy) that has negative pressure. An alternative approach to adding extra components to the Universe would be to modify the equations of Gravity. Although GR is supported by many current observations there are still alternative models that can be considered. Recently there have been many works attempting to test for modified gravity using the large scale clustering of galaxies, ISW, cluster abundance, RSD, 21cm observations, and weak lensing. In this work, we compare various modified gravity models using cosmic shear data from the Deep Lens Survey as well as data from CMB, SNe Ia, and BAO. We use the Bayesian Evidence to quantify the comparison robustly, which naturally penalizes complex models with weak data support. In this talk we present our methodology and preliminary results that show f(R) gravity is mildly disfavoured by the data.

• 천문학회지
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• 제37권5호
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• pp.447-454
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• 2004

During the hierarchical formation of large scale structure in the universe, the progressive collapse and merging of dark matter should inevitably drive shocks into the gas, with nonthermal particle acceleration as a natural consequence. Two topics in this regard are discussed, emphasizing what important things nonthermal phenomena may tell us about the structure formation (SF) process itself. 1. Inverse Compton gamma-rays from large scale SF shocks and non-gravitational effects, and the implications for probing the warm-hot intergalactic medium. We utilize a semi-analytic approach based on Monte Carlo merger trees that treats both merger and accretion shocks self-consistently. 2. Production of $^6Li$ by cosmic rays from SF shocks in the early Galaxy, and the implications for probing Galaxy formation and uncertain physics on sub-Galactic scales. Our new observations of metal-poor halo stars with the Subaru High Dispersion Spectrograph are highlighted.

• 천문학회지
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• 제37권5호
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• pp.465-470
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• 2004

I address the issue of nonthermal processes in the large scale structure of the universe. After reviewing the properties of cosmic shocks and their role as particle accelerators, I discuss the main observational results, from radio to $\gamma$-ray and describe the processes that are thought be responsible for the observed nonthermal emissions. Finally, I emphasize the important role of $\gamma$-ray astronomy for the progress in the field. Non detections at these photon energies have already allowed us important conclusions. Future observations will tell us more about the physics of the intracluster medium, shocks dissipation and CR acceleration.

• 천문학회지
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• 제37권5호
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• pp.361-369
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• 2004

Galaxy clusters as the densest and most prominent regions within the large-scale structure can be used as well characterizable laboratories to study astrophysical processes on the largest scales. X-ray observations provide currently the best way to determine the physical properties of galaxy clusters and the environmental parameters that describe them as laboratories. We illustrate this use of galaxy clusters and the precision of our understanding of them as laboratory environments with several examples. Their application to determine the matter composition of the Universe shows good agreement with results from other methods and is therefore a good test of our understanding. We test the reliability of mass measurements and illustrate the use of X-ray diagnostics to study the dynamical state of clusters. We discuss further studies on turbulence in the cluster ICM, the interaction of central AGN with the radiatively cooling plasma in cluster cooling cores and the lessons learned from the ICM enrichment by heavy elements.