• 천문학회보
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• 제45권1호
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• pp.38.2-38.2
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• 2020

Horizon Run 5 (HR5) is a cosmological hydrodynamical simulation which captures the properties of the Universe on a Gpc scale while achieving a resolution of 1 kpc. This enormous dynamic range allows us to simultaneously capture the physics of the cosmic web on very large scales and account for the formation and evolution of dwarf galaxies on much smaller scales. Inside the simulation box. we zoom-in on a high-resolution cuboid region with a volume of 1049 × 114 × 114 Mpc3. The subgrid physics chosen to model galaxy formation includes radiative heating/cooling, reionization, star formation, supernova feedback, chemical evolution tracking the enrichment of oxygen and iron, the growth of supermassive black holes and feedback from active galactic nuclei (AGN) in the form of a dual jet-heating mode. For this simulation we implemented a hybrid MPI-OpenMP version of the RAMSES code, specifically targeted for modern many-core many thread parallel architectures. For the post-processing, we extended the Friends-of-Friend (FoF) algorithm and developed a new galaxy finder to analyse the large outputs of HR5. The simulation successfully reproduces many observations, such as the cosmic star formation history, connectivity of galaxy distribution and stellar mass functions. The simulation also indicates that hydrodynamical effects on small scales impact galaxy clustering up to very large scales near and beyond the baryonic acoustic oscillation (BAO) scale. Hence, caution should be taken when using that scale as a cosmic standard ruler: one needs to carefully understand the corresponding biases. The simulation is expected to be an invaluable asset for the interpretation of upcoming deep surveys of the Universe.

• 천문학회보
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• 제46권1호
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• pp.35.2-35.2
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• 2021

Blue straggler stars (BSs) are "rejuvenated" main sequence stars first recognized by Allan Sandage from his observation of the prominent northern globular cluster M3 in the year of 1953. BSs are now known to be present in diverse stellar environments including open clusters, globular clusters, dwarf galaxies, and even the field populations of the Milky Way. This makes them a very useful tool in a wide range of astrophysical applications: Particularly BSs are considered to have a crucial role in the evolution of stellar clusters because they affect on the dynamics, the binary population, and the history of the stellar evolution of the cluster they belong to. Here we report a part of the preliminary results from our ongoing research on the BSs in the two metal-poor globular clusters (GCs) in the Large Magellanic Cloud (LMC), Hodge 11 and NGC1466. Using the high precision multi-band images obtained with the Advanced Camera for Survey (ACS) onboard the Hubble Space Telescope (HST), we extract time-series photometry to search for the signal of periodic variations in the luminosity of the BSs. Our preliminary results confirm that several BSs are intrinsic "short period (0.05 < P < 0.25 days)" variable stars with either pulsating or eclipsing types. We will discuss our investigation on the properties of those variable BS candidates in the context of the formation channels of these exotic main sequence stars, and their roles in the dynamical evolution of the host star clusters.

• 천문학논총
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• 제32권1호
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• pp.281-285
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• 2017

We present a summary of our spectroscopic redshift catalogue of 404 sources in the AKARI Deep Field South (ADF-S). We have used the AAOmega spectrograph to target mid-infrared and far-infrared sources selected primarily from AKARI observations in this field for which we were able to obtain optical counterparts. Our sources with identified redshifts include 316 with $H{\alpha}$ detections at $z{\leq}0.345$ and 15 sources at z > 1 with MgII or $Ly{\alpha}$ emission lines. About 13% of our $z{\leq}0.345$ sources are dominated by active galactic nuclei (AGN) emission, although many show emission from both star formation and AGNs. The median Balmer decrement is 5.9. Ultra-luminous infrared galaxies (ULIRGs) were found only in the higher-redshift sources. Optical and near infrared data will be available shortly, enabling calibration of the line luminosities and spectral energy distribution (SED) fitting for these sources.

• 천문학회보
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• 제41권1호
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• pp.39.4-40
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• 2016

We aim to investigate formation of satellite sub-galactic structures around isolated dwarf galaxies using cosmological hydrodynamic zoom simulations. For this, we modify a cosmological hydrodynamic code, GADGET-3, in a way that includes gas cooling down to T~10K, gas heating by universal reionization when z < 8.9, UV shielding for high density regions of $n_{shield}$ > $0.014cm^{-3}$, star formation in the dense regions ($n_H$ > $100cm^{-3}$), and supernova feedback. To get good statistics, we perform three different simulations for different target galaxies of the same mass of ${\sim}10^{10}M_{sun}$. Each simulation starts in a cubic box of a side length of 1Mpc/h with 17 million particles from z = 49. The mass of dark matter (DM) and gas particle is $M_{DM}=4.1{\times}10^3M_{sun}$ and $M_{gas}=7.9{\times}10^2M_{sun}$, respectively, thus each satellite sub-galactic structure can be resolved with more than hundreds or thousands particles. We analyze total 90 sub-galactic structures that have formed outside of the main halos but infall the main halos. We found that 1) mini halos that interact more with the other mini halos tend to accrete the more mass, 2) mini halos that interact more before the reionization tend to form more stars, 3) mini halos with the more interaction tend to approach closer to the galactic center and have the lower orbital circularity, 4) survivals even in the strong tidal fields evolve baryon dominated system, such as globular clusters.

• 천문학회보
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• 제46권2호
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• pp.45.1-45.1
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• 2021

Horizon Run 5 (HR5) is a cosmological hydrodynamical simulation which captures the properties of the Universe on aGpc scale while achieving a resolution of 1kpc. This enormous dynamic range allows us to simultaneously capture the physics of the cosmic web on very large scales and account for the formation and evolution of dwarf galaxies on much smaller scales. On the back of a remarkable achievement of this, we have finished to run follow-up simulations which have 2 times larger volume than before and are expected to complementary to some limitations of previous HR simulations both for the study on the large scale features and the expansion history in a distant Universe. For these simulations, we consider the sub-grid physics of radiative heating/cooling, reionization, star formation, SN/AGN feedbacks, chemical evolution and the growth of super-massive blackholes. In order to do this project, we implemented a hybrid MPI-OpenMP version of the RAMSES code, 'RAMSES-OMP', which is specifically designed for modern many-core many thread parallel systems. These simulation successfully reproduce various observation result and provide a large amount of statistical samples of Lyman-alpha emitters and protoclusters which are important to understand the formation and expansion history of early universe. These are invaluable assets for the interpretation of current ΛCDM cosmology and current/upcoming deep surveys of the Universe, such as the world largest narrow band imaging survey, ODIN (One-hundred-square-degree Dark energy camera Imaging in Narrow band).

• 천문학회보
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• 제40권1호
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• pp.79.3-80
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• 2015

Galaxy overdensities such as galaxy clusters and superclusters are the largest gravitationally bound systems in the Universe. Since they contain many different levels of local densities, they are excellent places to test galaxy evolution models in connection to the environments. The environment studies of galaxies at z ~ 1 are important because the environmental quenching seems to be an important mechanism to reduce star formation activities in galaxies at z < 1. However, there have been not many studies about high redshift galaxy clusters at z ~ 1 because of the lack of wide and deep multi-wavelength data. We have used the multi-wavelength data from the UKIDSS DXS (J and K band), the SWIRE (4 IRAC bands), and the PAN-STARRS (g, r, i, z, y bands) in the ELAIS-N1 field. We identified galaxy cluster candidates at 0.2 < z < 1.6 using the multi-wavelength data. We found several superclusters where cluster candidates are concentrated on few tens of Mpc scale. Interestingly, some of the supercluster candidates consist of galaxy clusters which have high blue galaxy. We will present high redshift galaxy cluster and supercluster candidates in ELAIS-N1 field and galaxy properties in different environments including dense clusters and fields.

• 천문학회보
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• 제35권1호
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• pp.74.2-74.2
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• 2010

We present a multi-wavelength study of a supercluster in the NEP region at z=0.087, using AKARI (Infrared space telescope) NEP-Wide (5.8 deg2) survey which has obtained an unique IR imaging dataset with contiguous wavelength coverage from 2 to $24{\mu}m$, overcoming the Spitzer limitation of imaging capability at $10-20{\mu}m$. The NEP-Wide survey is also covered in other wavelength such as X-ray, Radio, GALEX UV in the archive, optical (BRI from Maidanak 1.5m and CFHT's MegaPrime), and NIR imaging data (JH from KPNO 2.1m), with nearly 1900 optical spectra, mostly obtained by our group using MMT/Hectospec and WIYN/Hydra. Armed with the multiwavelength datasets, we investigate the connection between IR properties of galaxies and their environments as a tool to understand the evolution of galaxies in a supercluster environment. Specific attention will be given to MIR emission which can trace star formation activities and passive phases right after post-starbursts, and its relation to other wavelength data.

• 천문학회보
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• 제38권1호
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• pp.36.2-36.2
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• 2013

The SPICA (SPace Infrared Telescope for Cosmology & Astrophysics) project is a next-generation infrared space telescope optimized for mid- and far-infrared observation with a cryogenically cooled 3m-class telescope. The focal plane instruments onboard SPICA will enable us to resolve many astronomical key issues from the formation and evolution of galaxies to the planetary formation. The FPC-S (Focal Plane Camera - Sciecne) is a near-infrared instrument proposed by Korea as an international collaboration. Owing to the capability of both low-resolution imaging spectroscopy and wide-band imaging with a field of view of $5^{\prime}{\times}5^{\prime}$, it has large throughput as well as high sensitivity for diffuse light compared with JWST. In order to strengthen advantages of the FPC-S, we propose the studies of probing population III stars by the measurement of cosmic near-infrared background radiation and the star formation history at high redshift by the discoveries of active star-forming galaxies. In addition to the major scientific targets, to survey large area opens a new parameter space to investigate the deep Universe. The good survey capability in the parallel imaging mode allows us to study the rare, bright objects such as quasars, bright star-forming galaxies in the early Universe as a way to understand the formation of the first objects in the Universe, and ultra-cool brown dwarfs. Observations in the warm mission will give us a unique chance to detect high-z supernovae, ices in young stellar objects (YSOs) even with low mass, the $3.3{\mu}$ feature of shocked circumstance in supernova remnants. Here, we report the current status of SPICA/FPC project and its extragalactic sciences.

• 천문학회지
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• 제36권3호
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• pp.89-95
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• 2003

Observations of dark matter dominated dwarf and low surface brightness disk galaxies favor density profiles with a flat-density core, while cold dark matter (CDM) N-body simulations form halos with central cusps, instead. This apparent discrepancy has motivated a re-examination of the microscopic nature of the dark matter in order to explain the observed halo profiles, including the suggestion that CDM has a non-gravitational self-interaction. We study the formation and evolution of self-interacting dark matter (SIDM) halos. We find analytical, fully cosmological similarity solutions for their dynamics, which take proper account of the collisional interaction of SIDM particles, based on a fluid approximation derived from the Boltzmann equation. The SIDM particles scatter each other elastically, which results in an effective thermal conductivity that heats the halo core and flattens its density profile. These similarity solutions are relevant to galactic and cluster halo formation in the CDM model. We assume that the local density maximum which serves as the progenitor of the halo has an initial mass profile ${\delta}M / M {\propto} M^{-{\epsilon}$, as in the familiar secondary infall model. If $\epsilon$ = 1/6, SIDM halos will evolve self-similarly, with a cold, supersonic infall which is terminated by a strong accretion shock. Different solutions arise for different values of the dimensionless collisionality parameter, $Q {\equiv}{\sigma}p_br_s$, where $\sigma$ is the SIDM particle scattering cross section per unit mass, $p_b$ is the cosmic mean density, and $r_s$ is the shock radius. For all these solutions, a flat-density, isothermal core is present which grows in size as a fixed fraction of $r_s$. We find two different regimes for these solutions: 1) for $Q < Q_{th}({\simeq} 7.35{\times} 10^{-4}$), the core density decreases and core size increases as Q increases; 2) for $Q > Q_{th}$, the core density increases and core size decreases as Q increases. Our similarity solutions are in good agreement with previous results of N-body simulation of SIDM halos, which correspond to the low-Q regime, for which SIDM halo profiles match the observed galactic rotation curves if $Q {\~} [8.4 {\times}10^{-4} - 4.9 {\times} 10^{-2}]Q_{th}$, or ${\sigma}{\~} [0.56 - 5.6] cm^2g{-1}$. These similarity solutions also show that, as $Q {\to}{\infty}$, the central density acquires a singular profile, in agreement with some earlier simulation results which approximated the effects of SIDM collisionality by considering an ordinary fluid without conductivity, i.e. the limit of mean free path ${\lambda}_{mfp}{\to} 0$. The intermediate regime where $Q {\~} [18.6 - 231]Q_{th}$ or ${\sigma}{\~} [1.2{\times}10^4 - 2.7{\times}10^4] cm^2g{-1}$, for which we find flat-density cores comparable to those of the low-Q solutions preferred to make SIDM halos match halo observations, has not previously been identified. Further study of this regime is warranted.

• 천문학논총
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• 제27권4호
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• pp.123-128
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• 2012

An overview of the North Ecliptic Pole (NEP) deep multi-wavelength survey covering from X-ray to radio wavelengths is presented. The main science objective of this multi-wavelength project is to unveil the star-formation and AGN activities obscured by dust in the violent epoch of the Universe (z=0.5-2), when the star formation and black-hole evolution activities were much stronger than the present. The NEP deep survey with AKARI/IRC consists of two survey projects: shallow wide (8.2 sq. deg, NEP-Wide) and the deep one (0.6 sq. deg, NEP-Deep). The NEP-Deep provides us with a $15{\mu}m$ or $18{\mu}m$ selected sample of several thousands of galaxies, the largest sample ever made at these wavelengths. A continuous filter coverage at mid-IR wavelengths (7, 9, 11, 15, 18, and $24{\mu}m$) is unique and vital to diagnose the contribution from starbursts and AGNs in the galaxies at the violent epoch. The recent updates of the ancillary data are also provided: optical/near-IR magnitudes (Subaru, CFHT), X-ray (Chandra), FUV/NUV (GALEX), radio (WSRT, GMRT), optical spectra (Keck/DEIMOS etc.), Subaru/FMOS, Herschel/SPIRE, and JCMT/SCUBA-2.