arXiv subjects

Hyung Mok Lee

Publications and source records attributed to Hyung Mok Lee.

At least 109 records · Page 6Linked to original sources

arXiv2006

Far-IR Detection Limits II: Probing Confusion including Source Confusion

We present a comprehensive analysis for the determination of the confusion levels for the current and the next generation of far-infrared surveys assuming three different cosmological evolutionary scenarios. We include an extensive model for diffuse emission from infrared cirrus in order to derive absolute sensitivity levels taking into account the source confusion noise due to point sources, the sky confusion noise due to the diffuse emission, and instrumental noise. We use our derived sensitivities to suggest best survey strategies for the current and the future far-infrared space missions Spitzer, AKARI (ASTRO-F), Herschel, and SPICA. We discuss whether the theoretical estimates are realistic and the competing necessities of reliability and completeness. We find the best estimator for the representation of the source confusion and produce predictions for the source confusion using far-infrared source count models. From these confusion limits considering both source and sky confusions, we obtain the optimal, confusion limited redshift distribution for each mission. Finally, we predict the Cosmic Far-Infrared Background (CFIRB) which includes information about the number and distribution of the contributing sources.

astro-ph↗

arXiv2005

Discovery of Remote Star Clusters in the Halo of the Irregular Galaxy NGC 6822

We report the discovery of three new star clusters in the halo of the Local Group dwarf irregular galaxy NGC 6822. These clusters were found in the deep images taken with the MegaPrime at the CFHT, covering a total field of 2 deg x 2 deg. The most remote cluster is found to be located as far as 79 arcmin away from the center of NGC 6822. This distance is several times larger than the size of the region in NGC 6822 where star clusters were previously found. Morphological structures of the clusters and color-magnitude diagrams of the resolved stars in the clusters show that at least two of these clusters are proabably old globular clusters.

astro-ph↗

arXiv2005

Bonnor-type Black Dihole Solution in Brans-Dicke-Maxwell Theory

It was originally thought that Bonnor's solution in Einstein-Maxwell theory describes a singular point-like magnetic dipole. Lately, however, it has been demonstrated that indeed it may describe a black {\it dihole}, i.e., a pair of static, oppositely-charged extremal black holes with regular horizons. Motivated particularly by this new interpretation, in the present work, the construction and extensive analysis of a solution in the context of the Brans-Dicke-Maxwell theory representing a black dihole are attempted. It has been known for some time that the solution-generating algorithm of Singh and Rai produces stationary, axisymmetric, charged solutions in Brans-Dicke-Maxwell theory from the known such solutions in Einstein-Maxwell theory. Thus this algorithm of Singh and Rai's is employed in order to construct a Bonnor-type magnetic black dihole solution in Brans-Dicke-Maxwell theory from the known Bonnor solution in Einstein-Maxwell theory. The peculiar feature of the new solution including internal infinity nature of the symmetry axis and its stability issue have been discussed in full detail.

gr-qc↗

arXiv2005

Effects of External Tidal Field on the Evolution of Multi-mass Star Clusters

We present N-body simulations of realistic globular clusters containing initial mass function in the galaxy to study effects of tidal field systematically on the properties of outer parts of globular clusters. Using NBODY6 which takes into account the two-body relaxation correctly, we investigate general evolution of globular clusters in galactic tidal field. For simplicity, we have employed only spherical components (bulge and halo) of the galaxy. Total number of stars in our simulations was about 20,000. All simulations were done for several orbital periods in order to understand the development of the tidal tails. In our scaled down models, the relaxation time is sufficiently short to show the mass segregation effect, but we did not go far enough to see the core-collapse, and the fraction of stars lost from the cluster at the end of simulation is only about 10%. The radial distribution of extra-tidal stars can be described by a power law with a slope around -3. The direction of tidal tails are determined by the orbits and locations of the clusters. We find that the length of tidal tails increases towards the apogalacticon and decreases towards the perigalacticon. This is an anti-correlation with the strength of the tidal field, caused by the fact that the the time-scale for the stars to respond to the potential is similar to the orbital time-scale of the cluster. When the length of tidal tails decreases some of the stars in the tidal tails are recaptured by the host cluster. From the investigation of velocity anisotropy of the model clusters, we find that in the early stages of globular cluster evolution the clusters have radial anisotropy in the outermost parts, while clusters are nearly isotropic in the cental region. The radial anisotropy decreases with time.

astro-ph↗

arXiv2005

Pulsar Magnetosphere : A General Relativistic Treatment

A fully general relativistic description of the pulsar magnetosphere is provided. To be more concrete, a study of the pulsar magnetosphere is performed in the context of general relativistic magnetohydrodynamics (MHD) employing the so-called Grad-Shafranov approach. Not surprisingly, the resulting Grad-Shafranov equations and all the other related general relativistic MHD equations turn out to take essentially the same structures as those for the (rotating) black hole magnetosphere. Other different natures between the two cases including the structure of singular surfaces of MHD flows in each magnetosphere are essentially encoded in the different spacetime (metric) contents. In this way, the pulsar and the black hole magnetospheres can be described in an unified fashion. Particularly, the direction of poloidal currents circulating in the neutron star magnetosphere turns out to be the same as that of currents circulating in the black hole magnetosphere which, in turn, leads to the pulsar and the black hole spin-downs via the ``magnetic braking''.

astro-ph↗

arXiv2004

Far-IR Detection Limits I: Sky Confusion Due to Galactic Cirrus

Fluctuations in the brightness of the background radiation can lead to confusion with real point sources. Such background emission confusion will be important for infrared observations with relatively large beam sizes since the amount of fluctuation tends to increase with angular scale. In order to quantitively assess the effect of the background emission on the detection of point sources for current and future far-infrared observations by space-borne missions such as Spitzer, ASTRO-F, Herschel and SPICA, we have extended the Galactic emission map to higher angular resolution than the currently available data. Using this high resolution map, we estimate the sky confusion noise due to the emission from interstellar dust clouds or cirrus, based on fluctuation analysis and detailed photometry over realistically simulated images. We find that the confusion noise derived by simple fluctuation analysis agrees well with the result from realistic simulations. Although the sky confusion noise becomes dominant in long wavelength bands (> 100 um) with 60 - 90cm aperture missions, it is expected to be two order of magnitude smaller for the next generation space missions with larger aperture sizes such as Herschel and SPICA.

astro-ph↗

arXiv2004

Mass Segregation and Tidal Tails of the Globular Cluster NGC 7492

We present a wide field CCD photometric study of Galactic globular cluster NGC 7492. The derived VR color-magnitude diagram extends down to about 3.5 mag below the cluster main sequence turn-off. The field covers 42' X 42' about 3 times larger than the known tidal radius of this cluster. The sample of cluster member candidates obtained by CMD-mask process has been used to construct luminosity and mass functions and surface density map. NGC 7492 has a very flat mass function with very little variation in the slope with the distance from the cluster center. However, there is a clear evidence for the increase of the slope of the mass function from inner to outer regions, indicating a mass segregation of the cluster. The surface density map of NGC 7492 shows extensions toward the Galactic anticenter (northeast) and northwest from the cluster center. A comparison of luminosity function for stars in the tails with that for stars within the tidal radius suggests that the extensions shown in the surface density map could be a real feature. The overall shape of NGC 7492 is significantly flattened. If the flattened shape of the NGC 7492 is caused by its rotation, Galactic tidal field must have given important influences, since the initial rotation would have been almost completely removed by dynamical relaxation.

astro-ph↗

arXiv2004

Nuclear Spirals as Signatures of Supermassive Black Holes

Recent high resolution images of spiral galaxies show wide varieties of features including nuclear spirals in the central parts. Some of the galaxies show grand-design nuclear spirals. The morphology of grand-design spirals can be further divided by the openness of the arms: tightly wound ones with winding angle of around 3$π$ radian and open spirals with winding angle of around $π$ radian. Based on hydrodynamical simulations, we have investigated the mechanism responsible for the openness of nuclear spirals. Since the gas flow in the nuclear region is mainly governed by the central mass concentration near the nuclei and the sound speed of the gas, we have examined various models with different mass concentration represented by the mass of the central black hole and different sound speeds. We found that the tightly wound spirals can be formed when the mass of the black hole is large enough to remove the inner-inner Lindblad resonances and sound speeds lie between 15 - 20 km/sec. Thus, the presence of the tightly wound nuclear spiral could imply the presence of relatively massive black hole in the center.

astro-ph↗

arXiv2004

Dynamical Evolution of Rotating Stellar Systems: III. The Effect of Mass Spectrum

We have studied the dynamical evolution of rotating star clusters with mass spectrum using a Fokker-Planck code. As a simplest multi-mass model, we first investigated the two-component clusters. Rotation is found to accelerate the dynamical evolution through the transfer of angular momentum outward, as well as from the high masses to the low masses. However, the degree of acceleration depends sensitively on the assumed initial mass function since dynamical friction, which generates mass segregation, also tends to accelerate the evolution, and the combined effect of both is not linear or multiplicative. As long as dynamical friction dominates in the competition with angular momentum exchange the heavy masses lose random energy and angular momentum, sink towards the centre, but their remaining angular momentum is sufficient to speed them up rotationally. This is gravo-gyro instability. As a consequence, we find that the high mass stars in the central parts rotate faster than low mass stars. This leads to the suppression of mass segregation compared to the non-rotating clusters. From the study of multi-component models, we observe similar trends to the two-component models in almost all aspects. The mass function changes less drastically for clusters with rotation. Unlike non-rotating clusters, the mass function depends on $R$ and $z$. Our models are the only ones that can predict mass function and other quantities to be compared with new observations.

astro-ph↗

arXiv2003

Charges and Magnetic Fields of the Slowly-rotating Neutron Stars

In association with the Goldreich-Julian's magnetic braking mechanism to explain the luminosity of radio pulsars as a result of their spin-down, it is of some interest to explore how much of magnetic flux can actually penetrate the surface of the rotating neutron stars at least in idealized situations. In order to address this issue, one needs to figure out the amount of charge on and the structure of magnetic field around a rotating neutron star. In the present work, based on the solution-generating method given by Wald, the magnetic fields around both the uncharged and (slightly) charged neutron star have been obtained. Particularly for the charged neutron star, it has been demonstrated following again the argument by Wald that the neutron star will gradually accrete the charge until it reaches the equilibrium value $\tilde{Q}=2B_{0}J$. Then next, the magnetic flux through one half of the surface of the rotating neutron star has been computed as well. With the nonvanishing accretion charge having value in the range $0 < \tilde{Q} \leq 2B_{0}J$, the total magnetic flux through the neutron star has been shown to be greater than that without the accretion charge.

astro-ph↗

arXiv2003

ASTRO-F/FIS Observing Simulation: Detection Limits for Point Sources

We describe the observing simulation software FISVI (FIS Virtual Instrument), which was developed for the Far-Infrared Surveyor (FIS) that will be on the Japanese infrared astronomy mission ASTRO-F. The FISVI has two purposes: one is to check the specifications and performances of the ASTRO-F/FIS as a whole; the other is to prepare input data sets for the data analysis softwares prior to launch. In the FISVI, special care was taken by introducing the "Compiled PSF (Point Spread Function)" to optimise inevitable, but time-consuming, convolution processes. With the Compiled PSF, we reduce the computation time by an order of magnitude. The photon and readout noises are included in the simulations. We estimate the detection limits for point sources from the simulation of virtual patches of the sky mostly consisting of distant galaxies. We studied the importance of source confusion for simple power-law models for N(>S), the number of sources brighter than S. We found that source confusion plays a dominant role in the detection limits only for models with rapid luminosity evolution for the galaxy counts, the evolution of which is suggested by recent observations.

astro-ph↗

arXiv2002

Wide field CCD photometry of the globular cluster M92

We present wide field CCD photometry of a galactic globular cluster M92 obtained in the V and I bands with the CFH12K mosaic CCD at the Canada-France-Hawaii Telescope. A well-defined color-magnitude diagram is derived down to 5 magnitudes fainter than the cluster main sequence turn-off. After removing the background contribution, we obtain luminosity and mass functions, surface density profiles, and the surface number density maps of the stars belonging to the cluster. The surface density profile of all stars shows that the cluster's halo extends at least out to ~30' from the cluster center in agreement with previous study, but the profile of faint stars at the very outer region of the cluster shows a different gradient compared with that of bright stars. For a mass function of the cluster, we find that the inner region of the cluster has x = 1.2+-0.2, whereas the outer region has x = 1.8 +-0.3, clearly indicating a mass segregation of the cluster. An estimate of the photometric mass of the cluster implies that the remnant populations (white dwarfs and neutron stars) contribute at least 25% of the total cluster mass. The surface density map of M92 shows some evidence that the tidal tail of M92 may be oriented perpendicular to the direction toward the Galactic center.

astro-ph↗

arXiv2002

P-Cygni Type Lya from Starburst Galaxies

P-Cygni type Lya profiles exhibited in nearly half of starburst galaxies, both nearby and high-z, are believed to be formed by an expanding supershell surrounding a star-forming region. We apply the Monte Carlo code which was developed previously for static and plane-parallel medium to calculate the Lya line transfer in a supershell of neutral hydrogen which are expanding radially in a spherical bulk flow. We consider typical cases that the supershell has the Lya line-centre optical depth of $τ_0=10^5-10^7$, a radial expansion velocity of $V_exp = 300 km/s, and the turbulence of b=40 km/s. We find that there appear a few emission peaks at the frequencies corresponding to (2N-1) V_exp, where the order of back scatterings N > 1. As V_exp -> b, the emergent profiles become similar to those for the static medium and the peaks are less prominent. We also investigate the effects of column density of the supershell on the emergent Lya profiles. We find that the number and the flux ratios of emission peaks are determined by interplay of $τ_0$ and V_exp of the supershell. We discuss the effects of dust extinction and the implication of our works in relation to recent spectroscopic observations of starburst galaxies.

astro-ph↗

arXiv2001

Lyman alpha line formation in starbursting galaxies II. Extremely Thick, Dustless, and Static HI Media

The Lya line transfer in an extremely thick medium of neutral hydrogen is investigated by adopting an accelerating scheme in our Monte Carlo code to skip a large number of core or resonant scatterings. This scheme reduces computing time significantly with no sacrifice in the accuracy of the results. We applied this numerical method to the Lya transfer in a static, uniform, dustless, and plane-parallel medium. Two types of photon sources have been considered, the midplane source and the uniformly distributed sources. The emergent profiles show double peaks and absorption trough at the line-center. We compared our results with the analytic solutions derived by previous researchers, and confirmed that both solutions are in good agreement with each other. We investigated the directionality of the emergent Lya photons and found that limb brightening is observed in slightly thick media while limb darkening appears in extremely thick media. The behavior of the directionality is noted to follow that of the Thomson scattered radiation in electron clouds, because both Lya wing scattering and Thomson scattering share the same Rayleigh scattering phase function. The mean number of wing scatterings just before escape is in exact agreement with the prediction of the diffusion approximation. The Lya photons constituting the inner part of the emergent profiles follow the relationship derived from the diffusion approximation. We present a brief discussion on the application of our results to the formation of Lya broad absorption troughs and P-Cygni type Lya profiles seen in the UV spectra of starburst galaxies.

astro-ph↗

arXiv2001

Dynamical evolution of rotating stellar systems - II. Post-collapse, equal mass system

We present the first post core collapse models of initially rotating star clusters, using the numerical solution of an orbit-averaged 2D Fokker-Planck equation. Based on the code developed by Einsel & Spurzem (1999), we have improved the speed and the stability and included the steady three-body binary heating source. We have confirmed that rotating clusters, whether they are in a tidal field or not, evolve significantly faster than non-rotating ones. Consequences for observed shapes, density distribution, and kinematic properties of young and old star clusters are discussed. The results are compared with gaseous and 1D Fokker-Planck models in the non-rotating case.

astro-ph↗

arXiv2001

Lyman alpha line formation in starbursting galaxies I. Moderately thick, dustless, and static HI media

We investigate the Lyman alpha line transfer in nearby and high redshift starbursting galaxies, where the effect of high optical depths and the role of dust in the scattering medium are expected to be conspicuous and should be treated in a very careful manner. We present our first results in dustless, static, and uniform HI media with moderate Lyman alpha line center optical depths τ_0=10^{3-6}. We assume that the temperatures of media to be T=10^{1-4}K, and use a Monte Carlo technique. We investigate the basic processes of the line transfer and confirm the criterion of aτ_0>10^3 for the validity of diffusion approximation suggested by Neufeld in 1990, where $a$ is the Voigt parameter. Adopting the model suggested by Tenorio-Tagle et al., we performed calculations on the Lyman alpha line formation for each evolutionary stage of an expanding supershell. The emergent Lyman alpha profiles are characterized by the double peaks and the absorption trough at the line center. We found that the absorption troughs expected in most of the evolutionary stages are not wide enough to be observed with current instruments. However, the absorption trough in the Lyman alpha emission profile from an expanding recombining supershell can be marginally detected.

astro-ph↗

arXiv2000

N-Body Simulations of Compact Young Clusters near the Galactic Center

We investigate the dynamical evolution of compact young star clusters (CYCs) near the Galactic center (GC) using Aarseth's Nbody6 codes. The relatively small number of stars in the cluster (5,000-20,000) makes real-number N-body simulations for these clusters feasible on current workstations. Using Fokker-Planck (F-P) models, Kim, Morris, & Lee (1999) have made a survey of cluster lifetimes for various initial conditions, and have found that clusters with a mass <~ 2x10^4 Msun evaporate in ~10 Myr. These results were, however, to be confirmed by N-body simulations because some extreme cluster conditions, such as strong tidal forces and a large stellar mass range participating in the dynamical evolution, might violate assumptions made in F-P models. Here we find that, in most cases, the CYC lifetimes of previous F-P calculations are 5-30% shorter than those from the present N-body simulations. The comparison of projected number density profiles and stellar mass functions between N-body simulations and HST/NICMOS observations by Figer et al. (1999) suggests that the current tidal radius of the Arches cluster is ~1.0 pc, and the following parameters for the initial conditions of that cluster: total mass of 2x10^4 Msun and mass function slope for intermediate-to-massive stars of 1.75 (the Salpeter function has 2.35). We also find that the lower stellar mass limit, the presence of primordial binaries, the amount of initial mass segregation, and the choice of initial density profile (King or Plummer models) do not significantly affect the dynamical evolution of CYCs.

astro-ph↗

arXiv1999

Evaporation of Compact Young Clusters near the Galactic Center

We investigate the dynamical evolution of compact young clusters (CYCs) near the Galactic center (GC) using Fokker-Planck models. CYCs are very young (< 5 Myr), compact (< 1 pc), and only a few tens of pc away from the GC, while they appear to be as massive as the smallest Galactic globular clusters (~10^4 Msun). A survey of cluster lifetimes for various initial mass functions, cluster masses, and galactocentric radii is presented. Short relaxation times due to the compactness of CYCs, and the strong tidal fields near the GC make clusters evaporate fairly quickly. Depending on cluster parameters, mass segregation may occur on a time scale shorter than the lifetimes of most massive stars, which accelerates the cluster's dynamical evolution even more. When the difference between the upper and lower mass boundaries of the initial mass function is large enough, strongly selective ejection of lighter stars makes massive stars dominate even in the outer regions of the cluster, so the dynamical evolution of those clusters is weakly dependent on the lower mass boundary. The mass bins for Fokker-Planck simulations were carefully chosen to properly account for a relatively small number of the most massive stars. We find that clusters with a mass <~ 2x10^4 Msun evaporate in <~ 10 Myr. A simple calculation based on the total masses in observed CYCs and the lifetimes obtained here indicates that the massive CYCs comprise only a fraction of the star formation rate (SFR) in the inner bulge estimated from Lyman continuum photons and far-IR observations.

astro-ph↗