Hyung Mok Lee

The Infrared Astronomical Mission AKARI

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid- to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180 mu m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

The Disk Accretion of a Tidally Disrupted Star onto a Massive Black Hole

We consider the tidal disruption of a star by a moderately massive black hole such as might be found in the nucleus of a galaxy like M31. The initial eccentric orbit of the stellar debris will circularize near the tidal radius after experiencing strong shocks. We study the evolution of the accretion disk produced by this torus using a time dependent α-disk model. We find that the light-to-mass ratio of the disk-plus-black-hole exceeds unity for several thousand years after disruption. Some fraction of galaxies should be extremely bright at far UV wavelengths if they contain black holes of mass 106–8 M ⊙.

EFFECTS OF TIDAL SHOCKS ON THE EVOLUTION OF GLOBULAR CLUSTERS

We present new Fokker-Planck models of the evolution of globular clusters, including gravitational tidal shocks. We extend our calculations beyond the core collapse by adopting three-body binary heating. EUects of the shocks are included by adding the tidal shock diUusion coefficients to the ordinary Fokker-Planck equation: the —rst-order heating term, S*ET, and the second-order energy dispersion term, S*E2T. As an example, we investigate the evolution of models for the globular cluster NGC 6254. Using the Hipparcos proper motions, we are now able to construct orbits of this cluster in the Galaxy. Tidal shocks accelerate signi—cantly both core collapse and the evaporation of the cluster and shorten the destruction time from 24 to 18 Gyr. We examine various types of adiabatic corrections and —nd that they are critical for accurate calculation of the evolution. Without adiabatic corrections, the destruction time of the cluster is twice as short. We examine cluster evolution for a wide range of the concentration and tidal shock parameters and determine the region of the parameter space where tidal shocks domi- nate the evolution. We present —tting formulae for the core collapse time and the destruction time, covering all reasonable initial conditions. In the limit of strong shocks, the typical value of the core collapse time decreases from to or less, while the destruction time is just twice that number. 10t rh 3t rh The eUects of tidal shocks are rapidly self-limiting: as clusters lose mass and become more compact, the importance of the shocks diminishes. This implies that tidal shocks were more important in the past. Subject headings: celestial mechanics, stellar dynamicsglobular clusters: general ¨ globular clusters: individual (NGC 6254)

The Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal-plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 mu m, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcminutes, and the pixel pitch matches the diffraction limit of the telescope. Derived point-spread functions (PSFs) from observations of asteroids are similar to those given by the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and the performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

Cross sections for tidal capture binary formation and stellar merger

The cross sections for tidal capture binary formation are computed for polytropic models with indices n = 3/2, 2, and 3 to represent a wide range of stellar models. The computational procedure for the tidal capture cross sections is summarized, and the results of normal mode analyses and the capture cross sections are presented. It is estimated that 25 percent of the tidally captured binaries become mergers for a stellar system whose rms velocity is roughly 10 km/s (typical for globular clusters), while about 50 percent are expected to become mergers for rms velocity of roughly 300 km/s (typical for galactic nuclei). The implications of these results for the dynamical evolution of stellar systems influenced by tidally captured binaries are briefly discussed. 22 references.

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 parsecs away from the GC, while they appear to be as massive as the smallest Galactic globular clusters (~104 M?). 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 timescale shorter than the lifetimes of most massive stars, which accelerates the clusters 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 2 ? 104 M? evaporate in 10 Myr. Two CYCs observed near the GC?the Arches cluster (G0.121+0.17) and the Quintuplet cluster (AFGL 2004)?are interpreted in terms of the models; their central densities and apparent ages are consistent with the hypothesis that they represent successive stages of cluster evolution along a common track, with both undergoing rapid evaporation. A simple calculation based on the total masses in observed CYCs and the lifetimes obtained here indicates that the massive CYCs make up only a fraction of the star formation rate (SFR) in the inner bulge estimated from Lyman continuum photons and far-IR observations. This is consistent with the observation that many stars in the inner bulge form outside the large clusters.

Environmental dependence of local luminous infrared galaxies

Aims. We study the environmental dependence of local luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies (ULIRGs) found in the Sloan Digital Sky Survey (SDSS) data. Methods. The LIRG and ULIRG samples are constructed by cross-correlating spectroscopic catalogs of galaxies of the SDSS Data Release 7 and the Infrared Astronomical Satellite Faint Source Catalog. We examine the effects of the large-scale background density (∑ 5 ), galaxy clusters and the nearest neighbor galaxy on the properties of infrared galaxies (IRGs). Results. We find that the fraction of LIRGs plus ULIRGs among IRGs (f (U)LIRGs ) and the infrared luminosity (L IR ) of IRGs strongly depend on the morphology of and the distance to the nearest neighbor galaxy: the probability of an IRG being a (U)LIRG (f (U)LRGs ) and its L IR both increases as it approaches a late-type galaxy, but decreases as it approaches an early-type galaxy (within half the virial radius of its neighbor). We find no dependence of f (U)LIRGs on the background density (surface galaxy number density) at a fixed stellar mass of galaxies. The dependence of f (U)LIRCs on the distance to galaxy clusters is also found to be very weak, but in the highest density regions, such as the center of galaxy clusters, few (U)LIRGs are found. Conclusions. This environmental dependence of LIRGs and ULIRGs and the evolution of the star formation rate (SFR)-environment relation from high redshifts to low redshifts seem to support the idea that galaxy-galaxy interactions and merging play a critical role in triggering the star formation activity of LIRGs and ULIRGs.

P Cygni type Lyα from starburst galaxies

P Cygni type Lya emission is exhibited in the spectra of nearly half the starburst galaxies in either the nearby Universe or the high-z Universe. Such emission is presumably formed by an expanding supershell surrounding star-forming regions. We assume that the Lya photons from the central star-forming regions transfer in a spherical galactic supershell of uniform neutral hydrogen that is expanding radially in a bulk flow. We apply the Monte Carlo code developed previously for static and plane-parallel media. We consider typical cases in which the supershell has Lya line-centre optical depth τ 0 = 10 5 -10 7 , a radial expansion velocity V e x p 300 km s - 1 and turbulence b ≃ 40 km s - 1 . We find that a series of emission peaks appear redward of the systemic redshift, their frequencies being given by (2N - 1)V e x p , where the number of back-scatterings N is greater than zero. We also investigate how the emergent profiles are changed in accordance with the expansion velocity and the column density of the supershell. We find that the number and the flux ratios of emission peaks are determined by interplay of the two parameters. It is very important that the kinematics of the supershell is imprinted on both the widths of the peaks and the velocity differences between peaks. We discuss the effects of dust extinction and the implication of our work in relation to recent spectroscopic observations of starburst galaxies.

Mapping dusty star formation in and around a cluster at z= 0.81 by wide-field imaging with AKARI

We present environmental dependence of dusty star-forming activity in and around the cluster RXJ1716.4+6708 at z= 0.81 based on wide-field and multiwavelength observations with the Prime Focus Camera on the Subaru Telescope (Suprime-Cam) and the Infrared Camera onboard the AKARI satellite. Our optical data show that the optical colour distribution of galaxies starts to dramatically change from blue to red at the medium-density environment such as cluster outskirts, groups and filaments. By combining with the AKARI infrared data, we find that 15-μm-detected galaxies tend to have optical colours between the red sequence and the blue cloud with a tail into the red sequence, consistent with being dusty star-forming galaxies. The spatial distribution of the 15-μm-detected galaxies over ∼200 arcmin2 around the cluster reveals that few 15-μm galaxies are detected in the cluster central region. This is probably due to the low star-forming activity in the cluster core. However, interestingly, the fraction of 15-μm-detected galaxies in the medium-density environments is as high as in the low-density field, despite the fact that the optical colours start to change in the medium-density environments. Furthermore, we find that 15-μm-detected galaxies which have optically red colours (candidates for dusty red galaxies) and galaxies with high specific star formation rates are also concentrated in the medium-density environment. These results imply that the star-forming activity in galaxies in groups and filaments is enhanced due to some environmental effects specific to the medium-density environment (e.g. galaxy–galaxy interaction), and such a phenomenon is probably directly connected to the truncation of star-forming activity in galaxies seen as the dramatic change in optical colours in such environment.

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 using Aarseths Nbody6 codes. The relatively small number of stars in the cluster (5000-20,000) makes real-number N-body simulations for these clusters feasible on current workstations. Using Fokker-Planck (F-P) models, Kim, Morris, & Lee made a survey of cluster lifetimes for various initial conditions and have found that clusters with a mass 2 × 104 M☉ 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 Hubble Space Telescope/NICMOS observations by Figer and colleagues in 1999 suggests that the current tidal radius of the Arches cluster is ~1.0 pc and that the parameters for the initial conditions of that cluster are as follows: total mass of 2 × 104 M☉ 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.