arXiv subjects

Kwang-Il Seon

Publications and source records attributed to Kwang-Il Seon.

48 records · Page 3Linked to original sources

arXiv2013

Far-ultraviolet Observations of the Taurus-Perseus-Auriga Complex

We have constructed a far-ultraviolet (FUV) continuum map of the Taurus-Auriga-Perseus complex, one of the largest local associations of dark clouds, by merging the two data sets of GALEX and FIMS, which made observations at similar wavelengths. The FUV intensity varies significantly across the whole region, but the diffuse FUV continuum is dominated by dust scattering of stellar photons. A diffuse FUV background of $\sim$1000 CU is observed, part of which may be attributable to the scattered photons of foreground FUV light, located in front of the thick clouds. The fluorescent emission of molecular hydrogen constitutes $\sim$10% of the total FUV intensity throughout the region, generally proportional to the local continuum level. We have developed a Monte Carlo radiative transfer code and applied it to the present clouds complex to obtain the optical properties of dust grains and the geometrical structures of the clouds. The albedo and the phase function asymmetry factor were estimated to be $0.42^{+0.05}_{-0.05}$, and $0.47^{+0.11}_{-0.27}$, respectively, in accordance with theoretical estimations as well as recent observations. The distance and thickness of the four prominent clouds in this complex were estimated using a single slab model applied individually to each cloud. The results obtained were in good agreement with those from other observations in the case of the Taurus cloud, as its geometrical structure is rather simple. For other clouds, which were observed to have multiple components, the results gave distances and thicknesses encompassing all components of each cloud. The distance and thickness estimations were not crucially sensitive to the exact values of the albedo and the phase function asymmetry factor, while the locations of the bright field stars relative to the clouds as initial photon sources seem to be the most important factor in the process of fitting.

astro-ph.GA↗

arXiv2012

What determines the sizes of red early-type galaxies?

The sizes of galaxies are known to be closely related with their masses, luminosities, redshifts and morphologies. However, when we fix these quantities and morphology, we still find large dispersions in the galaxy size distribution. We investigate the origin of these dispersions for red early-type galaxies, using two SDSS-based catalogs. We find that the sizes of faint galaxies (log(M_dyn/M_sun) < 10.3 or M_r > -19.5, where M_r is the r-band absolute magnitude, k-corrected to z = 0.1) are affected more significantly by luminosity, while the sizes of bright galaxies (log(M_dyn/M_sun) > 11.4 or M_r < -21.4) are by dynamical mass. At fixed mass and luminosity, the sizes of low-mass galaxies (log(M_dyn/M_sun) ~ 10.45 and M_r ~ -19.8) are relatively less sensitive to their colors, color gradients and axis ratios. On the other hand, the sizes of intermediate-mass (log(M_dyn/M_sun) ~ 10.85 and M_r ~ -20.4) and high-mass (log(M_dyn/M_sun) ~ 11.25 and M_r ~ -21.0) galaxies significantly depend on those parameters, in the sense that larger red early-type galaxies have bluer colors, more negative color gradients (bluer outskirts) and smaller axis ratios. These results indicate that the sizes of intermediate- and high-mass red early-type galaxies are significantly affected by their recent minor mergers or rotations, whereas the sizes of low-mass red early-type galaxies are affected by some other mechanisms. Major dry mergers also seem to have influenced on the size growth of high-mass red early-type galaxies.

astro-ph.CO↗

arXiv2012

The Column Density Variance in Turbulent Interstellar Media: A Fractal Model Approach

Fractional Brownian motion (fBm) structures are used to investigate the dependency of column density variance (σ_{\ln N}^2) in the turbulent interstellar medium on the variance of three-dimensional density (σ_{\lnρ}^2) and the power-law slope of the density power spectrum. We provide quantitative expressions to infer the three-dimensional density variance, which is not directly observable, from the observable column density variance and spectral slope. We also investigate the relationship between the column density variance and sonic Mach number (Ms) in the hydrodynamic (HD) regime by assuming the spectral slope and density variance as functions of sonic Mach number, as obtained from the HD turbulence simulations. They are related by the expression σ_{\ln N}^2 = Aσ_{\lnρ}^2 = Aln(1+b^2M^2), suggested by Burkhart & Lazarian for the magnetohydrodynamic (MHD) case. The proportional constant A varies from ~ 0.2 to ~ 0.4 in the HD regime as the turbulence forcing parameter b increases from 1/3 (purely solenoidal forcing) to 1 (purely compressive forcing). It is also discussed that the parameter A is lowered in the presence of a magnetic field.

astro-ph.GA↗

arXiv2012

Far-Ultraviolet Spectral Images of the Vela Supernova Remnant: Supplements and Comparisons with other Wavelength Images

We present the improved far-ultraviolet (FUV) emission-line images of the entire Vela supernova remnant (SNR) using newly processed SPEAR/FIMS data. The incomplete C III λ977 and O VI λλ1032, 1038 images presented in the previous study are updated to cover the whole region. The C IV λλ1548, 1551 image with a higher resolution and new images at Si IV λλ1394, 1403, O IV] λ1404, He II λ1640.5, and O III] λλ1661, 1666 are also shown. Comparison of emission line ratios for two enhanced FUV regions reveals that the FUV emissions of the east enhanced FUV region may be affected by nonradiative shocks of another very young SNR, the Vela Jr. SNR (RX J0852.0-4622, G266.6-1.2). This result is the first FUV detection that is likely associated with the Vela Jr. SNR, supporting previous arguments that the Vela Jr. SNR is close to us. The comparison of the improved FUV images with soft X-ray images shows that a FUV filamentary feature forms the boundary of the northeast-southwest asymmetrical sections of the X-ray shell. The southwest FUV features are characterized as the region where the Vela SNR is interacting with slightly denser ambient medium within the dim X-ray southwest section. From a comparison with the Hα image, we identify a ring-like Hα feature overlapped with an extended hot X-ray feature of similar size and two local peaks of C IV emission. Their morphologies are expected when the Hα ring is in direct contact with the near or far side of the Vela SNR.

astro-ph.GA↗

arXiv2012

On the origins of the diffuse H-alpha emission: Ionized gas or dust-scattered H-alpha halos?

It is known that the diffuse H-alpha emission outside of bright H II regions not only are very extended, but also can occur in distinct patches or filaments far from H II regions, and the line ratios of [S II] 6716/H-alpha and [N II] 6583/H-alpha observed far from bright H II regions are generally higher than those in the H II regions. These observations have been regarded as evidence against the dust-scattering origin of the diffuse H-alpha emission (including other optical lines), and the effect of dust scattering has been neglected in studies on the diffuse H-alpha emission. In this paper, we reexamine the arguments against dust scattering and find that the dust-scattering origin of the diffuse H-alpha emission cannot be ruled out. As opposed to the previous contention, the expected dust- scattered H-alpha halos surrounding H II regions are, in fact, in good agreement with the observed H-alpha morphology. We calculate an extensive set of photoionization models by varying elemental abundances, ionizing stellar types, and clumpiness of the interstellar medium (ISM) and find that the observed line ratios of [S II]/H-alpha, [N II]/H-alpha, and He I 5876/H-alpha in the diffuse ISM accord well with the dust-scattered halos around H II regions, which are photoionized by late O- and/or early B-type stars. We also demonstrate that the H-alpha absorption feature in the underlying continuum from the dust-scattered starlight ("diffuse galactic light") and unresolved stars is able to substantially increase the [S II]/H-alpha and [N II]/H-alpha line ratios in the diffuse ISM.

astro-ph.GA↗

arXiv2012

Simulation Study of Dust-Scattered Far-Ultraviolet Emission in the Orion-Eridanus Superbubble

We present the results of dust scattering simulations carried out for the Orion Eridanus Superbubble region by comparing them with observations made in the far-ultraviolet. The albedo and the phase function asymmetry factor (g-factor) of interstellar grains were estimated, as were the distance and thickness of the dust layers. The results are as follows: 0.43$^{+0.02}_{-0.04}$ for the albedo and 0.45$^{+0.2}_{-0.2}$ for the g-factor, in good agreement with previous determinations and theoretical predictions. The distance of the assumed single dust layer, modeled for the Orion Molecular Cloud Complex, was estimated to be ~110 pc and the thickness ranged from ~130 at the core to ~50 pc at the boundary for the region of the present interest, implying that the dust cloud is located in front of the superbubble. The simulation result also indicates that a thin (~10 pc) dust shell surrounds the inner X-ray cavities of hot gas at a distance of ~70-90 pc.

astro-ph.GA↗

arXiv2011

Comparison of the Diffuse Halpha and FUV Continuum Backgrounds: On the Origins of the Diffuse Halpha Background

We compare the diffuse H$α$ map of our Galaxy with the FUV (1370-1710Å) continuum map. The H$α$ intensity correlates well with the FUV intensity. The H$α$/FUV intensity ratio increases in general with the H$α$ intensity and the FUV hardness ratio (1370-1520Å to 1560-1710Å), implying that late OB stars may be the main source of the H$α$ recombination line at high latitudes. The variation of the H$α$ intensity as a function of the Galactic latitude is also very similar to that of the FUV intensity. The results likely suggest that not only the original radiation sources of the H$α$ and FUV backgrounds but also the radiative transfer mechanisms responsible for the diffuse backgrounds are largely common. Therefore, we propose a scenario wherein the H$α$ background at high latitudes is mostly composed of two components, H$α$ photons produced by in-situ recombination at the ionized regions around late OB stars and dust-scattered light of the H$α$ photons originating from late OB stars.

astro-ph.GA↗

arXiv2011

AKARI Near-infrared Spectral Observations of Shocked H2 Gas of the Supernova Remnant IC 443

We present near-infrared (2.5 - 5.0 um) spectra of shocked H2 gas in the supernova remnant IC 443, obtained with the satellite AKARI. Three shocked clumps-known as B, C, and G-and one background region were observed, and only H2 emission lines were detected. Except the clump B, the extinctioncorrected level population shows the ortho-to-para ratio of ~ 3.0. From the level population of the clumps C and G-both AKARI's only and the one extended with previous mid-infrared observations-we found that the v = 0 levels are more populated than the v > 0 levels at a fixed level energy, which cannot be reproduced by any combination of H2 gas in Local Thermodynamic Equilibrium. The populations are described by the two-density power-law thermal admixture model, revised to include the collisions with H atoms. We attributed the lower (n(H2)=10^(2.8-3.8) cm-3) and higher (n(H2)=10^(5.4-5.8) cm-3) density gases to the shocked H2 gas behind C-type and J-type shocks, respectively, based on several arguments including the obtained high H I abundance n(H I)/n(H2)=0.01. Under the hierarchical picture of molecular clouds, the C-type and J-type shocks likely propagate into "clumps" and "clouds" (interclump media), respectively. The power-law index b of 1.6 and 3.5, mainly determined by the lower density gas, is attributed to the shock-velocity diversity, which may be a natural result during shock-cloud interactions. According to our results, H2 v = 1 - 0 S(1) emission is mainly from J-shocks propagating into interclump media. The H2 emission was also detected at the background region, and this diffuse H2 emission may originate from collisional process in addition to the ultraviolet photon pumping.

astro-ph.HE↗

arXiv2009

Can the Lyman Continuum leaked out of H II regions explain Diffuse Ionized Gas?

We present an attempt to explain the diffuse H_alpha emission of a face-on galaxy M 51 with the ``standard'' photoionization model, in which the Lyman continuum (Lyc) escaping from H II regions propagates large distances into the diffuse interstellar medium (ISM). The diffuse H_alpha emission of M 51 is analyzed using thin slab models and exponential disk models in the context of the ``on-the-spot'' approximation. The scale height of the ionized gas needed to explain the diffuse H_alpha emission with the scenario is found to be of the order of ~ 1-2 kpc, consistent with those of our Galaxy and edge-on galaxies. The model also provides a vertical profile, when the galaxy is viewed edge-on, consisting of two-exponential components. However, it is found that an incredibly low absorption coefficient of kappa_0 ~ 0.4-0.8 kpc^{-1} at the galactic plane, or, equivalently, an effective cross-section as low as sigma_eff ~ 10^{-5} of the photoionization cross-section at 912A is required to allow the stellar Lyc photons to travel through the H I disk. Such a low absorption coefficient is out of accord with the properties of the ISM. Furthermore, we found that even the model that has the DIG phase only and no H I gas phase shows highly concentrated H_alpha emissions around H II regions, and can account for only < 26% of the H_alpha luminosity of the DIG. This result places a strong constraint on the ionizing source of the DIG. We also report that the H_alpha intensity distribution functions not only of the DIG, but also of H II regions in M 51, appear to be lognormal.

astro-ph.GA↗

arXiv2007

Smoothing of an All-sky Survey Map with a Fisher-von Mises Function

A convolution of the all-sky survey data with a smoothing function is crucial in calculating the smooth surface brightness of the sky survey data. The convolution is usually performed using a spherical version of the convolution theorem. However, a Gaussian function, applicable only in the flat-sky approximation, has been usually adopted as a smoothing kernel. In this paper, we present an exact analytic solution of the spherical-harmonics transformation of a Fisher-von Mises function, the mathematical version of a Gaussian function in spherical space. We also obtain the approximate solutions exp[-l(l + 1)/2k]. The exact and the approximate solutions may be useful when an astrophysical survey map is convolved with a smoothing function of k>1.

astro-ph↗

arXiv2006

SPEAR Far Ultraviolet Spectral Images of the Cygnus Loop

We present far-ultraviolet (FUV) spectral images, measured at C IV 1550, He II 1640, Si IV+O IV] 1400, and O III] 1664, of the entire Cygnus Loop, observed with the Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR) instrument, also known as FIMS. The spatial distribution of FUV emission generally corresponds with a limb-brightened shell, and is similar to optical, radio and X-ray images. The features found in the present work include a ``carrot'', diffuse interior, and breakout features, which have not been seen in previous FUV studies. Shock velocities of 140-160 km/s is found from a line ratio of O IV] to O III], which is insensitive not only to resonance scattering but also to elemental abundance. The estimated velocity indicates that the fast shocks are widespread across the remnant. By comparing various line ratios with steady-state shock models, it is also shown that the resonance scattering is widespread.

astro-ph↗

arXiv2006

The SPEAR Instrument and On-Orbit Performance

The SPEAR (or 'FIMS') instrumentation has been used to conduct the first large-scale spectral mapping of diffuse cosmic far ultraviolet (FUV, 900-1750 AA) emission, including important diagnostics of interstellar hot (10^4 K - 10^6 K) and photoionized plasmas, H_2, and dust scattered starlight. The instrumentation's performance has allowed for the unprecedented detection of astrophysical diffuse far UV emission lines. A spectral resolution of 550 and an imaging resolution of 5' is achieved on-orbit in the Short (900 - 1175 AA) and Long (1335 - 1750 AA) bandpass channels within their respective 7.4 deg x 4.3' and 4.0 deg x 4.6' fields of view. We describe the SPEAR imaging spectrographs, their performance, and the nature and handling of their data.

astro-ph↗