• Journal of Astronomy and Space Sciences
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• 제2권2호
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• pp.101-115
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• 1985

Using Brandt model the mess distribution of the late type spiral galaxy NGC 6946 was derived, and the total mass was reestimated to understand the M/L ratio of this galaxy. Two kinds of the rotation curve with shape parameter n=1 and 3.3 were examined. The following are the main results; (1) The total masses of NGC 6946 are $3.1\times$$10^{11}$ M(n=1) AND $2.8\times$$10^{11}$ M(n=3.3) respectively. and the corresponding M/L are about 17 and 16 for both cases. (2) The optical image in the blue light, whose radius is 9.6 kpc, has 8$\times$$10^{10}$ M and 1.4$\times$$10^{11}$ M. These give the value of M/L about 5 and 8 respectively. (3) The masses and M/L of the nuclear region within 1.2 kpc are 4.0$\times$$10^{9}$ M 4.7$\times$$10^9$ M and 3, 4 for both cases. Those of the disk from 1.2 kpc to 9.6 kpc are 7.6$\times$1$10^{10}$M, 1.4$\times$$10^{11}$M, and 5, 8. (4) The masses of the outer halo extended to few hundreds kiloparsecs are 2.3$\times$$10^{11}$ M and 1.4$\times$$10^{11}$M. The corresponding M/L are about 62 and 37.

• 천문학회보
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• 제36권2호
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• pp.78.1-78.1
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• 2011

We report on a photometric study of star clusters in the circumnuclear star-forming region of nearby barred spiral galaxy NGC 1672. We use FUV to NIR imaging data (FUV, U, B, V, R, $H{\alpha}$, I, and H) in the Hubble Space Telescope (HST) archive. We have selected 89 star clusters with V<21.8 (MV<-9) mag in the central 600 pc region, and have derived their age & mass by comparing their colors with theoretical population synthesis models. Most of the star clusters are mildly reddened (E(B-V)~0.2 on average), but some star clusters suffer from severe reddening (E(B-V)>0.6). The mass of the star clusters are in the range from $10^4\;M_{\odot}$ to $10^7\;M_{\odot}$. About half of them are more massive than $10^5\;M_{\odot}$. Their ages range from 1 Myr to several 100 Myr, and concentrate at ~6 Myr and ~40 Myr. It indicates that the episodic starburst in the circumnuclear star-forming region lasted at least several 100 Myr. We find no obvious azimuthal age gradient, but we find a radial age gradient in some regions. We discuss these properties with theoretical expectations.

• 천문학회보
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• 제39권1호
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• pp.38.1-38.1
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• 2014

A non-axisymmetric mass distribution in the galactic bulge (or bar) causes gas flow from the disk to the nuclear region, inducing intense star formation in the nucleus. We investigate the relation between the ellipticity of the bulge and the presence of a nuclear starburst by using a volume-limited sample of galaxies. We use 1,680 spiral galaxies with Mr < -19.5 at 0.02 <= z < 0.05 in the Sloan Digital Sky Survey Data Release 7. We find that the occurrence of nuclear starburst has a moderate correlation with bulge ellipticity in intermediate-type spiral galaxies (morphology classes Sab~Sb) in low galaxy number density environments. In high galaxy number density environments, close encounters and mergers between galaxies can cause gas inflow to the nuclear region even without the presence of non-axisymmetric bulges.

• 천문학회보
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• 제40권1호
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• pp.77.2-77.2
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• 2015

Galaxies are known to evolve passively in the cluster environment. Indeed, much evidence for HI stripping has been found in cluster galaxies to date, which is likely to be connected to their low star formation rate. What is still puzzling however, is that the molecular gas, which is believed to be more directly related to star formation, shows no significant difference in its fraction between the cluster population and the field galaxies. Therefore, HI stripping alone does not seem to be enough to fully understand how galaxies become passive in galaxy clusters. Intriguingly, our recent high resolution CO study of a subsample of Virgo spirals which are undergoing strong ICM pressure has revealed a highly disturbed molecular gas morphology and kinematics. The morphological and kinematical peculiarities in their CO data have many properties in common with those of HI gas in the sample, indicating that strong ICM pressure in fact can have impacts on dense gas deep inside of a galaxy. This implies that it is the molecular gas conditions rather than the molecular gas stripping which is more responsible for quenching of star formation in cluster galaxies. In this study, using multi transitions of 12CO and 13CO, we investigate the density and temperature distributions of CO gas of a Virgo spiral galaxy, NGC 4402 to probe the physical and chemical properties of molecular gas and their relations to star formation activities.

• 천문학회지
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• 제30권1호
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• pp.95-106
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• 1997

We investigate the velocity distribution of dark matter in the disk of a galaxy like the Milky Way at the solar radius. Using N-body simulations with the total mass and z-component of angular momentum conserved, we calculate the response of a dissipationless dark matter galactic halo during the dissipational collapse of the baryonic matter in spiral galaxy formation. The initial distribution of dark matter and baryonic particles is assumed to be a homogeneous mixture based on a King model. The baryonic matter is assumed to contract, forming the final luminous components of the galaxy, namely the disk and, in some cases, a bulge and central point. Both slow and fast growth of the luminous components are considered. We find that the velocity distribution of dark matter particles in a reference frame rotating slowly about the galaxy center in the plane of the disk is similar to a Maxwellian, but it is somewhat boxier, being flatter at the peak and truncated in the tails of the distribution. We tabulate parameters for the best-fitting Maxwellian and modified-Maxwellian distributions. There is no significant difference between slow collapse and fast collapse for all these results. We were unable to detect any effect of disk formation on the z-dependence of the dark matter density distribution.

• 천문학회보
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• 제36권1호
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• pp.87.1-87.1
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• 2011

We analyze the I-GALFA HI 21-cm line survey data in order to study the characteristics of interstellar turbulence in the neutral hydrogen medium in the Galaxy. We select several regions of 4.3 deg x 4.3 deg area near or far from the galactic plane both in the inner and outer Galaxy, transform the power of those regions into Fourier planes and derive one- and two-dimensional power spectra of HI emission. Our Fourier-analysis shows that the iso-power contours generally elongate along the latitude direction more in the outermost spiral arm, which indicates that the HI structure is "filamentary" and mainly aligned along the longitude. At high latitudes or in the interarm region, on the other hand, the iso-power contours are close to circles implying that the HI structures are randomly distributed or "clumpy". In the inner Galaxy, we derive two-dimensional spectra both far from and near the arm and explore the nature of the turbulence.

• 천문학논총
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• 제30권2호
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• pp.463-464
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• 2015

It has been suggested that only the most luminous AGNs ($L{\gtrsim}10^{45}erg/s$) are triggered by galaxy mergers, while less luminous AGNs ($L{\sim}10^{43}erg/s$) are driven by other internal processes. The lack of merging features in low luminosity AGN host galaxies has been a primary argument against the idea of merger triggering of low luminosity AGNs. But a merger, especially a rather minor one, might still have played an important role in low luminosity AGNs, as minor merging features at low luminosities are more difficult to identify than major merging features. Using SNUCAM on the 1.5 m telescope at Maidanak observatory, we obtained deep optical images of NGC 7743, a barred spiral galaxy classified as a Seyfert 2 AGN with a low bolometric luminosity of $5{\times}10^{42}erg/s$. Surprisingly, we discovered a merging feature around the galaxy, which indicates past merging activity in the galaxy. This example indicates that the merging fraction of low luminosity AGNs may be much higher than previously thought, hinting at the importance of galaxy mergers even in low luminosity AGNs.

• 천문학회보
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• 제41권1호
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• pp.39.3-39.3
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• 2016

About one tenth of dwarf elliptical galaxies found in the Virgo cluster have a disk component, and some of them even possess substructures such as bars, lens, and spiral arms. We use N-body simulations to study the formation of these non-axisymmetric features in disky dwarf elliptical galaxies. By mimicking VCC 856, a bulgeless dwarf galaxy with embedded faint spiral arms, we construct 11 sets of initial conditions with slight dynamical variations based on observational data. Our standard model starts slowly to form a bar at ~3 Gyr and then undergoes buckling instability that temporarily weakens the bar although the bar strength continues to grow afterward. We find 9 of our models are unstable to bar formation and undergo buckling instability. This suggests that disky dwarf elliptical galaxies are intrinsically unstable to form bars, accounting for a population of barred dwarf galaxies in the outskirts of the Virgo cluster. To understand the origin of the faint grand-design spiral arms, we additionally construct 6 sets of models that undergo tidal interactions with their neighbors. We find that faint spiral arms consistent with observations develop when tidal forcing is relatively weak although strong encounter still results in bar formation. We discuss our results in light of the dynamical evolution of dwarf elliptical galaxies including mergers.

• 천문학회보
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• 제39권1호
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• pp.37.2-37.2
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• 2014

Gas in disk galaxies interacts nonlinearly with a underlying stellar spiral potential to form galactic spiral shocks. Numerical simulations typically show that these shocks are unstable to the wiggle instability, forming non-axisymmetric structures with high vorticity. While previous studies suggested that the wiggle instability may arise from the Kelvin-Helmholtz instability or orbit crowding of gas elements near the shock, its physical nature remains uncertain. It was even argued that the wiggle instability is of numerical origin, caused by the inability of a numerical code to resolve a shock that is inclined to numerical grids. In this work, we perform a normal-mode linear stability analysis of galactic spiral shocks as a boundary-value problem. We find that the wiggle instability originates physically from the potential vorticity generation at a distorted shock front. As the gas follows galaxy rotation, it periodically passes through multiple shocks, successively increasing its potential vorticity. This sets up a normal-mode that grows exponentially, with a growth rate comparable to the orbital angular frequency. We show that the results of our linear stability analysis are in good agreement with the those of local hydrodynamic simulations of the wiggle instability.

• 천문학회지
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• 제25권1호
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• pp.11-21
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• 1992

Applying mass model to disk galaxy NGC 300, since the observed rotation curve of NGC 300 is flatter than Toomre's mass model n = 1, two cases are used; obtaining parameters $a^n$ and $b^n$ from the polynomial fitting of the observed rotation curve (case A) and from the least square fitting between the observed rotation curve and model rotation curve (case B). In any case, n bas a fixed value of 1. Brandt's mass model is also discussed. which has a shape parameter n = 1.4. Calculated total mass and total mass to luminosity ratio are $3.3{\times}10^{10}M_{\odot}$, l2.1 for case A and $2.8{\times}10^{10}M_{\odot}$, 10.3 for case B. In case of Brandt's model, the values are $4.2{\times}10^{10}M_{\odot}$ and 15.4. The rise in the local mass to luminosity ratio in the outer part of NGC 300 implies existence of massive halo. Other dynamical properties are also discussed.