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RISS 인기검색어
- 검색키워드
- 주제어 : ISM: structure-turbulence (검색결과 5 건)
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TURBULENCE IN THE OUTSKIRTS OF THE MILKY WAY
Sanchez-Salcedo, F.J.,Santillan, A.,Franco, Jose The Korean Astronomical Society 2007 Journal of The Korean Astronomical Society Vol.40 No.4
In external galaxies, the velocity dispersion of the atomic hydrogen gas shows a remarkably flat distribution with the galactocentric radius. This has been a long-standing puzzle because if the gas velocity dispersion is due to turbulence caused by supernova explosions, it should decline with radius. After a discussion on the role of spiral arms and ram pressure in driving interstellar turbulence in the outer parts of galactic disks, we argue that the constant bombardment by tiny high-velocity halo clouds can be a significant source of random motions in the outer disk gas. Recent observations of the flaring of H I in the Galaxy are difficult to explain if the dark halo is nearly spherical as the survival of the streams of tidal debris of Sagittarius dwarf spheroidal galaxy suggests. The radial enhancement of the gas velocity dispersion (at R > 25 kpc) due to accretion of cloudy gas might naturally explain the observed flaring in the Milky Way. Other motivations and implications of this scenario have been highlighted.
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FRACTAL DIMENSIONS OF INTERSTELLAR MEDIUM: I. THE MOLECULAR CLOUDS IN THE ANTIGALACTIC CENTER
YOUNGUNG LEE 한국천문학회 2004 Journal of The Korean Astronomical Society Vol.37 No.5
We have estimated the fractal dimension of the molecular clouds in the Antigalactic Center based onthe12CO (J = 1 - 0) and13CO (J = 1- 0) database obtained using the 14m telescope at Taeduk RadioAstronomy Observatory. Using a developed code within IRAF, we were able to identify slice-clouds,and determined the dispersions of two spatial coordinates as well as perimeters and areas. The fractaldimension of the target region was estimated to beD = 1 :34 for low resolution12CO (J = 1- 0)database, and D = 1.4 for higher resolution12CO (J = 1- 0) and13CO (J = 1 0) database, whereP ∝ A^D/ 2. The sampling rate (spatial resolution) of observed data must be an important parameterwhen estimating fractal dimension. Our database with higher resolution of 1 arcminute, which iscorresponding to 0.2 pc at a distance of 1.1 kpc, gives us the same estimate of fractal dimensionto that of local dark clouds. Fractal dimension is apparently invariant when varying the thresholdtemperatures applied to cloud identication. According to the dispersion pattern of longitudes andlatitudes of identied slice-clouds, there is no preference of elongation direction.
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THE FRACTAL DIMENSION OF THE $\rho$ OPHIUCUS MOLECULAR CLOUD COMPLEX
Yongung Lee,Di Li,Y. S. Kim,J. H. Jung,H. W. Kang,C. H. Lee,I. S. Yim,H. G. Kim 한국천문학회 2016 Journal of The Korean Astronomical Society Vol.49 No.6
We estimate the fractal dimension of the $\rho$ Ophiuchus Molecular Cloud Complex, associated with star forming regions. We selected a cube ($v, l, b$) database, obtained with $J=1-0$ transition lines of \co and \tco at a resolution of $22''$ using a multibeam receiver system on the 14-m telescope of the Five College Radio Astronomy Observatory. Using a code developed within IRAF, we identified slice-clouds with two threshold temperatures to estimate the fractal dimension. With threshold temperatures of 2.25 K (3$\sigma$) and 3.75 K (5$\sigma$), the fractal dimension of the target cloud is estimated to be $D = 1.52$--1.54, where $P \propto A^{D/2}$ , which is larger than previous results. We suggest that the sampling rate (spatial resolution) of observed data must be an important parameter when estimating the fractal dimension, and that narrower or wider dispersion around an arbitrary fit line and the intercepts at NP = 100 should be checked whether they relate to rms noise level or characteristic structure of the target cloud. This issue could be investigated by analysing several high resolution databases with different quality (low or moderate sensitivity).
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Statistics of velocity centroids: effects of density–velocity correlations and non-Gaussianity
Esquivel, A.,Lazarian, A.,Horibe, S.,Cho, J.,Ossenkopf, V.,Stutzki, J. Blackwell Publishing Ltd 2007 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.381 No.4
<P>ABSTRACT</P><P>We continue with our previous work on statistics of velocity centroids, to retrieve information about the scaling properties of an underlying turbulent velocity field from spectroscopic observations. We use synthetic data sets with extreme effects of velocity–density correlations that we create artificially, which also have a non-Gaussian distribution of fluctuations. We confirm that centroids can be used to obtain the scaling properties of the turbulent velocity when the ratio of the density dispersion to the mean density is less than unity, regardless of velocity–density correlations and non-Gaussianity. It was found that extreme velocity–density correlations can distort the statistics of velocity centroids, impeding the recovery of the turbulent velocity spectral index from centroids. We show that such correlations introduce high-order moments to the maps of centroids, which we disregarded in previous work, but that they are only important when the density dispersion is large in comparison with the mean density. It was also found that non-Gaussian velocity and/or density distort the statistics of centroids too, but to a lower degree than extreme cross-correlations.</P>
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FRACTAL DIMENSIONS OF INTERSTELLAR MEDIUM: I. THE MOLECULAR CLOUDS IN THE ANTIGALACTIC CENTER
LEE YOUNGUNG The Korean Astronomical Society 2004 Journal of The Korean Astronomical Society Vol.37 No.4
We have estimated the fractal dimension of the molecular clouds in the Antigalactic Center based on the $^{12}CO$ (J = 1- 0) and $^{13}CO$ (J = 1- 0) database obtained using the 14m telescope at Taeduk Radio Astronomy Observatory. Using a developed code within IRAF, we were able to identify slice-clouds, and determined the dispersions of two spatial coordinates as well as perimeters and areas. The fractal dimension of the target region was estimated to be D = 1.34 for low resolution $^{12}CO$ (J = 1 - 0) database, and D = 1.4 for higher resolution $^{12}CO$ (J = 1 - 0) and $^{13}CO$ (J = 1 - 0) database, where $P {\propto} A^{D/2}$. The sampling rate (spatial resolution) of observed data must be an important parameter when estimating fractal dimension. Our database with higher resolution of 1 arcminute, which is corresponding to 0.2 pc at a distance of 1.1 kpc, gives us the same estimate of fractal dimension to that of local dark clouds. Fractal dimension is apparently invariant when varying the threshold temperatures applied to cloud identification. According to the dispersion pattern of longitudes and latitudes of identified slice-clouds, there is no preference of elongation direction.
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