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Interstellar matter. Galaxies

77

Properties of the interstellar gas clouds
I.V. Gosachinskij

St.Petersburg branch of Special Astrophysical observatory, St.Petersburg, Russia The characteristics of interstellar gas clouds play the main role in any theories of star formation. The dense molecular cloud cores are probably immediate parents of stars. However, the molecular clouds themselves arise obviously from some structures of atomic componentof interstellar medium, because namely in the atomic gas may arise two-phase system: clouds and intercloud medium, namely neutral atomic clouds are able to increase their masses and densities due to inelastic collisions and rapid cooling. So the neutral atomic clouds may be named as "grand parents" of stars and their properties play genetic role for whole chain of matter transformations in the galaxies. The RATAN-600 has greatly high sensitivitytolow contrast details of emission observed on the bright complex background. That's whywewere able to create the unique database of HI clouds on the base of HI RATAN-600 Survey. The angular resolution of this Survey was 2 40 1300,velocity resolution was 6.3 km s, r.m.s. uctuations of antenna temperature was 0.25 K. All clouds with kinematical distances 1 0kpc were rejected because their relative distance errors are very high. Moreover, clouds with 0 75 K 3 times r.m.s. errors and line widths 6 3 km s were rejected too. Diameters, masses, gas densities and velocity dispersions of about 7600 HI clouds were determined in the second and third quadrants of galactic longitudes in 180 260 , ,15 +15 100 150 , ,10 +10 . Some selection e ects were discovered and our statistic results were corrected for them.
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Fig. 1


78 It is than shap 16

Interstellar matter. Galaxies found that mean clouds linear diameters along Galactic plane are 2.5 times greater in transverse direction. The cloud diameter spectrum Fig. 1 has a bimodal power e with spectral indexes of 1.9 0.5 between 1 16 pc, and 3.9 0.5 between 45pc.
H

HI gas density spectrum in the range of 1.0 to 300 cm,3 Fig. 2 is not a power-law, but it has a maximum at n =10 40 cm,3 depending on galactic latitude. The lowest and highest densities observed in the clouds are very important parameters for theories of thermal instability and formation of molecules.

Fig. 2 The mass spectrum in the form of MNlog M was obtained in the mass range of 0.6 to 2.5104 M Fig. 3. It consists of at least three parts. In the range of 2 to 600 M the spectrum has a spectral index of 0.8 0.1, in the range of 0.6 to 2 M the spectral index is 3.0 1, and in the range of 600 to 2104 M the spectral index is 0.7 0.3. These data show that the process of coalescence in cloud-cloud collisions predominates in the middle mass range but the clouds with low masses are evaporated probably due to very hot ISM component. In the very high mass range the number of neutral gas clouds may be decreased because of gravitational instability or and molecularization.

Fig. 3


Interstellar matter. Galaxies
n d
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The relation between HI concentrations and cloud diameters is con rmed in the form ,1 25 0 01 Fig. 4 regardless of selection e ects. The correlation coe cient of between and is equal 0.87.
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Fig. 4 It is shown that other important dependence, in particular velocity dispersion versus cloud diameters, that is well de ned for molecular clouds, is completely absent in the case of HI clouds Fig. 5. Probably this is due to negligible role of intrinsic turbulence in the HI clouds.

Fig. 5


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Interstellar matter. Galaxies

It is found that 36 of HI clouds have systematic velocity gradients across cloud disks that is may be due to the rotation of clouds Fig. 6. This phenomenon may explain rather high velocity widths of HI line pro les in the observable clouds. The mean value of clouds angular rotational velocity is about 10,13 s,1 , the mean rotational energy is about 1048 ergs, that is comparable to the energy of random cloud motions. Finally, observable quantities of clouds with opposite directions of rotation are equal within 5 in both galactic quadrants investigated.

Fig. 6 The author is very grateful to his colleagues who provided the RATAN-600 HI Survey during manyyears: N.A. Yudaeva, M.E. Makrosova, Z.A. Alferova and T.V. Monastyreva and especially to V.V. Morosova who made all computer reduction and evaluation of HI clouds database.
This research was supp orted by RFFI grant 96-02-16565.