## 4. The model for NGC 6946We consider galaxies to be differentially rotating turbulent disks
embedded in a plasma of given conductivity (Elstner et al. 1990). In
the simplest case the "plasma" is vacuum and the conductivity
therefore vanishes. Thickness and height of the galactic disk are
determined by the density scales, see Sect. 4.3. The calculated volume
is restricted to the radius ## 4.1. Differential rotationWe describe the differentially rotating gas by a Brandt-type law By selecting the values =
72 Gyr
The shape of rotation curves is still a topic of investigation.
Sofue (1996) claimed that HI data compared to observations of the
molecular gas (CO) yield too small velocity values, especially at
small radii. An approximation to his (CO+HI) rotation curve for
NGC 6946 is also shown in Fig. 2 (dashed line). For this rotation
curve we choose the parameters =
220 Gyr ## 4.2. Non-axisymmetrySince investigations of the H The turbulence intensity can be measured in principle by the velocity dispersion of spectral lines, e.g. HI (Boulanger & Viallefond 1992). However, other effects like unresolved velocity gradients or vertical gas motions will also increase the observed velocity dispersion. On the other hand, a strong arm/interarm contrast in turbulence intensity, as assumed in several previous papers (Shukurov 1998; Rohde & Elstner 1998; Moss 1998; Schreiber & Schmitt 1999) should be detectable through a significant difference in line widths. No such effect has been found in any galaxy so far. In particular, HI data for NGC 6946 do not show any difference in linewidth between arm and interarm regions near the minor axis (Kamphuis 1993 and Kamphuis, priv.comm.). Consequently, is also assumed to be axisymmetric in our models. We introduce a spiral profile attached The arm/interarm contrast is described via the shape of a logarithmic spiral varying between 1 and ## 4.3. Vertical stratification and radial scale lengthsLacking knowledge about the vertical stratification of the gas
density in NGC 6946 we adopt the empirical HI distribution of
Dickey & Lockmann (1990) for our Galaxy: a combination of two
Gaussians with central densities 0.395 and 0.107 cm We further introduce exponential scale lengths for the different
components of the gas disk of NGC 6946 (Tacconi & Young
1986): 10 kpc (HI), 3.5 kpc (HII) and 3 kpc
(H Based on the given density stratification we adopt the common simplification of the vertical momentum equation as a possibility to calculate the turbulence intensity: The used potential is essentially due to a self-gravitating isothermal sheet of stars with constant thickness and we assume Lacking knowledge on the exact density distribution in
NGC 6946 and for the sake of simplicity we neglect radial
dependencies within the potential .
We set an average value =
21.5 km s The velocity dispersion in HI shows a radial dependency decreasing
outwards from approximately 16 km s
© European Southern Observatory (ESO) 1999 Online publication: October 4, 1999 |