Australia Telescope National Facility

An HI Study of the NGC 6744 System
Stuart D. Ryder , Wilfred Walsh , David Malin, PASA, 16 (1), in press.

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Rotation Curve and Dark Matter Model

Using both the VELFIT task within MIRIAD and the GAL task within AIPS, we have attempted to model the velocity field of Figure 3. By fixing only the rotation centre, we find a best-fitting systemic velocity

$V_{\rm sys} = 838\pm2$ km ${\rm s}^{-1}$ , an inclination

$i=(50\pm4)^{\circ}$ , and a line-of-nodes position angle

$\theta=(16\pm2)^{\circ}$ . Fixing all of these parameters then yields the rotation curve in Figure 4, where the error bars reflect the differences from analysing separately the receding and approaching halves of the disk. As can be seen, the rotational velocity rises fairly quickly within the first 3 kpc (due to beam smearing, the actual rise is probably steeper), and then stays just below 200 km ${\rm s}^{-1}$ over the rest of the disk. There is the hint of a rise in the last few points, but the analysis is complicated here by the influence of NGC 6744A, and apparent streaming motions along the arms.

**Figure 4:** Rotation curve for NGC 6744, derived from a kinematical analysis of the HI disk. The points with error bars correspond to the range of values found by analysing both the entire disk, and the receding and approaching halves separately. The dotted curve indicates the rotation due to the atomic gas only, the dashed curve indicates the rotation due to the stellar disk, the dash-dot line is rotation due to the inferred dark matter component, and the solid curve is the sum of all three components.
$\begin{figure} \begin{center} \centerline{\psfig{figure=n6744rc.eps,height=18cm,width=13cm}}\end{center}\end{figure}$

In order to derive the contribution of a dark matter halo to the rotation curve of NGC 6744, we must first compute the contributions of the stellar and gaseous components. Using a 2K x 2K CCD on the Mount Stromlo and Siding Spring Observatories' 40'' reflector, we have constructed a B-band mosaic of NGC 6744 covering some

35' x 37', which has allowed us to determine the run of mean surface brightness with radius in the disk. As a starting point, we assume M/L_B=1 (although this is almost certainly a lower limit) which then gives the radial distribution of stellar surface density. We derive the radial atomic gas mass profile by scaling the HI surface density by 4/3 (to account for He). The relative contributions of the gas, stars, and dark matter to the rotation curve of NGC 6744 have been computed (assuming that the stellar disk dominates in the very inner disk), and the results are shown by the various curves in Figure 4.

There are a number of points to note about this mass model. First, the model curve cannot account for all of the structure in the observed rotation curve, and has particular difficulty in the central 10 kpc. Also note that for the first 20 kpc or so, the rotation velocity of the gas is negative. This comes about as a result of the concentration of HI in the ring producing a net outward force, so that the balance of gravitational and centripetal forces requires that the square of the circular velocity, $V_{\rm c}^{2}$ be negative. $V_{\rm c}$ must therefore be imaginary, but by convention is plotted as the negative of the real part. The implied isothermal dark halo core radius is 1.83 kpc, which is low but not unreasonable, while the halo central density is

$2.10\times10^{-3}$ M $_{\odot}$ pc^-3. This mass model must be considered preliminary until we can better constrain the total HI mass in NGC 6744, and measure the inner rotation curve at higher resolution.

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