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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Results

In Figure 2, we have superimposed contours of the HI column density on an optical image of NGC 6744 extracted from the Digitised Sky Survey. As can be seen, the gas disk extends over some 30', or 1.5 times the size of the stellar disk. A ``ring'' of gas underlies the optical spiral arms, with comparatively little HI in the bulge and inner disk. Two gaseous spiral arms emanate from either end of this ring, and wind for almost $270^{\circ}$ each before fading out. The peak gas column densities occur where these arms join the ring, and near the location of NGC 6744A, the irregular galaxy visible to the NW.

**Figure 2:** Contours of the HI column density (resolution $\sim 55''$ ) overlaid on a blue image of NGC 6744 from the *Digitised Sky Survey*. The IB(s)m galaxy NGC 6744A can be seen near (
$19^{\rm h}~09^{\rm m}$ ,

$-63^{\circ}~44'$ ).
$\begin{figure} \begin{center} \centerline{\psfig{figure=N6744H1ONDSS.PS,width=10cm}}\end{center}\end{figure}$

A better impression of the structure of the HI disk, together with its velocity field, can be had from Figure 3. The rotation pattern is basically that expected for gas in uniform circular motion, but distortions are evident, particularly in the gaseous spiral arms. and around NGC 6744A. By examining the HI datacube, we have been able to kinematically resolve NGC 6744A from the HI arm behind it, and find a redshift for this galaxy of $765\pm10$ km ${\rm s}^{-1}$ . As Figure 3 shows, we have also detected a cloud of HI to the SE. According to the NASA/IPAC Extragalactic Database, this cloud coincides with the location of the SABm galaxy ESO 104-g44. However, we observe HI line-of-sight velocities in this cloud of $846\pm3$ km ${\rm s}^{-1}$ (with virtually no gradient), in contrast with the HI heliocentric velocity of 779 km ${\rm s}^{-1}$ measured by Longmore et al. (1982) using the Parkes 64 m antenna. The difference is most likely due to spillover into the beam from blue-shifted gas in the disk of NGC 6744.

**Figure 3:** Isovelocity contours for the HI in NGC 6744, overlaid on a grey-scale of the same HI distribution as in Figure 2. Notice the severe distortion in the contours due to NGC 6744A. The isolated patch of HI to the SE is associated with ESO 104-g44.
$\begin{figure} \begin{center}\centerline{\psfig{figure=N6744L5VFONH1.PS,width=10cm}}\end{center}\end{figure}$

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Contents Page: Volume 16, Number 1

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