Results

The HI observations were carried out in April-May 1995 with the ATCA in three configurations: 1.5B, 1.5C, 6C. The integration time for each configuration was 12 hrs. We observed with a 8 MHz bandwidth and 512 channels, centred at a frequency of 1407 MHz (corresponding to 2830 km s). The data were calibrated using the standard ATCA procedures in the MIRIAD environment. The cubes were produced from the data using a robust weighting with robustness set to 0.3, and a channel width of 10 km s. The noise in each channel map of the high resolution data is 0.8 mJy beam. The continuum was observed at a frequency of 1380 MHz, and we determined a total flux for NGC 4650A of 5.4 mJy.
The new ATCA observations shows in addition to the HI detected by vGSK87 a faint, more extended HI structure. We detect additional filaments and/or detached HI clouds out to 4' north and south of the galaxy centre. The total HI flux in the map is 23 Jy km s. The inner parts of the HI distribution are very elongated, the projected axis ratio of the HI contours is which implies a nearly edge-on HI distribution out to from the galaxy centre, while it becomes more face-on in the outer regions (), see Figure 1.

 
Figure 1: Contours of HI column density for NGC 4650A from the high resolution data superposed on the Digitalised Sky Survey image. Contours are drawn from atoms cm or 1.62 pc, in steps of atoms cm or 12.6 pc. The size of the synthesised beam is with P.A. =, and is shown by the small shaded ellipse in the lower left corner.

The HI intensity map shows several secondary peaks and wiggles in the central distribution, with spatial scales of . The very regular velocity field is shown in Figure 2, superposed on the ESO NTT B-band image.

 
Figure 2: High resolution 2-D velocity field superposed on the ESO NTT B-band image; iso-velocity contours are drawn in step of 10 km s. Where the polar structure is clearly seen in front of the S0 there are perturbations in the velocity contours, and no peaks in the HI distribution.

The new high resolution HI data, combined with the optical and NIR images allow us to test the proposed models for the PR galaxy NGC 4650A. Models for this galaxy should simultaneously account for the morphology of the polar structure in the optical and NIR, plus the HI distribution and kinematics. We must discuss published geometries for the PR which were formulated prior the acquisition of the NIR images and the high resolution HI data. SS suggested a warped geometry for the PR which assumed a linear change in inclination beginning at to face-on at the center and becoming edge-on at a distance of 50''. S94 proposed a descriptive warp model based on the assumption that the bisymmetric knots seen at in the PR B-band image are caused by orbit crowding and superposition, and in rough agreement with the axis ratio of the HI contours of the vGSK87 data at large radii: the S94 warp model assumes that all orbits are polar, but that their inclinations to the sky plane vary linearly with galactocentric radius along the ring major axis such that the ring is from edge-on at the center, is edge-on at 30'', and from edge-on at r=90''. As stated by S94 in Section 4.1 this warp differs only slightly from that assumed by SS, and we test this last one against our new data. This model is in agreement with the 2-D HI velocity field, but fails to reproduce the clear ``S'' shape morphology of the PR in the NIR K-band image (Arnaboldi et al. 1996). The comparison of the HI map with the new NTT B-band image obtained in good seeing (FWHM=1'') indicates that the HI peaks do not follow peaks in the light, but they coincide with absorption features (Arnaboldi et al. 1996), contrary to what is expected from projected warp geometries (see for example the case of NGC 660, van Driel et al. 1995, Arnaboldi & Galletta 1993). The HI data, the optical and NIR images can be reconciled by the presence of spiral arms in the edge-on polar disk (Arnaboldi et al. 1996): a very deep image of the field around NGC 4650A obtained by David Malin shows low surface brightness features NW of the PR which follows the proposed spiral arm structure by Arnaboldi et al. 1996.
These new ATCA HI observations contradict the basic hypothesis assumed so far in the modelling of the dynamics of NGC 4650A i.e. that the polar structure is a ring and its dynamics is driven by the potential generated by the mass distribution (luminous + dark) associated with the S0. The presence of spiral arms in the polar structure indicates that polar disk is very massive. These new results do strengthen the relationship between wide PR galaxies and spirals as previously pointed out by Arnaboldi et al. (1995) and Combes & Arnaboldi (1996). The scenarios for the formation of such a polar disk will have serious implications about the nature of the dark matter which must be investigated: for an accretion event to produce such a flat disk, dissipative dark matter might be essential.

© Copyright Astronomical Society of Australia 1997