ATCA Observations

NGC 6744 was observed with a total of 5 different configurations of the ATCA in the period between April and November of 1996. Further details of the observations are given in Table 1. To be sure of covering the entire HI disk of NGC 6744 and its nearest neighbours, we mapped a 20 pointing mosaic covering almost

120' x 100', as indicated in Figure 1. The calibrator PKS B1934-638 served as both the primary and secondary flux calibrator, on account of its close proximity () to NGC 6744. The data for each configuration were edited and calibrated with AIPS. Mosaicing, imaging, and deconvolution were carried out using the MIRIAD suite of software (Sault, Teuben & Wright 1995), which implements the ``joint approach'' to interferometric mosaicing discussed in Sault et al. (1996).

A maximum entropy-based deconvolution task was used to deconvolve the mosaiced dirty image. While being well-suited to imaging of extended objects, the algorithm leads to an inherently positive bias in the flux. This bias can be minimised if the total source flux is known a priori, but of course, one of the reasons for doing a mosaic of NGC 6744 in the first place was to determine the total extent and mass of HI. Since the total flux is not yet well-constrained, each of the channel maps in the cleaned HI cube exhibits a ``pedestal'' of emission underneath the dominant gaseous component at each velocity. For this reason, we confine the present analysis mainly to the kinematical aspects of NGC 6744, which are less affected by this positive bias.

Table 1: ATCA observing parameters for NGC 6744.

Parameter	Value
(J2000 pointing centre)
(J2000 pointing center)
Distance adopted	10.4 Mpc
Configurationsa	0.375, 0.75A, 0.75D, 1.5A, 1.5D
Natural-weighted beam FWHP	54'' x 51'' ( kpc)
Channel map rms noise	3.6 mJy beam-1
Channel increment	9.9 km

^aFor further details about the configurations, consult the ATCA Users' Guide.

Figure 1: Mosaic pattern on the sky for NGC 6744, as traced by the ATCA antennas. Integrations of s were averaged before moving on to the next position.

© Copyright Astronomical Society of Australia 1997