2 Observations and data reduction

 

 

Table 1: Details of ATCA 375-m array observations.

Observing Date	1996, Dec. 8-9
Net Observing Time	11:00 h
Shortest Baseline	31 m
Longest Baseline$^{\rm a}$	459 m
Flux Calibrator	1934-638
Phase Calibrator	0537-441
Primary Beam ( FWHM)	33'
Restored Beam ( FWHM)	$127\hbox{$.\!\!^{\prime\prime}$ }9\times77\hbox{$.\!\!^{\prime\prime}$ }3$
Bandwidth (helio. vel.)	620-1675 km s-1
Velocity Resolution	6.6 km s-1
Sensitivity per Channel$^{\rm b}$	5 mJy beam-1
$S_{\rm HI}/T_{\rm B}$ Conversion Factor$^{\rm c}$	62.2 Jy/K
Pointing1 ( $\alpha,\delta(2000)$)	05:05:13.8, -37:58:49
Pointing2 ( $\alpha,\delta(2000)$)	05:05:40.9, -37:35:00
Pointing3 ( $\alpha,\delta(2000)$)	05:06:52.8, -37:40:06
Pointing4 ( $\alpha,\delta(2000)$)	05:06:52.9, -37:49:28
Pointing5 ( $\alpha,\delta(2000)$)	05:07:15.6, -37:54:33
Pointing6 ( $\alpha,\delta(2000)$)	05:07:42.1, -37:30:44

Notes to Table 1:

a) Discarding data from antenna 6.

b) 1-$\sigma$ rms at the centre of the field of view.

c) Conversion from antenna brightness temperature to flux
density units for the angular resolution of our data of
$127\hbox{$.\!\!^{\prime\prime}$ }9\times77\hbox{$.\!\!^{\prime\prime}$ }3$ (ATCA Users Guide; October 1999).

Our observations were carried out in late 1996; the most important dates and parameters are listed in Table 1. Of the 12:45 hours about 1:45 h were spent on flux, bandpass and phase calibration. The rest of the time was split evenly among six pointing directions. One was centred on NGC1792, one on NGC1808 and the remaining four between the two galaxies, as specified in the table.

All data were calibrated in the usual way, using the MIRIAD package installed at the Australia Telescope National Facility. We assumed a continuum flux density at 1.42 GHz of PKSB1934-638 of 15.01 Jy. PKSB0537-441, which served as phase calibrator, was observed at regular intervals during the observations. The data for the six pointings were separated from each other in the uv plane of the visibilities and calibrated separately. Subsequently the data were recombined using the MIRIAD task UVAVER and then jointly cleaned, using the task MOSSDI, and restored with a common angular resolution (Full Width at Half Maximum) of $FWHM = 127\hbox{$.\!\!^{\prime\prime}$ }9\times77\hbox{$.\!\!^{\prime\prime}$ }3$. For our attempt to detect extended low surface brightness emission, two original line channels each were averaged, resulting in a velocity resolution of 6.6 km s^-1. The resulting data cube has 160 channels of 6.6 km s^-1width each and covers a field of view (FOV) of approximately $1\hbox{$.\!\!^\circ$ }4\times1\hbox{$.\!\!^\circ$ }2$. The major axis of this elongated FOV is oriented along the line connecting NGC1792 and NGC1808. The data were also corrected for the effect of primary beam attenuation across the FOV.

Continuum subtraction was performed by subtracting the average of the line-free channels from each of the channels containing line emission in each of the sub-datasets.