Observations

HI observations were carried out on 1997 January 3 in the A configuration of the Very Large Array (VLA)¹ using flux calibrators, J1331+305 (3C286) and J0137+331 (3C48), and phase calibrator, J0957+553. Appropriate uv restrictions were applied during calibration. The field center was placed 13 arcseconds south of the quasar to avoid baseline-dependent map errors which tend to show up at the field center and, for highest sensitivity, the 4IF mode was used. The central observing velocity was set to 974 km s^-1, which is between the velocities expected for a flat rotation curve and a Keplerian fall-off for NGC 3079, and encompasses both possibilities with a total bandwidth of 163.4 km s^-1. The band contained 63 spectral channels which, with on-line Hanning smoothing, gave a final channel width (= spectral resolution) of 2.59 km s^-1.

The primary flux density calibrators were also used as bandpass calibrators and were observed 4 times during the 10 hours of observations. The calibration observation closest in time to the source observation was applied. Examination of the average bandpass for each time showed that the maximum temporal change in the bandpass over the 4 scans was of order 0.2%.

UV data for each channel were Fourier Transformed and cleaned using uniform weighting to create a total emission (RA-DEC-Velocity) cube. A continuum image was also made by averaging together the uv data for all low-noise channels before Fourier Transformation. The data were phase-only and then phase+amplitude self-calibrated until no improvement in rms resulted.

A continuum-subtracted cube was made by subtracting a linear fit to the visibilities from each low-noise total emission channel. The resulting rms noise in the continuum-subtracted maps is 0.58 0.01 mJy beam^-1, where the error represents the 1 variation over all channels. Channel-to-channel variations due to the bandpass calibration are smaller than this. The spectral dynamic range, i.e. the ratio of peak continuum intensity to rms noise in the channel maps is 300/1.

Figure 1: Left: Optical image of NGC 3079, showing the position of the background quasar, Q 0957+561 (large star). The small star denotes the outermost point at which HI is observed in emission around NGC 3079 (Irwin & Seaquist 1991). Right: Continuum image of Q 0957+561, labelled according to Greenfield et al.(1985) and Avruch et al.(1997). Contours are at -0.26, 0.26 (2), 0.3, 0.4, 0.5, 0.8, 1.5, 2.5, 5, 10, 20, 30, 50, 75, 120, and 170 mJy beam^-1 and the beam size is 1.31

x 1.15

at position angle, -87.31.

© Copyright Astronomical Society of Australia 1997