2 Observations

 

2.1 Sample

The sample includes 51 well classified (from their optical spectra) Seyfert galaxies south of declination $0\hbox{$^\circ$}$ and with redshift cz<3600 km s^-1. More details about selection and optical observations of this sample are presented in Tsvetanov et al. (in preparation), Tsvetanov et al. (1998).

The 29 sources listed in Table 1 were observed with either the VLA or ATCA, depending on their declination. One source, NGC 3393, was observed with both arrays. The array configurations and frequencies were chosen to obtain a resolution of $\sim1$$^{\prime\prime}$ in order to match the resolution of the optical data. In particular, the VLA was used in its B and $B_{\rm N}A$ configurations, providing a resolution of $\sim$ 1$^{\prime\prime}$ at 6 cm (4.9 GHz). To obtain similar resolution with the ATCA required an observing frequency of 3.5 cm (8.6 GHz) with the 6 km configuration. Owing to missing short baselines in either array, our data are more sensitive to compact emission from the circumnuclear region but less sensitive to extended, diffuse emission. In addition, faint, diffuse emission from active spirals tends to have very steeply falling radio spectra with increasing frequency; at observing wavelengths of 3.5 and 6 cm any diffuse emission will probably have dropped below our surface brightness detection limit.

  

Table 1: VLA observations (6 cm)

Many of those sources lying north of $\delta \sim -30^\circ$ have been observed in previous studies. The data available in the literature will be included in our discussion in Sect. 4.

Thoughout the paper we adopt a Hubble constant H₀=75 km s^-1 Mpc^-1.

2.2 VLA observations

VLA snapshot observations were obtained for eight of the survey Seyferts in the declination range $-30\hbox{$^\circ$}<\delta<0\hbox{$^\circ$}$.The VLA observations are summarized in Table 1. The observations were carried out using the standard 6 cm (4.9 GHz) continuum mode, that is, with two 50 MHz-wide channels at bandwidth-separated frequencies (4.835 & 4.885 GHz). Three sources were observed using the B-array (8 Apr. 1993); the remaining five were observed using the $B_{\rm N}$ array (1 Feb. 1993).

Data reduction followed standard procedures using the NRAO software package AIPS. The calibration sequence includes: (1) calibrating the flux scale against observations of the flux standards 3C 48 & 3C 286, adopting the standard scaling of Baars et al. (1977) and adjusting for calibrator variability; (2) bootstrapping this flux scale to observations of the phase calibrators; (3) performing station-based phase and amplitude calibrations on the phase calibrators; and (4) applying the phase calibrator solutions to the target sources. Four iterations of phase-only self-calibration were then applied to each source. Solution intervals were typically 5-10 minutes for each iteration of self-calibration.

2.3 ATCA observations

The ATCA observing parameters are summarized in Table 2. Twenty-two objects in the most southern part of this sample ($\delta < -30^\circ$) were observed at 3.5 cm (8.6 GHz) with the ATCA in 6 km configuration. The only overlap with the VLA observations is NGC 3393.

  

Table 2: ATCA observations (3.5 cm)

The observations occurred during July, 1995. Data were taken simultaneously at 8.256 and 8.896 GHz, each through a bandwidth of 128 MHz. These dual-frequency observations improved the coverage of the (u,v)-plane and the sensitivity. Use of dual-frequency synthesis substantially improves the image quality generated by data from sparse arrays.

We used the MIRIAD package (Sault et al. 1995) for data reduction. The flux density scale was calibrated against observations of PKS 1934-638, assumed to be 2.84 Jy at 8.6 GHz according to the latest analysis by Reynolds (1996). Each source was observed in 8 scans with a duration of roughly 20 min each. The scans were spread throughout a 12 hour observing period in order to optimize the (u,v) coverage within the available integration time. Sources with particularly weak or interesting structure were repeated in order to improve the signal-to-noise and image quality.