2 Observations and data reduction

2.1 Australia Telescope Compact Array observations

 

 

Table 1: ATCA observation details.

Galaxy	Arraya	Date	Int. time
			[h:mm]
NGC1511	750A	1998, May 03	9:00
	750B	1997, Aug. 10	9:30
	750C	1997, Oct. 20	9:30
	1.5D	1998, Oct. 19	4:50
	1.5D	1998, Oct. 20	6:00
	1.5C	1999, Apr. 10	8:50
NGC7090	750A	1998, May 02	9:30
	750B	1997, Aug. 06	9:30
	750C	1997, Oct. 18	9:30
	1.5D	1998, Oct. 17	5:20
	1.5D	1998, Oct. 20	3:00
	1.5B	1999, Apr. 02	8:40
NGC7462	750A	1998, May 04	9:00
	750B	1997, Aug. 08	9:30
	750C	1997, Oct. 19	9:30
	1.5D	1998, Oct. 18	2:30
	1.5D	1998, Oct. 20	1:00

Note to Table 1:

^a) Full aperture synthesis for each ATCA array is obtained by
combining data from 4 configurations, named A, B, C and D.
Three configurations already provide good uv coverage.

The galaxies observed by us with the ATCA are NGC1511, NGC7090 and NGC7462. Each was observed with three different configurations of the 750 m array and two different configurations of the 1.5 km array, see Table 1. The individual observing runs were 13 hours long (including time for calibration), providing almost full 12 hour aperture syntheses in each configuration. The total on-source integration times range from 32 h (NGC7462) to 47 h (NGC1511). The ATCA is capable of observing at two frequencies simultaneously. We observed the radio continuum at 2.45 GHz and 1.43 GHz (13 cm and 21 cm wavelength, respectively). All data are polarisation calibrated. Due to technical problems part of the 1.5 km array observations of NGC7462 could not be used.

1934-638 was used as the primary flux calibrator, 0407-658 (for NGC1511) and 2106-413 (for both NGC7090 and NGC7462) as polarisation and phase calibrators. The adopted flux of 1934-638 is 11.14 (14.94) Jy at 2.45 (1.43) GHz. The data reduction was performed in a standard fashion, using the software package MIRIAD.

With angular extents of their radio emission of 4' (see below), the observed galaxies are so small that they fit easily into the primary beam of the ATCA's 22-m antennae (Full Width at Half Maximum FWHM = 20'/34' at 2.45/1.43 GHz, respectively) and no primary beam correction is necessary. With a shortest spacing of 45.9 m and good uv coverage, flux losses due to missing short spacings are negligible.

2.2 VLA data

 

 

Table 2: VLA observation details.

Galaxy	Arraya	Date	Int. time
			[h:mm]
NGC1055	D	1999, Mar. 25/29	3:30
NGC1406	DnC	1999, Feb. 16	2:45
NGC1421	D	1999, Apr. 02	2:45
NGC2748	D	1999, Apr. 04	3:30
NGC2820	D	1999, Apr. 04	3:05
NGC3175	DnC	1999, Feb. 18	2:00
NGC3437	D	1999, Mar. 29	3:00
NGC3717	DnC	1999, Feb. 18	2:00
NGC4527	D	1999, May 28	1:30
NGC4700	D	1999, May 28	1:35
NGC5073	D	1999, May 28	1:35
NGC7541	D	1999, Apr. 02	2:05

Note to Table 2:

^a) Southern galaxies ( $\delta \leq 30^\circ$) where observed with a hybrid,
i.e., with an extended northern arm of the array.

Those galaxies observed with the VLA are listed in Table 2. Two IFs of 50 MHz bandwith were used, centred at 1.465 GHz and 1.385 GHz, respectively. Primary flux calibrators are either 0137+331 (3C 48; 15.62 Jy at 1.465 GHz and 16.32 Jy at 1.385 GHz) or 1331+305 (3C 286; 14.55/14.94 Jy, respectively) or both, if available, adopting the flux scale by Baars et al. (1977). The combined final maps have a centre frequency of 1.425 GHz. The data reduction was performed in a standard way, utilizing the NRAO software package AIPS.

Due to the shortness of most of the observing runs, no polarisation calibration could be obtained (owing to insufficient parallactic angle coverage).

For galaxies with an extent of 5' or larger a primary beam correction was performed. This affects NGC2820 (and its partners), NGC3717 and NGC4527. For NGC3717, one of the most extended objects in our sample, a test was performed on the influence of the primary beam correction on the total flux measurement, which turned out to be negligible (0.1 mJy with respect to a total flux of 235 mJy).

For investigations of the z height of the observed radio emission it is important to have the highest possible resolution in the direction perpendicular to the disk plane of the sample galaxies, while normally interpretation of the results is easiest with a circular beam. In those cases where the clean beam was elongated roughly along the minor axis of the observed galaxy it was restored circular by slightly re-weighting the visibilities. In a few cases, where the beam was elongated in a direction close to the major axis of the target galaxy, it was left elliptical, but restored with its major axis oriented exactly along the galaxy disk. This leaves the beam's minor axis, and thus the highest resolution, perpendicular to the galaxy disk, while keeping the data analysis in the z direction straight-forward.

Figure 1: VLA 1.43 GHz radio continuum contour map of NGC1055, superimposed on a Digital Sky Survey (DSS) image. The contour levels displayed are -0.23, -0.16, 0.16 (=$2\sigma$), 0.23, 0.32, 0.45, ..., 80.2 mJy beam-1, with a contour spacing of a factor of $\sqrt {2}$.