4 Radio observations and data analysis

4.1 Observations and reduction

The Australia Telescope Compact Array (ATCA) consists of five 22 m antennas on a 3 km east-west railway track, and a sixth antenna 3 km from the western end of the track. Each cluster was observed in 1999 February for 12 hours with the ATCA in the 6C configuration, giving baselines ranging from 153 m to 6 km. Simultaneous observations at 1.384 GHz and 2.496 GHz were made for each cluster.

The radio data were processed using standard MIRIAD (Sault et al. 1995) software and techniques. The primary flux density calibrator was PKS B1934-638, with PKS B1245-197 as the secondary phase calibrator. The data were then CLEANed and RESTORed. Table 10 lists details of the ATCA observations, including the size and orientation of the elliptical Gaussian restoring beams and the rms noise in the final images.

4.2 Data analysis

The AIPS task VSAD was used to generate a radio source list for each cluster, above a nominal cutoff of 0.5 mJy at 1384 MHz. An elliptical Gaussian was fitted to each source as described by Condon (1997). For sources which were extended or complex, the integrated flux density was estimated by using kview (Gooch 1996) to sum inside a rectangular region defined around the source. The radio sources for Abell 1451 and RXJ1314-25 within one Abell radius ( $R_{\rm A} = 1.7/z$ arcmin) of the cluster centre are listed in Table 11. Positions are measured from the 1384 MHz image except for sources R1 and R9 in Abell 1451 which are measured at 2496 MHz; quoted errors are the quadratic combination of the formal VSAD error and a nominal 0.5 $^{\prime\prime}$ calibration uncertainty. Errors in flux density are taken directly from the VSAD output; if no error is quoted for $S_{\rm int}$ the measurement was made using kview.

Figure 7: Abell 1451: Optical V-band image from the Danish 1.5-m telescope overlaid with HRI X-ray contours (heavy) and 20 cm ATCA radio contours (lightweight). The X-ray contours run from 0.006 to 0.02 counts s-1 arcmin-2 in logarithmic steps; the innermost cluster contour is at 0.0134 counts s-1 arcmin-2. The peak of the extended X-ray emission, used for the profile measurements, is marked with a cross. The radio contour levels are 0.2, 0.3, 0.5, 1, 2, 5 and 10 mJy/beam; the rms noise level is 80 $\mu$Jy/beam.

Figure 8: RXJ1314-25: optical I-band image from the Danish 1.5-m telescope overlaid with HRI X-ray contours (heavy) and 20 cm ATCA contours (lightweight). The X-ray contours run from 0.004 to 0.065 counts s-1 arcmin-2 in logarithmic steps; the innermost cluster contour is at 0.012 counts s-1 arcmin-2. The centre of the X-ray emission taken for the profile measurements is marked with a cross. The radio contour levels are 0.35, 0.5, 0.7, 1, 1.5 and 2 mJy/beam; the rms noise level is 90 $\mu$Jy/beam.

A search was carried out for optical identifications of the radio sources in Table 11 using the SuperCOSMOS catalogue (Hambly et al. 2001). A search radius of 10 arcsec was used. The results are shown in Table 12.

4.2.1 Abell 1451

The 1.384 GHz (20 cm) radio contours overlaid on the DSS image of the cluster field are shown in Fig. 11; the 1.384 GHz (20 cm) radio contours for the central part of the cluster are shown in Fig. 7 together with the X-ray contours. The central radio source (R7 in Table 11) is extended north-south with a peak that best matches galaxy #40 from Table 3. The apparent wide-angle tail (WAT) morphology relies on the lowest (2.5$\sigma$) contours and is therefore uncertain. Such morphologies are usually associated with the central dominant cluster member. An alternative interpretation, suggested by the 13 cm observation shown only for the central part of the cluster in Fig. 2, is that R7 is a head-tail source identified either with galaxy #40 or the brighter galaxy #39 which is $\sim$ $10\hbox{$^{\prime\prime}$ }$ north. Higher resolution radio observations are needed to settle this issue.

Apart from R7, there are no other cross-identifications between the spectroscopic catalogue and radio source list. However, there are several likely cluster identifications, including the head-tail source (R1) located $\sim$8 $\hbox{$^\prime$ }$ NW of the cluster centre. The weakest source in the field, R11, is a clear AGN candidate - it is a strong X-ray source with a flat radio spectrum; SuperCOSMOS classifies the optical counterpart as a galaxy. R9 is an extended source with no obvious optical counterpart. We initially considered it as a possible relic source (Enßlin et al. 1998), but the 13 cm image suggests an identification with a very faint object visible on both the B and R sky survey images. The QSO (#32, z=1.17) is radio quiet.

4.2.2 RXJ1314-25

The 1.384 GHz (20 cm) radio contours overlaid on the DSS image of the cluster field are shown in Fig. 12; the central region of RXJ1314-25, together with X-ray contours, are shown in Fig. 8 overlaid on the optical image from Fig. 3. Background noise in the central region of the 20 cm image is affected by sidelobes of the 200 mJy source NVSS J1314-2522 located SW of the cluster centre, just outside one Abell radius. For this reason the lowest contour level in Fig. 8 has been set at $\sim$4$\sigma$.

As shown in Table 11, the projected radio source density is lower than for Abell 1451. There are no positive identifications with spectroscopically confirmed cluster members, although there is a weak source just below the 0.5 mJy threshold, very close to the brightest cluster galaxy BCG2 (galaxy #48; see Fig. 8). There is also a striking mirror symmetry of sources R2 and R5 (Table 11) with respect to this galaxy, resembling the lobes of an FR II radio galaxy (see e.g. Fig. 8). At the redshift of the galaxy, z=0.2466, the linear size of $6\hbox{$^\prime$ } 15\hbox{$^{\prime\prime}$ }$ would correspond to $\sim$1 Mpc, a typical scale for a giant radio galaxy (Schoenmakers et al. 2001). Such an interpretation is questionable, however, as the galaxy is clearly not located in an underdense environment (see Fig. 6), as required for the growth of giant sources. Neither source has an obvious optical counterpart in the 20 cm image, but R5 appears double at 13 cm, with a plausible optical identification midway between the components. On the other hand, with its extended radio emission, steep spectrum and position in the cluster, R5 is an excellent candidate for a relic source. Its true extent is difficult to judge from the ATCA 20 cm image because of sidelobe confusion, but there is a suggestion of low surface brightness, extended emission in the vicinity of BGC#1 and the western X-ray peak.