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Spectral Index Distribution

Figure 8 shows the 0.35 to 1.4 GHz spectral index distribution inferred from comparing the WENSS ([Rengelink et al. 1997]) image with our BnC configuration image convolved to the same resolution. The northern and southern warm spots have spectral indices5 tex2html_wrap_inline1346 of -0.6 and -0.55, not unusual for the hot spots of FR II sources in this frequency regime. The background sources also exhibit spectral indices that are quite typical of extragalactic sources (A, -0.68; B, -1.0; F, -0.7). Near the centers of the north and south lobes of NVSS 2146+82, however, this comparison shows regions of unusually "flat'' spectral index (tex2html_wrap_inline1346 ~ -0.3 ± 0.02 in the north lobe, tex2html_wrap_inline1346 ~ -0.4 ± 0.03 in the south lobe).

Figure 8: Gray scale representation of the spectral index distribution derived from VLA measurements at 1.4 GHz and the 0.35 GHz WENSS image with superimposed contours from the VLA image. The resolution is 54'' (FWHM), illustrated in the lower-left and the bar at the top gives the gray scale values.

The spectral index variations across the lobes can also be studied from our 1.36 and 1.63 GHz data. Due to the low surface brightness the data were tapered to 55'' resolution before imaging for this comparison. To eliminate any complication from the mosaicing technique, only data derived from the pointing on a given lobe were used to determine the spectral index variations for that lobe. Thus, the data from two pointings were imaged independently at 1.36 and 1.63 GHz, corrected for the antenna power pattern, and spectral index images were derived independently for the two lobes. These results are shown in Figure 9. The close spacing of the frequencies makes determining the spectral index more difficult; but this is compensated to some extent by the nearly identical imaging properties at the two frequencies, which reduce systematic errors. These data sets are fully independent of those used for the spectral index image in Figure 8, but also reveal symmetric regions of unusually flat spectral index, tex2html_wrap_inline1354 ~ -0.3±0.08, in both lobes.

Figure 9: Gray scale representation of the spectral index at 55'' resolution derived from the 1.36 and 1.63 GHz data with superimposed contours of the 1.63 GHz total intensity at the same resolution. The bar at the top gives the gray scale values and the resolution is shown in the lower-left corner. The north lobe is shown in the left and the south lobe on the right.

We conclude that four independent data sets show evidence for regions with tex2html_wrap_inline1358 ~ -0.3 in regions of relatively high signal to noise ratio. These regions are not artifacts of "lumpiness'' in the zero levels of the images.

5 tex2html_wrap_inline1342, as used here is given by tex2html_wrap_inline1344.

next up previous
Next: Source Alignment Up: Radio Observations Previous: Polarimetry

Fri Feb 4 16:02:06 EST 2000