The Images

This section presents the final images produced via the procedure described above. All images were corrected for primary beam attenuation using a polynomial approximation of the GMRT primary beam. As mentioned earlier, the resolution in these images changes from image to image due to a combination of declination dependent uv-coverage, changes in the number of available antennas and the flagging of bad data. Images of fields with large angular size sources are presented at a few arcmin resolution. Higher resolution images of some of the fields were also made where necessary (due to the presence of small angular size sources of interest in the field, e.g. the field containing G003.6-0.1).

Fig. 4.4 shows the GMRT image of the field containing the SNR G001.4-0.0 at the centre of the image. Other well known sources (SNRs and H II) regions in the Galactic Centre region are clearly visible in this image. The RMS noise is relatively high, possible due to the Galactic Center which lies at the south-western edge of the primary beam. Few of the GMRT antennas had servo related errors due to which there were small oscillations in the antenna pointing while tracking. This, in the presence of strong sources at the edge of the fields, results in short time scale differential gain variations which are not easy to correct later and also results in a higher RMS noise.

Fig. 4.5 shows the full primary beam corrected images of the field containing the SNRs G004.7-0.2, G003.8+0.3 and the unclassified source G003.6-0.1. The low resolution image in the left panel was made using a single facet, while the higher resolution image was made using a grid of $4\times 4$ facets. The lower surface brightness SNR G003.8+0.3 is better discerned in the low resolution image.

The dominant extended source in Fig. 4.6 is a known Ultra Compact H II region (Becker et al.1994). A small angular size SNR G004.2-0.0 was reported by Gray (1994a) in this field at the centre of this image. However there is no indication of this source at the level of 10 mJy/beam in this image. It is, however detected as a compact flat spectrum source in the low resolution image. This sources is unlikely to be an SNR.

Fig. 4.7 shows the GMRT 325-MHz image of the shell type SNR G004.8+6.2. The strong, marginally resolved source due west of this SNR is the well known Kepler's SNR (Fig. 5.14). G004.8+6.2 is again clearly detected in the NVSS image of this region (Fig. 5.7). This SNR is also detected in the image made from a 327-MHz VLA observation of a region close to this source (Fig. 5.8).

Fig. 4.8 shows the field containing the barrel shaped SNR G356.2-1.5. The 843-MHz image of this SNR by Gray (1994a) was severely affected by artifacts due to the grating response of nearby sources. This SNR is however clearly detected in the GMRT 325-MHz image. A marginally extended source of emission is also visible in this image in the north-eastern direction.

Fig. 4.9 shows the GMRT image of the shell type SNR G356.2+4.5. This SNR is also clearly visible in the NVSS image of this region (Fig. 5.10). The quality of NVSS images close the Galactic plane is usually poor. However, a few degrees away from the plane, low surface brightness SNRs are often easily visible in NVSS fields (Bhatnagar2000; Green2001; Trushkin1999). A careful examination of the NVSS fields, few degrees away from the plane is therefore likely to result in the identification of more, hitherto unknown SNRs. Deep imaging of such objects can then be followed up with the GMRT/VLA. Detailed multi frequency imaging of a number of high Galactic latitude SNRs can be used to possibly deduce the distribution of ionized gas and examine the statistical significance of the $\Sigma$-$D$-$z$ relation (Caswell & Lerche1979).

Fig. 4.10 shows the GMRT image of the incomplete shell of the SNR G358.0+3.8. This is a low surface brightness SNR, but also detected in the NVSS image (Fig. 5.11).

Figure 4.4: Full primary beam image of the field containing the sources G001.4$-$0.0 at the centre of the image. The resolution in the image is $\approx 3\times 2 {\mathrm{arcmin^2}}$. The RMS noise in the images is $\sim30$ mJy/beam. The Galactic Centre sources (LaRosa et al.2000) Sgr D HII region and Sgr D SNR pair, the SNR G000.9$+$0.1, Sgr B1 and Sgr B2 are the dominant sources in the south-east direction.

Figure 4.5: Full primary beam image of the field containing the sources G003.6$-$0.1, G003.7-0.2, and G003.8+0.3. The resolution in the image in the left panel is this image $\approx 98\times 29 {\mathrm{arcsec^2}}$ while that in the image in the right panel is $\approx 20\times11 {\mathrm{arcsec^2}}$. The RMS noise in the images is $\sim 5$ mJy/beam. The extended emission is more clearly visible in the low resolution image, which was generated using a single facet. The higher resolution image was made with multiple facets.

Figure 4.6: High resolution, multi facet image of the field containing G004.2$-$0.0. The resolution in the images is $\approx 15\times10 {\mathrm{arcsec^2}}$ and the RMS noise $\approx 5$ mJy/beam. The dominant extended source in the field is an Ultra Compact H II region. There is a hint of a emission at the location of G004.2$-$0.0, but much below the expected level, probably indicative of thermal source (see Figure 5.5 for a low resolution image where G004.2$-$0.0 is detected as compact source).

Figure 4.7: Full primary beam corrected image of the field containing G004.8+6.2. The weaker source, due east of the centre of the image is the SNR G004.8+6.2. The strong source due west of the image centre is the 38 Jy Kepler's SNR. The resolution in the image $\approx2.3\times 1.4 {\mathrm{arcmin^2}}$ and the RMS noise is $\approx 23$ mJy/beam. Apart from other unknown sources of noise, the higher RMS noise in this image is also due to the wobble in the antenna pointing for some of the antennas used for this observation.

Figure 4.8: Multi facet image of the field containing G356.2-1.5. The resolution in the image on the left is $\approx 1.7\times 0.8 {\mathrm{arcmin^2}}$ and the RMS noise is $\approx 5$ mJy/beam. Notice the extended emission north-east of the barrel shaped SNR in the centre of the field. The panel on the right shows a high resolution image of this extended source.

Figure 4.9: Full primary beam corrected image of the field containing G356.2+4.5. The resolution in the image is $\approx 3\times 1.5 {\mathrm{arcmin^2}}$ and the RMS noise is $\approx 10$ mJy/beam.

Figure 4.10: Full primary beam corrected image of the field containing G358.0+3.8. The resolution in the image is $\approx 2.6 \times 1.5 {\mathrm{arcmin^2}}$ and the RMS noise is $\approx 15$ mJy/beam.