Introduction

The Galactic longitude of all the observations presented in this dissertation were within $\pm5{^\circ}$. The Galactic latitude of three out of the six independent pointings was within $\pm0{^\circ}.2$, while the rest were at $>\pm1{^\circ}$. Most of the emission from the Galactic plane at radio comes from many sources - (1) normal HII regions (Lockman1989; Wink et al.1982), (2) bright Ultra compact H II (UC H II) regions with a lower surface brightness extended emission (Wood & Churchwell1989b; Kurtz et al.1994), (3) compact HII regions with no detectable extended emission (Kuchar & Clark1997; Wink et al.1983), (4) SNRs (Green2000), (5) background sources seen through the Galactic disk, and (6) the large scale Galactic background emission (Haslam et al.1982). The $\sim1{^\circ}.4$ field of view of GMRT observations at 327 MHz therefore reveal a number of compact as well as a few extended sources of emission.

The physical mechanism for the radio emission from HII regions and SNRs is, however, quite different. Radio emission from HII regions is due to the interaction between electrons and ions. The resulting thermal spectrum is almost flat (with a spectral index of $\sim 0.1$) at frequencies typically greater than $\sim1$ GHz and turns over below this frequency due to free-free absorption by the intervening material. High resolution imaging of many compact HII regions reveal a halo surrounding a compact core (Wood & Churchwell1989b). Detailed models of HII regions which include such temperature gradients and departure from LTE have been developed (Wilson & Jaeger1987). However, these models have been applied only to objects where resolved images at a number of frequencies are available. High resolution multi frequency observations of HII regions, particularly of a larger sample of compact HII regions, will test these models more rigorously and provide improved estimates of the physical parameters. Most HII regions are also sources of radio recombination line (RRL) emission which has been detected at high radio frequencies (Lockman1989; Caswell & Haynes1987). Detection of high frequency RRL emission is one of the signatures used to identify H II regions. Emission from SNRs on the other hand is synchrotron radiation from free electrons accelerated to relativistic energies in the supernova shock front or due to the transfer of rotation energy from the neutron star, to the surrounding medium. The magnetic field in which the electrons move is also amplified. The spectral index is typically negative with no associated thermal or RRL emission.

The measured flux densities of all sources detected at 327 MHz, along with the 1420-MHz flux densities from NVSS images, are listed here. Two objects, namely G004.4$+$0.1 and G003.7$-$0.1, both catalogued as HII regions, reveal the core-halo morphology in the GMRT image and these GMRT observations constitute the first resolved low frequency images of these, possibly compact or UC H II regions.

Table 6.1: List of point sources and their 327- and 1400-MHz flux densities in the field of G003.7$-$0.1

Name	${\mathrm{RA}_{J2000}}$	${\mathrm{Dec}_{J2000}}$	$S_{327}$	$S_{1400}$
	($h m s$)	( ${^\circ}  {^\prime} {^{\prime\prime}}$)	(mJy)	(mJy)
1756-2549	17 56 41.4	-25 49 10.9	$99.5\pm10$	$26.9\pm1$
1756-2542	17 56 38.1	-25 42 15.0	$221.8\pm10$	$70.7\pm1$
1755-2540	17 55 39.2	-25 40 44.0	$207.9\pm10$	$50.0\pm1$
1755-2535	17 55 22.3	-25 35 44.0	$232.1\pm10$	$6.0\pm1$
1755-2537	17 55 08.4	-25 37 24.0	$108.1\pm10$	$148.4\pm1$
1754-2539	17 54 56.3	-25 39 31.8	$<10$	$17.2\pm1$
1755-2543	17 55 10.2	-25 43 59.9	$<10$	$79.6\pm1$
1754-2534	17 54 39.4	-25 34 43.4	$192.2\pm10$	$31.2\pm1$
1754-2536	17 54 27.6	-25 36 23.2	$177.7\pm10$	$13.2\pm1$
1754-2540	17 54 10.7	-25 40 22.7	$<10$	$44.3\pm1$
1754-2544	17 54 10.4	-25 44 06.6	$<10$	$44.3\pm1$
1754-2556	17 54 46.4	-25 56 03.6	$<10$	$21.9\pm1$
1755-2557	17 55 19.9	-25 57 20.0	$114.7\pm20$	$16.1\pm1$
1755-2556	17 55 19.0	-25 56 56.0	$78.1\pm20$	$21.7\pm1$
1755-2551	17 55 09.6	-25 51 27.9	$100.5\pm30$	$11.4\pm1$
1755-2549	17 55 40.4	-25 49 52.0	$<10$	$11.6\pm1$
1754-2609	17 54 38.9	-26 13 47.5	$183.3\pm10$	$47.0\pm1$
1754-2609	17 54 36.4	-26 13 26.5	$44.8\pm10$	$47.0\pm1$

Table 6.2: List of point sources and their 327- and 1400-MHz flux densities in the field of G004.8$+$6.2

Name	${\mathrm{RA}_{J2000}}$	${\mathrm{Dec}_{J2000}}$	$S_{327}$	$S_{1400}$
	($h m s$)	( ${^\circ}  {^\prime} {^{\prime\prime}}$)	(mJy)	(mJy)
1732-21	17 32 53.7	-21 24 43.3	$416\pm60$	$22.7\pm0.7$
1732-22	17 32 19.4	-22 06 43.8	$530\pm50$	$274.0\pm0.7$
1734-21	17 34 04.0	-21 46 25.8	$1120\pm65$	$100.2\pm1$
1733-21	17 33 56.2	-21 42 41.3	$716\pm70$	$66.5\pm0.7$

Table 6.3: List of point sources and their 327- and 1400-MHz flux densities in the field of G356.2$+$4.5

Name	${\mathrm{RA}_{J2000}}$	${\mathrm{Dec}_{J2000}}$	$S_{327}$	$S_{1400}$
	($h m s$)	( ${^\circ}  {^\prime} {^{\prime\prime}}$)	(mJy)	(mJy)
1715-29	17 15 04.7	-29 12 21.1	$194.0\pm20$	$297.1\pm2$
1716-29	17 16 11.6	-29 20 01.6	$274.8\pm20$	$242.5\pm2$
1715-29	17 15 14.3	-29 43 17.1	$213.8\pm20$	$157.1\pm2$
1715-29	17 16 52.5	-29 48 59.4	$373.9\pm20$	$124.5\pm2$
1757-30	17 17 57.6	-30 00 43.17	$1053.9\pm20$	$315.6\pm2$
1757-30	17 18 12.6	-30 01 43.97	$812.5\pm20$	$183.1\pm2$
1719-29	17 19 46.4	-29 52 49.1	$91.6\pm20$	$17.7\pm2$
1721-29	17 21 43.5	-29 35 16.9	$294.0\pm20$	$127.9\pm2$

Table 6.4: List of point sources and their 327- and 1400-MHz flux densities in the field of G356.2$-$1.5

Name	${\mathrm{RA}_{J2000}}$	${\mathrm{Dec}_{J2000}}$	$S_{327}$	$S_{1400}$
	($h m s$)	( ${^\circ}  {^\prime} {^{\prime\prime}}$)	(mJy)	(mJy)
1740-3228	17 40 53.6	-32 28 13.9	$177.9\pm15$	$26.4\pm5$
1740-3334	17 40 56.3	-33 34 44.8	$131.6\pm15$	$51.3\pm2$
1741-3141	17 41 54.1	-32 41 30.1	$140.6\pm15$	$35.6\pm5$
1741-3227	17 41 38.7	-32 27 29.9	$263.9\pm15$	$38.3\pm5$
1741-3245	17 41 38.6	-32 45 30.0	$92.4\pm15$	$29.6\pm5$
1741-3314	17 41 31.3	-33 14 00.0	$119.1\pm15$	$32.1\pm5$
1742-3313	17 42 49.0	-33 13 44.8	$219.6\pm20$	$57.1\pm5$
1742-3222	17 42 27.2	-32 22 14.9	$631.6\pm33$	$191.4\pm5$
1742-3333	17 42 32.4	-33 33 30.4	$71.8\pm15$	$19.8\pm2$
1743-3309	17 43 04.5	-33 09 59.0	$606.5\pm20$	$173.7\pm5$
1743-3313	17 43 05.8	-33 13 14.3	$77.6\pm20$	$33.1\pm5$
1744-3251	17 44 22.4	-32 51 19.3	$106.8\pm15$	$37.3\pm2$
1746-3259	17 46 18.8	-32 59 44.0	$487.3\pm15$	$120.5\pm2$
1740-3251	17 40 26.9	-32 51 02.7	$103.4\pm15$	$48.6\pm2$
1748-3241	17 48 31.1	-32 41 00.8	$1211.0\pm15$	$412.5\pm2$
1743-3238	17 43 06.6	-32 38 06.4	$91.1\pm15$	$10.2\pm2$
1742-3241	17 42 26.9	-32 41 00.7	$293.6\pm15$	$243.0\pm2$

Table 6.5: List of point sources and their 327- and 1400-MHz flux densities in the field of G358.3$+$3.8

Name	${\mathrm{RA}_{J2000}}$	${\mathrm{Dec}_{J2000}}$	$S_{327}$	$S_{1400}$
	($h m s$)	( ${^\circ}  {^\prime} {^{\prime\prime}}$)	(mJy)	(mJy)
1728-28	17 28 28.9	-28 46 03.5	$1605.4\pm25$	$567.2\pm3$
1727-28	17 27 01.9	-28 15 55.5	$87.4\pm25$	$104.5\pm3$
1725-28	17 25 20.3	-28 05 19.8	$386.4\pm25$	$130.2\pm3$
1724-28	17 24 17.7	-28 06 05.6	$812.1\pm25$	$167.2\pm3$
1724-29	17 24 19.5	-29 01 05.8	$244.9\pm25$	$125.4\pm3$
1727-28	17 27 39.8	-28 59 06.2	$299.5\pm25$	$117.1\pm3$
1727-28	17 27 42.3	-28 40 42.1	$154.8\pm25$	$34.7\pm3$
1727-28	17 27 22.9	-28 19 42.9	$153.9\pm25$	$98.5\pm3$
1727-28	17 27 34.6	-28 15 18.4	$114.2\pm25$	$53.7\pm3$