Future directions

This work demonstrated and extensively used the imaging capabilities of the GMRT at low frequencies. The GMRT provides high resolution along with sensitivity to relatively large angular scales ($\sim25$ arcmin) - both of which are of crucial importance when mapping complex fields like the Galactic plane. All maps reveal a rich variety of objects in the field. For many of them, these observations constitute the first low frequency observations and reveal features not known from higher frequency observations. Sensitive high resolution observations in the Galactic plane with the GMRT is therefore expected to give rich scientific dividends. Spectral information is often a crucial ingredient for understanding the physical properties of these objects. As a future, possibly Observatory Project, a high resolution multi frequency survey of the Galactic plane with the GMRT will be of great interest.

It is clear that the role of magnetic field is as important as the kinetic energy of the relativistic electrons in explaining the emission from typical SNR. Measurements of the magnitude of the magnetic field for some of the SNRs reveal magnetic field amplification of the ambient by $2-3$ orders of magnitude. However these observations are few and far-in-between (e.g. Brogan et al.2000; Claussen et al.1997). High resolution images of many SNRs reveal compact sources embedded in the diffused larger scale emission from the SNR. If these objects are background sources, rotation measure (RM) measurements against these sources can provide a measure of the magnetic field for a larger sample of SNRs, which will be valuable input to the theoretical models to explain the magnetic field amplification.

Distances to many Galactic SNRs are poorly determined. Errors of a factor of 2 or more are frequently found in the quoted distance (often using the $\Sigma$-$D$ relation (Case & Bhattacharya1998, and references therein)). Distance estimated using the velocity structure of H I absorption profiles towards SNRs in combination with the Galactic rotation model, often provides a more reliable distance estimates. Such observations with the GMRT to determine the distance to a large sample of SNRs will be very desirable.

The 327 MHz images of the UC H II regions in the field of view of some of the images presented here reveal, somewhat unexpectedly, reasonably extended and strong emission around the compact cores. Such extended emission has also been recently detected at higher frequencies using the VLA (Kim & Koo2001; Kurtz et al.1999). The exact nature and the mechanism that could sustain such large scale (few pc) emission is far from clear. The GMRT images are the first low frequency images of these objects at resolutions comparable to those at higher frequencies. Such observations of a sample of UC H II, preferably at 327, 610 and possibly at 1420 MHz, which are readily doable with the GMRT in its present state, are highly desirable and will almost certainly pay rich scientific dividends in this field. In particular, the proposed model to explain the extended emission till low frequencies (see Chapter 6 and Kim & Koo (2001)) appears to have ``preferred arrangement'' where a massive star is formed off-centre from a hot core which is in-turn embedded in a lower density molecular cloud. If a large fraction of UC H II do show extended emission, a more refined model may be required which can relax the requirement of this preferred arrangement of the components.

Measurement of the low frequency turn-over below 100 MHz, due to free-free absorption by the wide spread extended low density warm ionized medium (ELDWIM) in the Galaxy, provides information about the continuum optical depth towards various lines of sight in the Galaxy. The path lengths and filling factors derived from the low frequency radio recombination lines (RRL), detected in almost every direction in the inner Galaxy (Roshi & Anantharamaiah2000), suggests that the gas may be the extended low density HII envelopes surrounding higher density HII regions (Anantharamaiah1985b; Anantharamaiah1986). Also, the parameters of this gas, derived from RRL observations and the low frequency turn-over in the SNR spectra, are very similar, suggesting that the same gas is responsible for the RRL emission and the low frequency turn-over in SNR continuum spectra. If we assume that the properties of the ionized gas derived from these observations is typical, then we can expect to detect RRLs through stimulated emission due to the background radiation of SNRs at 330 and 1420 MHz. Such a detection will directly give the (negative) line optical depths at the two frequencies. High resolution RRL and continuum observations at low frequencies towards extended but strong SNRs will make tremendous advance in this area. All previous studies either had poor sensitivity or poor resolution (or both!) making it impossible to connect the absorption or RRL emission to known thermal sources. The GMRT and the VLA provides the required resolution at 327, 610 and 1420 MHz and are sensitive enough for these observations.