We studied the total magnetic field strength in normal star-forming galaxies estimated using energy equipartition assumption. Using the well known radio--far infrared correlation we demonstrate that the equipartition assumption is valid in galaxies a
t sub-kpc scales. We find that the magnetic field strength is strongly correlated with the surface star formation rate in the galaxies NGC 6946 and NGC 5236. Further, we compare the magnetic field energy density to the total (thermal + turbulent) energy densities of gas (neutral + ionized) to identify regions of efficient field amplification in the galaxy NGC 6946. We find that in regions of efficient star formation, the magnetic field energy density is comparable to that of the total energy density of various interstellar medium components and systematically dominates in regions of low star formation efficiency.
We report discovery of a shell like structure G354.4+0.0 of size 1.6 that shows morphology of a shell supernova remnant. Part of the structure show polarized emission in NRAO VLA sky survey (NVSS) map. Based on 330 MHz, 1.4 GHz Giant Metrewave Radio
Telescope (GMRT) observations and existing observations at higher frequencies, we conclude the partial shell structure showing synchrotron emission is embedded in an extended HII region of size ~4. The spectrum of the diffuse HII region turns over between 1.4 GHz and 330 MHz. HI absorption spectrum shows it to be located more than 5 kpc away from Sun. Based on morphology, non-thermal polarized emission and size, this object is one of the youngest supernova remnants discovered in the Galaxy with an estimated age of about 100-500 years.
We have observed the Galactic Center (GC) region at 0.154 and 0.255 GHz with the GMRT. A total of 62 compact likely extragalactic sources are detected. Their scattering sizes go down linearly with increasing angular distance from the GC up to about 1
deg. The apparent scattering sizes of sources are more than an order of magnitude down than predicted earlier by the NE2001 model of Galactic electron distribution within 359.5 deg < l < 0.5 deg and -0.5 deg <b <0.5 deg (Hyperstrong scattering region) of the Galaxy. High free-free optical depths are observed towards most of the extended nonthermal sources within 0.6 deg from the GC. Significant variation of optical depth indicate the absorbing medium is patchy at an angular scale of 10 and electron density is ~10 per cc that matches with the NE2001 model. This model predicts the extragalactic (EG) sources to be resolved out from 1.4 GHz interferometric surveys. However, 8 likely EG sources out of 10 expected in the region are present in 1.4 GHz catalog. Ionized interfaces of dense molecular clouds to the ambient medium are most likely responsible for strong scattering and low radio frequency absorption. However, dense GC clouds traced by CS $J=1-0$ emission are found to have a narrow distribution of ~0.2 deg across the Galactic plane. Angular distribution of most of the EG sources seen through the so called Hyperstrong scattering region are random in $b$, and typically ~7 out of 10 sources will not be seen through to the dense molecular clouds, and it explains why most of them are not scatter broadened at 1.4 GHz.
We report detection of strong circularly polarized emission from the transient bursting source GCRT J1745-3009 based on new analysis of 325 MHz GMRT observations conducted on 28 September 2003. We place 8 Solar radius as the upper limit on the size o
f the emission region. The implied high brightness temperature required for an object beyond 1 pc and the high fraction of circular polarization firmly establish the emission as coherent. Electron cyclotron or plasma emission from a highly subsolar magnetically dominated dwarf located less than 4 kpc away could have given rise to the GCRT radio emission.
To determine the properties of the Faraday screen and the magnetic field near the central region of the Galaxy, we measured the Faraday rotation measure (RM) towards 60 background extragalactic source components through the -6 deg < l <6 deg, -2 deg
< b < 2 deg region of the Galaxy using the 4.8 and 8.5 GHz bands of the ATCA and VLA. Here we use the measured RMs to estimate the systematic and the random components of the magnetic fields. The measured RMs are found to be mostly positive for the sample sources in the region. This is consistent with either a large scale bisymmetric spiral magnetic fields in the Galaxy or with fields oriented along the central bar of the Galaxy. The outer scale of the RM fluctuation is found to be about 40 pc, which is much larger than the observed RM size scales towards the non-thermal filaments (NTFs). The RM structure function is well-fitted with a power law index of 0.7 +/- 0.1 at length scales of 0.3 to 100 pc. If Gaussian random processes in the ISM are valid, the power law index is consistent with a two dimensional Kolmogorov turbulence. If there is indeed a strong magnetic field within 1 degree (radius 150 pc) from the GC, the strength of the random field in the region is estimated to be 20 microGauss. Given the highly turbulent magnetoionic ISM in this region, the strength of the systematic component of the magnetic fields would most likely be close to that of the random component. This suggests that the earlier estimated milliGauss magnetic field near the NTFs is localised and does not pervade the central 300 pc of the Galaxy.
We have observed a sample of 64 small diameter sources towards the central -6 degree < l< 6 degree, -2 degree < b < 2 degree of the Galaxy with the aim of studying the Faraday rotation measure near the Galactic Centre (GC) region. All the sources wer
e observed at 6 and 3.6 cm wavelengths using the ATCA and the VLA. Fifty nine of these sources are inferred to be extragalactic. The observations presented here constitute the first systematic study of the radio polarisation properties of the background sources towards this direction and increases the number of known extragalactic radio sources in this part of the sky by almost an order of magnitude. Based on the morphology, spectral indices and lack of polarised emission, we identify four Galactic HII regions in the sample.
