Subhashis Roy

My research interests lie in several fields involving the interstellar medium (ISM) of the central region of the Milky Way, supernova remnants in the Milky Way, and magnetic fields in our galaxy and nearby galaxies. More detail on each research area is provided below:

The interstellar medium and magnetic fields near the centre of the Milky Way:

The central kilo-parsec region of the Milky Way harbors a variety of activity unique to that region. Because of a significantly increased gravitation potential, the gaseous medium in the central few hundred pcs is characterised by high density, large velocity dispersions, comparatively higher temperatures and magnetic fields. My multi-frequency GMRT observations led to the detection of the central compact radio source (Sgr A*) for the first time at 610 MHz . From observations at 240 and 150 MHz, I have identified a large number (~62) of compact extragalactic radio sources. The ionised gas in the region scatters electromagnetic radiation passing through it and the amount of scattering gives information on the electron density and its fluctuation in the region. I have measured the angular sizes of the background sources at 150, 240 and 1400 MHz and showed for the first time presence of an enhanced scattering screen within a degree from the Galactic Centre. We have also shown that the fluctuating component of the magnetic field in the Galactic Centre region is ~ 20 micro-Gauss. In this turbulent region, the systematic field will have similar or lower magnitude, implying that its strength is relatively low, tens of micro-gauss, significantly lower than earlier estimates of milli-Gauss fields. Missing supernova remnants near the Galactic Centre:

Missing supernova remnants near the Galactic Centre:

Due to the central turbulent environment in our Galaxy, heavier stars are preferentially formed close to the centre of the Milky Way. Also, our line of sight through this region passes through the longest path length in the Galaxy, which maximises the probability of finding a supernova remnant (SNR). In the Milky Way, the number of expected SNRs is more than 1000, but only about 270 have been discovered. Most of the missing SNRs are believed to be concentrated in the inner Galaxy where identification becomes difficult due to increased source density and confusion (for larger remnants), while the smaller remnants are missed due to finite angular resolutions of surveys. The GMRT offers the highest sensitivity and angular resolution at metre wavelengths. From our search, we have confirmed the nature of 5 candidate SNRs in the region out of 7 observed systems. From one of these, I identified a small shell-like structure and re-observed this with the GMRT at 325 MHz and 1.4 GHz. We recently confirmed this system as one of the youngest SNRs in the Milky Way, with an age of 100 to 500 years.

Magnetic fields in galaxies:

We have studied magnetic fields in the disks of 6 nearby normal galaxies. Assuming equipartition of energy between magnetic fields and cosmic ray particles, we estimated the field strength from the intensity of synchrotron emission (emission from high-energy electrons gyrating in the magnetic fields). We obtained field strengths of ~20-25 micro-Gauss at the galaxy centres, with a systematic decrease to the outer parts, where the field strengths are ~10 micro-Gauss. We found that the energy density in the magnetic fields and the gas are within a factor of two, indicating equipartition of energy between them. We also showed that the slope of the radio flux density to the far infrared flux density in these galaxies is the same at low and high frequencies in the ``arm'' regions. However, the slope steepens for the inter-arm regions, when observations are done at high frequencies (> 1 GHz). This is due to the longer propagation length of the comparatively lower energy electrons radiating below 1 GHz, as compared to their high energy counterparts radiating at frequencies above 1 GHz.