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  • Bhavesh Rajpoot

HI 21 cm Line: Significance and Detection

The ‘Neutral Hydrogen’, also known as H1, comprises 25% of all the baryonic matter. This

cold neutral gas is present throughout our galaxy. Although as it is neutral it gives off neither visible radiation nor it can participate in standard radio wave generation

method. But the H1 does something special. It’s electron’s transits not from one orbit to

another but it’s spin flips from parallel spin state to anti-parallel spin state with respect to

the nucleus, giving away a hyperfine line of 21.106 cm and 1420.405 MHz, the most

important radio spectral line. The HI line has width and shape which are determined by

the gas kinetic temperature and any internal motions. The line is sensitive to both

temperature and amount of gas. Also, this line is not impeded by interstellar dust.

Therefore, by detecting and analysing the HI line we can infer brightness temperature,

structure and dynamics of our galaxy, temperature and Doppler shifts of various

astronomical bodies, structure and dynamics of radio galaxies, and whatnot.


For the detection, a 4m Simple Radio Telescope was used. The Telescope has comprised of Parabolic Dish with a Dipole Antenna inside FeedHorn which was fixed at the focus of the dish. The FeedHorn was attached to the bandpass filters, low noise amplifiers and superheterodyne receiver. The telescope was first calibrated using some defined calibrated sources and then HI line was observed of different deep-sky objects. The data was recorded in the units of volts/doppler shift which was then converted for graphing and analysis. The analysis gave the magnitude of Doppler shifts from different parts of a galaxy which helped us to study its dynamics and evolution. The measurement of the velocity component towards the earth of the neutral hydrogen gas clouds along with some estimate to their distances can help us to plot the galactic rotation curve. The nearly flat- rotation curve gave the sense of dark matter present in the galaxy.


Here is the video of my poster presentation at Frontiers in Physics - XIII at Fergusson College, Pune where I presented my work on the detection of HI line emission during Radio Astronomy Winter School 2018 at National Centre for Radio Astrophysics - TIFR and Inter-University Centre for Astronomy and Astrophysics, Pune.


The video is shot at Amphitheatre of Fergusson College, Pune by Simran Joharle on OnePlus6.






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