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H \alpha spectral line profiles

Starting from ground zero, the H \alpha (Balmer) emission line is observed at a wavelength of 656.3 NM. It is one of the most prominent emission lines observed in galaxies and it was one of the lines that I was looking for with Prof. Jeremy Mould as part of a summer project. Over the course of our work, we found something interesting. Given the differences in inclination of galaxies (from our point of view i.e face-on or side-on point of view), differences in distribution of gas in the galaxy and the galaxy's rotation curve, the shape of the H \alpha line differs a lot! In the lab, it'd be a simple Gaussian function but in practice, it ends up being a sum of multiple Gaussian functions with different mean and variance. Interestingly, it might happen that the spectral line has two peaks instead of one.

Think of a case where you are observing a galaxy side-on i.e one of the arms of the galaxy is moving towards you and the other, moving away. Using the same principle as Doppler shift of sound, light from two arms of the galaxy will be blue-shifted and red-shifted about the central wavelength. A lopsided or biased gas distribution in the galaxy i.e more gas away from the central region also helps such a profile, along with a large rotation curve. Such double peaked profiles are usually observed in the HI emission line profiles of galaxies.

My intentions now are to simulate such a double-peaked profile by assuming an inclination, a gas distribution and a rotation curve. I am of the idea that the double-peaked profile in itself might reveal interesting features about a galaxy using relevant models. I have a thesis by a Erica Nelson as reference on 'HI line profiles of galaxies : Tilted ring models'. I could try extend this to understand how the H \alpha line would look under similar assumptions. This is an interesting tangent of my project that I would like to pursue ...

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