The transition between atomic and molecular gas in the spiral arm of a Milky Way-like galaxy

  • Adarsh Ranjan

Student thesis: Master of Science by Research


I have performed line radiative transfer to see emission at the frequency of 115 GHz for CO 1 → 0 line (under large velocity gradient (LVG) approximation) and the 1.5 GHz emission produced by the spin-flip transition of atomic hydrogen (HI) (under local thermodynamic equilibrium (LTE) assumption). I look at emission from different spiral arms (including inter-arm filaments) on a galactic model which is similar to the Milky Way. The line radiative transfer has been performed by using the RADMC-3D software. The input data for the modelling has been taken from a galactic model created by Dr. Rowan Smith using the AREPO moving mesh code. The emission data was further used to calculate intrinsic inter-stellar medium (ISM) gas properties like integrated emission and velocity dispersion. I then compared these properties as seen for different species of gas (like CO and HI) as well as for different position profiles of the same gas (like emitting from a galactic arm or an inter-arm filament). CO is used as a tracer to estimate molecular hydrogen and I made maps of observable molecular hydrogen and CO-dark molecular gas in the model. I further used output data in combination with the binary input files available to me to draw various interesting conclusions about the galactic model.
Date of Award1 Aug 2017
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorRowan Smith (Supervisor) & Gary Fuller (Supervisor)


  • molecular clouds, CO 1->0 line, HI 21cm line, CO-dark gas
  • inter-stellar medium, galactic ISM, radiative transfer simulations, RADMC3D

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