Finding The Dark Gas In The Milky Way

Abstract

The interstellar medium is the medium that occupies the space between stars, and it is the host for new star formation. Molecular hydrogen H$_2$ is the most abundant molecule of interstellar gas. However, due to the lack of permanent dipole moment, the H$_2$ cannot be detected directly and is traced by the second most abundant molecule CO. H$_2$ self-shields itself from UV radiation. In contrast, the CO fails to self-shield itself, and the gas phase carbon will be present in C or C$^+$. Thus, some amount of H$_2$ has not been traced by CO, known as "CO-dark gas". 10\(\textup{M}_\odot\) resolution region of the fiducial Milky Way model, which is produced by the high-resolution simulations using a moving AREPO code, has been adopted to perform this thesis which aims to investigate the amount of CO-dark gas mass present in this region at 50pc, 100pc,250pc,500pc, and 1kpc spatial resolution of the AREPO image. \\ It is observed that as the spatial resolution is increased, more CO is detected for H$_{2}$ column densities $\geq$ 10$^{20}$ cm$^{-2}$. This affects the globally averaged CO-H$_{2}$ conversion factor (X$_{\textrm{CO}}$) up to a factor of 3. The standard value for CO integrated intensity threshold (W$_{\textrm{CO,th}}$, threshold to distinguish between CO dark and CO bright gas) less than 10 K kms$^{-1}$ does not hold good for 50pc, 100pc, and 250pc spatial resolutions. At these spatial resolutions, CO tends to be bright before reaching the W$_{\textrm{CO,th}}$ and new CO integrated intensity threshold (W$_{\textrm{CO,newth}}$) less than 1 K kms$^{-1}$ provides the better estimation of CO-dark gas fraction. CO-dark gas fraction varies along with the spatial resolution of the AREPO image. At high resolution, we miss 1$\%$ - 34$\%$ (50pc), 15$\%$ - 32$\%$ (100pc), 10$\%$ - 24$\%$ (250pc), 22$\%$(500pc) and 36$\%$(1kpc) of H$_{2}$ mass the 10\(\textup{M}_\odot\) resolution region. For the subregions, the amount of H$_{2}$ mass that is missed varies on the spatial resolution.\\ CO-dark gas fraction found at $10^{15}$ cm$^{-2}$ $

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Gary Fuller (Supervisor) & Rowan Smith (Supervisor)