| Abstract : | Superthin galaxies are a class of low surface brightness, bulgeless, disc galaxies, with strikingly high values of planar-to-vertical axes (a/b) ratios of the stellar disc, possibly indicating the presence of an ultra-cold stellar disc, the dynamical stability of which continues to be a puzzle. In this thesis, we explore the origin of the superthin galaxies by constructing dynamical models of a sample of five superthin galaxies (8 < a/b < 16) and two extremely thin galaxies (a/b >20) using AGAMA i.e. Action-based Galaxy Modelling Architecture of Vasilev (2018). We use stellar photometry and HI 21cm radio-synthesis observations as constraints and employ a Markov Chain Monte Carlo method to determine the best-fitting model. Our results indicate that the superthin galaxies host ultra-cold stellar discs with central vertical velocity dispersion equal to 10 - 20 km/s comparable to the 25 km/s for stars in the thinnest disc of the Milky Way. Further, the ratio of the vertical velocity dispersion to the total circular velocity in case of superthins is equal to 0.1 compared to 0.3 for nearby spirals. We also find that despite their ultra-cold stellar discs, superthins are highly stable against the growth of axi-symmetric instabilities with a two-component stability parameter equal to 5 compared to 2.2 for nearby spirals. Finally, we carry out a Principal Component Analysis of the the dynamical parameters possibly regulating the stellar vertical structure of our sample of galaxies: specific angular momentum of the stellar disc, 2-component disc-dynamical stability, ratio of vertical-to-radial velocity dispersion, ratio of asymptotic rotation velocity to compactness parameter along with the major axis to minor axes ratio. Interestingly, we find that the large a/b ratio is primarily driven by a dense and compact dark matter halo followed by a high value of the disc-dynamical stability.
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