How ‘cold’ are the stellar discs of superthin galaxies?

Abstract

ABSTRACT Superthin galaxies are a class of bulgeless, low surface brightness galaxies with strikingly high values of planar-to-vertical axes ratio $\rm (b/a\gt 10\!-\!20)$, possibly indicating the presence of an ultracold stellar disc. Using the multicomponent galactic disc model of gravitationally coupled stars and gas in the force field of the dark matter halo as well as the stellar dynamical code AGAMA (Action-based Galaxy Modelling Architecture), we determine the vertical velocity dispersion of stars and gas as a function of galactocentric radius for five superthin galaxies (UGC 7321, IC 5249, FGC 1540, IC2233, and UGC00711) using observed stellar and atomic hydrogen (H i) scale heights as constraints, using a Markov chain Monte Carlo Method. We find that the central vertical velocity dispersion for the stellar disc in the optical band varies between σ0s ∼ 10.2−18.4 $\rm {kms}\,^{-1}$ and falls off with an exponential scale length of 2.6–3.2 Rd where Rd is the exponential stellar disc scale length. Interestingly, in the 3.6 μm, the same, averaged over the two components of the stellar disc, varies between 5.9 and 11.8 $\rm {kms}\,^{-1}$, both of which confirm the presence of ‘ultracold’ stellar discs in superthin galaxies. Interestingly, the global median of the multicomponent disc dynamical stability parameter QN of our sample superthins is found to be 5 ± 1.5, which higher than the global median value of 2.2 ± 0.6 for a sample of spiral galaxies.