Angular momentum regulates HI gas content and HI central hole size in the discs of spirals

Abstract

The neutral atomic hydrogen (H I) content of spiral galaxies has been observed to vary with environment, with more H I-deficient spirals residing in high-density environments. This can be attributed to environmental effects such as ram pressure stripping and tidal interactions, which remove H I from the discs of galaxies. However, some spirals in low-density environments have also been observed to have relatively low H I mass fractions. The low densities of the intergalactic medium and lack of nearby galaxies in such environments make ram pressure stripping and tidal interactions unlikely candidates of gas removal. What then could be making these spirals H I deficient? Obreschkow et al. introduced a parameter-free model for the neutral atomic gas fraction (fatm), in a symmetric equilibrium disc as a function of the global atomic stability parameter (q), which depends on specific angular momentum. In order to examine if this model accounts for H I-deficient galaxies in low-density environments, we have used the MHI−MR scaling relation to select six H I-deficient spiral galaxies and observed them with the Australia Telescope Compact Array. By measuring their fatm and q values we find that the galaxies owe their observed H I deficiencies to low specific angular momenta. Additionally, we also find that the central H I hole sizes of our sample galaxies are related to their q values, following the prediction of Obreschkow et al. This result brings to light the importance of angular momentum in understanding the physics of the interstellar medium in the discs of galaxies and consequently their evolution.