Disc heating: possible link between weak bars and superthin galaxies

TL;DR

This study uses N-body simulations to explore how weak bars in galaxies can help maintain the superthin structure of stellar discs over cosmic time, contrasting with the thickening caused by strong bars.

Contribution

It demonstrates that weak bars cause minimal stellar heating, allowing superthin galaxies to preserve their thinness during secular evolution.

Findings

Strong bars heat stellar discs efficiently, leading to thickening.

Weak bars cause slow heating, preserving thinness.

Superthin galaxies may host weak bars to maintain their structure.

Abstract

The extreme flatness of stellar discs in superthin galaxies is puzzling and the apparent dearth of these objects in cosmological simulation poses challenging problem to the standard cold dark matter paradigm. Irrespective of mergers or accretion that a galaxy might be going through, stars are heated as they get older while they interact with the spirals and bars which are ubiquitous in disc galaxies -- leading to a puffed up stellar disc. It remains unclear how superthin galaxies maintain their thinness through the cosmic evolution. We follow the internal evolution of a sample of 16 initially extremely thin stellar discs using collisionless N-body simulation. All of these discs eventually form a bar in their central region. Depending on the initial condition, some of these stellar discs readily form strong bars while others grow weak bars over secular evolution time scale. We show that galaxies with strong bars heat the stars very efficiently, eventually making their stellar discs thicker. On the other hand, stars are heated very slowly by weak bars -- as a result, galaxies hosting weak bars are able to maintain their thinness over several billion years, if left isolated. We suggest that some of the superthin galaxies might as well be forming weak bars and thereby prevent any strong vertical heating which in turn helps maintaining their thinness during the course of secular evolution.