GMC formation by agglomeration and self gravity

TL;DR

This study explores how giant molecular clouds (GMCs) form in spiral galaxies through cloud agglomeration and self-gravity, revealing the conditions under which each process dominates and their resulting properties.

Contribution

It provides a comparative analysis of GMC formation via agglomeration and self-gravity using two-fluid simulations, highlighting the influence of galactic conditions on each process.

Findings

Agglomeration dominates when ISM is stable against gravitational instabilities.

GMCs formed by agglomeration are often unbound and disperse outside spiral arms.

Self gravity leads to more massive, bound GMCs with higher angular momentum.

Abstract

We investigate the formation of GMCs in spiral galaxies through both agglomeration of clouds in the spiral arms, and self gravity. The simulations presented include two-fluid models, which contain both cold and warm gas, although there is no heating or cooling between them. We find agglomeration is predominant when both the warm and cold components of the ISM are effectively stable to gravitational instabilities. In this case, the spacing (and consequently mass) of clouds and spurs along the spiral arms is determined by the orbits of the gas particles and correlates with their epicyclic radii (or equivalently spiral shock strength). Notably GMCs formed primarily by agglomeration tend to be unbound associations of many smaller clouds, which disperse upon leaving the spiral arms. These GMCs are likely to be more massive in galaxies with stronger spiral shocks or higher surface densities. GMCs formed by agglomeration are also found to exhibit both prograde and retrograde rotation, a consequence of the clumpiness of the gas. At higher surface densities, self gravity becomes more important in arranging both the warm and cold gas into clouds and spurs, and determining the properties of the most massive GMCs. These massive GMCs can be distinguished by their higher angular momentum, exhibit prograde rotation and are more bound. For a 20 M$_{\odot}$ pc$^{-2}$ disc, the spacing between the GMCs fits both the agglomeration and self gravity scenarios, as the maximum unstable wavelength of gravitational perturbations in the warm gas is similar to the spacing found when GMCs form solely by agglomeration.