Giant Molecular clouds: what are they made from, and how do they get there?

TL;DR

This study uses simulations to explore the formation, properties, and lifecycle of giant molecular clouds in galaxies, revealing their unique gas conditions and the flow patterns driving their evolution.

Contribution

It provides new insights into the gas properties before and after GMC formation, and links flow patterns to cloud formation mechanisms, using a Lagrangian simulation approach.

Findings

GMC-forming gas differs from typical ISM at least 50 Myr before formation.

Gas takes about 10 Myr to reach mean cloud densities before formation.

GMC gas takes at least 50 Myr to disperse and return to typical ISM conditions.

Abstract

We analyse the results of four simulations of isolated galaxies: two with a rigid spiral potential of fixed pattern speed, but with different degrees of star-formation induced feedback, one with an axisymmetric galactic potential and one with a `live' self-gravitating stellar component. Since we use a Lagrangian method we are able to select gas that lies within giant molecular clouds (GMCs) at a particular timeframe, and to then study the properties of this gas at earlier and later times. We find that gas which forms GMCs is not typical of the interstellar medium at least 50 Myr before the clouds form and reaches mean densities within an order of magnitude of mean cloud densities by around 10 Myr before. The gas in GMCs takes at least 50 Myr to return to typical ISM gas after dispersal by stellar feedback, and in some cases the gas is never fully recycled. We also present a study of the two-dimensional, vertically-averaged velocity fields within the ISM. We show that the velocity fields corresponding to the shortest timescales (that is, those timescales closest to the immediate formation and dissipation of the clouds) can be readily understood in terms of the various cloud formation and dissipation mechanisms. Properties of the flow patterns can be used to distinguish the processes which drive converging flows (e.g.\ spiral shocks, supernovae) and thus molecular cloud formation, and we note that such properties may be detectable with future observations of nearby galaxies.