The exciting lives of giant molecular clouds

Abstract

We present a detailed study of the evolution of GMCs in a galactic disc simulation. We follow individual GMCs (defined in our simulations by a total column density criterion), including their level of star formation, from their formation to dispersal. We find the evolution of GMCs is highly complex. GMCs often form from a combination of smaller clouds and ambient ISM, and similarly disperse by splitting into a number of smaller clouds and ambient ISM. However some clouds emerge as the result of the disruption of a more massive GMC, rather than from the assembly of smaller clouds. Likewise in some cases, clouds accrete onto more massive clouds rather than disperse. Because of the difficulty of determining a precursor or successor of a given GMC, determining GMC histories and lifetimes is highly non-trivial. Using a definition relating to the continuous evolution of a cloud, we obtain lifetimes typically of 4-25 Myr for > 10 5 M GMCs, over which time the star formation efficiency is about 1 %. We also relate the lifetime of GMCs to their crossing time. We find that the crossing time is a reasonable measure of the actual lifetime of the cloud, although there is considerable scatter. The scatter is found to be unavoidable because of the complex and varied shapes and dynamics of the clouds. We study cloud dispersal in detail and find both stellar feedback and shear contribute to cloud disruption. We also demonstrate that GMCs do not behave as ridge clouds, rather massive spiral arm GMCs evolve into smaller clouds in inter-arm spurs.

A 500 pc filamentary gas wisp in the disk of the Milky Way

“…In our case, the physical length of the filamentary gas wisp ( 500 pc) is much larger than the scale-height of the Milky Way molecular disk. This is consistent with the cloud-formation scenario by Pringle et al (2001) and Dobbs & Pringle (2013). Such large-scale structures are also observed in other numerical simulations of galactic disks (Tan 2000;Kim & Ostriker 2002;Shetty & Ostriker 2006;Tasker & Tan 2009;Ceverino et al 2012).…”

“…The second scenario has been proposed by Pringle et al (2001) and Dobbs & Pringle (2013). In this scenario, the gas is already relatively dense and cold prior to becoming a giant molecular cloud.…”

“…Theoretically, the structure of the multi-phased interstellar medium in a galactic disk has been studied through simulating the whole disk with different approaches (Kim & Ostriker 2002;Shetty & Ostriker 2006;Tasker 2011;Dobbs et al 2011;Van Loo et al 2013), complemented by analytical calculations (Lee & Shu 2012). It was found that filaments or spurs can be created through the combination of gravitational instability of a galactic disk, galactic shear, and frequent Appendices are available in electronic form at http://www.aanda.org encounters/agglomeration between molecular clouds (Pringle et al 2001;Dobbs & Pringle 2013).…”

The formation and destruction of molecular clouds and galactic star formation

“…The sink term on the righthand side of this equation corresponds to the destruction rate of molecular clouds in the sense of ensemble average. If the dynamical effects, such as shear and tidal stresses, contribute to the cloud destruction (e.g., Koda et al 2009;Dobbs & Pringle 2013), we should modify T d in this equation. Since the lefthand side of this equation should be regarded as the ensemble average, the term 1/T d represents the sum of the destruction rate of all the possible processes.…”

Star formation in the Gaia era

“…The resolution of the clouds can be increased by adding extra particles distributed according to the SPH kernel. His results show that the velocity field inherited from the galactic scale simulations can have a major im- (Dobbs & Pringle, 2013) showing two selected molecular clouds extracted from the simulation with their resolution increased to facilitate more detailed simulations (Rey-Raposo et al, 2015). In the lower left the method for increasing the resolution of the molecular clouds is shown, whereby extra particles are distributed according using the SPH kernel.…”