Star Formation in Disk Galaxies. I. Formation and Evolution of Giant Molecular Clouds via Gravitational Instability and Cloud Collisions

Abstract

We investigate the formation and evolution of giant molecular clouds (GMCs) in a Milky-Way-like disk galaxy with a flat rotation curve. We perform a series of 3D adaptive mesh refinement (AMR) numerical simulations that follow both the global evolution on scales of ∼ 20 kpc and resolve down to scales 10 pc with a multiphase atomic interstellar medium (ISM). In this first study, we omit star formation and feedback, and focus on the processes of gravitational instability and cloud collisions and interactions. We define clouds as regions with n H ≥ 100 cm −3 and track the evolution of individual clouds as they orbit through the galaxy from their birth to their eventual destruction via merger or via destructive collision with another cloud. After ∼ 140 Myr a large fraction of the gas in the disk has fragmented into clouds with masses ∼ 10 6 M ⊙ and a mass spectrum similar to that of Galactic GMCs. The disk settles into a quasi steady state in which gravitational scattering of clouds keeps the disk near the threshold of global gravitational instability. The cloud collision time is found to be a small fraction, ∼ 1/5, of the orbital time, and this is an efficient mechanism to inject turbulence into the clouds. This helps to keep clouds only moderately gravitationally bound, with virial parameters of order unity. Many other observed GMC properties, such as mass surface density, angular momentum, velocity dispersion, and vertical distribution, can be accounted for in this simple model with no stellar feedback.

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

“…Global galactic simulations have shown that in a flat rotation curve disk, GMC collision timescales are relatively frequent, at  t t 0.2 coll orbit (Tasker & Tan 2009;Dobbs et al 2015).…”

“…It is an attractive mechanism because it is a process that is expected to create ∼parsec-scale dense gas clumps that are prone to gravitational instability and are the precursors to star clusters, while at the same time being sensitive to global galactic dynamics, such as the shear rate (Tan 2000(Tan , 2010Tasker & Tan 2009;Suwannajak et al 2014) and the presence of spiral arms (Dobbs 2008). Such a connection to orbital shear naturally explains the dynamical Kennicutt-Schmidt relation (Kennicutt 1998;Leroy et al 2008), S µ S W…”

Galaxy formation hydrodynamics: From cosmic flows to star-forming clouds

“…The high-density tail is where star formation takes place. Li et al 2005;Agertz et al 2009b;Tasker & Tan 2009). They have highlighted the role of gravity and turbulence in driving GMC formation.…”

Star Formation and the Properties of Giant Molecular Clouds in Global Simulations

“…Details of the disc initial conditions for simulations without star formation are described in Tasker & Tan (2009). Our highest resolution simulation has a smallest cell size of 7.8 pc and star particles were formed with typical masses of 10 3 M from gas with n H ,c > 100 cm −3 (i.e.…”

“…They were tracked through the simulation by comparing outputs at 1 Myr intervals. Details of the cloud identification and tracking scheme are also described in Tasker & Tan (2009).…”