GMC formation by agglomeration and self gravity

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.

16 pages, 12 figures, accepted for publication in MNRAS. Version with higher resolution figures available at http://www.astro.ex.ac.uk/people/dobbs/Dobbsgr.pdf

References in corpus (25)

• Theory of Star Formation
• SPLASH: An interactive visualisation tool for Smoothed Particle Hydrodynamics simulations
• Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
• Turbulent Structure of a Stratified Supernova-Driven Interstellar Medium
• Simulating the formation of molecular clouds. II. Rapid formation from turbulent initial conditions
• The impact of magnetic fields on single and binary star formation
• From the warm magnetized atomic medium to molecular clouds
• On the Rapid Collapse and Evolution of Molecular Clouds
• The Birth of Molecular Clouds: Formation of Atomic Precursors in Colliding Flows
• Cooling, Gravity and Geometry: Flow-driven Massive Core Formation
• Formation of Spiral-Arm Spurs and Bound Clouds in Vertically Stratified Galactic Gas Disks
• The formation of molecular clouds in spiral galaxies
• Kinematics of Spiral Arm Streaming in M51
• Spurs and feathering in spiral galaxies
• The Generation and Dissipation of Interstellar Turbulence - Results from Large Scale High Resolution Simulations
• The ISM in spiral galaxies: can cooling in spiral shocks produce molecular clouds?
• An HST Archival Survey of Feathers in Spiral Galaxies
• Cloud and Star Formation in Disk Galaxy Models with Feedback
• Simulations of spiral galaxies with an active potential: molecular cloud formation and gas dynamics
• Instabilities of Spiral Shocks -- II. A quasi-steady State in the multi-phase inhomogeneous ISM
• Core Mass Function: The Role of Gravity
• Galactic Spiral Shocks with Thermal Instability
• Magnetic fields and the dynamics of spiral galaxies
• Cold HI in Turbulent Eddies and Galactic Spiral Shocks
• Simulating the ISM in Global Disk Galaxies

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