Cloud and Star Formation in Spiral Arms

We present the results from simulations of GMC formation in spiral galaxies. First we discuss cloud formation by cloud-cloud collisions, and gravitational instabilities, arguing that the former is prevalent at lower galactic surface densities and the latter at higher. Cloud masses are also limited by stellar feedback, which can be effective before clouds reach their maximum mass. We show other properties of clouds in simulations with different levels of feedback. With a moderate level of feedback, properties such as cloud rotations and virial parameters agree with observations. Without feedback, an unrealistic population of overly bound clouds develops. Spiral arms are not found to trigger star formation, they merely gather gas into more massive GMCs. We discuss in more detail interactions of clouds in the ISM, and argue that these are more complex than early ideas of cloud-cloud collisions. Finally we show ongoing work to determine whether the Milky Way is a flocculent or grand design spiral.Comment: 10 pages, 5 figures, to be published in Seychelles conference "Lessons from the Local Group", ed. K. C. Freeman, B. G. Elmegreen, D. L. Block, and M. Woolway (Dordrecht: Springer), 201

Iron and silicate dust growth in the Galactic interstellar medium: clues from element depletions

The interstellar abundances of refractory elements indicate a substantial depletion from the gas phase, that increases with gas density. Our recent model of dust evolution, based on hydrodynamic simulations of the lifecycle of giant molecular clouds (GMCs) proves that the observed trend for [Si$_{gas}$/H] is driven by a combination of dust growth by accretion in the cold diffuse interstellar medium (ISM) and efficient destruction by supernova (SN) shocks (Zhukovska et al. 2016). With an analytic model of dust evolution, we demonstrate that even with optimistic assumptions for the dust input from stars and without destruction of grains by SNe it is impossible to match the observed [Si$_{gas}$/H]$-n_H$ relation without growth in the ISM. We extend the framework developed in our previous work for silicates to include the evolution of iron grains and address a long-standing conundrum: ``Where is the interstellar iron?'. Much higher depletion of Fe in the warm neutral medium compared to Si is reproduced by the models, in which a large fraction of interstellar iron (70%) is locked as inclusions in silicate grains, where it is protected from sputtering by SN shocks. The slope of the observed [Fe$_{gas}$/H]$-n_H$ relation is reproduced if the remaining depleted iron resides in a population of metallic iron nanoparticles with sizes in the range of 1-10nm. Enhanced collision rates due to the Coulomb focusing are important for both silicate and iron dust models to match the observed slopes of the relations between depletion and density and the magnitudes of depletion at high density.Comment: Accepted for publication in the ApJ, 15 pages, 9 figure

Spiral arm triggering of star formation

We present numerical simulations of the passage of clumpy gas through a galactic spiral shock, the subsequent formation of giant molecular clouds (GMCs) and the triggering of star formation. The spiral shock forms dense clouds while dissipating kinetic energy, producing regions that are locally gravitationally bound and collapse to form stars. In addition to triggering the star formation process, the clumpy gas passing through the shock naturally generates the observed velocity dispersion size relation of molecular clouds. In this scenario, the internal motions of GMCs need not be turbulent in nature. The coupling of the clouds' internal kinematics to their externally triggered formation removes the need for the clouds to be self-gravitating. Globally unbound molecular clouds provides a simple explanation of the low efficiency of star formation. While dense regions in the shock become bound and collapse to form stars, the majority of the gas disperses as it leaves the spiral arm.Comment: 6 pages, 4 figures: IAU 237, Triggering of star formation in turbulent molecular clouds, eds B. Elmegreen and J. Palou

Gas dynamics in whole galaxies: SPH

I review the progress of SPH calculations for modelling galaxies, and resolving gas dynamics on GMC scales. SPH calculations first investigated the response of isothermal gas to a spiral potential, in the absence of self gravity and magnetic fields. Surprisingly though, even these simple calculations displayed substructure along the spiral arms. Numerical tests indicate that this substructure is still present at high resolution (100 million particles, ~10 pc), and is independent of the initial particle distribution. One interpretation of the formation of substructure is that smaller clouds can agglomerate into more massive GMCs via dissipative collisions. More recent calculations have investigated how other processes, such as the thermodynamics of the ISM, and self gravity affect this simple picture. Further research has focused on developing models with a more realistic spiral structure, either by including stars, or incorporating a tidal interaction.Comment: 8 pages, 6 figures, review talk for IAU 270 symposium 'Computational star formation