The simulation of molecular clouds formation in the Milky Way

Abstract

Using 3D hydrodynamic calculations we simulate formation of molecular clouds in the Galaxy. The simulations take into account molecular hydrogen chemical kinetics, cooling and heating processes. Comprehensive gravitational potential accounts for contributions from the stellar bulge, two and four armed spiral structure, stellar disk, dark halo and takes into account self-gravitation of the gaseous component. Gas clouds in our model form in the spiral arms due to shear and wiggle instabilities and turn into molecular clouds after t > ∼ 100 Myr. At the times t ∼ 100 − 300 Myr the clouds form hierarchical structures and agglomerations with the sizes of 100 pc and greater. We analyze physical properties of the simulated clouds and find that synthetic statistical distributions like mass spectrum, "mass-size" relation and velocity dispersion are close to those observed in the Galaxy. The synthetic l − v (galactic longitude -radial velocity) diagram of the simulated molecular gas distribution resembles observed one and displays a structure with appearance similar to Molecular Ring of the Galaxy. Existence of this structure in our modelling can be explained by superposition of emission from the galactic bar and the spiral arms at ∼3-4 kpc.

The impact of galactic disc environment on star-forming clouds

“…This is akin to a density wave spiral (Lin & Shu 1964). The adopted potential is based on the form of Wada & Koda (2004); Khoperskov et al (2013) and is included as a perturbation to the potential of the stellar disc:…”

A Kennicutt–Schmidt relation at molecular cloud scales and beyond

“…Based on our simple model for H2 chemical kinetics (Khoperskov et al 2013) we expand the Nelson & Langer (1999) network by several reactions needed for hydrogen ionization and recombination. For H2 and CO photodissociation we use the approach described by .…”

“…The other cooling and heating rates are presented in detail in Table 2 (Appendix B in Khoperskov et al 2013). Here we simply provide a list of it: cooling due to recombination and collisional excitation and free-free emission of hydrogen (Cen 1992), molecular hydrogen cooling (Galli & Palla 1998), cooling in the fine structure and metastable transitions of carbon, oxygen and silicon (Hollenbach & McKee 1989), energy transfer in collisions with the dust particles (Wolfire et al 2003) and recombination cooling on the dust (Bakes & Tielens 1994), photoelectric heating on the dust particles (Bakes & Tielens 1994;Wolfire et al 2003), heating due to H2 formation on the dust particles, and the H2 photodissociation (Hollenbach & McKee 1979) and the ion-ization heating by cosmic rays (Goldsmith & Langer 1978).…”

Giant molecular cloud scaling relations: the role of the cloud definition

“…We assume that a gas has solar metallicity with the abundances given in Asplund et al (2005) For cooling and heating processes we extend our previous model (Khoperskov et al 2013) by CO and OH cooling rates (Hollenbach & McKee 1979) and CI fine structure cooling rate (Hollenbach & McKee 1989). The other cooling and heating rates are presented in detail in Table 2 (Appendix B in Khoperskov et al 2013).…”