The SILCC project

Simulating the Life-Cycle of molecular Clouds

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 Universität zu Köln Team Cologne S. Walch-Gassner D. Derigs Team Garching P. Girichidis T. Naab A. Gatto T. Peters Universität Heidelberg Team Heidelberg S.C.O. Glover R.S. Klessen C. Baczynski Team Prague R. Wünsch Cardiff University Team Cardiff P.C. Clark

Introduction

Star formation takes place in the densest and coldest gas in a galaxy, in so-called molecular clouds (MCs). MCs do not evolve in isolation but are highly dynamical objects, which are born, fed, heated, and stirred from their turbulent environment into which they eventually dissolve. They form in regions where the hot or warm, ionized and atomic interstellar medium (ISM) condenses into cold ($T < 300K$), molecular gas. Often concentrated to the midplane of galactic disks, this process involves metallicity-dependent, non-equilibrium chemistry and molecule formation, heating and cooling, turbulence, self-gravity, and magnetic fields. Once formed, MCs further collapse to form stars and star clusters.

Less than 1% of all new-born stars are more massive than 8 solar masses, but these are particularly important for galaxy evolution. The life and death of massive stars differ intriguingly from those of their low-mass counterparts. Such stars affect their environment dramatically through their strong UV radiation, their energetic stellar wind, and their final explosion as a supernova (SN). These ’feedback’ processes generate turbulence in the parental molecular cloud, dissociate, ionize, and eventually destroy them from within, thereby preventing further star formation. Stellar feedback is thus thought to regulate the star formation efficiency in molecular clouds leading to a self-regulation of star formation on galactic scales.

In the framework of the Gauss project "SILCC" (Simulating the Life Cycle of Molecular Clouds) run on SuperMUC, the peta-scale machine at the Leibniz Rechenzentrum Garching, scientists from from Cologne, Garching, Heidelberg, Prague and Zurich model representative regions of disk galaxies using adaptive, three-dimensional simulations with the necessary physical complexity to follow the full life-cycle of molecular clouds. These simulations include self-gravity, magnetic fields, heating and cooling at different gas metallicities, molecule formation and dissociation, and stellar feedback. The ultimate goal of the SILCC project is to provide a self-consistent answer as to how stellar feedback regulates the star formation efficiency of a galaxy, how molecular clouds are formed and destroyed, and how galactic outflows are driven.

Available simulations

The SILCC project is split into several sets of simulations including different physical processes and different numerical realisations. Future simulations will be made public together with the corresponding scientific publication.

In the first set of simulations (see below) we show the impact of different supernova positioning and different (but constant in time) supernova rates on the structural evolution of the ISM in a galactic disc with a gas surface density of 10 Msun/pc² . For more information on the simulations please check the SILCC Paper.

SILCC I. Chemical evolution of the supernova-driven ISM

No self-gravity

S10-KS-rand-nsg 15 random driving

Different driving schemes and supernova rates

S10-lowSN-rand 5 random driving
S10-KS-rand 15 random driving
S10-highSN-rand 45 random driving
S10-lowSN-peak 5 peak driving
S10-KS-peak 15 peak driving
S10-highSN-peak 45 peak driving
S10-lowSN-mix 5 mixed driving, ratio 1:1
S10-KS-mix 15 mixed driving, ratio 1:1
S10-highSN-mix 45 mixed driving, ratio 1:1
S10-KS-clus2 15 clustered driving; Type II SNe
S10-KS-clus 15 clustered driving; 20% of all SNe is Type Ia

MHD runs with B0 = 3 microGauss

S10-KS-clus-mag3 15 clustered driving; 20% of all SNe is Type Ia
• SN = supernova
• random driving = randomly placed supernovae
• peak driving = Supernovae placed at global density maxima
• mixed driving = mixed 50:50 (random vs. peak)
• clustered driving = 50% of all supernovae are in randomly placed clusters, which contain between 5 and 40 supernovae. 30% are single random supernovae, and 20% are Type Ia's, which have a larger scale height of 320 pc.
• KS = Kennnicutt-Schmidt. The KS relation was converted to the expected SN rate for a disk with gas surface density 10 Msun/pc² .

SILCC IV. Impact of dissociating and ionising radiation on the interstellar medium and Halpha emisssion as a tracer of the star formation rate

Type
Supernovae
Supernovae, Stellar winds

random driving
mixed driving
peak driving

Publications

The SILCC (SImulating the LifeCycle of molecular Clouds) project:I. Chemical evolution of the supernova-driven ISM

S. Walch, P. Girichidis, T. Naab, A. Gatto, S. C. O. Glover, R. Wünsch, R. S. Klessen, P. C. Clark, T. Peters, D. Derigs, C. Baczynski

The SILCC (SImulating the Life-Cycle of molecular Clouds) project aims to self-consistently understand the small-scale structure of the interstellar medium (ISM) and its link to galaxy evolution. We simulate the evolution of the multiphase ISM in a $(500{\rm~pc})^2 \times \pm 5 {\rm~kpc}$ region of a galactic disc, with a gas surface density of $\Sigma _{_{\rm GAS}} = 10 \;{\rm M}_{\odot }\,{\rm pc}^{-2}$. The $\mathtt{FLASH}$ 4 simulations include an external potential, self-gravity, magnetic fields, heating and radiative cooling, time-dependent chemistry of ${\rm H}_2$ and ${\rm CO}$ considering (self-) shielding, and supernova (SN) feedback but omit shear due to galactic rotation. We explore SN explosions at different rates in high-density regions (peak), in random locations with a Gaussian distribution in the vertical direction (random), in a combination of both (mixed), or clustered in space and time (clus/clus2). Only models with self-gravity and a significant fraction of SNe that explode in low-density gas are in agreement with observations. Without self-gravity and in models with peak driving the formation of ${\rm H}_2$ is strongly suppressed. For decreasing SN rates, the ${\rm H}_2$ mass fraction increases significantly from $<10$ per cent for high SN rates, i.e. 0.5 dex above Kennicutt–Schmidt, to 70–85 per cent for low SN rates, i.e. 0.5 dex below KS. For an intermediate SN rate, clustered driving results in slightly more ${\rm H}_2$ than random driving due to the more coherent compression of the gas in larger bubbles. Magnetic fields have little impact on the final disc structure but affect the dense gas ($n \gtrsim 10{\rm~cm}^{-3}$) and delay ${\rm H}_2$ formation. Most of the volume is filled with hot gas ($\sim 80$ per cent within $\pm 150{\rm~pc}$). For all but peak driving a vertically expanding warm component of atomic hydrogen indicates a fountain flow. We highlight that individual chemical species populate different ISM phases and cannot be accurately modelled with temperature-/density-based phase cut-offs.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (November 21, 2015) 454 (1): 246-276. A copy of the preprint is also available on the astro-ph server (arXiv:1412.2749).

The SILCC (SImulating the LifeCycle of molecular Clouds) project:II. Dynamical evolution of the supernova-driven ISM and the launching of outflows

P. Girichidis, S. Walch, T. Naab, A. Gatto, S. C. O. Glover, R. Wünsch, R. S. Klessen, P. C. Clark, T. Peters, D. Derigs, C. Baczynski

The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a more self-consistent understanding of the interstellar medium (ISM) on small scales and its link to galaxy evolution. We present three-dimensional (magneto)hydrodynamic simulations of the ISM in a vertically stratified box including self-gravity, an external potential due to the stellar component of the galactic disc, and stellar feedback in the form of an interstellar radiation field and supernovae (SNe). The cooling of the gas is based on a chemical network that follows the abundances of ${\rm H}^{+}$, ${\rm H}$, ${\rm H}_2$, ${\rm C}^{+}$, and ${\rm CO}$ and takes shielding into account consistently. We vary the SN feedback by comparing different SN rates, clustering and different positioning, in particular SNe in density peaks and at random positions, which has a major impact on the dynamics. Only for random SN positions the energy is injected in sufficiently low-density environments to reduce energy losses and enhance the effective kinetic coupling of the SNe with the gas. This leads to more realistic velocity dispersions ($\sigma_{HI} $$~\sim~$$ 0.8\sigma_{(300 - 8000{\rm~K})} $$~\sim~$$ 10-20{\rm~km/s}$, $\sigma_{H\alpha} $$~\sim~$$ 0.6\sigma_{(8000 - 3 \cdot 10^5 {\rm~K})} $$~\sim~$$ 20-30{\rm~km/s}$), and strong outflows with mass loading factors of up to 10 even for solar neighbourhood conditions. Clustered SNe abet the onset of outflows compared to individual SNe but do not influence the net outflow rate. The outflows do not contain any molecular gas and are mainly composed of atomic hydrogen. The bulk of the outflowing mass is dense ($\rho $$~\sim~$$ 10^{-25} - 10^{-24} {\rm~g/cc}$) and slow ($v $$~\sim~$$ 20-40 {\rm~km/s}$) but there is a high-velocity tail of up to $v $$~\sim~$$ 500{\rm~km/s}$ with $\rho $$~\sim~$$ 10^{-28} - 10^{-27} {\rm~g/cc}$.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (March 11, 2016) 456 (4): 3432-3455. A copy of the preprint is also available on the astro-ph server (arXiv:1508.06646).

Impact of supernova and cosmic-ray driving on the surface brightness of the galactic halo in soft X-rays

T. Peters, P. Girichidis, A. Gatto, T. Naab, S. Walch, R. Wünsch, S. C. O. Glover, P. C. Clark, R. S. Klessen, C. Baczynski

The halo of the Milky Way contains a hot plasma with a surface brightness in soft X-rays of the order $10^{-12}{\rm~erg} {\rm~cm}^{-2} {\rm~s}^{-1} {\rm~deg}^{-2}$. The origin of this gas is unclear, but so far numerical models of galactic star formation have failed to reproduce such a large surface brightness by several orders of magnitude. In this paper, we analyze simulations of the turbulent, magnetized, multi-phase interstellar medium including thermal feedback by supernova explosions as well as cosmic-ray feedback. We include a time-dependent chemical network, self-shielding by gas and dust, and self-gravity. Pure thermal feedback alone is sufficient to produce the observed surface brightness, although it is very sensitive to the supernova rate. Cosmic rays suppress this sensitivity and reduce the surface brightness because they drive cooler outflows. Self-gravity has by far the largest effect because it accumulates the diffuse gas in the disk in dense clumps and filaments, so that supernovae exploding in voids can eject a large amount of hot gas into the halo. This can boost the surface brightness by several orders of magnitude. Although our simulations do not reach a steady state, all simulations produce surface brightness values of the same order of magnitude as the observations, with the exact value depending sensitively on the simulation parameters. We conclude that star formation feedback alone is sufficient to explain the origin of the hot halo gas, but measurements of the surface brightness alone do not provide useful diagnostics for the study of galactic star formation.

This paper has been published in the The Astrophysical Journal Letters, Volume 813, Issue 2, article id. L27, 7 pp. (2015). A copy of the preprint is also available on the astro-ph server (arXiv:1510.06563).

Acknowledgments

This work is supported by
 The DFG Priority Programme 1573 'Physics of the ISM' Leibniz-Rechenzentrum Garching Gauss Center for Supercomputing: Link to a short project description on the GCS site Max Planck Computing and Data Facility (MPCDF)