# The SILCC project

Simulating the Life-Cycle of molecular Clouds

## People

 Universität zu Köln Team Cologne S. Walch-Gassner D. Seifried A. Franeck S. Haid D. Derigs F. Dinnbier Team Garching T. Naab T.-E. Rathjen T. Peters (former member) A. Gatto (former member) Universität Heidelberg Team Heidelberg R.S. Klessen S.C.O. Glover E. Pellegrini C. Baczynski (former member) Team Prague R. Wünsch Cardiff University Team Cardiff P.C. Clark Team Potsdam P. Girichidis

## 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 Potsdam 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

### SILCC 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

(2015, MNRAS, 454, 238)

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).

#### Launching Cosmic-Ray-driven Outflows from the Magnetized Interstellar Medium

P. Girichidis, T. Naab, S. Walch, M. Hanasz, M.-M. Mac Low, J. P. Ostriker, A. Gatto, T. Peters, R. Wünsch, S. C. O. Glover, R. S. Klessen, P. C. Clark, C. Baczynski

(2016, APJL, 816, L19)

We present a hydrodynamical simulation of the turbulent, magnetized, supernova (SN)-driven interstellar medium (ISM) in a stratified box that dynamically couples the injection and evolution of cosmic rays (CRs) and a self-consistent evolution of the chemical composition. CRs are treated as a relativistic fluid in the advection-diffusion approximation. The thermodynamic evolution of the gas is computed using a chemical network that follows the abundances of H${\rm H}^{+}$, ${\rm H}$, ${\rm H}_2$, ${\rm CO}$, and ${\rm C}^{+}$, and free electrons and includes (self-)shielding of the gas and dust. We find that CRs perceptibly thicken the disk with the heights of 90% (70%) enclosed mass reaching ~1.5 kpc (~0.2 kpc). The simulations indicate that CRs alone can launch and sustain strong outflows of atomic and ionized gas with mass loading factors of order unity, even in solar neighborhood conditions and with a CR energy injection per SN of 10^50 erg, 10% of the fiducial thermal energy of an SN. The CR-driven outflows have moderate launching velocities close to the midplane (~100 km/s) and are denser ($\rho$~1e-24 - 1e-26 g/cm^3), smoother, and colder than the (thermal) SN-driven winds. The simulations support the importance of CRs for setting the vertical structure of the disk as well as the driving of winds.

This paper has been published in the The Astrophysical Journal Letters, Volume 816, Issue 2. A copy of the preprint is also available on the astro-ph server (arXiv:1509.07247).

#### 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

(2016, MNRAS, 456, 3432)

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).

#### The SILCC project:III. Regulation of star formation and outflows by stellar winds and supernovae

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

(2016, MNRAS, 466, 1903)

We study the impact of stellar winds and supernovae on the multi-phase interstellar medium using three-dimensional hydrodynamical simulations carried out with FLASH. The selected galactic disc region has a size of (500 pc)$^2$ x $\pm$ 5 kpc and a gas surface density of 10 M$_{\odot}$/pc$^2$. The simulations include an external stellar potential and gas self-gravity, radiative cooling and diffuse heating, sink particles representing star clusters, stellar winds from these clusters which combine the winds from indi- vidual massive stars by following their evolution tracks, and subsequent supernova explosions. Dust and gas (self-)shielding is followed to compute the chemical state of the gas with a chemical network. We find that stellar winds can regulate star (cluster) formation. Since the winds suppress the accretion of fresh gas soon after the cluster has formed, they lead to clusters which have lower average masses (10$^2$ - 10$^{4.3}$ M$_{\odot}$) and form on shorter timescales (10$^{-3}$ - 10 Myr). In particular we find an anti-correlation of cluster mass and accretion time scale. Without winds the star clusters easily grow to larger masses for ~5 Myr until the first supernova explodes. Overall the most massive stars provide the most wind energy input, while objects beginning their evolution as B-type stars contribute most of the supernova energy input. A significant outflow from the disk (mass loading $\gtrsim$ 1 at 1 kpc) can be launched by thermal gas pressure if more than 50% of the volume near the disc mid-plane can be heated to T > 3x10$^5$ K. Stellar winds alone cannot create a hot volume-filling phase. The models which are in best agreement with observed star formation rates drive either no outflows or weak outflows.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (December 12, 2016), Volume 466, Issue 2, p. 1903-1924. A copy of the preprint is also available on the astro-ph server (arXiv:1606.05346).

#### The SILCC project:IV. Impact of dissociating and ionizing radiation on the interstellar medium and Hα emission as a tracer of the star formation rate

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

(2016, MNRAS, 466, 3293)

We present three-dimensional radiation-hydrodynamical simulations of the impact of stellar winds, photoelectric heating, photodissociating and photoionising radiation, and supernovae on the chemical composition and star formation in a stratified disc model. This is followed with a sink-based model for star clusters with populations of individual massive stars. Stellar winds and ionising radiation regulate the star formation rate at a factor of ~10 below the simulation with only supernova feedback due to their immediate impact on the ambient interstellar medium after star formation. Ionising radiation (with winds and supernovae) significantly reduces the ambient densities for most supernova explosions to rho < 10^-25 g cm^-3, compared to 10^-23 g cm^-3 for the model with only winds and supernovae. Radiation from massive stars reduces the amount of molecular hydrogen and increases the neutral hydrogen mass and volume filling fraction. Only this model results in a molecular gas depletion time scale of 2 Gyr and shows the best agreement with observations. In the radiative models, the Halpha emission is dominated by radiative recombination as opposed to collisional excitation (the dominant emission in non-radiative models), which only contributes ~1-10 % to the total Halpha emission. Individual massive stars ($M >= 30 M_\odot$) with short lifetimes are responsible for significant fluctuations in the Halpha luminosities. The corresponding inferred star formation rates can underestimate the true instantaneous star formation rate by factors of ~10.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (December 10, 2016), Volume 466, Issue 3, p. 3293-3308. A copy of the preprint is also available on the astro-ph server (arXiv:1610.06569).

#### The turbulent life of dust grains in the supernova-driven, multiphase interstellar medium

T. Peters, S. Zhukovska, T. Naab, P. Girichidis, S. Walch, S. C. O. Glover, R. S. Klessen, P. C. Clark, D. Seifried

(2017, MNRAS, 467, 4322)

Dust grains are an important component of the interstellar medium (ISM) of galaxies. We present the first direct measurement of the residence times of interstellar dust in the different ISM phases, and of the transition rates between these phases, in realistic hydrodynamical simulations of the multi-phase ISM. Our simulations include a time-dependent chemical network that follows the abundances of H${}^+$, H, H${}_2$, C${}^+$ and CO and take into account self-shielding by gas and dust using a tree-based radiation transfer method. Supernova explosions are injected either at random locations, at density peaks, or as a mixture of the two. For each simulation, we investigate how matter circulates between the ISM phases and find more sizeable transitions than considered in simple mass exchange schemes in the literature. The derived residence times in the ISM phases are characterised by broad distributions, in particular for the molecular, warm and hot medium. The most realistic simulations with random and mixed driving have median residence times in the molecular, cold, warm and hot phase around 17, 7, 44 and 1 Myr, respectively. The transition rates measured in the random driving run are in good agreement with observations of Ti gas-phase depletion in the warm and cold phases in a simple depletion model, although the depletion in the molecular phase is under-predicted. ISM phase definitions based on chemical abundance rather than temperature cuts are physically more meaningful, but lead to significantly different transition rates and residence times because there is no direct correspondence between the two definitions.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (June 1, 2017), Volume 467, Issue 4, p.4322-4342. A copy of the preprint is also available on the astro-ph server (arXiv:1610.06579).

#### SILCC-Zoom: the dynamic and chemical evolution of molecular clouds

D. Seifried, S. Walch, P. Girichidis, T. Naab, R. Wünsch, R. S. Klessen, S. C. O. Glover, T. Peters, P. Clark

(2017, MNRAS, 472, 4797)

We present 3D zoom-in simulations of the formation of two molecular clouds out of the galactic interstellar medium. We model the clouds – identified from the SILCC simulations – with a resolution of up to 0.06 pc using adaptive mesh refinement in combination with a chemical network to follow heating, cooling and the formation of $\rm~H_2$ and $\rm~CO$ including (self-) shielding. The two clouds are assembled within a few million years with mass growth rates of up to $\sim 10^{−2}$ ${\rm M}_{\odot} \rm~yr^{−1}$ and final masses of $\sim 50000 \rm~M_{\odot}$. A spatial resolution of $\leqslant 0.1 \rm~pc$ is required for convergence with respect to the mass, velocity dispersion and chemical abundances of the clouds, although these properties also depend on the cloud definition such as based on density thresholds, $\rm~H_2$ or $\rm~CO$ mass fraction. To avoid grid artefacts, the progressive increase of resolution has to occur within the free-fall time of the densest structures ($1–1.5 \rm~Myr$) and $\geqslant 200$ time-steps should be spent on each refinement level before the resolution is progressively increased further. This avoids the formation of spurious, large-scale, rotating clumps from unresolved turbulent flows. While $\rm~CO$ is a good tracer for the evolution of dense gas with number densities $\rm{n} \geqslant 300 \rm~cm^{−3}$, $\rm~H_2$ is also found for $\rm{n} \leqslant 30 \rm~cm^{−3}$ due to turbulent mixing and becomes dominant at column densities around $30–50 \rm{M}_{\odot} \rm{pc}^{−2}$. The $\rm~CO$-to-$\rm~H_2$ ratio steadily increases within the first $2 \rm~Myr$, whereas $\rm{X_{CO}} \simeq 1–4 \times 10^{20} \rm~cm^{−2} (K\ km\ s^{−1})^{−1}$ is approximately constant since the CO(1−0) line quickly becomes optically thick.

This paper was published in the Monthly Notices of the Royal Astronomical Society (December 21, 2017), Volume 472, Issue 4, p. 4797-4818. A copy of the preprint is also available on the astro-ph server (arXiv:1704.06487).

#### The SILCC project – V. The impact of magnetic fields on the chemistry and the formation of molecular clouds

P. Girchidis, D. Seifried, T. Naab, T. Peters, S. Walch, R. Wünsch, S. C. O. Glover, R. S. Klessen

(2018, MNRAS, 480, 3511)

Magnetic fields are ubiquitously observed in the interstellar medium (ISM) of present-day star-forming galaxies with dynamically relevant energy densities. Using three-dimensional magnetohydrodynamic (MHD) simulations of the supernova-driven ISM in the flux-freezing approximation (ideal MHD), we investigate the impact of the magnetic field on the chemical and dynamical evolution of the gas, fragmentation, and the formation of molecular clouds. We follow the chemistry with a network of six species ($H^+$, $H$, $H_2$, $C^+$, $CO$, and free electrons) including local shielding effects. We find that magnetic fields thicken the disc by a factor of a few to a scale height of $\sim 100$ pc⁠, delay the formation of dense (and molecular) gas by $\sim 25$ Myr⁠, and result in differently shaped gas structures. The magnetized gas fragments into fewer clumps, which are initially at subcritical mass-to-flux ratios, $M/\Phi \approx 0.3(M/Φ)_{crit}$, and accrete gas preferentially parallel to the magnetic field lines until supercritial mass-to-flux ratios of up to the order of 10 are reached. The accretion rates onto molecular clouds scale with $\dot{M} \propto M^{1.5}$⁠. The median of the intercloud velocity dispersion is $\sim 2−5$ $kms^{−1}$ and lower than the internal velocity dispersion in the clouds (⁠$\sim 3−7$ $kms^{−1}$⁠). However, individual cloud–cloud collisions occur at speeds of a few $10$ $kms^{−1}$⁠.

This paper was published in the Monthly Notices of the Royal Astronomical Society (November 1, 2018), Volume 480, Issue 3, p. 3511-3540. A copy of the preprint is also available on the astro-ph server (arXiv:1808.05222).

#### SILCC-Zoom: Polarisation and depolarisation in molecular clouds

D. Seifried, S. Walch, S. Reissl, J. C. Ibáñez-Mejía

(2019, MNRAS, 482, 2697)

We present synthetic dust polarisation maps of 3D magneto-hydrodynamical simulations of molecular clouds embedded in their galactic environment performed within the SILCC-Zoom project. The radiative transfer is carried out with POLARIS for wavelengths from 70 $\mu$m to 3 mm at a resolution of 0.12 pc, and includes self-consistently calculated alignment efficiencies for radiative torque alignment. We explore the reason of the observed depolarisation in the center of molecular clouds: We find that dust grains remain well aligned even at high densities (n > 103 cm$^{−3}$) and visual extinctions (A$_V$ > 1). The depolarisation is rather caused by strong variations of the magnetic field direction along the LOS due to turbulent motions. The observed magnetic field structure thus resembles best the mass-weighted, line-of-sight averaged field structure. Furthermore, it differs by only a few 1$^{\circ}$ for different wavelengths and is little affected by the spatial resolution of the synthetic observations. Noise effects can be reduced by convolving the image. Doing so, for $\lambda \geq 160$ $\mu$m the observed magnetic field traces reliably the underlying field in regions with intensities I $\geq$ 3 times the noise level and column densities above 1 M$_{sun}$ pc$^{−2}$. Here, typical deviations are $\leq$ 10$^{\circ}$. The observed structure is less reliable in regions with low polarisation degrees and possibly in regions with large column density gradients. Finally, we show that the simplified and widely used method by Wardle & Konigl (1990) without self-consistent dust alignment efficiencies can provide a good representation of the observable field structure with deviations below 5$^{\circ}$.

This paper was published in the Monthly Notices of the Royal Astronomical Society (January 11, 2019), Volume 482, Issue 2, p. 2697-2716. A copy of the preprint is also available on the astro-ph server (arXiv:1804.10157).

#### SILCC-Zoom: The early impact of ionizing radiation on forming molecular clouds

S. Haid, S. Walch, D. Seifried, R. Wünsch, F. Dinnbier, T. Naab

(2019, MNRAS, 482, 4062)

As part of the SILCC-Zoom project, we present our first sub-parsec resolution radiation-hydrodynamic simulations of two molecular clouds self-consistently forming from a turbulent, multiphase ISM. The clouds have similar initial masses of few $10^4 M_{\odot}$, escape velocities of $5 km s^{-1}$, and a similar initial energy budget. We follow the formation of star clusters with a sink-based model and the impact of radiation from individual massive stars with the tree-based radiation transfer module TREERAY. Photoionizing radiation is coupled to a chemical network to follow gas heating, cooling, and molecule formation and dissociation. For the first 3 Myr of cloud evolution, we find that the overall star formation efficiency is considerably reduced by a factor of $4$ to global cloud values of $\leq 10$ per cent as the mass accretion of sinks that host massive stars is terminated after $\leq 1$ Myr. Despite the low efficiency, star formation is triggered across the clouds. Therefore, a much larger region of the cloud is affected by radiation and the clouds begin to disperse. The time-scale on which the clouds are dispersed sensitively depends on the cloud sub-structure and in particular on the amount of gas at high visual extinction. The damage of radiation done to the highly shielded cloud ($MC_1$) is delayed. We also show that the radiation input can sustain the thermal and kinetic energy of the clouds at a constant level. Our results strongly support the importance of ionizing radiation from massive stars for explaining the low-observed star formation efficiency of molecular clouds.

This paper was published in the Monthly Notices of the Royal Astronomical Society (January 21, 2019), Volume 482, Issue 3, p. 4062-4083. A copy of the preprint is also available on the astro-ph server (arXiv:1810.08210).

### Related project publications

#### Modelling the supernova-driven ISM in different environments

A. Gatto, S. Walch, M.-M. Mac Low, T. Naab, P. Girichidis, S. C. O. Glover, R. Wünsch, R. S. Klessen, P. C. Clark, C. Baczynski, T. Peters, J. P. Ostriker, J. C. Ibáñez-Mejía, S. Haid

(2015, MNRAS, 449, 1057)

We present three-dimensional magneto-hydrodynamical simulations of the self-gravitating interstellar medium (ISM) in a periodic (256 pc)$^3$ box with a mean number density of 0.5 cm$^{-3}$. At a fixed supernova rate we investigate the multi-phase ISM structure, H$_{2}$ molecule formation and density-magnetic field scaling for varying initial magnetic field strengths (0, $6\times 10^{-3}$, 0.3, 3 $\mu$G). All magnetic runs saturate at mass weighted field strengths of $\sim$ 1 $-$ 3 $\mu$G but the ISM structure is notably different. With increasing initial field strengths (from $6\times 10^{-3}$ to 3 $\mu$G) the simulations develop an ISM with a more homogeneous density and temperature structure, with increasing mass (from 5% to 85%) and volume filling fractions (from 4% to 85%) of warm (300 K $<$ T $<$ 8000 K) gas, with decreasing volume filling fractions (VFF) from $\sim$ 35% to $\sim$ 12% of hot gas (T $> 10^5$ K) and with a decreasing H$_{2}$ mass fraction (from 70% to $<$ 1%). Meanwhile the mass fraction of gas in which the magnetic pressure dominates over the thermal pressure increases by a factor of 10, from 0.07 for an initial field of $6\times 10^{-3}$ $\mu$G to 0.7 for a 3 $\mu$G initial field. In all but the simulations with the highest initial field strength self-gravity promotes the formation of dense gas and H$_{2}$, but does not change any other trends. We conclude that magnetic fields have a significant impact on the multi-phase, chemical and thermal structure of the ISM and discuss potential implications and limitations of the model.

This paper has been published in the Monthly Notices of the Royal Astronomical Society, Volume 449, Issue 1, p.1057-1075. A copy of the preprint is also available on the astro-ph server (arXiv:1411.0009).

#### 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

(2015, APJL, 813, L27)

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).

#### Soft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM

M. Tanga, P. Schady, A. Gatto, J. Greiner, M. G. H. Krause, R. Diehl, S. Savaglio, S. Walch

(2016, AA, 595, A24)

Two-thirds of long duration gamma-ray bursts (GRBs) show soft X-ray absorption in excess of the Milky Way. The column densities of metals inferred from UV and optical spectra differ from those derived from soft X-ray spectra, at times by an order of magnitude, with the latter being higher. The origin of the soft X-ray absorption excess observed in GRB X-ray afterglow spectra remains a heavily debated issue, which has resulted in numerous investigations on the effect of hot material both internal and external to the GRB host galaxy on our X-ray afterglow observations. Nevertheless, all models proposed so far have either only been able to account for a subset of our observations (i.e. at $z > 2$), or they have required fairly extreme conditions to be present within the absorbing material. In this paper, we investigate the absorption of the GRB afterglow by a collisionally ionised and turbulent interstellar medium (ISM). We find that a dense (3 per cubic centimeters) collisionally ionised ISM could produce UV/optical and soft X-ray absorbing column densities that differ by a factor of 10, however the UV/optical and soft X-ray absorbing column densities for such sightlines and are 2-3 orders of magnitude lower in comparison to the GRB afterglow spectra. For those GRBs with a larger soft X-ray excess of up to an order of magnitude, the contribution in absorption from a turbulent ISM as considered here would ease the required conditions of additional absorbing components, such as the GRB circumburst medium and intergalactic medium.

This paper has been published in the Astronomy & Astrophysics, Volume 595, id.A24, 10. A copy of the preprint is also available on the astro-ph server (arXiv:1603.05123).

#### Supernova blast waves in wind-blown bubbles, turbulent, and power-law ambient media

S. Haid, S. Walch, T. Naab, D. Seifried, J. Mackey, A. Gatto

(2016, MNRAS, 460, 2962)

Supernova (SN) blast waves inject energy and momentum into the interstellar medium (ISM), control its turbulent multiphase structure and the launching of galactic outflows. Accurate modelling of the blast wave evolution is therefore essential for ISM and galaxy formation simulations. We present an efficient method to compute the input of momentum, thermal energy, and the velocity distribution of the shock-accelerated gas for ambient media (densities of 0.1 $\geq$ n$_0$ [cm$^{-3}$] $\geq$ 100) with uniform (and with stellar wind blown bubbles), power-law, and turbulent (Mach numbers M from 1 - 100) density distributions. Assuming solar metallicity cooling, the blast wave evolution is followed to the beginning of the momentum conserving snowplough phase. The model recovers previous results for uniform ambient media. The momentum injection in wind-blown bubbles depend on the swept-up mass and the efficiency of cooling, when the blast wave hits the wind shell. For power-law density distributions with n(r) $\sim$ r$^{-2}$ (for n(r) $\gt$ n$_{floor}$) the amount of momentum injection is solely regulated by the background density n$_{floor}$ and compares to $n_{uni} = n_{floor}$. However, in turbulent ambient media with lognormal density distributions the momentum input can increase by a factor of 2 (compared to the homogeneous case) for high Mach numbers. The average momentum boost can be approximated as $p_{turb}/p_{0} =23.07 (\frac{n_{0,turb}}{1 cm^{-3}})^{-0.12} + 0.82 (ln (1+b^2M^2))^{1.49}(\frac{n_{0,turb}}{1 cm^{-3}})^{-1.6}$. The velocity distributions are broad as gas can be accelerated to high velocities in low-density channels. The model values agree with results from recent, computationally expensive, three-dimensional simulations of SN explosions in turbulent media.

This paper has been published in the Monthly Notices of the Royal Astronomical Society, Volume 460, Issue 3, p.2962-2978 (2016). A copy of the preprint is also available on the astro-ph server (arXiv:1604.04395).

#### The impact of magnetic fields on the chemical evolution of the supernova-driven ISM

A. Pardi, P. Girichidis, T. Naab, S. Walch, T. Peters, F. Heitsch, S. C. O. Glover, R. S. Klessen, R. Wünsch, A. Gatto

(2017, MNRAS, 465, 4611)

We present three-dimensional magneto-hydrodynamical simulations of the self-gravitating interstellar medium (ISM) in a periodic (256 pc)$^3$ box with a mean number density of 0.5 cm$^{-3}$. At a fixed supernova rate we investigate the multi-phase ISM structure, H$_{2}$ molecule formation and density-magnetic field scaling for varying initial magnetic field strengths (0, $6\times 10^{-3}$, 0.3, 3 $\mu$G). All magnetic runs saturate at mass weighted field strengths of $\sim$ 1 $-$ 3 $\mu$G but the ISM structure is notably different. With increasing initial field strengths (from $6\times 10^{-3}$ to 3 $\mu$G) the simulations develop an ISM with a more homogeneous density and temperature structure, with increasing mass (from 5% to 85%) and volume filling fractions (from 4% to 85%) of warm (300 K $<$ T $<$ 8000 K) gas, with decreasing volume filling fractions (VFF) from $\sim$ 35% to $\sim$ 12% of hot gas (T $> 10^5$ K) and with a decreasing H$_{2}$ mass fraction (from 70% to $<$ 1%). Meanwhile the mass fraction of gas in which the magnetic pressure dominates over the thermal pressure increases by a factor of 10, from 0.07 for an initial field of $6\times 10^{-3}$ $\mu$G to 0.7 for a 3 $\mu$G initial field. In all but the simulations with the highest initial field strength self-gravity promotes the formation of dense gas and H$_{2}$, but does not change any other trends. We conclude that magnetic fields have a significant impact on the multi-phase, chemical and thermal structure of the ISM and discuss potential implications and limitations of the model.

This paper has been published in the Monthly Notices of the Royal Astronomical Society, Volume 465, Issue 4, p.4611-4633. A copy of the preprint is also available on the astro-ph server (arXiv:1611.00585).

#### The origin of dust polarization in molecular outflows

S. Reissl, D. Seifried, S. Wolf, R. Banerjee, R. S. Klessen

(2017, AA, 603, A71)

Aims: Polarization measurements of dust grains aligned with the magnetic field direction are an established technique for tracing large-scale field structures. In this paper we present a case study to investigate the conditions that need to be met to detect a characteristic magnetic field substructure that is embedded in such a large-scale field. A helical magnetic field with a surrounding hourglass-shaped field is expected from theoretical predictions and self-consistent magnetohydrodynamical (MHD) simulations to be present in the specific case of protostellar outflows. Hence, such an outflow environment is the perfect environment for our study.
Methods: We present synthetic polarization maps in the infrared and millimeter regime of simulations of protostellar outflows. The simulations were performed with the newly developed radiative transfer and polarization code POLARIS. The code is the first to include a self-consistent description of various alignment mechanisms such as the imperfect Davis-Greenstein (IDG) and the radiative torque (RAT) alignment. We investigated the effects of the grain size distribution, inclination, and applied alignment mechanism.
Results: We find that the IDG mechanism cannot produce any measurable polarization degree ($\geq$1%), whereas the RAT alignment produced polarization degrees of a few percent. Furthermore, we developed a method for identifying the origin of the polarization. We show that the helical magnetic field in the outflow can only be observed close to the outflow axis and at its tip, whereas in the surrounding regions the hourglass field in the foreground dominates the polarization. Furthermore, the polarization degree in the outflow lobe is lower than in the surroundings, in agreement with observations. We also find that the orientation of the polarization vector flips around at about a few hundred micrometers because of the transition from dichroic extinction to thermal re-emission. In order to avoid ambiguities when interpreting polarization data, we therefore suggest to observe in the far-infrared and millimeter regime. The actual grain size distribution has only little effect on the emerging polarization maps. Finally, we show that it is possible to observe the polarized radiation emerging from protostellar outflows with ALMA.

This paper has been published in the Astronomy & Astrophysics, Volume 603, id.A71, 14 pp.. A copy of the preprint is also available on the astro-ph server (arXiv:1703.02932).

#### A simple method to convert sink particles into stars

M. C. Sormani, R. G. Treß, R. S. Klessen, S. C. O. Glover

(2017, MNRAS, 466, 407)

Hydrodynamical simulations of star formation often do not possess the dynamic range needed to fully resolve the build-up of individual stars and star clusters, and thus have to resort to sub-grid models. A popular way to do this is by introducing Lagrangian sink particles, which replace contracting high-density regions at the point where the resolution limit is reached. A common problem then is how to assign fundamental stellar properties to sink particles, such as the distribution of stellar masses. We present a new and simple statistical method to assign stellar contents to sink particles. Once the stellar content is specified, it can be used to determine a sink particle's radiative output, supernovae rate or other feedback parameters that may be required in the calculations. Advantages of our method are: (I) it is simple to implement; (II) it guarantees that the obtained stellar populations are good samples of the initial mass function; (III) it can easily deal with infalling mass accreted at later times; and (IV) it does not put restrictions on the sink particles' masses in order to be used. The method works very well for sink particles that represent large star clusters and for which the stellar mass function is well sampled, but can also handle the transition to sink particles that represent a small number of stars.

This paper has been published in the Monthly Notices of the Royal Astronomical Society, Volume 466, Issue 1, p.407-412 (2017). A copy of the preprint is also available on the astro-ph server (arXiv:1610.02538).

#### Winds and radiation in unison: a new semi-analytic feedback model for cloud dissolution

D. Rahner, E. W. Pellegrini, S. C. O. Glover, R. S. Klessen

(2017, MNRAS, 470, 4453)

Star clusters interact with the interstellar medium (ISM) in various ways, most importantly in the destruction of molecular star-forming clouds, resulting in inefficient star formation on galactic scales. On cloud scales, ionizing radiation creates H II regions, while stellar winds and supernovae (SNe) drive the ISM into thin shells. These shells are accelerated by the combined effect of winds, radiation pressure, and SN explosions, and slowed down by gravity. Since radiative and mechanical feedback is highly interconnected, they must be taken into account in a self-consistent and combined manner, including the coupling of radiation and matter. We present a new semi-analytic 1D feedback model for isolated massive clouds ($\geq$105 M$_{\odot}$) to calculate shell dynamics and shell structure simultaneously. It allows us to scan a large range of physical parameters (gas density, star formation efficiency, and metallicity) and to estimate escape fractions of ionizing radiation f$_{esc, I}$, the minimum star formation efficiency $\epsilon_{min}$ required to drive an outflow, and recollapse time-scales for clouds that are not destroyed by feedback. Our results show that there is no simple answer to the question of what dominates cloud dynamics, and that each feedback process significantly influences the efficiency of the others. We find that variations in natal cloud density can very easily explain differences between dense-bound and diffuse-open star clusters. We also predict, as a consequence of feedback, a 4-6 Myr age difference for massive clusters with multiple generations.

This paper has been published in the Monthly Notices of the Royal Astronomical Society, Volume 470, Issue 4, p.4453-4472 (2017). A copy of the preprint is also available on the astro-ph server (arXiv:1704.04240).

#### Forming clusters within clusters: how 30 Doradus recollapsed and gave birth again

D. Rahner, E. W. Pellegrini, S. C. O. Glover, R. S. Klessen

(2018, MNRAS, 473, L11)

The 30 Doradus nebula in the Large Magellanic Cloud (LMC) contains the massive starburst cluster NGC 2070 with a massive and probably younger stellar sub clump at its centre: R136. It is not clear how such a massive inner cluster could form several million years after the older stars in NGC 2070, given that stellar feedback is usually thought to expel gas and inhibit further star formation. Using the recently developed 1D feedback scheme WARPFIELD to scan a large range of cloud and cluster properties, we show that an age offset of several million years between the stellar populations is in fact to be expected given the interplay between feedback and gravity in a giant molecular cloud with a density $\geq$ 500 cm$^{-3}$ due to re-accretion of gas on to the older stellar population. Neither capture of field stars nor gas retention inside the cluster have to be invoked in order to explain the observed age offset in NGC 2070 as well as the structure of the interstellar medium around it.

This paper has been published in the Monthly Notices of the Royal Astronomical Society: Letters, Volume 473, Issue 1, p.L11-L15 (2018). A copy of the preprint is also available on the astro-ph server (arXiv:1710.02747).

#### A theoretical explanation for the Central Molecular Zone asymmetry

M. C. Sormani, R. G. Treß, M. Ridley, S. C. O. Glover, R. S. Klessen, J. Binney, J. Magorrian, R. Smith

(2018, MNRAS, 475, 2383)

It has been known for more than 30 yr that the distribution of molecular gas in the innermost 300 parsecs of the Milky Way, the Central Molecular Zone, is strongly asymmetric. Indeed, approximately three quarters of molecular emission come from positive longitudes, and only one quarter from negative longitudes. However, despite much theoretical effort, the origin of this asymmetry has remained a mystery. Here, we show that the asymmetry can be neatly explained by unsteady flow of gas in a barred potential. We use high-resolution 3D hydrodynamical simulations coupled to a state-of-the-art chemical network. Despite the initial conditions and the bar potential being point symmetric with respect to the Galactic Centre, asymmetries develop spontaneously due to the combination of a hydrodynamical instability known as the `wiggle instability' and the thermal instability. The observed asymmetry must be transient: observations made tens of megayears in the past or in the future would often show an asymmetry in the opposite sense. Fluctuations of amplitude comparable to the observed asymmetry occur for a large fraction of time in our simulations, and suggest that the present is not an exceptional moment in the life of our Galaxy.

This paper has been published in the Monthly Notices of the Royal Astronomical Society, Volume 475, Issue 2, p.2383-2402 (2018). A copy of the preprint is also available on the astro-ph server (arXiv:1707.03650).

#### Is Molecular Cloud Turbulence Driven by External Supernova Explosions?

D. Seifried, S. Walch, S. Haid, P. Girichidis, T. Naab

(2018, APJ, 855, 4797)

We present high-resolution ($\sim$0.1 pc), hydrodynamical and magnetohydrodynamical simulations to investigate whether the observed level of molecular cloud (MC) turbulence can be generated and maintained by external supernova (SN) explosions. The MCs are formed self-consistently within their large-scale galactic environment following the non-equilibrium formation of H2 and CO, including (self-) shielding and important heating and cooling processes. The MCs inherit their initial level of turbulence from the diffuse ISM, where turbulence is injected by SN explosions. However, by systematically exploring the effect of individual SNe going off outside the clouds, we show that at later stages the importance of SN-driven turbulence is decreased significantly. This holds for different MC masses as well as for MCs with and without magnetic fields. The SN impact also decreases rapidly with larger distances. Nearby SNe (d $\sim$ 25 pc) boost the turbulent velocity dispersions of the MC by up to 70% (up to a few km s$^{‑1}$). For d $\geq$ 50 pc, however, their impact decreases fast with increasing d and is almost negligible. For all probed distances the gain in velocity dispersion decays rapidly within a few 100 kyr. This is significantly shorter than the average timescale for an MC to be hit by a nearby SN under solar neighborhood conditions ($\sim$2 Myr). Hence, at these conditions SNe are not able to sustain the observed level of MC turbulence. However, in environments with high gas surface densities and SN rates, like the Central Molecular Zone, observed elevated MC dispersions could be triggered by external SNe.

This paper has been published in the The Astrophysical Journal, Volume 855, Issue 2, article id. 81, 9 pp. (2018). A copy of the preprint is also available on the astro-ph server (arXiv:1802.00973).

#### The relative impact of photoionizing radiation and stellar winds on different environments

S. Haid, S. Walch, D. Seifried, R. Wünsch, F. Dinnbier, T. Naab

(2018, MNRAS, 478, 4799)

Photoionizing radiation and stellar winds from massive stars deposit energy and momentum into the interstellar medium (ISM). They might disperse the local ISM, change its turbulent multi-phase structure, and even regulate star formation. Ionizing radiation dominates the massive stars' energy output, but the relative effect of winds might change with stellar mass and the properties of the ambient ISM. We present simulations of the interaction of stellar winds and ionizing radiation of 12, 23, and 60 M$_{\odot}$ stars within a cold neutral (CNM, n$_0$ = 100 cm$^{−3}$), warm neutral (WNM, n$_0$ = 1, 10 cm$^{−3}$) or warm ionized (WIM, n$_0$ = 0.1 cm$^{−3}$) medium. The FLASH simulations adopt the novel tree-based radiation transfer algorithm TreeRay. With the On-the-Spot approximation and a temperature-dependent recombination coefficient, it is coupled to a chemical network with radiative heating and cooling. In the homogeneous CNM, the total momentum injection ranges from $1.6 \times 10^4$ to $4 \times 10^5$ M$_{\odot}$ km s$_{−1}$ and is always dominated by the expansion of the ionized HII region. In the WIM, stellar winds dominate ($2 \times 10^2$ to $5 \times 10^3$ M$_{\odot}$ km s$^{−1}$), while the input from radiation is small ($\sim 10^2$ M$_{\odot}$ km s$^{−1}$). The WNM (n$_0$ = 1 cm$^{−3}$) is a transition regime. Energetically, stellar winds couple more efficiently to the ISM ($\sim$ 0.1 percent of wind luminosity) than radiation (< 0.001 percent of ionizing luminosity). For estimating the impact of massive stars, the strongly mass-dependent ratios of wind to ionizing luminosity and the properties of the ambient medium have to be considered.

This paper has been published in the Monthly Notices of the Royal Astronomical Society, Volume 478, Issue 4, p.4799-4815 (2018). A copy of the preprint is also available on the astro-ph server (arXiv:1804.10218).

## 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)