arXiv subjects

Ulrich P. Steinwandel

Publications and source records attributed to Ulrich P. Steinwandel.

50 records · Page 3Linked to original sources

arXiv2022

Dust in the Wind with Resonant Drag Instabilities: I. The Dynamics of Dust-Driven Outflows in GMCs and HII Regions

Radiation-dust driven outflows, where radiation pressure on dust grains accelerates gas, occur in many astrophysical environments. Almost all previous numerical studies of these systems have assumed that the dust was perfectly-coupled to the gas. However, it has recently been shown that the dust in these systems is unstable to a large class of resonant drag instabilities (RDIs) which de-couple the dust and gas dynamics and could qualitatively change the nonlinear outcome of these outflows. We present the first simulations of radiation-dust driven outflows in stratified, inhomogeneous media, including explicit grain dynamics and a realistic spectrum of grain sizes and charge, magnetic fields and Lorentz forces on grains (which dramatically enhance the RDIs), Coulomb and Epstein drag forces, and explicit radiation transport allowing for different grain absorption and scattering properties. In this paper we consider conditions resembling giant molecular clouds (GMCs), HII regions, and distributed starbursts, where optical depths are modest, single-scattering effects dominate radiation-dust coupling, Lorentz forces dominate over drag on grains, and the fastest-growing RDIs are similar, such as magnetosonic and fast-gyro RDIs. These RDIs generically produce strong size-dependent dust clustering, growing nonlinear on timescales that are much shorter than the characteristic times of the outflow. The instabilities produce filamentary and plume-like or 'horsehead' nebular morphologies that are remarkably similar to observed dust structures in GMCs and HII regions. Additionally, in some cases they strongly alter the magnetic field structure and topology relative to filaments. Despite driving strong micro-scale dust clumping which leaves some gas behind, an order-unity fraction of the gas is always efficiently entrained by dust.

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arXiv2022

The role of physical and numerical viscosity in hydrodynamical instabilities

The evolution of the Kelvin-Helmholtz Instability (KHI) is widely used to assess the performance of numerical methods. We employ this instability to test both the smoothed particle hydrodynamics (SPH) and the meshless finite mass (MFM) implementation in OpenGadget3. We quantify the accuracy of SPH and MFM in reproducing the linear growth of the KHI with different numerical and physical set-ups. Among them, we consider: $i)$ numerical induced viscosity, and $ii)$ physically motivated, Braginskii viscosity, and compare their effect on the growth of the KHI. We find that the changes of the inferred numerical viscosity when varying nuisance parameters such as the set-up or the number of neighbours in our SPH code are comparable to the differences obtained when using different hydrodynamical solvers, i.e. MFM. SPH reproduces the expected reduction of the growth rate in the presence of physical viscosity and recovers well the threshold level of physical viscosity needed to fully suppress the instability. In the case of galaxy clusters with a virial temperature of $3\times10^7$ K, this level corresponds to a suppression factor of $\approx10^{-3}$ of the classical Braginskii value. The intrinsic, numerical viscosity of our SPH implementation in such an environment is inferred to be at least an order of magnitude smaller (i.e. $\approx10^ {-4}$), re-ensuring that modern SPH methods are suitable to study the effect of physical viscosity in galaxy clusters.

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arXiv2022

Robust field-level inference with dark matter halos

We train graph neural networks on halo catalogues from Gadget N-body simulations to perform field-level likelihood-free inference of cosmological parameters. The catalogues contain $\lesssim$5,000 halos with masses $\gtrsim 10^{10}~h^{-1}M_\odot$ in a periodic volume of $(25~h^{-1}{\rm Mpc})^3$; every halo in the catalogue is characterized by several properties such as position, mass, velocity, concentration, and maximum circular velocity. Our models, built to be permutationally, translationally, and rotationally invariant, do not impose a minimum scale on which to extract information and are able to infer the values of $Ω_{\rm m}$ and $σ_8$ with a mean relative error of $\sim6\%$, when using positions plus velocities and positions plus masses, respectively. More importantly, we find that our models are very robust: they can infer the value of $Ω_{\rm m}$ and $σ_8$ when tested using halo catalogues from thousands of N-body simulations run with five different N-body codes: Abacus, CUBEP$^3$M, Enzo, PKDGrav3, and Ramses. Surprisingly, the model trained to infer $Ω_{\rm m}$ also works when tested on thousands of state-of-the-art CAMELS hydrodynamic simulations run with four different codes and subgrid physics implementations. Using halo properties such as concentration and maximum circular velocity allow our models to extract more information, at the expense of breaking the robustness of the models. This may happen because the different N-body codes are not converged on the relevant scales corresponding to these parameters.

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arXiv2022

On the impact of runaway stars on dwarf galaxies with resolved interstellar medium

About ten to 20 percent of massive stars may be kicked out of their natal clusters before exploding as supernovae. These "runaway stars" might play a crucial role in driving galactic outflows and enriching the circumgalactic medium with metals. To study this effect, we carry out high resolution dwarf galaxy simulations that include velocity kicks to massive O/B stars above 8 M$_{\odot}$. We consider two scenarios, one that adopts a power law velocity distribution for kick velocities, resulting in more stars with high velocity kicks, and a more moderate scenario with a Maxwellian velocity distribution. We explicitly resolve the multi-phase interstellar medium (ISM), and include non-equilibrium cooling and chemistry channels. We adopt a resolved feedback scheme (\textsc{Griffin}) where we sample individual massive stars from an IMF. We follow the lifetime of these stars and add their photoionising radiation, their UV radiation field, and their photoelectric heating rate to the surrounding gas. At the end of their lifetime we explode the massive population as core collapse supernovae (CCSN). In the simulations with runaway massive stars, we add additional (natal) velocity kicks that mimic two and three body interactions that cannot be fully resolved in our simulations. We find that the inclusion of runaway or walkaway star scenarios has an impact on mass, metal, momentum and energy outflows as well as the respective loading factors. We find an increase in mass, metal and momentum loading by a factor of 2-3, whereas we find an increase in the mean energy loading by a factor of 5 in the runaway case and a factor of 3 in the walkaway case. However, we find that the peak values are increased by a factor of up to 10, independent of the adopted velocity kick model. We conclude that the inclusion of runaway stars could have a significant impact on the global outflow properties of dwarf galaxies.

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arXiv2022

The origin of the [CII]-deficit in a simulated dwarf galaxies starburst

We present [CII] synthetic observations of smoothed particle hydrodynamics (SPH) simulations of a dwarf galaxy merger. The merging process varies the star-formation rate by more than three orders of magnitude. Several star clusters are formed, the feedback of which disperses and unbinds the dense gas through expanding HII regions and supernova (SN) explosions. For galaxies with properties similar to the modelled ones, we find that the [CII] emission remains optically thin throughout the merging process. We identify the Warm Neutral Medium ($3<\log T_{\rm gas}<4$ with $χ_{\rm HI}>2χ_{\rm H2}$) to be the primary source of [CII] emission ($\sim58\%$ contribution), although at stages when the HII regions are young and dense (during star cluster formation or SNe in the form of ionized bubbles) they can contribute $\gtrsim50\%$ to the total [CII] emission. We find that the [CII]/FIR ratio decreases due to thermal saturation of the [CII] emission caused by strong FUV radiation fields emitted by the massive star clusters, leading to a [CII]-deficit medium. We investigate the [CII]-SFR relation and find an approximately linear correlation which agrees well with observations, particularly those from the Dwarf Galaxy Survey. Our simulation reproduces the observed trends of [CII]/FIR versus $Σ_{\rm SFR}$ and $Σ_{\rm FIR}$, and it agrees well with the Kennicutt relation of SFR-FIR luminosity. We propose that local peaks of [CII] in resolved observations may provide evidence for ongoing massive cluster formation.

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arXiv2022

The CAMELS project: public data release

The Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) project was developed to combine cosmology with astrophysics through thousands of cosmological hydrodynamic simulations and machine learning. CAMELS contains 4,233 cosmological simulations, 2,049 N-body and 2,184 state-of-the-art hydrodynamic simulations that sample a vast volume in parameter space. In this paper we present the CAMELS public data release, describing the characteristics of the CAMELS simulations and a variety of data products generated from them, including halo, subhalo, galaxy, and void catalogues, power spectra, bispectra, Lyman-$α$ spectra, probability distribution functions, halo radial profiles, and X-rays photon lists. We also release over one thousand catalogues that contain billions of galaxies from CAMELS-SAM: a large collection of N-body simulations that have been combined with the Santa Cruz Semi-Analytic Model. We release all the data, comprising more than 350 terabytes and containing 143,922 snapshots, millions of halos, galaxies and summary statistics. We provide further technical details on how to access, download, read, and process the data at \url{https://camels.readthedocs.io}.

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arXiv2021

On the optical properties of Resonant Drag Instabilities: Variability of Asymptotic Giant Branch and R Coronae Borealis stars

In dusty cool-star outflow or ejection events around AGB or RCB-like stars, dust is accelerated by radiation from the star and coupled to the gas via collisional drag forces. But it has recently been shown that such dust-gas mixtures are unstable to a super-class of instabilities called the resonant drag instabilities (RDIs), which promote dust clustering. We therefore consider idealized simulations of the RDIs operating on a spectrum of dust grain sizes subject to radiative acceleration (allowing for different grain optical properties), coupled to the gas with a realistic drag law, including or excluding the effects of magnetic fields and charged grains, and calculate for the first time how the RDIs could contribute to observed variability. We show that the RDIs naturally produce significant variations ($\sim 10-20\%$ $1σ$-level) in the extinction, corresponding to $\sim 0.1-1\,$mag level in the stellar types above, on timescales of order months to a year. The fluctuations are surprisingly robust to finite source-size effects as they are dominated by large-scale modes, which also means their spatial structure could be resolved in some nearby systems. We also quantify how this produces variations in the line-of-sight grain size-distribution. All of these variations are similar to those observed, suggesting that the RDIs may play a key role driving observed variability in dust extinction within dusty outflow/ejection events around cool stars. We further propose that the measured variations in grain sizes could directly be used to identify the presence of the RDIs in close by systems with observations.

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arXiv2021

The challenge of simulating the star cluster population of dwarf galaxies with resolved interstellar medium

We present results on the star cluster properties from a series of high resolution smoothed particles hydrodynamics (SPH) simulations of isolated dwarf galaxies as part of the GRIFFIN project. The simulations at sub-parsec spatial resolution and a minimum particle mass of 4 $\mathrm{M_\odot}$ incorporate non-equilibrium heating, cooling and chemistry processes, and realise individual massive stars. All the simulations follow feedback channels of massive stars that include the interstellar-radiation field, that is variable in space and time, the radiation input by photo-ionisation and supernova explosions. Varying the star formation efficiency per free-fall time in the range $ε_\mathrm{ff}$ = 0.2 - 50$\%$ neither changes the star formation rates nor the outflow rates. While the environmental densities at star formation change significantly with $ε_\mathrm{ff}$, the ambient densities of supernovae are independent of $ε_\mathrm{ff}$ indicating a decoupling of the two processes. At low $ε_\mathrm{ff}$, more massive, and increasingly more bound star clusters are formed, which are typically not destroyed. With increasing $ε_\mathrm{ff}$ there is a trend for shallower cluster mass functions and the cluster formation efficiency $Γ$ for young bound clusters decreases from $50 \%$ to $\sim 1 \%$ showing evidence for cluster disruption. However, none of our simulations form low mass ($< 10^3$ $\mathrm{M_\odot}$) clusters with structural properties in perfect agreement with observations. Traditional star formation models used in galaxy formation simulations based on local free-fall times might therefore not be able to capture low mass star cluster properties without significant fine-tuning.

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arXiv2021

On the small scale turbulent dynamo in the intracluster medium: A comparison to dynamo theory

We present non-radiative, cosmological zoom-simulations of galaxy cluster formation with magnetic fields and (anisotropic) thermal conduction of one very massive galaxy cluster with a mass at redshift zero that corresponds to $M_\mathrm{vir} \sim 2 \times 10^{15} M_{\odot}$. We run the cluster on three resolution levels (1X, 10X, 25X), starting with an effective mass resolution of $2 \times 10^8M_{\odot}$, subsequently increasing the particle number to reach $4 \times 10^6M_{\odot}$. The maximum spatial resolution obtained in the simulations is limited by the gravitational softening reaching $ε=1.0$ kpc at the highest resolution level, allowing to resolve the hierarchical assembly of the structures in very fine detail. All simulations presented, have been carried out with the SPMHD-code Gadget-3 with a heavily updated SPMHD prescription. The primary focus is to investigate magnetic field amplification in the Intracluster Medium (ICM). We show that the main amplification mechanism is the small scale-turbulent-dynamo in the limit of reconnection diffusion. In our two highest resolution models we start to resolve the magnetic field amplification driven by this process and we explicitly quantify this with the magnetic power-spectra and the magnetic tension that limits the bending of the magnetic field lines consistent with dynamo theory. Furthermore, we investigate the $\nabla \cdot \mathbf{B}=0$ constraint within our simulations and show that we achieve comparable results to state-of-the-art AMR or moving-mesh techniques, used in codes such as Enzo and Arepo. Our results show for the first time in a fully cosmological simulation of a galaxy cluster that dynamo action can be resolved in the framework of a modern Lagrangian magnetohydrodynamic (MHD) method, a study that is currently missing in the literature.

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arXiv2020

Driving galactic winds with magnetic fields at low and high redshift

Although playing a key role for our understanding of the evolution of galaxies, the exact way how observed galactic outflows are driven is still far from being understood and therefore our understanding of associated feedback mechanisms that control the evolution of galaxies is still plagued by many enigmas. In this work we present a simple toy model that can provide insight on how non-axis-symmetric instabilities in galaxies (bars, spiral-arms, warps) can lead to local exponential magnetic field growth by a radial flows beyond the equipartition value by at least two orders of magnitude on a time-scale of a few $100$ Myr. Our predictions show that the process can lead to galactic outflows in barred spiral galaxies with a mass loading factor $η\approx 0.1$, in agreement with our numerical simulations. Moreover, our outflow mechanism could contribute to an understanding of the large fraction of bared spiral galaxies that show signs of galactic outflows in the CHANG-ES survey. Extending our model shows the importance of such processes in high redshift galaxies by assuming equipartition between magnetic energy and turbulent energy. Simple estimates for the star formation rate (SFR) in our model together with cross-correlated masses from the star-forming main-sequence at redshifts $z\sim2$ allow us to estimate the outflow rate and mass loading factors by non-axis-symmetric instabilities and a subsequent radial inflow dynamo, giving mass loading factors of $η\approx 0.1$ for galaxies in the range of M$_{\star}=10^9 - 10^{12}$ M$_{\odot}$, in good agreement with recent results of Sinfoni and KMOS$^{3\mathrm{D}}$.

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arXiv2020

Hot phase generation by supernovae: resolution, chemistry and thermal conduction

Supernovae (SN) generate hot gas in the interstellar medium (ISM), help setting the ISM structure and support the driving of outflows. It is important to resolve the hot gas generation for galaxy formation simulations at solar mass and sub-parsec resolution which realise individual supernova (SN) explosions with ambient densities varying by several orders of magnitude in a realistic multi-phase ISM. We test resolution requirements by simulating SN blast waves at three metallicities ($Z = 0.01, 0.1$ and $1 Z_{\odot}$), six densities and their respective equilibrium chemical compositions ($n=0.001$ cm$^{-3}$ - $100$ cm$^{-3}$), and four mass resolutions ($0.1$ - $100$ M$_{\odot}$), in three dimensions. We include non-equilibrium cooling and chemistry, a homogenous interstellar radiation field, and shielding with a modern pressure-energy smoothed particle hydrodynamics (SPH) method including isotropic thermal conduction and a meshless-finite-mass (MFM) solver. We find stronger resolution requirements for chemistry and hot phase generation than for momentum generation. While at $10$ M$_{\odot}$ the radial momenta at the end of the Sedov phase start converging, the hot phase generation and chemistry require higher resolutions to represent the neutral to ionised hydrogen fraction at the end of the Sedov phase correctly. Thermal conduction typically reduces the hot phase by $0.2$ dex and has little impact on the chemical composition. In general, our $1$, and $0.1$ M$_{\odot}$ results agree well with previous numerical and analytic estimates. We conclude that for the thermal energy injection SN model presented here resolutions higher than $10$ M$_{\odot}$ are required to model the chemistry, momentum and hot phase generation in a multi-phase ISM.

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arXiv2020

The GRIFFIN project -- Formation of star clusters with individual massive stars in a simulated dwarf galaxy starburst

We describe a population of young star clusters (SCs) formed in a hydrodynamical simulation of a gas-rich dwarf galaxy merger resolved with individual massive stars at sub-parsec spatial resolution. The simulation is part of the GRIFFIN (Galaxy Realizations Including Feedback From INdividual massive stars) project. The star formation environment during the simulation spans seven orders of magnitude in gas surface density and thermal pressure, and the global star formation rate surface density ($Σ_\mathrm{SFR}$) varies by more than three orders of magnitude during the simulation. Young SCs more massive than $M_{\mathrm{*,cl}}\sim 10^{2.5}\,M_{\odot}$ form along a mass function with a power-law index $α\sim-1.7$ ($α\sim-2$ for $M_{\mathrm{*,cl}}\gtrsim10^{3}\,M_{\odot}$) at all merger phases, while the normalization and the highest SC masses (up to $\sim 10^6 M_{\odot}$) correlate with $Σ_\mathrm{SFR}$. The cluster formation efficiency varies from $Γ\sim20\%$ in early merger phases to $Γ\sim80\%$ at the peak of the starburst and is compared to observations and model predictions. The massive SCs ($\gtrsim10^4\,M_{\odot}$) have sizes and mean surface densities similar to observed young massive SCs. Simulated lower mass clusters appear slightly more concentrated than observed. All SCs form on timescales of a few Myr and lose their gas rapidly resulting in typical stellar age spreads between $σ\sim0.1-2$ Myr ($1σ$), consistent with observations. The age spreads increase with cluster mass, with the most massive cluster ($\sim10^6\, M_{\odot}$) reaching a spread of $5\, \mathrm{Myr}$ once its hierarchical formation finishes. Our study shows that it is now feasible to investigate the SC population of entire galaxies with novel high-resolution numerical simulations.

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arXiv2019

On the origin of magnetic driven winds and the structure of the galactic dynamo in isolated galaxies

We investigate the build-up of the galactic dynamo and subsequently the origin of a magnetic driven outflow. We use a setup of an isolated disc galaxy with a realistic circum-galactic medium (CGM). We find good agreement of the galactic dynamo with theoretical and observational predictions from the radial and toroidal components of the magnetic field as function of radius and disc scale height. We find several field reversals indicating dipole structure at early times and quadrupole structure at late times. Together with the magnetic pitch angle and the dynamo control parameters $R_α$, $R_ω$ and $D$ we present strong evidence for an $α^2$-$Ω$ dynamo. The formation of a bar in the centre leads to further amplification of the magnetic field via adiabatic compression which subsequently drives an outflow. Due to the Parker-Instability the magnetic field lines rise to the edge of the disc, break out and expand freely in the CGM driven by the magnetic pressure. Finally, we investigate the correlation between magnetic field and star formation rate. Globally, we find that the magnetic field is increasing as function of the star formation rate surface density with a slope between $0.3$ and $0.45$ in good agreement with predictions from theory and observations. Locally, we find that the magnetic field can decrease while star formation increases. We find that this effect is correlated with the diffusion of magnetic field from the spiral arms to the inter-arm regions which we explicitly include by solving the induction equation and accounting for non-linear terms.

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arXiv2018

Magnetic buoyancy in simulated galactic discs with a realistic circum galactic medium

We present simulations of isolated disc galaxies in a realistic environment performed with the Tree-SPMHD-Code Gadget-3. Our simulations include a spherical circum-galactic medium (CGM) surrounding the galactic disc, motivated by observations and the results of cosmological simulations. We present three galactic models with different halo masses between 10e10 Msol and 10e12 Msol, and for each we use two different approaches to seed the magnetic field, as well as a control simulation without a magnetic field. We find that the amplification of the magnetic field in the centre of the disc leads to a biconical magnetic outflow of gas that magnetizes the CGM. This biconical magnetic outflow reduces the star formation rate (SFR) of the galaxy by roughly 40 percent compared to the simulations without magnetic fields. As the key aspect of our simulations, we find that small scale turbulent motion of the gas in the disc leads to the amplification of the magnetic field up to tens of 10e-6 G, as long as the magnetic field strength is low. For stronger magnetic fields turbulent motion does not lead to significant amplification but is replaced by an alpha-omega dynamo. The occurance of a small scale turbulent dynamo becomes apparent through the magnetic power spectrum and analysis of the field lines' curvature. In accordance with recent observations we find an anti-correlation between the spiral structure in the gas density and in the magnetic field due to a diffusion term added to the induction equation.

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