arXiv subjects

Thorsten Naab

Publications and source records attributed to Thorsten Naab.

At least 127 records · Page 7Linked to original sources

arXiv2018

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

Magnetic fields are ubiquitously observed in the interstellar medium (ISM) of present-day star-forming galaxies with dynamically relevant energy densities. Using three-dimensional magneto-hydrodynamic (MHD) simulations of the supernova (SN) 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, 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 $\sim100\,\mathrm{pc}$, delay the formation of dense (and molecular) gas by $\sim25\,\mathrm{Myr}$ and result in differently shaped gas structures. The magnetised gas fragments into fewer clumps, which are initially at subcritical mass-to-flux ratios, $M/Φ\approx0.3(M/Φ)_\mathrm{crit}$, and accrete gas preferentially parallel to the magnetic field lines until supercritial mass-to-flux ratios of up to order 10 are reached. The accretion rates onto molecular clouds scale with $\dot{M}\propto M^{1.5}$. The median of the inter-cloud velocity dispersion is $\sim2-5\,\mathrm{km\,s}^{-1}$ and lower than the internal velocity dispersion in the clouds ($\sim3-7\,\mathrm{km\,s}^{-1}$). However, individual cloud-cloud collisions occur at speeds of a few $10\,\mathrm{km\,s}^{-1}$.

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arXiv2018

The imprint of cosmic ray driven outflow on Lyman-alpha spectra

Recent magneto-hydrodynamic simulations of the star-forming interstellar medium (ISM) with parsec scale resolution indicate that relativistic cosmic rays support the launching of galactic outflows on scales of a few kpc. If these fundamental constituents of the ISM are injected at the sites of supernova (SN) explosions, the outflows are smoother, colder, and denser than the highly structured, hot-phase driven outflows forming, e.g., by thermal SN energy injection alone. In this Letter we present computations of resonant Lyman-$α$ (Ly$α$) radiation transfer through snapshots of a suite of stratified disk simulations from the SILCC project. For a range of thermal, radiative, and kinetic feedback models only simulations including non-thermal cosmic rays produce Ly$α$ spectra with enhanced red peaks and strong absorption at line center -- similar to observed systems. The absence of cosmic ray feedback leads to spectra incompatible with observations. We attribute this to the smoother neutral gas distribution of cosmic ray supported outflows within a few kpc from the disk midplane.

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arXiv2018

Cooler and smoother -- the impact of cosmic rays on the phase structure of galactic outflows

We investigate the impact of cosmic rays (CRs) on galactic outflows from a multi-phase interstellar medium with solar neighbourhood conditions. The three-dimensional magneto-hydrodynamical simulations include CRs as a relativistic fluid in the advection-diffusion approximation. The thermal and chemical state of the ISM is computed with a non-equilibrium chemical network. We find that CRs (injected with 10 \% of the supernova energy) efficiently support the launching of outflows and strongly affect their phase structure. Outflows leaving the midplane are denser ($ρ\sim 10^{-26}\,\mathrm{g\,cm}^{-3}$), colder ($\sim 10^4\,\mathrm{K}$), and slower ($\sim 30\,\mathrm{km\,s}^{-1}$) if CRs are considered in addition to thermal SNe. The CR supported outflows are also smoother, in particular at larger heights ($> 1\,\mathrm{kpc}$ above the midplane) without the direct impact of SN explosions. Approximately $5\% - 25\%$ of the injected CR energy is lost via hadronic cooling. Smaller diffusion coefficients lead to slightly larger hadronic losses but allow for steeper CR pressure gradients, stronger outflows and larger accelerations. Up to a height of $z \sim1\,\mathrm{kpc}$ there are large volumes in approximate pressure equilibrium between thermal and CR component. At larger altitudes the CR pressure is $10-100$ times as large as the thermal counterpart. More than $\sim 1\,\mathrm{kpc}$ away from the midplane, CRs provide the dominant gas acceleration mechanism.

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arXiv2018

Momentum-driven Winds from Radiatively Efficient Black Hole Accretion and Their Impact on Galaxies

We explore the effect of momentum-driven winds representing radiation pressure driven outflows from accretion onto supermassive black holes in a set of numerical hydrodynamical simulations. We explore two matched sets of cosmological zoom-in runs of 24 halos with masses ~$10^{12.0}-10^{13.4}$ M_sun run with two different feedback models. Our `NoAGN' model includes stellar feedback via UV heating, stellar winds and supernovae, photoelectric heating and cosmic X-ray background heating from a meta-galactic background. Our fiducial `MrAGN' model is identical except that it also includes a model for black hole seeding and accretion, as well as heating and momentum injection associated with the radiation from black hole accretion. Our MrAGN model launches galactic outflows which result in both `ejective' feedback - the outflows themselves which drive gas out of galaxies - and `preventative' feedback, which suppresses the inflow of new and recycling gas. As much as 80 % of outflowing galactic gas can be expelled, and accretion can be suppressed by as much as a factor of 30 in the MrAGN runs when compared with the NoAGN runs. The histories of NoAGN galaxies are recycling-dominated, with ~70% of material that leaves the galaxy eventually returning, and the majority of outflowing gas re-accretes on 1 Gyr timescales without AGN feedback. Outflowing gas in the MrAGN runs has higher characteristic velocity (500 - 1,000 km/s versus 100-300 km/s for outflowing NoAGN gas) and travels as far as a few Mpcs. Only ~10% of ejected material is re-accreted in the MrAGN galaxies.

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arXiv2018

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

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 H$_{\text{II}}$ 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.

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arXiv2018

Mass density slope of elliptical galaxies from strong lensing and resolved stellar kinematics

We discuss constraints on the mass density distribution (parameterized as $ρ\propto r^{-γ}$) in early-type galaxies provided by strong lensing and stellar kinematics data. The constraints come from mass measurements at two `pinch' radii. One `pinch' radius $r_1=2.2 R_{Einst}$ is defined such that the Einstein (i.e. aperture) mass can be converted to the spherical mass almost independently of the mass-model. Another `pinch' radius $r_2=R_{opt}$ is chosen so that the dynamical mass, derived from the line-of-sight velocity dispersion, is least sensitive to the anisotropy of stellar orbits. We verified the performance of this approach on a sample of simulated elliptical galaxies and on a sample of 15 SLACS lens galaxies at $0.01 \leq z \leq 0.35$, which have already been analysed in Barnabe et al. (2011) by the self-consistent joint lensing and kinematic code. For massive simulated galaxies the density slope $γ$ is recovered with an accuracy of $\sim 13\%$, unless $r_1$ and $r_2$ happen to be close to each other. For SLACS galaxies, we found good overall agreement with the results of Barnabe et al. (2011) with a sample-averaged slope $γ=2.1\pm0.05$. While the two-pinch-radii approach has larger statistical uncertainties, it is much simpler and uses only few arithmetic operations with directly observable quantities.

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arXiv2018

The fate of the Antennae galaxies

We present a high-resolution smoothed particle hydrodynamics simulation of the Antennae galaxies (NGC 4038/4039) and follow the evolution $3$ Gyrs beyond the final coalescence. The simulation includes metallicity dependent cooling, star formation, and both stellar feedback and chemical enrichment. The simulated best-match Antennae reproduces well both the observed morphology and the off-nuclear starburst. We also produce for the first time a simulated two-dimensional metallicity map of the Antennae and find good agreement with the observed metallicity of off-nuclear stellar clusters, however the nuclear metallicities are overproduced by $\sim 0.5$ dex. Using the radiative transfer code SKIRT we produce multi-wavelength observations of both the Antennae and the merger remnant. The $1$ Gyr old remnant is well fitted with a Sérsic profile of $n=7.07$, and with an $r$-band effective radius of $r_{\mathrm{e}}= 1.6$ kpc and velocity dispersion of $σ_{\mathrm{e}}=180$ km$/$s the remnant is located on the fundamental plane of early-type galaxies (ETGs). The initially blue Antennae remnant evolves onto the red sequence after $\sim 2.5$ Gyr of secular evolution. The remnant would be classified as a fast rotator, as the specific angular momentum evolves from $λ_{\mathrm{Re}}\approx0.11$ to $λ_{\mathrm{Re}}\approx0.14$ during its evolution. The remnant shows ordered rotation and a double peaked maximum in the mean 2D line-of-sight velocity. These kinematical features are relatively common among local ETGs and we specifically identify three local ETGs (NGC 3226, NGC 3379 and NGC 4494) in the ATLAS$^\mathrm{3D}$ sample, whose photometric and kinematic properties most resemble the Antennae remnant.

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arXiv2017

The Co-Evolution of Total Density Profiles and Central Dark Matter Fractions in Simulated Early-Type Galaxies

We present evidence from cosmological hydrodynamical simulations for a co-evolution of the slope of the total (dark and stellar) mass density profile, gamma_tot, and the dark matter fraction within the half-mass radius, f_DM, in early-type galaxies. The relation can be described as gamma_tot = A f_DM + B for all systems at all redshifts. The trend is set by the decreasing importance of gas dissipation towards lower redshifts and for more massive systems. Early-type galaxies are smaller, more concentrated, have lower f_DM and steeper gamma_tot at high redshifts and at lower masses for a given redshift; f_DM and gamma_tot are good indicators for growth by "dry" merging. The values for A and B change distinctively for different feedback models, and this relation can be used as a test for such models. A similar correlation exists between gamma_tot and the stellar mass surface density Sigma_*. A model with weak stellar feedback and feedback from black holes is in best agreement with observations. All simulations, independent of the assumed feedback model, predict steeper gamma_tot and lower f_DM at higher redshifts. While the latter is in agreement with the observed trends, the former is in conflict with lensing observations, which indicate constant or decreasing gamma_tot. This discrepancy is shown to be artificial: the observed trends can be reproduced from the simulations using observational methodology to calculate the total density slopes.

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arXiv2017

Synthetic nebular emission from massive galaxies I: origin of the cosmic evolution of optical emission-line ratios

Galaxies occupy different regions of the [OIII]$\lambda5007$/H$β$-versus-[NII]$\lambda6584$/H$α$ emission-line ratio diagram in the distant and local Universe. We investigate the origin of this intriguing result by modelling self-consistently, for the first time, nebular emission from young stars, accreting black holes (BHs) and older, post-asymptotic-giant-branch (post-AGB) stellar populations in galaxy formation simulations in a full cosmological context. In post-processing, we couple new-generation nebular-emission models with high-resolution, cosmological zoom-in simulations of massive galaxies to explore which galaxy physical properties drive the cosmic evolution of the optical-line ratios [OIII]$\lambda5007$/H$β$, [NII]$\lambda6584$/H$α$, [SII]$λ\lambda6717,6731$/H$α$ and [OI]$\lambda6300$/H$α$. The line ratios of simulated galaxies agree well with observations of both star-forming and active local SDSS galaxies. Towards higher redshifts, at fixed galaxy stellar mass, the average [OIII]/H$β$ increases and [NII]/H$α$, [SII]/H$α$ and [OI]/H$α$ decrease -- widely consistent with observations. At fixed stellar mass, we identify star formation history, which controls nebular emission from young stars via the ionization parameter, as the primary driver of the cosmic evolution of [OIII]/H$β$ and [NII]/H$α$. For [SII]/H$α$ and [OI]/H$α$, this applies only to redshifts above $z=1.5$, the evolution at lower redshift being driven in roughly equal parts by nebular emission from AGN and post-AGB stars. Instead, changes in the hardness of ionizing radiation, ionized-gas density, the prevalence of BH accretion relative to star formation and the dust-to-metal mass ratio (whose impact on the gas-phase N/O ratio we model at fixed O/H) play at most a minor role in the cosmic evolution of simulated galaxy line ratios.

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arXiv2017

Integral-field kinematics and stellar populations of early-type galaxies out to three half-light radii

We observed twelve nearby HI -detected early-type galaxies (ETGs) of stellar mass $\sim 10^{10}M\odot \leq M_* \leq \sim 10^{11}M\odot$ with the Mitchell Integral-Field Spectrograph, reaching approximately three half-light radii in most cases. We extracted line-of-sight velocity distributions for the stellar and gaseous components. We find little evidence of transitions in the stellar kinematics of the galaxies in our sample beyond the central effective radius, with centrally fast-rotating galaxies remaining fast-rotating and centrally slow-rotating galaxies likewise remaining slow-rotating. This is consistent with these galaxies having not experienced late dry major mergers; however, several of our objects have ionised gas that is misaligned with respect to their stars, suggesting some kind of past interaction. We extract Lick index measurements of the commonly-used H$β$, Fe5015, Mg\, b, Fe5270 and Fe5335 absorption features, and we find most galaxies to have flat H$β$ gradients and negative Mg, b gradients. We measure gradients of age, metallicity and abundance ratio for our galaxies using spectral fitting, and for the majority of our galaxies find negative age and metallicity gradients. We also find the stellar mass-to-light ratios to decrease with radius for most of the galaxies in our sample. Our results are consistent with a view in which intermediate-mass ETGs experience mostly quiet evolutionary histories, but in which many have experienced some kind of gaseous interaction in recent times.

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arXiv2017

Physics of Galactic Metals: Evolutionary Effects due to Production, Distribution, Feedback & Interaction with Black Holes

We ask how the inclusion of various physical heating processes due to the metal content of gas affect the evolution of massive galaxies and compute a suite of cosmological hydrodynamical simulations that follow these systems and their supermassive black holes. We use a smoothed particle hydrodynamics code with a pressure-entropy formulation and a more accurate treatment of the metal production, turbulent diffusion and cooling rate based on individual element abundances. The feedback models include (1) AGN feedback via high velocity BAL winds and Compton/photoionization heating, (2) explicit stellar feedback from multiple processes including powerful winds from supernova events, stellar winds from young massive stars and AGB stars as well as radiative heating within Stromgren spheres around massive stars, and (3) additional heating effects due to the presence of metals including grain photoelectric heating, metallicity dependent X-ray heating by nearby accreting black holes and from the cosmic X-ray background, which are the major improvement in our feedback model. With a suite of zoom-in simulations of 30 halos with $M_{vir} \sim 10^{12-13.4}$, we show that energy and momentum budget from all feedback effects generate realistic galaxy properties. We explore the detailed role of each feedback model with three additional sets of simulations with varying input physics. We show that the metal induced heating mechanisms reduce the fraction of accreted stellar material by mainly suppressing the growth of diffuse small stellar systems at high redshift but overall have a relatively minor effect on the final stellar and gas properties of massive galaxies. The inclusion of AGN feedback significantly improves the ability of our cosmological simulations to yield realistic gas and stellar properties of massive galaxies with reasonable fraction of the final stellar mass accreted from other galaxies.

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arXiv2017

EMERGE - An empirical model for the formation of galaxies since $z\sim10$

We present EMERGE, an Empirical ModEl for the foRmation of GalaxiEs, describing the evolution of individual galaxies in large volumes from $z\sim10$ to the present day. We assign a star formation rate to each dark matter halo based on its growth rate, which specifies how much baryonic material becomes available, and the instantaneous baryon conversion efficiency, which determines how efficiently this material is converted to stars, thereby capturing the baryonic physics. Satellites are quenched following the delayed-then-rapid model, and they are tidally disrupted once their subhalo has lost a significant fraction of its mass. The model is constrained with observed data extending out to high redshift. The empirical relations are very flexible, and the model complexity is increased only if required by the data, assessed by several model selection statistics. We find that for the same final halo mass galaxies can have very different star formation histories. Nevertheless, the average star formation and accretion rates are in good agreement with models following an abundance matching strategy. Galaxies that are quenched at $z=0$ typically have a higher peak star formation rate compared to their star-forming counterparts. The accretion of stars can dominate the total mass of massive galaxies, but is insignificant for low-mass systems, independent of star-formation activity. EMERGE predicts stellar-to-halo mass ratios for individual galaxies and introduces scatter self-consistently. We find that at fixed halo mass, passive galaxies have a higher stellar mass on average. The intra-cluster-mass in massive haloes can be up to 8 times larger than the mass of the central galaxy. Clustering for star-forming and quenched galaxies is in good agreement with observational constraints, indicating a realistic assignment of galaxies to haloes.

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arXiv2017

Post-Newtonian dynamical modeling of supermassive black holes in galactic-scale simulations

We present KETJU, a new extension of the widely-used smoothed particle hydrodynamics simulation code GADGET-3. The key feature of the code is the inclusion of algorithmically regularized regions around every supermassive black hole (SMBH). This allows for simultaneously following global galactic-scale dynamical and astrophysical processes, while solving the dynamics of SMBHs, SMBH binaries and surrounding stellar systems at sub-parsec scales. The KETJU code includes Post-Newtonian terms in the equations of motions of the SMBHs which enables a new SMBH merger criterion based on the gravitational wave coalescence timescale pushing the merger separation of SMBHs down to $\sim 0.005$ pc. We test the performance of our code by comparison to NBODY7 and rVINE. We set up dynamically stable multi-component merger progenitor galaxies to study the SMBH binary evolution during galaxy mergers. In our simulation sample the SMBH binaries do not suffer from the final-parsec problem, which we attribute to the non-spherical shape of the merger remnants. For bulge-only models, the hardening rate decreases with increasing resolution, whereas for models which in addition include massive dark matter halos the SMBH binary hardening rate becomes practically independent of the mass resolution of the stellar bulge. The SMBHs coalesce on average 200 Myr after the formation of the SMBH binary. However, small differences in the initial SMBH binary eccentricities can result in large differences in the SMBH coalescence times. Finally, we discuss the future prospects of KETJU, which allows for a straightforward inclusion of gas physics in the simulations.

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arXiv2017

AGN feedback, quiescence and CGM metal enrichment in early-type galaxies

We present three-dimensional hydrodynamical simulations showing the effect of kinetic and radiative AGN feedback on a model galaxy representing a massive quiescent low-redshift early-type galaxy of $M_* = 8.41\times 10^{10} M_\odot$, harbouring a $M_\mathrm{BH} = 4\times 10^8 M_\odot $ black hole surrounded by a cooling gaseous halo. We show that, for a total baryon fraction of $\sim 20\%$ of the cosmological value, feedback from the AGN can keep the galaxy quiescent for about 4.35 Gyr and with properties consistent with black hole mass and X-ray luminosity scaling relations. However, this can only be achieved if the AGN feedback model includes both kinetic and radiative feedback modes. The simulation with only kinetic feedback fails to keep the model galaxy fully quiescent, while one with only radiative feedback leads to excessive black-hole growth. For higher baryon fractions (e.g. 50\% of the cosmological value), the X-ray luminosities exceed observed values by at least one order of magnitude, and rapid cooling results in a star-forming galaxy. The AGN plays a major role in keeping the circumgalactic gas at observed metallicities of $Z/Z_\odot \gtrsim 0.3 $ within the central $\sim 30$ kpc by venting nuclear gas enriched with metals from residual star formation activity. As indicated by previous cosmological simulations, our results are consistent with a model for which the black hole mass and the total baryon fraction are set at higher redshifts $z > 1$ and the AGN alone can keep the model galaxy on observed scaling relations. Models without AGN feedback violate both the quiescence criterion as well as CGM metallicity constraints.

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arXiv2017

Variable interstellar radiation fields in simulated dwarf galaxies: supernovae versus photoelectric heating

We present high-resolution hydrodynamical simulations of isolated dwarf galaxies including self-gravity, non-equilibrium cooling and chemistry, interstellar radiation fields (IRSF) and shielding, star formation, and stellar feedback. This includes spatially and temporally varying photoelectric (PE) heating, photoionization, resolved supernova (SN) blast waves and metal enrichment. A new flexible method to sample the stellar initial mass function allows us to follow the contribution to the ISRF, the metal output and the SN delay times of individual massive stars. We find that SNe play the dominant role in regulating the global star formation rate, shaping the multi-phase interstellar medium (ISM) and driving galactic outflows. Outflow rates (with mass-loading factors of a few) and hot gas fractions of the ISM increase with the number of SNe exploding in low-density environments where radiative energy losses are low. While PE heating alone can suppress star formation slightly more (a factor of a few) than SNe alone can do, it is unable to drive outflows and reproduce the multi-phase ISM that emerges naturally when SNe are included. These results are in conflict with recent results of Forbes et al. who concluded that PE heating is the dominant process suppressing star formation in dwarfs, about an order of magnitude more efficient than SNe. Potential origins for this discrepancy are discussed. In the absence of SNe and photoionization (mechanisms to disperse dense clouds), the impact of PE heating is highly overestimated owing to the (unrealistic) proximity of dense gas to the radiation sources. This leads to a substantial boost of the infrared continuum emission from the UV-irradiated dust and a far infrared line-to-continuum ratio too low compared to observations. Though sub-dominant in regulating star formation, the ISRF controls the abundance of molecular hydrogen via photodissociation.

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arXiv2017

Modeling for Stellar Feedback in Galaxy Formation Simulations

Various heuristic approaches to model unresolved supernova (SN) feedback in galaxy formation simulations exist to reproduce the formation of spiral galaxies and the overall inefficient conversion of gas into stars. Some models, however, require resolution dependent scalings. We present a sub-resolution model representing the three major phases of supernova blast wave evolution $-$free expansion, energy conserving Sedov-Taylor, and momentum conserving snowplow$-$ with energy scalings adopted from high-resolution interstellar-medium simulations in both uniform and multiphase media. We allow for the effects of significantly enhanced SN remnant propagation in a multiphase medium with the cooling radius scaling with the hot volume fraction, $f_{\mathrm{hot}}$, as $(1 - f_{\mathrm{hot}})^{-4/5}$. We also include winds from young massive stars and AGB stars, Strömgren sphere gas heating by massive stars, and a gas cooling limiting mechanism driven by radiative recombination of dense HII regions. We present initial tests for isolated Milky-Way like systems simulated with the GADGET based code SPHgal with improved SPH prescription. Compared to pure thermal SN input, the model significantly suppresses star formation at early epochs, with star formation extended both in time and space in better accord with observations. Compared to models with pure thermal SN feedback, the age at which half the stellar mass is assembled increases by a factor of 2.4, and the mass loading parameter and gas outflow rate from the galactic disk increase by a factor of 2. Simulation results are converged for a two order of magnitude variation in particle mass in the range (1.3$-$130)$\times 10^4$ solar masses.

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arXiv2016

Theoretical Challenges in Galaxy Formation

Numerical simulations have become a major tool for understanding galaxy formation and evolution. Over the decades the field has made significant progress. It is now possible to simulate the formation of individual galaxies and galaxy populations from well defined initial conditions with realistic abundances and global properties. An essential component of the calculation is to correctly estimate the inflow to and outflow from forming galaxies since observations indicating low formation efficiency and strong circum-glactic presence of gas are persuasive. Energetic 'feedback' from massive stars and accreting super-massive black holes - generally unresolved in cosmological simulations - plays a major role for driving galactic outflows, which have been shown to regulate many aspects of galaxy evolution. A surprisingly large variety of plausible sub-resolution models succeeds in this exercise. They capture the essential characteristics of the problem, i.e. outflows regulating galactic gas flows, but their predictive power is limited. In this review we focus on one major challenge for galaxy formation theory: to understand the underlying physical processes that regulate the structure of the interstellar medium, star formation and the driving of galactic outflows. This requires accurate physical models and numerical simulations, which can precisely describe the multi-phase structure of the interstellar medium on the currently unresolved few hundred parsecs scales of large scale cosmological simulations. Such models ultimately require the full accounting for the dominant cooling and heating processes, the radiation and winds from massive stars and accreting black holes, an accurate treatment of supernova explosions as well as the non-thermal components of the interstellar medium like magnetic fields and cosmic rays.

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arXiv2016

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

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 $μ$G). All magnetic runs saturate at mass weighted field strengths of $\sim$ 1 $-$ 3 $μ$G but the ISM structure is notably different. With increasing initial field strengths (from $6\times 10^{-3}$ to 3 $μ$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 $6\times 10^{-3}$ $μ$G to 0.7 for a 3 $μ$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.

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