arXiv subjects

Klaus Dolag

Publications and source records attributed to Klaus Dolag.

At least 181 records · Page 10Linked to original sources

arXiv2019

Black hole mass of central galaxies and cluster mass correlation in cosmological hydro-dynamical simulations

Recently, relations connecting the SMBH mass of central galaxies and global properties of the hosting cluster, such as temperature and mass, were observed. We investigate the correlation between SMBH mass and cluster mass and temperature, their establishment and evolution. We compare their scatter to that of the classical $M_{\rm BH}-M_{\rm BCG}$ relation. We study how gas accretion and BH-BH mergers contribute to SMBH growth across cosmic time. We employed 135 groups and clusters with a mass range $1.4\times 10^{13}M_{\odot}-2.5\times 10^{15} M_{\odot}$ extracted from a set of 29 zoom-in cosmological hydro-dynamical simulations where the baryonic physics is treated with various sub-grid models, including feedback by AGN. In our simulations we find that $M_{\rm BH}$ correlates well with $M_{500}$ and $T_{500}$, with the scatter around these relations compatible within $2σ$ with the scatter around $M_{\rm BH}-M_{\rm BCG}$ at $z=0$. The $M_{\rm BH}-M_{500}$ relation evolves with time, becoming shallower at lower redshift as a direct consequence of hierarchical structure formation. On average, in our simulations the contribution of gas accretion to the total SMBH mass dominates for the majority of the cosmic time ($z>0.4$), while in the last 2 Gyr the BH-BH mergers become a larger contributor. During this last process, substructures hosting SMBHs are disrupted in the merger process with the BCG and the unbound stars enrich the diffuse stellar component rather than increase BCG mass. From the results obtained in our simulations with simple sub-grid models we conclude that the scatter around the $M_{\rm BH}-T_{500}$ relation is comparable to the scatter around the $M_{\rm BH}-M_{\rm BCG}$ relation and that, given the observational difficulties related to the estimation of the BCG mass, clusters temperature and mass can be a useful proxy for the SMBHs mass, especially at high redshift.

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arXiv2019

Simulations of core formation for frequent dark matter self-interactions

We present the first $N$-body simulations that adapt the equations of smoothed particle hydrodynamics to capture the effect of dark matter self-interactions which are too frequent to be resolved explicitly. The relevant energy transfer equations are derived, the appropriate thermal conductivity is determined and the effects of different smoothing kernels are studied. We apply our framework to simulate the formation of isothermal cores in isolated dark matter haloes and determine the core growth rate as a function of the self-scattering cross section. Our approach may be combined with explicit simulations of rare scatterings in order to simulate accurately the effects of arbitrary dark matter self-interactions in future cosmological simulations.

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arXiv2019

Using X-Ray Morphological Parameters to Strengthen Galaxy Cluster Mass Estimates via Machine Learning

We present a machine learning approach for estimating galaxy cluster masses, trained using both Chandra and eROSITA mock X-ray observations of 2,041 clusters from the Magneticum simulations. We train a random forest regressor, an ensemble learning method based on decision tree regression, to predict cluster masses using an input feature set. The feature set uses core-excised X-ray luminosity and a variety of morphological parameters, including surface brightness concentration, smoothness, asymmetry, power ratios, and ellipticity. The regressor is cross-validated and calibrated on a training sample of 1,615 clusters (80% of sample), and then results are reported as applied to a test sample of 426 clusters (20% of sample). This procedure is performed for two different mock observation series in an effort to bracket the potential enhancement in mass predictions that can be made possible by including dynamical state information. The first series is computed from idealized Chandra-like mock cluster observations, with high spatial resolution, long exposure time (1 Ms), and the absence of background. The second series is computed from realistic-condition eROSITA mocks with lower spatial resolution, short exposures (2 ks), instrument effects, and background photons modeled. We report a 20% reduction in the mass estimation scatter when either series is used in our random forest model compared to a standard regression model that only employs core-excised luminosity. The morphological parameters that hold the highest feature importance are smoothness, asymmetry, and surface brightness concentration. Hence, these parameters, which encode the dynamical state of the cluster, can be used to make more accurate predictions of cluster masses in upcoming surveys, offering a crucial step forward for cosmological analyses.

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arXiv2019

Gone after one orbit: How cluster environments quench galaxies

The effect of galactic orbits on a galaxy's internal evolution within a galaxy cluster environment has been the focus of heated debate in recent years. To understand this connection, we use both the $(0.5 \,$Gpc)$^3$ and the Gpc$^3$ boxes from the cosmological hydrodynamical simulation set Magneticum Pathfinder. We investigate the velocity-anisotropy, phase space, and the orbital evolution of up to $\sim 5 \cdot 10^{5}$ resolved satellite galaxies within our sample of 6776 clusters with $M_{\mathrm{vir}} > 10^{14} \, \mathrm{M_{\odot}}$ at low redshift, which we also trace back in time. In agreement with observations, we find that star-forming satellite galaxies inside galaxy clusters are characterised by more radially dominated orbits, independent of cluster mass. Furthermore, the vast majority of star-forming satellite galaxies stop forming stars during their first passage. We find a strong dichotomy both in line-of-sight and radial phase space between star-forming and quiescent galaxies, in line with observations. The tracking of individual orbits shows that the star-formation of almost all satellite galaxies drops to zero within $1 \, \mathrm{Gyr}$ after in-fall. Satellite galaxies that are able to remain star-forming longer are characterised by tangential orbits and high stellar mass. All this indicates that in galaxy clusters the dominant quenching mechanism is ram-pressure stripping.

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arXiv2019

WVTICs -- SPH initial conditions for everyone

We present a novel and fast application to generate glass-like initial conditions for Lagrangian hydrodynamic schemes (e.g. Smoothed Particle Hydrodynamics (SPH)) following arbitrary density models based on weighted Voronoi tessellations and combine it with improved initial configurations and an additional particle reshuffling scheme. We show our application's ability to sample different kinds of density features and to converge properly towards the given model density as well as a glass-like particle configuration. We analyse convergence with iterations as well as with varying particle number. Additionally, we demonstrate the versatility of the implemented algorithms by providing an extensive test suite for standard (magneto-) hydrodynamic test cases as well as a few common astrophysical applications. We indicate the potential to bridge further between observational astronomy and simulations as well as applicability to other fields of science by advanced features such as describing a density model using gridded data for exampling from an image file instead of an analytic model.

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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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arXiv2019

The Outer Stellar Halos of Galaxies: how Radial Merger Mass Deposition, Shells and Streams depend on Infall-Orbit Configurations

Galaxy mergers are a fundamental part of galaxy evolution. To study the resulting mass distributions of different kinds of galaxy mergers, we present a simulation suite of 36 high-resolution isolated merger simulations, exploring a wide range of parameter space in terms of mass ratios (mu = 1:5, 1:10, 1:50, 1:100) and orbital parameters. We find that mini mergers deposit a higher fraction of their mass in the outer halo compared to minor mergers, while their contribution to the central mass distribution is highly dependent on the orbital impact parameter: for larger pericentric distances we find that the centre of the host galaxy is almost not contaminated by merger particles. We also find that the median of the resulting radial mass distribution for mini mergers differs significantly from the predictions of simple theoretical tidal-force models. Furthermore, we find that mini mergers can increase the size of the host disc significantly without changing the global shape of the galaxy, if the impact occurs in the disc plane, thus providing a possible explanation for extended low-surface brightness disks reported in observations. Finally, we find clear evidence that streams are a strong indication of nearly circular infall of a satellite (with large angular momentum), whereas the appearance of shells clearly points to (nearly) radial satellite infall.

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arXiv2019

Dependency of halo concentration on mass, redshift and fossilness in Magneticum hydrodynamic simulations

We study the dependency of the concentration on mass and redshift using three large N-body cosmological hydrodynamic simulations carried out by the Magneticum project. We constrain the slope of the mass-concentration relation with an unprecedented mass range for hydrodynamic simulations and find a negative trend on the mass-concentration plane and a slightly negative redshift dependency, in agreement with observations and other numerical works. We also show how the concentration correlates with the fossil parameter, defined as the stellar mass ratio between the central galaxy and the most massive satellite, in agreement with observations. We find that haloes with high fossil parameter have systematically higher concentration and investigate the cause in two different ways. First we study the evolution of haloes that lives unperturbed for a long period of time, where we find that the internal region keeps accreting satellites as the fossil parameter increases and the scale radius decreases (which increases the concentration). We also study the dependency of the concentration on the virial ratio and the energy term from the surface pressure $E_s$. We conclude that fossil objects have higher concentration because they are dynamically relaxed, with no in-fall/out-fall material and had time to accrete their satellites.

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arXiv2019

Cosmic evolution of supermassive black holes: A view into the next two decades

The discoveries made over the past 20 years by Chandra and XMM-Newton surveys in conjunction with multiwavelength imaging and spectroscopic data available in the same fields have significantly changed the view of the supermassive black hole (SMBH) and galaxy connection. These discoveries have opened up several exciting questions that are beyond the capabilities of current X-ray telescopes and will need to be addressed by observatories in the next two decades. As new observatories peer into the early Universe, we will begin to understand the physics and demographics of SMBH infancy (at $z>6$) and investigate the influence of their accretion on the formation of the first galaxies ($§$ 2.1). We will also be able to understand the accretion and evolution over the cosmic history (at $z\sim$1-6) of the full population of black holes in galaxies, including low accretion rate, heavily obscured AGNs at luminosities beyond the reach of current X-ray surveys ($§$2.2 and $§$2.3), enabling us to resolve the connection between SMBH growth and their environment.

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arXiv2019

Open Questions in Cosmic-Ray Research at Ultrahigh Energies

We review open questions and prospects for progress in ultrahigh-energy cosmic ray (UHECR) research, based on a series of discussions that took place during the `The High-Energy Universe: Gamma-Ray, Neutrino, and Cosmic-ray Astronomy' MIAPP workshop in 2018. Specifically, we overview open questions on the origin of the bulk of UHECRs, the UHECR mass composition, the origin of the end of the cosmic-ray spectrum, the transition from Galactic to extragalactic cosmic rays, the effect of magnetic fields on the trajectories of UHECRs, anisotropy expectations for specific astrophysical scenarios, hadronic interactions, and prospects for discovering neutral particles as well as new physics at ultrahigh energies. We also briefly present upcoming and proposed UHECR experiments and discuss their projected science reach.

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arXiv2019

The Three Hundred Project: Ram pressure and gas content of haloes and subhaloes in the phase-space plane

We use TheThreeHundred project, a suite of 324 resimulated massive galaxy clusters embedded in a broad range of environments, to investigate (i) how the gas content of surrounding haloes correlates with phase-space position at $z=0$, and (ii) to investigate the role that ram pressure plays in this correlation. By stacking all 324 normalised phase-space planes containing 169287 haloes and subhaloes, we show that the halo gas content is tightly correlated with phase-space position. At $\sim\,1.5-2\,\text{R}_{\text{200}}$ of the cluster dark matter halo, we find an extremely steep decline in the halo gas content of infalling haloes and subhaloes irrespective of cluster mass, possibly indicating the presence of an accretion shock. We also find that subhaloes are particularly gas-poor, even in the cluster outskirts, which could indicate active regions of ongoing pre-processing. By modelling the instantaneous ram pressure experienced by each halo and subhalo at $z=0$, we show that the ram pressure intensity is also well correlated with phase-space position, which is again irrespective of cluster mass. In fact, we show that regions in the phase-space plane with high differential velocity between a halo or subhalo and its local gas environment, are almost mutually exclusive with high halo gas content regions. This suggests a causal link between the gas content of objects and the instantaneous ram pressure they experience, where the dominant factor is the differential velocity.

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arXiv2018

Chandra centres for COSMOS X-ray galaxy groups: Differences in stellar properties between central dominant and offset brightest group galaxies

We present the results of a search for galaxy clusters and groups in the $\sim2$ square degree of the COSMOS field using all available X-ray observations from the XMM-Newton and Chandra observatories. We reach an X-ray flux limit of $3\times10^{-16}\;ergs\;cm^{-2}\;s^{-1}$ in 0.5--2 keV range, and identify 247 X-ray groups with $M_{200c}=8\times10^{12}-3\times10^{14}\;M_{\odot}$ at a redshift range of $0.08\leq z<1.53$, using the multiband photometric redshift and the master spectroscopic redshift catalogues of the COSMOS. The X-ray centres of groups are determined using high-resolution Chandra imaging. We investigate the relations between the offset of the brightest group galaxies (BGGs) from halo X-ray centre and group properties and compare with predictions from semi-analytic models and hydrodynamical simulations. We find that BGG offset decreases with both increasing halo mass and decreasing redshift with no strong dependence on the X-ray flux and SNR. We show that the BGG offset decreases as a function of increasing magnitude gap with no considerable redshift dependent trend. The stellar mass of BGGs in observations extends over a wider dynamic range compared to model predictions. At $z<0.5$, the central dominant BGGs become more massive than those with large offsets by up to 0.3dex, in agreement with model prediction. The observed and predicted lognormal scatter in the stellar mass of both low- and large-offset BGGs at fixed halo mass is $\sim0.3$dex.

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arXiv2018

Exploiting the Space Filling Curve Ordering of Particles in the Neighbour Search of Gadget3

Gadget3 is nowadays one of the most frequently used high performing parallel codes for cosmological hydrodynamical simulations. Recent analyses have shown t\ hat the Neighbour Search process of Gadget3 is one of the most time-consuming parts. Thus, a considerable speedup can be expected from improvements of the u\ nderlying algorithms. In this work we propose a novel approach for speeding up the Neighbour Search which takes advantage of the space-filling-curve particle ordering. Instead of performing Neighbour Search for all particles individually, nearby active particles can be grouped and one single Neighbour Search can be performed to obta\ in a common superset of neighbours. Thus, with this approach we reduce the number of searches. On the other hand, tree walks are performed within a larger searching radius. There is an optimal size of grouping that maximize the speedup, which we found by numerical experiments. We tested the algorithm within the boxes of the Magneticum project. As a result we obtained a speedup of $1.65$ in the Density and of $1.30$ in the Hydrodynamics computation, respectively, and a total speedup of $1.34.$

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arXiv2018

A web portal for hydrodynamical, cosmological simulations

This article describes a data center hosting a web portal for accessing and sharing the output of large, cosmological, hydro-dynamical simulations with a broad scientific community. It also allows users to receive related scientific data products by directly processing the raw simulation data on a remote computing cluster. The data center has a multi-layer structure: a web portal, a job control layer, a computing cluster and a HPC storage system. The outer layer enables users to choose an object from the simulations. Objects can be selected by visually inspecting 2D maps of the simulation data, by performing highly compounded and elaborated queries or graphically by plotting arbitrary combinations of properties. The user can run analysis tools on a chosen object. These services allow users to run analysis tools on the raw simulation data. The job control layer is responsible for handling and performing the analysis jobs, which are executed on a computing cluster. The innermost layer is formed by a HPC storage system which hosts the large, raw simulation data. The following services are available for the users: (I) {\sc ClusterInspect} visualizes properties of member galaxies of a selected galaxy cluster; (II) {\sc SimCut} returns the raw data of a sub-volume around a selected object from a simulation, containing all the original, hydro-dynamical quantities; (III) {\sc Smac} creates idealised 2D maps of various, physical quantities and observables of a selected object; (IV) {\sc Phox} generates virtual X-ray observations with specifications of various current and upcoming instruments.

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arXiv2018

The Three Hundred project: a large catalogue of theoretically modelled galaxy clusters for cosmological and astrophysical applications

We introduce the THE THREE HUNDRED project, an endeavour to model 324 large galaxy clusters with full-physics hydrodynamical re-simulations. Here we present the data set and study the differences to observations for fundamental galaxy cluster properties and scaling relations. We find that the modelled galaxy clusters are generally in reasonable agreement with observations with respect to baryonic fractions and gas scaling relations at redshift z = 0. However, there are still some (model-dependent) differences, such as central galaxies being too massive, and galaxy colours (g - r) being bluer (about 0.2 dex lower at the peak position) than in observations. The agreement in gas scaling relations down to 10^{13} h^{-1} M_{\odot} between the simulations indicates that particulars of the sub-grid modelling of the baryonic physics only has a weak influence on these relations. We also include - where appropriate - a comparison to three semi-analytical galaxy formation models as applied to the same underlying dark-matter-only simulation. All simulations and derived data products are publicly available.

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arXiv2018

Kinematics of Simulated Galaxies I: Connecting Dynamical and Morphological Properties of Early-Type Galaxies at Different Redshifts

State-of-the-art integral field surveys like $\mathrm{ATLAS^{3D}}$, SLUGGS, CALIFA, SAMI, and MaNGA provide large data sets of kinematical observations of early-type galaxies (ETGs), yielding constraints on the formation of ETGs. Using the cosmological hydrodynamical \textit{Magneticum Pathfinder} simulations, we investigate the paradigm of fast and slow rotating ETGs in a fully cosmological context. We show that the ETGs within the \textit{Magneticum} simulation are in remarkable agreement with the observations, revealing fast and slow rotators quantified by the angular momentum proxy $λ_{\mathrm{R}}$ and the flattening $ε$ with the observed prevalence. Taking full advantage of the three-dimensional data, we demonstrate that the dichotomy between fast and slow rotating galaxies gets enhanced, showing an upper and lower population separated by an underpopulated region in the edge-on $λ_{\mathrm{R}}$-$ε$ plane. Following the evolution of the $λ_{\mathrm{R}}$-$ε$ plane through cosmic time, we find that, while the upper population is already in place at $z=2$, the lower population gets statistically significant below $z=1$ with a gradual increase. At least $50\%$ of the galaxies transition from fast to slow rotators on a short timescale, in most cases associated to a significant merger event. Furthermore, we connect the $M_{*}$-$j_{*}$ plane, quantified by the $b$-value, with the $λ_{\mathrm{R}}$-$ε$ plane, revealing a strong correlation between the position of a galaxy in the $λ_{\mathrm{R}}$-$ε$ plane and the $b$-value. Going one step further, we classify our sample based on features in their velocity map, finding all five common kinematic groups, also including the recently observed group of prolate rotators, populating distinct regions in the $λ_{\mathrm{R}}$-$b$ plane.

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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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arXiv2018

Cosmological simulations of black hole growth II: how (in)significant are merger events for fuelling nuclear activity?

Which mechanism(s) are mainly driving nuclear activity in the centres of galaxies is a major unsettled question. In this study, we investigate the statistical relevance of galaxy mergers for fuelling gas onto the central few kpc of a galaxy, potentially resulting in an active galactic nucleus (AGN). To robustly address that, we employ large-scale cosmological hydrodynamic simulations from the Magneticum Pathfinder set, including models for BH accretion and AGN feedback. Our simulations predict that for luminous AGN ($L_{\rm AGN} > 10^{45} {\rm erg/s}$) at $z = 2$, more than 50 per cent of their host galaxies have experienced a merger in the last 0.5~Gyr. These high merger fractions, however, merely reflect the intrinsically high merger fractions of massive galaxies at $z=2$, in which luminous AGN preferentially occur. Apart from that, our simulations suggest that merger events are not the statistically dominant fuelling mechanism for nuclear activity over a redshift range $z=0-2$: irrespective of AGN luminosity, less than 20 per cent of AGN hosts have on average undergone a recent merger, in agreement with a number of observational studies. The central ISM conditions required for inducing AGN activity can be, but are not necessarily caused by a merger. Despite the statistically minor relevance of mergers, at a given AGN luminosity and stellar mass, the merger fractions of AGN hosts can be by up to three times higher than that of inactive galaxies. Such elevated merger fractions still point towards an intrinsic connection between AGN and mergers, consistent with our traditional expectation.

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