Alexander M. Beck

CONNECTING ANGULAR MOMENTUM AND GALACTIC DYNAMICS: THE COMPLEX INTERPLAY BETWEEN SPIN, MASS, AND MORPHOLOGY

The evolution and distribution of the angular momentum of dark matter halos have been discussed in several studies over the last decades. In particular, the idea arose that angular momentum conservation should allow to infer the total angular momentum of the entire dark matter halo from measuring the angular momentum of the baryonic component, which is populating the center of the halo, especially for disk galaxies. To test this idea and to understand the connection between the angular momentum of the dark matter halo and its galaxy, we use a state-of-the-art, hydrodynamical cosmological simulation taken from the set of Magneticum Pathfinder Simulations. Thanks to the inclusion of the relevant physical processes, the improved underlying numerical methods and high spatial resolution, we successfully produce populations of spheroidal and disk galaxies self-consistently. Thus, we are able to study the dependance of galactic properties on their morphology. We find that: (I) The specific angular momentum of stars in disk and spheroidal galaxies as function of their stellar mass compares well with observational results; (II) The specific angular momentum of the stars in disk galaxies is slightly smaller compared to the specific angular momentum of the cold gas, in good agreement with observations; (III) Simulations including the baryonic component show a dichotomy in the specific stellar angular momentum distribution when splitting the galaxies according to their morphological type. This dichotomy can also be seen in the spin parameter, where disk galaxies populate halos with slightly larger spin compared to spheroidal galaxies; (IV) Disk galaxies preferentially populate halos in which the angular momentum vector of the dark matter component in the central part shows a better alignment to the angular momentum vector of the entire halo; (V) The specific angular momentum of the cold gas in disk galaxies is approximately 40% smaller than the specific angular momentum of the total dark matter halo and shows a significant scatter. Subject headings: dark matter – galaxies: evolution – galaxies: formation – galaxies: halos

On the nature of hydrostatic equilibrium in galaxy clusters

In this paper we investigate the level of hydrostatic equilibrium (HE) in the intra-cluster medium of simulated galaxy clusters, extracted from state-of-the-art cosmological hydrodynamical simulations performed with the Smoothed-Particle-Hydrodynamic code GADGET-3. These simulations include several physical processes, among which stellar and AGN feedback, and have been performed with an improved version of the code that allows for a better description of hydrodynamical instabilities and gas mixing processes. Evaluating the radial balance between the gravitational and hydrodynamical forces, via the gas accelerations generated, we effectively examine the level of HE in every object of the sample, its dependence on the radial distance from the center and on the classification of the cluster in terms of either cool-coreness or dynamical state. We find an average deviation of 10-20% out to the virial radius, with no evident distinction between cool-core and non-cool-core clusters. Instead, we observe a clear separation between regular and disturbed systems, with a more significant deviation from HE for the disturbed objects. The investigation of the bias between the hydrostatic estimate and the total gravitating mass indicates that, on average, this traces very well the deviation from HE, even though individual cases show a more complex picture. Typically, in the radial ranges where mass bias and deviation from HE are substantially different, the gas is characterized by a significant amount of random motions (>~30 per cent), relative to thermal ones. As a general result, the HE-deviation and mass bias, at given interesting distance from the cluster center, are not very sensitive to the temperature inhomogeneities in the gas.

Cool core clusters from cosmological simulations

We present results obtained from a set of cosmological hydrodynamic simulations of galaxy clusters, aimed at comparing predictions with observational data on the diversity between cool-core (CC) and non-cool-core (NCC) clusters. Our simulations include the effects of stellar and AGN feedback and are based on an improved version of the smoothed particle hydrodynamics code GADGET-3, which ameliorates gas mixing and better captures gas-dynamical instabilities by including a suitable artificial thermal diffusion. In this Letter, we focus our analysis on the entropy profiles, the primary diagnostic we used to classify the degree of cool-coreness of clusters, and on the iron profiles. In keeping with observations, our simulated clusters display a variety of behaviors in entropy profiles: they range from steadily decreasing profiles at small radii, characteristic of cool-core systems, to nearly flat core isentropic profiles, characteristic of non-cool-core systems. Using observational criteria to distinguish between the two classes of objects, we find that they occur in similar proportions in both simulations and in observations. Furthermore, we also find that simulated cool-core clusters have profiles of iron abundance that are steeper than those of NCC clusters, which is also in agreement with observational results. We show that the capability of our simulations to generate a realistic cool-core structure in the cluster population is due to AGN feedback and artificial thermal diffusion: their combined action allows us to naturally distribute the energy extracted from super-massive black holes and to compensate for the radiative losses of low-entropy gas with short cooling time residing in the cluster core.

nIFTy galaxy cluster simulations – I. Dark matter and non-radiative models

We have simulated the formation of a galaxy cluster in a Ʌ cold dark matter universe using 13 different codes modelling only gravity and non-radiative hydrodynamics (RAMSES, ART, AREPO, HYDRA and nine incarnations of GADGET). This range of codes includes particle-based, moving and fixed mesh codes as well as both Eulerian and Lagrangian fluid schemes. The various GADGET implementations span classic and modern smoothed particle hydrodynamics (SPH) schemes. The goal of this comparison is to assess the reliability of cosmological hydrodynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at z = 0, global properties such as mass and radial profiles of various dynamical and thermodynamical quantities. The underlying gravitational framework can be aligned very accurately for all the codes allowing a detailed investigation of the differences that develop due to the various gas physics implementations employed. As expected, the mesh-based codes RAMSES, ART and AREPO form extended entropy cores in the gas with rising central gas temperatures. Those codes employing classic SPH schemes show falling entropy profiles all the way into the very centre with correspondingly rising density profiles and central temperature inversions. We show that methods with modern SPH schemes that allow entropy mixing span the range between these two extremes and the latest SPH variants produce gas entropy profiles that are essentially indistinguishable from those obtained with grid-based methods.

Neutral hydrogen in galaxy clusters: impact of AGN feedback and implications for intensity mapping

By means of zoom-in hydrodynamic simulations, we quantify the amount of neutral hydrogen (H I) hosted by groups and clusters of galaxies. Our simulations, which are based on an improved formulation of smoothed particle hydrodynamics, include radiative cooling, star formation, metal enrichment and supernova feedback, and can be split into two different groups, depending on whether feedback from active galactic nuclei (AGN) is turned on or off. Simulations are analysed to account for HI self-shielding and the presence of molecular hydrogen. We find that the mass in neutral hydrogen of dark matter haloes monotonically increases with the halomass and can be well described by a power law of the form M-H I (M, z) proportional to M-3/4. Our results point out that AGN feedback reduces both the total halo mass and its HI mass, although it is more efficient in removing HI. We conclude that AGN feedback reduces the neutral hydrogen mass of a given halo by similar to 50 per cent, with a weak dependence on halo mass and redshift. The spatial distribution of neutral hydrogen within haloes is also affected by AGN feedback, whose effect is to decrease the fraction of HI that resides in the halo inner regions. By extrapolating our results to haloes not resolved in our simulations, we derive astrophysical implications from the measurements of Omega(H) (I)(z): haloes with circular velocities larger than similar to 25 km s(-1) are needed to host HI in order to reproduce observations. We find that only the model with AGN feedback is capable of reproducing the value of Omega(HI)b(HI) derived from available 21 cm intensity mapping observations.

Strong magnetic fields and large rotation measures in protogalaxies from supernova seeding

We present a model for the seeding and evolution of magnetic fi elds in protogalaxies. Super- nova (SN) explosions during the assembly of a protogalaxy self-consistently provide magnetic seed fields, which are subsequently amplified by compression , shear flows and random mo- tions. Our model explains the origin of strong magnetic field s ofG amplitude within the first starforming protogalactic structures shortly after the fir st stars have formed. We implement the model into the MHD version of the cosmological N-body / SPH simulation code GADGET and we couple the magnetic seeding directly to the underlying multi-phase description of star formation. We perform simulations of Milky Way-like galactic halo for- mation using a standardCDM cosmology and analyse the strength and distribution of the subsequent evolving magnetic field. Within starforming regions and given typical dimensions and magnetic field strengths in canonical SN remnants, we inject a dipole-shape magnetic fie ld at a rate of �10 9 G Gyr 1 . Subsequently, the magnetic field strength increases expone ntially on timescales of a few ten million years within the innermost regions of the halo. Furt hermore, turbulent diffusion, shocks and gas motions transport the magnetic field towards t he halo outskirts. At redshift z�0, the entire galactic halo is magnetized and the field amplit ude is of the order of a fewG in the center of the halo and �10 9 G at the virial radius. Additionally, we analyse the intrinsic rotation measure (R M) of the forming galactic halo over redshift. The mean halo intrinsic RM peaks between redshifts z�4 and z�2 and reaches abso- lute values around 1000 rad m 2 . While the halo virializes towards redshift z�0, the intrinsic RM values decline to a mean value below 10 rad m 2 . At high redshifts, the distribution of individual starforming and thus magnetized regions is widespread. This leads to a widespread distribution of large intrinsic RM values. In our model for the evolution of galactic magnetic fields, th e seed magnetic field amplitude and distribution is no longer a free parameter, but determin ed self-consistently by the star formation process occuring during the formation of cosmic structures. Thus, it provides a solution to the seed field problem.

On the magnetic fields in voids

We study the possible magnetization of cosmic voids by void galaxies. Recently, observations revealed isolated starforming galaxies within the voids. F urthermore, a major fraction of a voids volume is expected to be filled with magnetic fields of a m inimum strength of about 10 15 G on Mpc scales. We estimate the transport of magnetic energy by cosmic rays (CR) from the void galaxies into the voids. We assume that CRs and winds are able to leave small isolated void galaxies shortly after they assembled, and th en propagate within the voids. For a typical void, we estimate the magnetic field strength and vo lume filling factor depending on its void galaxy population and possible contributions of st rong active galactic nuclei (AGN) which border the voids. We argue that the lower limit on the void magnetic field can be recovered, if a small fraction of the magnetic energy contained in the void galaxies or void bordering AGNs is distributed within the voids.

nIFTy galaxy cluster simulations – IV. Quantifying the influence of baryons on halo properties

Building on the initial results of the nIFTy simulated galaxy cluster comparison, we compare and contrast the impact of baryonic physics with a single massive galaxy cluster, run with 11 state-of-the-art codes, spanning adaptive mesh, moving mesh, classic and modern smoothed particle hydrodynamics (SPH) approaches. For each code represented we have a dark-matter-only (DM) and non-radiative (NR) version of the cluster, as well as a full physics (FP) version for a subset of the codes. We compare both radial mass and kinematic profiles, as well as global measures of the cluster (e.g. concentration, spin, shape), in the NR and FP runs with that in the DM runs. Our analysis reveals good consistency ⪅20 per cent) between global properties of the cluster predicted by different codes when integrated quantities are measured within the virial radius R200. However, we see larger differences for quantities within R2500, especially in the FP runs. The radial profiles reveal a diversity, especially in the cluster centre, between the NR runs, which can be understood straightforwardly from the division of codes into classic SPH and non-classic SPH (including the modern SPH, adaptive and moving mesh codes); and between the FP runs, which can also be understood broadly from the division of codes into those that include active galactic nucleus feedback and those that do not. The variation with respect to the median is much larger in the FP runs with different baryonic physics prescriptions than in the NR runs with different hydrodynamics solvers.

The history of chemical enrichment in the intracluster medium from cosmological simulations

The distribution of metals in the intracluster medium (ICM) of galaxy clusters provides valuable information on their formation and evolution, on the connection with the cosmic star formation and on the effects of different gas processes. By analysing a sample of simulated galaxy clusters, we study the chemical enrichment of the ICM, its evolution, and its relation with the physical processes included in the simulation and with the thermal properties of the core. These simulations, consisting of re-simulations of 29 Lagrangian regions performed with an upgraded version of the smoothed particle hydrodynamics (SPH) GADGET-3 code, have been run including two different sets of baryonic physics: one accounts for radiative cooling, star formation, metal enrichment and supernova (SN) feedback, and the other one further includes the effects of feedback from active galactic nuclei (AGN). In agreement with observations, we find an anti-correlation between entropy and metallicity in cluster cores, and similar radial distributions of heavy-element abundances and abundance ratios out to large clustercentric distances (similar to R-180). In the outskirts, namely outside of similar to 0.2 R-180, we find a remarkably homogeneous metallicity distribution, with almost flat profiles of the elements produced by either SNIa or SNII. We investigated the origin of this phenomenon and discovered that it is due to the widespread displacement of metal-rich gas by early (z > 2-3) AGN powerful bursts, acting on small high-redshift haloes. Our results also indicate that the intrinsic metallicity of the hot gas for this sample is on average consistent with no evolution between z = 2 and z = 0, across the entire radial range.

nIFTy galaxy cluster simulations - II. Radiative models

We have simulated the formation of a massive galaxy cluster (M