The Formation of Dark Matter Halos and High-Redshift Galaxies

In the concordance LCDM cosmological model, galaxies form in the centers of dark matter halos and merge with one another following the mergers of their host halos. Thus, we set out to quantify the growth mechanisms of dark matter halos. For this purpose, we analyze several large N-body simulations of the growth of cosmic structure. We devise a novel merger tree construction algorithm that properly takes into account halo fragmentations. We find that the merger rate evolves rapidly with redshift but depends weakly on mass, and that the proportions between mergers of different mass ratios, e.g. major and minor mergers, are universal. We also show that the merger rate per progenitor halo (related to future mergers and to galaxy pair counting) is smaller than that per descendant halo (related to past mergers and galaxy disturbed morphplogies), and that their redshift and mass dependencies are different. We find that only ~60% of the mass accreted onto halos arrives in mergers that are resolved in our simulations. Moreover, the functional form of the merger rate suggests that the merger contribution saturates at that value. Using full particle histories, we confirm that smoothly-accreted particles make a significant fraction of dark matter halos. This has important implications for the smoothness of gas accretion. Disk galaxies at z~2 are rapidly star-forming, but show regular rotation, indicating little merger activity. We use a large dark matter simulation to show that even non-merging z~2 halos grow fast enough to explain observed high star-formation rates. We also follow those halos to z=0, finding that many do not undergo major mergers at all. The z~2 disks also show high velocity dispersions and irregular clumpy morphologies. We run "zoom-in" cosmological hydrodynamical simulations focusing on the formation of individual z~2 galaxies. We find that the clumpy morphologies are a result of gravitational instability, where the high random motions make the (turbulent) Jeans scales as large as the observed giant clumps. Star-formation feedback in our model is implemented as galactic winds with high mass-loading factors. We find that the high mass-loading factors prevent the clumps from virializing. Within roughly half a disk orbital time, they lose a large fraction of their mass, such that they stop collapsing and disrupt. Thus, their lifetimes are short and they do not migrate to the galaxy centers as has been proposed in the literature so far. We compare simulated galaxies to observations using radiative transfer calculations, and by creating mock SINFONI/VLT data cubes with realistic "observing conditions". We find good agreement between "observed" simulated galaxies and real observed ones, in terms of their luminosities, colors, morphologies and kinematics. With this comparison, we conclude that the galaxies formed in our simulations are plausibly realistic

Zooming in on accretion - II. Cold Circumgalactic Gas Simulated with a super-Lagrangian Refinement Scheme

In this study we explore the complex multi-phase gas of the circumgalactic medium (CGM) surrounding galaxies. We propose and implement a novel, super-Lagrangian 'CGM zoom' scheme in the moving-mesh code AREPO, which focuses more resolution into the CGM and intentionally lowers resolution in the dense ISM. We run two cosmological simulations of the same galaxy halo, once with a simple 'no feedback' model, and separately with a more comprehensive physical model including galactic-scale outflows as in the Illustris simulation. Our chosen halo has a total mass of ~10^12 Msun at z ~ 2, and we achieve a median gas mass (spatial) resolution of ~2,200 solar masses (~95 parsecs) in the CGM, six-hundred (fourteen) times better than in the Illustris-1 simulation, a higher spatial resolution than any cosmological simulation at this mass scale to date. We explore the primary channel(s) of cold-phase CGM gas production in this regime. We find that winds substantially enhance the amount of cold gas in the halo, also evidenced in the covering fractions of HI and the equivalent widths of MgII out to large radii, in better agreement with observations than the case without galactic winds. Using a tracer particle analysis to follow the thermodynamic history of gas, we demonstrate how the majority of this cold, dense gas arises due to rapid cooling of the wind material interacting with the hot halo, and how large amounts of cold, ~10^4 K gas can be produced and persist in galactic halos with Tvir ~ 10^6 K. At the resolutions presently considered, the quantitative properties of the CGM we explore are not appreciably affected by the refinement scheme.Comment: MNRAS submitted, comments welcome. High-res version at http://www.mpa-garching.mpg.de/~dnelson/papers/Suresh19_zooming2.pd

Reducing noise in moving-grid codes with strongly-centroidal Lloyd mesh regularization

A method for improving the accuracy of hydrodynamical codes that use a moving Voronoi mesh is described. Our scheme is based on a new regularization scheme that constrains the mesh to be centroidal to high precision while still allowing the cells to move approximately with the local fluid velocity, thereby retaining the quasi-Lagrangian nature of the approach. Our regularization technique significantly reduces mesh noise that is attributed to changes in mesh topology and deviations from mesh regularity. We demonstrate the advantages of our method on various test problems, and note in particular improvements obtained in handling shear instabilities, mixing, and in angular momentum conservation. Calculations of adiabatic jets in which shear excites Kelvin Helmholtz instability show reduction of mesh noise and entropy generation. In contrast, simulations of the collapse and formation of an isolated disc galaxy are nearly unaffected, showing that numerical errors due to the choice of regularization do not impact the outcome in this case.Comment: 9 pages, 14 figures, MNRAS submitte

Following the flow: tracer particles in astrophysical fluid simulations

We present two numerical schemes for passive tracer particles in the hydrodynamical moving-mesh code AREPO, and compare their performance for various problems, from simple setups to cosmological simulations. The purpose of tracer particles is to allow the flow to be followed in a Lagrangian way, tracing the evolution of the fluid with time, and allowing the thermodynamical history of individual fluid parcels to be recorded. We find that the commonly-used `velocity field tracers', which are advected using the fluid velocity field, do not in general follow the mass flow correctly, and explain why this is the case. This method can result in orders-of-magnitude biases in simulations of driven turbulence and in cosmological simulations, rendering the velocity field tracers inappropriate for following these flows. We then discuss a novel implementation of `Monte Carlo tracers', which are moved along with fluid cells, and are exchanged probabilistically between them following the mass flux. This method reproduces the mass distribution of the fluid correctly. The main limitation of this approach is that it is more diffusive than the fluid itself. Nonetheless, we show that this novel approach is more reliable than what has been employed previously and demonstrate that it is appropriate for following hydrodynamical flows in mesh-based codes. The Monte Carlo tracers can also naturally be transferred between fluid cells and other types of particles, such as stellar particles, so that the mass flow in cosmological simulations can be followed in its entirety.Comment: Accepted for publication in MNRAS, minor updates to match accepted version. 19 pages, 14 figure

A physical model for cosmological simulations of galaxy formation: multi-epoch validation

We present a multi-epoch analysis of the galaxy populations formed within the cosmological hydrodynamical simulations presented in Vogelsberger et al. (2013). These simulations explore the performance of a recently implemented feedback model which includes primordial and metal line radiative cooling with self-shielding corrections; stellar evolution with associated mass loss and chemical enrichment; feedback by stellar winds; black hole seeding, growth and merging; and AGN quasar- and radio-mode heating with a phenomenological prescription for AGN electro-magnetic feedback. We illustrate the impact of the model parameter choices on the resulting simulated galaxy population properties at high and intermediate redshifts. We demonstrate that our scheme is capable of producing galaxy populations that broadly reproduce the observed galaxy stellar mass function extending from redshift z=0 to z=3. We also characterise the evolving galactic B-band luminosity function, stellar mass to halo mass ratio, star formation main sequence, Tully-Fisher relation, and gas-phase mass-metallicity relation and confront them against recent observational estimates. This detailed comparison allows us to validate elements of our feedback model, while also identifying areas of tension that will be addressed in future work.Comment: 22 pages, 10 figures, submitted to MNRAS. Volume-rendering movies and high-resolution images can be found at http://www.cfa.harvard.edu/itc/research/arepogal

Zooming in on accretion - I. The structure of halo gas

We study the properties of gas in and around 10^12 solar mass halos at z=2 using a suite of high-resolution cosmological hydrodynamic 'zoom' simulations. We quantify the thermal and dynamical structure of these gaseous reservoirs in terms of their mean radial distributions and angular variability along different sightlines. With each halo simulated at three levels of increasing resolution, the highest reaching a baryon mass resolution of ~10,000 solar masses, we study the interaction of filamentary inflow and the quasi-static hot halo atmosphere. We highlight the discrepancy between the spatial resolution available in the halo gas as opposed to within the galaxy itself, and find that stream morphologies become increasingly complex at higher resolution, with large coherent flows revealing density and temperature structure at progressively smaller scales. Moreover, multiple gas components co-exist at the same radius within the halo, making radially averaged analyses misleading. This is particularly true where the hot, quasi-static, high entropy halo atmosphere interacts with cold, rapidly inflowing, low entropy accretion. We investigate the process of gas virialization and identify different regimes for the heating of gas as it accretes from the intergalactic medium. Haloes at this mass have a well-defined virial shock, associated with a sharp jump in temperature and entropy at ~1.25 r_vir. The presence, radius, and radial width of this boundary feature, however, vary not only from halo to halo, but also as a function of angular direction, covering roughly ~85% of the 4pi sphere. Our findings are relevant for the proper interpretation of observations pertaining to the circumgalactic medium, including evidence for large amounts of cold gas surrounding massive haloes at intermediate redshifts.Comment: High-res PDF and simulation movies available at http://www.cfa.harvard.edu/~dnelson/#research (MNRAS submitted, comments welcome

Large-scale mass distribution in the Illustris simulation

Observations at low redshifts thus far fail to account for all of the baryons expected in the Universe according to cosmological constraints. A large fraction of the baryons presumably resides in a thin and warm-hot medium between the galaxies, where they are difficult to observe due to their low densities and high temperatures. Cosmological simulations of structure formation can be used to verify this picture and provide quantitative predictions for the distribution of mass in different large-scale structure components. Here we study the distribution of baryons and dark matter at different epochs using data from the Illustris simulation. We identify regions of different dark matter density with the primary constituents of large-scale structure, allowing us to measure mass and volume of haloes, filaments and voids. At redshift zero, we find that 49 % of the dark matter and 23 % of the baryons are within haloes more massive than the resolution limit of $2\times 10^8$ M$_\odot$. The filaments of the cosmic web host a further 45 % of the dark matter and 46 % of the baryons. The remaining 31 % of the baryons reside in voids. The majority of these baryons have been transported there through active galactic nuclei feedback. We note that the feedback model of Illustris is too strong for heavy haloes, therefore it is likely that we are overestimating this amount. Categorizing the baryons according to their density and temperature, we find that 17.8 % of them are in a condensed state, 21.6 % are present as cold, diffuse gas, and 53.9 % are found in the state of a warm-hot intergalactic medium.Comment: 12 pages, 15 figure