Jeffrey P. Gardner

The Formation of a Realistic Disk Galaxy in Λ-dominated Cosmologies

We simulate the formation of a realistic disk galaxy within the hierarchical scenario of structure formation and study its internal properties to the present epoch. We use a set of smoothed particle hydrodynamic (SPH) simulations, with a high dynamical range and force resolution, that include cooling, star formation, supernovae (SNe) feedback and a redshift-dependent UV background. We compare results from a Λ cold dark matter (ΛCDM) simulation to a Λ warm dark matter (ΛWDM) (2 keV) simulation that forms significantly less small-scale structure. We show how high mass and force resolution in both the gas and dark-matter components play an important role in solving the angular momentum catastrophe claimed from previous simulations of galaxy formation within the hierarchical framework. Hence, a large disk forms without the need of strong energy injection, the z = 0 galaxies lie close to the I band Tully-Fisher relation, and the stellar material in the disk component has a final specific angular momentum equal to 40% and 90% of the dark halo in the ΛCDM and ΛWDM models, respectively. If rescaled to the Milky Way, the ΛCDM galaxy has an overabundance of satellites, with a total mass in the stellar halo 40% of that in the bulge+disk system. The ΛWDM galaxy has a drastically reduced satellite population and a negligible stellar spheroidal component. Encounters with satellites play only a minor role in disturbing the disk. Satellites possess a variety of star formation histories linked to mergers and pericentric passages along their orbit around the primary galaxy. In both cosmologies, the galactic halo retains most of the baryons accreted and builds up a hot gas phase with a substantial X-ray emission. Therefore, while we have been successful in creating a realistic stellar disk in a massive galaxy within the ΛCDM scenario, energy injection emerges as necessary ingredient to reduce the baryon fraction in galactic halos, independent of the cosmology adopted.

Cooling Radiation and the Lyα Luminosity of Forming Galaxies

We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Ly? line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z ? 2. However, in contrast to the classical picture of gas cooling from the ~106 K virial temperature of a typical dark matter halo, we find that most of the cooling radiation is emitted by gas with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation emerges in the Ly? line. While a galaxys cooling luminosity is usually smaller than the ionizing continuum luminosity of its young stars, the two are comparable in the most massive systems, and the cooling radiation is produced at larger radii, where the Ly? photons are less likely to be extinguished by dust. We suggest, in particular, that cooling radiation could explain the two large (~100 kpc), luminous (LLy? ~ 1044 ergs s-1) blobs of Ly? emission found in the narrowband survey of a z = 3 protocluster by Steidel and collaborators. Our simulations predict objects of the observed luminosity at about the right space density, and radiative transfer effects can account for the observed sizes and line widths. We discuss observable tests of this hypothesis for the nature of the Ly? blobs, and we present predictions for the contribution of cooling radiation to the Ly? luminosity function of galaxies as a function of redshift.

Metal Enrichment of the Intergalactic Medium in Cosmological Simulations

Observations have established that the diffuse intergalactic medium (IGM) at z ~ 3 is enriched to ~10-2.5 solar metallicity and that the hot gas in large clusters of galaxies (ICM) is enriched to - Z☉ at z = 0. Metals in the IGM may have been removed from galaxies (in which they presumably form) during dynamical encounters between galaxies, by ram-pressure stripping, by supernova-driven winds, or as radiation-pressure-driven dust efflux. This study develops a method of investigating the chemical enrichment of the IGM and of galaxies, using already completed cosmological simulations. To these simulations we add dust and (gaseous) metals, assuming instantaneous recycling and distributing the dust and metals in the gas according to three simple parameterized prescriptions, one for each enrichment mechanism. These prescriptions are formulated to capture the basic ejection physics, and calibrated when possible with empirical data. Our method allows exploration of a large number of models, yet for each model yields a specific (not statistical) realization of the cosmic metal distribution that can be compared in detail to observations. Our results indicate that dynamical removal of metals from 108.5 M☉ galaxies cannot account for the observed metallicity of low column density Lyα absorbers and that dynamical removal from 1010.5 M☉ galaxies cannot account for the ICM metallicities. Dynamical removal also fails to produce a strong enough mass-metallicity relation in galaxies. In contrast, either wind or radiation-pressure ejection of metals from relatively large galaxies can plausibly account for all three sets of observations (though it is unclear whether metals can be distributed uniformly enough in the low-density regions without overly disturbing the IGM and whether clusters can be enriched quite as much as observed). We investigate in detail how our results change with variations in our assumed parameters and how results for the different ejection processes compare.

Haloes gone MAD: The Halo-Finder Comparison Project

We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends-of-friends, spherical-overdensity and phase-space-based algorithms. We

Evolution of the mass function of dark matter haloes

We use a high resolutionCDM numerical simulation to calculate the mass function of dark matter haloes down to the scale of dwarf galaxies, back to a redshift of fifteen, in a 50 h −1 Mpc volume containing 80 million particles. Our low redshift results allow us to probe low σ density fluctuations significantly beyond the range of previous cosmological simulations. The Sheth and Tormen mass function provides an excellent match to all of our data except for redshifts of ten and higher, where it overpredicts halo numbers increasingly with redshift, reaching roughly 50 percent for the 10 10 − 10 11 M⊙ haloes sampled at redshift 15. Our results confirm previous findings that the simulated halo mass function can be described solely by the variance of the mass distribution, and thus has no explicit redshift dependence. We provide an empirical fit to our data that corrects for the overprediction of extremely rare objects by the Sheth and Tormen mass function. This overprediction has implications for studies that use the number densities of similarly rare objects as cosmological probes. For example, the number density of high redshift (z ≃ 6) QSOs, which are thought to be hosted by haloes at 5σ peaks in the fluctuation field, are likely to be overpredicted by at least a factor of 50%. We test the sensitivity of our results to force accuracy, starting redshift, and halo finding algorithm.

The Origin and Evolution of the Mass-Metallicity Relationship for Galaxies: Results from Cosmological N-Body Simulations

We examine the origin and evolution of the mass-metallicity relationship (MZR, M*-Z) for galaxies using high-resolution cosmological smoothed particle hydrodynamics (SPH) + N-body simulations that include a physically motivated description of supernova feedback and subsequent metal enrichment. We discriminate between two sources that may contribute to the origin of the MZR: (1) metal and baryon loss due to gas outflow or (2) inefficient star formation at the lowest galaxy masses. Our simulated galaxies reproduce the observed MZR in shape and normalization at both z = 0 and 2. We find that baryon loss occurs due to UV heating before star formation turns on in galaxies with Mbar < 108 M☉, but that some gas loss due to supernova-induced winds is required to subsequently reproduce the low effective chemical yield observed in low-mass galaxies. Despite this, we show that low star formation efficiencies, regulated by supernova feedback, are primarily responsible for the lower metallicities of low-mass galaxies and the overall M*-Z trend. We find that the shape of the MZR is relatively constant with redshift but that its normalization increases with time. Simulations with no energy feedback from supernovae overproduce metals at low galaxy masses by rapidly transforming a large fraction of their gas into stars. Despite the fact that our low-mass galaxies have lost a majority of their baryons, they are still the most gas-rich objects in our simulations due to their low star formation efficiencies.

Metal enrichment of the intergalactic medium at z=3 by galactic winds

Studies of quasar absorption lines reveal that the low-density intergalactic medium (IGM) at z ~ 3 is enriched to between 10-3 and 10-2 solar metallicity. This enrichment may have occurred in an early generation of Population III stars at redshift z 10, by protogalaxies at 6 z 10, or by larger galaxies at 3 z 6. This paper addresses the third possibility by calculating the enrichment of the IGM at z 3 by galaxies of baryonic mass 108.5 M☉. We use already completed cosmological simulations, to which we add a prescription for chemical evolution and metal ejection by winds, assuming that the winds have properties similar to those observed in local starbursts and Lyman break galaxies. Results are given for a number of representative models, and we also examine the properties of the galaxies responsible for the enrichment as well as the physical effects responsible for wind escape and propagation. We find that winds of velocity 200-300 km s-1 are capable of enriching the IGM to the mean level observed, although many low-density regions would remain metal free. Calibrated by observations of Lyman break galaxies, our calculations suggest that most galaxies at z 3 should drive winds that can escape and propagate to large radii. The primary effect limiting the enrichment of low-density intergalactic gas in our scenario is then the travel time from high- to low-density regions, implying that the metallicity of low-density gas is a strong function of redshift.

Scientific Computing in the Cloud

Large, virtualized pools of computational resources raise the possibility of a new, advantageous computing paradigm for scientific research. To help achieve this, new tools make the cloud platform behave virtually like a local homogeneous computer cluster, giving users access to high-performance clusters without requiring them to purchase or maintain sophisticated hardware.

Dark matter subhaloes in numerical simulations

We use cosmologicalCDM numerical simulations to model the evolution of the substructure population in sixteen dark matter haloes with resolutions of up to seven million particles within the virial radius. The combined substructure circular velocity distribution function (VDF) for hosts of 10 11 to 10 14 M⊙ at redshifts from zero to two or higher has a self-similar shape, is independent of host halo mass and redshift, and follows the relation dn/dv = (1/8)(vcmax/vcmax,host) −4 . Halo to halo variance in the VDF is a factor of roughly two to four. At high redshifts, we find preliminary evidence for fewer large substructure haloes (subhaloes). Specific angular momenta are significantly lower for subhaloes nearer the host halo centre where tidal stripping is more effective. The radial distribution of subhaloes is marginally consistent with the mass profile for r > � 0.3rvir, where the possibility of artificial numerical disruption of subhaloes can be most reliably excluded by our convergence study, although a subhalo distribution that is shallower than the mass profile is favoured. Subhalo masses but not circular velocities decrease toward the host centre. Subhalo velocity dispersions hint at a positive velocity bias at small radii. There is a weak bias toward more circular orbits at lower redshift, especially at small radii. We additionally model a cluster in several power law cosmologies of P / k n , and demonstrate that a steeper spectral index, n, results in significantly less substructure.

Simulations of Damped Lyα and Lyman Limit Absorbers in Different Cosmologies: Implications for Structure Formation at High Redshift

We use hydrodynamic cosmological simulations to study damped Ly� (DLA) and Lyman limit (LL) absorption at redshifts z = 2−4 in five variants of the cold dark matter scenario: COBE-normalized (CCDM), cluster-normalized (SCDM), and tilted (n = 0.8) m = 1 models; and open (OCDM) and flat (LCDM) m = 0.4 models. Our standard simulations resolve the formation of dense concentrations of neutral gas in halos with circular velocity vc ≥ vc,res ≈ 140 km s −1 for m = 1 and 90 km s −1 for m = 0.4, at z = 2; an additional LCDM simulation resolves halos down to vc,res ≈ 50 km s −1 at z = 3. We find a clear relation between HI column density and projected distance to the center of the nearest galaxy, with DLA absorption usually confined to galactocentric radii less than 10 − 15 kpc and LL absorption arising out to projected separations of 30 kpc or more. If we consider only absorption in the halos resolved by our standard simulations, then all five models fall short of reproducing the observed abundance of DLA and LL systems at these redshifts. To estimate the absorption from lower mass