`Search for large-scale structure at high redshift'
We present new and exciting results on our search for large-scale structure at high redshift. Specifically, we have just completed a detailed analysis of the area surrounding the cluster cl0016+16 (z=0.546) and have the most compelling evidence yet that this cluster resides in the middle of a supercluster. The minimum dimension of this superstructure is tens of Mpc and it looks very similar - both in morphology and density - to the Great Wall in the CfA survey.
`Large- and Superlarge-Scale Structure in the LCRS and in N-body Simulations'
(A. Doroshkevich, D. Fong, H.Lin, and D.Tucker)
LCRS stands for Las Campanas Redshift Survey, for which we find, using 17,869 of the galaxies in the catalogue, two scales of Large-Scale Structure or galaxy filaments of 11 and 30 h-1 Mpc and a scale of 91 h-1 Mpc for sheet-like, i.e. Superlarge-Scale, structures, with as large a fraction as 60% of galaxies in these Superlarge-Scale structures, for which the overdensity is of order 10-20! Similar results are just this very moment also being found in an N-body simulation catalogue produced to mimic the possible results of the coming 250,000 galaxy redshift survey using the 2 degree facility now being tested on the Anglo-Australian Telescope.
`Statistics and dynamics of gravitational clustering'
Since the appearance of the classical paper of Lifshitz almost half a century ago, linear stability analysis of cosmological models is textbook knowledge. Until recently, however little was known about the behavior of higher than linear order terms in the perturbative expansion. These terms become important in the weakly nonlinear regime of gravitational clustering, when the rms mass density contrast is only slightly smaller than unity. In the past, theorists showed little interest in studying this regime, and for a good reason: only a decade ago, it would have been an academic excercise -- at scales, large enough to probe the weakly nonlinear regime, all measures of clustering were dominated by noise. This is no longer the case with present data. The purpose of this talk is to provide a brief summary of recent advances in weakly nonlinear perturbation theory. We present analytical perturbative results together with results of N-body experiments, conducted to test their accuracy. We compare perturbative predictions with measurements from galaxy surveys. Such comparisons can be used to test the gravitational instability theory and to constrain possible deviations from Gaussian statistics in the initial mass distribution; they can be also used to study the nature of physical processes that govern galaxy formation (``biasing''). We also show how future studies of velocity field statistics can provide a new way to determine the density parameter, $\Omega$. The opportunities to apply the analysis, presented here will be particularly favorable in new generation surveys.
`Higher Order Statistics from the EDSGC Survey'
Counts in cells are used to analyse the higher order properties of the statistics of the EDSGC survey. The probability distribution is obtained from an equal area projection source catalog with infinite oversampling over the range $0.015^\circ-2^\circ$. The factorial moments of the resulting distribution and the $s_N$'s characterizing the non-Gaussian nature of the distribution are extracted. These results are compared to previous results from the APM survey, and to theoretical results from perturbation theory. The deprojected 3D values corresponding to the $s_N$'s are also determined. We find that the 3D values match the scaling relation for strongly nonlinear clustering found in N-body simulations remarkably well, suggesting that the galaxies are reliable tracers of the underlying mass distribution. In particular, the predictions of extended perturbation theory (EPT) are in agreement with the EDSGC data.
`Cosmological Perturbations: Entering the Non-linear Regime'
I discuss new results concerning the evolution of statistical quantities such as the bispectrum and skewness due to gravitational instability from gaussian initial conditions using one-loop perturbation theory. Particular attention is paid to the transition from weakly non-linear scales to the non-linear regime at small scales. Comparison with fully non-linear numerical simulations is made to assess the regime of validity of the perturbative approach.
`Nonlinear gravitational clustering in cosmology'
The strongly nonlinear regime of gravitational clustering in cosmology is considered. The stable clustering hypothesis predicts the nonlinear autocorrelation function $\xi$, and suggests a form for the higher order correlations. Tests of its validity via N-body simulations show it to be valid on scales with $200 \simlt \xi \simlt 2000$. An analytical model, connecting $\xi$ with the density profiles and mass function of collapsed objects, is used to estimate the amplitude and spectrum dependence of $\xi$. Applications of these results, and connections to higher order statistics are outlined.
`Cosmic Shock Waves on Large Scales of the Universe'
In the standard theory of the large scale structure formation, matter accretes onto high density perturbations via gravitational instability. Collisionless dark matter forms caustics around such structures, while collisional baryonic matter forms accretion shocks which then halt and heat the infalling gas. Here, we study the characteristics, roles, and observational consequences of these accretion shocks encompassing the nonlinear structures such as supergalactic sheets, filaments, and clusters of galaxies. Firstly, we discuss the typical strength and scale of the shocks found in numerical simulations of various cosmological models. Secondly, we consider the possibility that the seeds of cosmological magnetic field are created by the Biermann battery mechanism at the shocks. Thirdly, we consider the possibility that the accretion shocks may serve as sites for the generation of ultra high energy cosmic rays above $10^{18}$ eV. Finally, we consider the detectability of the accreting upstream flows in the absorption systems of quasar emission lines.
`The Structure of a Cosmic String Wake'
The clustering of baryons and cold dark matter induced by a single moving string is analyzed numerically making use of a new three-dimensional Eulerian cosmological hydro code$^{1)}$ which is based on the PPM method to track the baryons and the PIC method to evolve the dark matter particles. A long straight string moving with a speed comparable to $c$ induces a planar overdensity (a``wake"). Since the initial perturbation is a velocity kick towards the plane behind the string and there is no initial Newtonian gravitational line source, the baryons are trapped in the center of the wake, leading to an enhanced baryon to dark matter ratio, leading to a possible resolution of the ``cluster baryon crisis". The cold coherent flow leads to very low post--shock temperatures of the baryonic fluid. In contrast, long strings with a lot of small-scale structure (which can be described by adding a Newtonian gravitational line source) move slowly and form filamentary objects. The large central pressure due to the gravitational potential causes the baryons to be expelled from the central regions and leads to a relative deficit in the baryon to dark matter ratio. In this case, the velocity of the baryons is larger, leading to high post-shock temperatures.
`First Structure Formation Cosmic Strings + HDM ?'
We use a 1-dim and 2-dim finite difference Eulerian cosmological hydrodynamics simulations to investigate the collapse of wakes of long straight and 'wiggly' cosmic strings in a flat universe dominated by Hot Dark Matter. The applied code solves the ideal non-visocus fluid equations, for the cosmological expansion, the non-equilibirum chemistry of the primordial gas with the following 9 species, H, H+, H-, He, He+, He++, H2+, and H2, and the optically thin cooling. Furthermore, it utilizes a logarithmic grid along the collapsing direction that allows to resolve the central 1.4 pc of a comoving 0.4 Mpc large box. We present a parameter study from 1-D simulations of the hydry-dynamics of strings that impart velocity boosts of 20, 15, 10, and 5 km/s at redshifts of 900, 100, and 50. In neither of these cases the wake aquires large overdensities. We find that even for very large inititial velocity boosts of ~20 km/s at z=900 from an infinite long straight string, its wake is not able to aquire higher overdensities than ~200 by redshift 5. For wiggly strings NO fragemtation due to a cooling instability is observed.
`Formation of Voids by Primordial Magnetic Fields'
The possible existence of a primordial magnetic field in the universe has been previously investigated in many articles. Studies involving the influence of a magnetic field in the nucleosyntesis era, studies considering the effects in the formation of structures during the radiation era and the matter era have been considered. We here assume the existence of a primordial magnetic field and study its effect, in particular, in the formation of voids. The study is twofold: to put constraints on the strength of the magnetic field during the recombination era and to preview its effects on the formation of voids.
` The influence of large scale perturbations of gravitational potential on the spatial distribution of galaxies and dark matter'
The influence of large scale perturbations of gravitational potential on the spatial distribution of galaxies and dark matter is examined. For appropriate parameters of the power spectrum, the cosmological model, and the mass of dark matter particles such perturbations produce the wall-like Super Large Scale Structure with parameters similar to the observed once. The bimodal character of the observed structure formed by superposition of filamentary and wall-like elements finds here a natural explanation. When appropriate reheating mechanism is included the same perturbations modulate the spatial distribution of the cold gas producing a large scale bias.
`Demonstrating discreteness and collision error in cosmological n-body simulations of dark matter gravitational clustering'
Two-body scattering is unimportant in cosmological gravitational clustering in most scenarios, as the dark matter has a small particle mass. The collective field should determine evolution: Two--body scattering in simulations violates the physics that is being modeled. We test for this, noting that a collisionless code will preserve the one--dimensional character of plane wave collapse. P$^3$M, the workhorse of most cosmological N--body simulations, fails miserably unless its softening parameter is so large it becomes a PM code -- which passes the test easily. This error calls into question all ``high resolution" results in which forces are resolved below the mean interparticle separation. Since dark matter usually dominates the gravitational field in cosmological hydrodynamic simulations, these too will be affected. Some approaches to solving the problem are suggested, mostly involving greater computer power, PM--based nested grid codes, and a more conservative approach to resolution claims. We test a nested--grid code which achieves high resolution without becoming collision--dominated.
` Simulations of galaxy clusters'
We use high resolution shock-capturing techniques in order to follow the fully nonlinear evolution of the hot gas filling the cluster core. We try to do a good estimate of the Sunyaev-Zel'dovich effect produced by rather realistic clusters and we compute their luminosities and other physical quantities.
`The Soft X-Ray background vs. distribution of galaxies'
I describe large scale (several degree, ~10 Mpc) correlations between the X-ray background as measured by ROSAT and distribution of galaxies with magnitudes between 10 and ~18.5.
`On the Generation of Vorticity during Large-Scale Structure Formation'
We explore the generation of vorticity during large-scale structure formation due to the violation of Kelvin theorem. Through a series of numerical simulations of gravitational instability and fragmentation of cosmological sheets (or pancakes) subject to symmetric perturbation modes, we show that significant vorticity can be generated behind curved, oblique, cosmological shocks. Vorticity is also generated in the postshock region due to a non-zero baroclinic term. Our simulations show that the first vortices to form have a spin angular momentum of a few $\times 10^{31}({\rm \lambda_{p,Mpc}\,cm^2s^{-1}})$ per unit mass where ${\rm \lambda_{p,Mpc}}$ is the pancake wavelength in Mpc. This is sufficient to account for the observational value of $10^{29}-10^{31} \,{\rm cm^2s^{-1}}$ in typical spiral galaxies. This mechanism of vorticity generation fits naturally within the gravitational instability model of structure formation in which the nonlinear growth of structure is accompanied by the formation of shocks and does not depend on a specific cosmological model (e.g. CDM, HDM or CHDM) for initial conditions.
`Possible Detection of the Hot Intergalactic Medium'
We have measured the extragalactic 0.7~keV X-ray background by observing the X-ray shadow of a neutral gas cloud in the Magellanic Bridge region. Two Rosat PSPC observations of total 104~ks were complemented by a detailed HI mapping of the cloud with both the Parkes 64~m telescope and the Australia Telescope Compact Array. >From the detected anti-correlation between the observed background intensity and the HI column density of the cloud, we derived the unabsorbed extragalactic background intensity as $\sim 28 {\rm~keV~s^{-1}~cm^{-2}~keV^{-1}} {\rm~sr^{-1}}$ at $\sim 0.7$~keV. The 95\% confidence lower limit $18 {\rm~keV~s^{-1}~cm^{-2}~keV^{-1}} {\rm~sr^{-1}}$ is greater than the expected point-like source contribution $\simlt 14 {\rm~keV~s^{-1}~cm^{-2}~keV^{-1}} {\rm~sr^{-1}}$, constrained by the mean source spectrum together with the total background intensity in the 1-2~keV band. A significant fraction of the 0.7~keV background likely arises in a diffuse hot intergalactic medium of a few million degrees, as has been predicted in hydrodynamic simulations of cosmological structure formation.
In addtion, we have detected a large-scale X-ray-emitting complex near the cluster Abell 2125. This complex extends about 10 Mpc, if at the same redshift as the cluster (z=0.25), including at least two smaller clusters. We speculate that much of the emission represents the hot intergalactic medium associated with a cosmic caustic.
`Structure In The Las Campanas Redshift Survey'
We cite a short history of the percolation parameters developed to detect structure in and characterize the topology of galaxy distributions. The application of this method to the Las Campanas Redshift Survey is explained and recent results are presented.
`Nonlinear Evolution of the Genus Statistics of the Large Scale Structure'
As a statistical measure to quantify the topological structure of large-scale structure in the universe, the genus number is calculated for a number of non-Gaussian distributions in which the density field is characterized by a nontrivial function of some Gaussian-distributed random numbers. As a specific example, the formulae for the lognormal and the chi-square distributions are given and compared with the results of N-body simulations together with the previously known formulae for the Gaussian distribution and second-order perturbation theory. It is shown that the lognormal formula fits most of the simulation data the best. It is also shown that using Poincare-Hopf theorem in differential geometry we can explicitly calculate genus statistics in the Zel'dovich approximation.
`Is the large peculiar motion of the Cen30 cluster real?'
In collaborationm with Marc Davis, Barry Madore, and Wendy Freedman, we have used HST to detect Cepheids in a spiral galaxy in the Cen30 cluster, one of those with a large peculiar motion which is not accounted for well by the density field derived from IRAS or optical galaxy distributions. Our preliminary result is in agreement with the Tully-Fisher distance, and thus a large peculiar motion. Although subsequent observations are scheduled which will refine our Cepheid distance determination, the immediate consequences that Tully-Fisher can not simply be dismissed and the longer term possibilities of determining Cepheid distances for a number of galaxies at ~3,000 km/s may both be of interest.