Hierarchical Clustering Scenario Research Articles

The Lyman-Alpha Forest in the Cold Dark Matter Model

Cosmological simulations with gas provide a detailed description of the intergalactic medium, which makes possible predictions of neutral hydrogen absorption in the spectra of background QSOs. We present results from a high-resolution calculation of an Ω = 1 cold dark matter model. Our simulation reproduces many of the observed properties of the Lyα forest surprisingly well. The distribution of H I column densities agrees with existing data to within a factor of ~2 over most of the range from 1014 cm-2 to 1022 cm-2; i.e., from unsaturated Lyα forest lines to damped Lyα systems. The equivalent width distribution matches the observed exponential form with a characteristic width W* ≈ 0.3 A. The distribution of b-parameters appears consistent with that derived from QSO spectra. Most of the low column density absorption arises in large, flattened structures of moderate or even relatively low overdensity, so there is no sharp distinction between the Lyα forest and the Gunn-Peterson absorption produced by the smooth intergalactic medium. Our results demonstrate that a Lyα forest like that observed develops naturally in a hierarchical clustering scenario with a photoionizing background. Comparison between simulations and high-resolution QSO spectra should open a new regime for testing theories of cosmic structure formation.

Large-scale structure formation for power spectra with broken scale invariance

We have simulated the formation of large-scale structure arising from COBE-normalized spectra computed by convolving a primordial double-inflation perturbation spectrum with the CDM transfer function. Due to the broken scale invariance ('BSI') characterizing the primordial perturbation spectrum, this model has less small-scale power than the (COBE-normalized) standard CDM model. The particle-mesh code (with $512^3$ cells and $256^3$ particles) includes a model for thermodynamic evolution of baryons in addition to the usual gravitational dynamics of dark matter. It provides an estimate of the local gas temperature. In particular, our galaxy-finding procedure seeks peaks in the distribution of gas that has cooled. It exploits the fact that ``cold" particles trace visible matter better than average and thus provides a natural biasing mechanism. The basic picture of large-scale structure formation in the BSI model is the familiar hierarchical clustering scenario. We obtain particle in cell statistics, the galaxy correlation function, the cluster abundance and the cluster-cluster correlation function and statistics for large and small scale velocity fields. We also report here on a semi-quantitative study of the distribution of gas in different temperature ranges. Based on confrontation with observations and comparison with standard CDM, we conclude that the BSI scenario could represent a promising modification of the CDM picture capable of describing many details of large-scale structure formation.

A search for 'failed clusters' of galaxies

We describe a search for a new type of object - large clouds of hot gas with no visible galaxies - which we call failed clusters of galaxies. We calculate the expected X-ray luminosity, temperature, and angular diameter of such objects as a function of total cloud mass and convert the results to expected X-ray fluxes from failed clusters at different redshifts. Using the Einstein Imaging Proportional Counter (IPC) database, we establish a strategy to search for candidate failed clusters. From this initial screening of 1435 IPC fields, 17 candidates are selected for more detailed analysis, which indicates that 10 of these are very probably extended X-ray sources. Optical follow-up on the 10 prime candidates finds eight clusters of galaxies (including six reproted for the first time in this paper), one stellar identification, and one without an obvious optical counterpart (the candidate with the weakest evidence for X-ray extent). Investigation of several candidates with less evidence for X-ray extent yields two additional new clusters of galaxies. A conservative comparison of our results with the Einstein Extended Medium Sensitivity Survey demonstrates that failed clusters are a relatively unimportant contributor to the mass density of the universe. Our inability to find failed clusters is consistent with the hierarchical clustering scenario for the formation of galaxies and clusters.

Subclustering and luminous-dark matter segregation in galaxy clusters

We have performed a series of N-body experiments on Connection Machine-5 in order to simulate the formation of galaxy clusters gravitationally dominated by a massive dark background. In accordance with previous authors we find an extremely inhomogeneous evolution where subcondensations are continually formed and merged. The final distribution of galaxies is more centrally condensed than that of dark matter particles. We have analyzed the origin of this galaxy-dark matter segregation and also the origin of subclustering leading to this effect. We have then analysed, analytically and numerically, the dependence of final segregation on the physical parameters characterizing the model of protocluster. We also find that such a segregation persists even when inelastic encounters of dark halos around galaxies are taken into account. We conclude that this effect cannot be in general avoided in any hierarchical clustering scenario.

Mergers of collisionless systems

The hierarchical clustering scenario is examined by merging haloes of collisionless particles under controlled conditions. Each simulation contains 16 000 particles, which gives a finer spatial resolution than in previous work. We find that dark matter haloes can grow homologously, obeying simple scaling laws. All the mergers tend towards a single density profile, which in our case closely resembles that given by de Vaucouleurs. The high resolution of our simulations also allows us to look at the resultant core structure of our haloes. We find that the central density is close to the maximum value allowed by phase-space constraints, indicating a smaller core radius in the dark matter than is generally assumed. The expected deviations from our results, given more realistic initial conditions, are discussed.

The formation and evolution of galaxies within merging dark matter haloes*

We construct semi-analytic models for galaxy formation within the framework of a hierarchical clustering scenario for structure formation in the Universe. We use the algorithm of Kauffmann & White to generate ensembles of merging histories for present-day dark matter haloes with a wide range of circular velocities. A galaxy is assumed to form from gas which cools and turns into stars at the centre of a halo until that halo merges with a more massive object. At this time the galaxy loses its source of new gas and becomes a non-dominant object within a larger group or cluster. Our methods thus enable us tolook insidepresent dark matter haloes and investigate the formation, evolution and merging of the galaxies that they contain

Cooling flows and cluster evolution

The X-ray emission from bimodal clusters is analyzed to determine whether or not there is evidence for cooling in the center of the clusters. All of the 14 clusters have long cooling times. When compared with the frequency of clusters with short cooling times in nearby rich clusters, this suggests a significant lack of cooling in the bimodal clusters. Tidal forces between the components of the bimodal clusters are insufficient to disrupt an existing cooling flow; apparently one has not yet formed. The clusters in the bimodal systems individually have luminosities comparable to nearby clusters and have relatively short merge times so that they represent an intermediate evolutionary stage in the formation of a luminous, rich cluster in a hierarchical clustering scenario

Higher order correlations of IRAS galaxies

view Abstract Citations (58) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Higher Order Correlations of IRAS Galaxies Meiksin, Avery ; Szapudi, Istvan ; Szalay, Alexander Abstract We derive the higher order irreducible angular correlation functions, up to the eight-point function, for a sample of 4654 IRAS galaxies, flux- limited at 1.2 Jy in the 60 micron band. The correlations are generally found to be somewhat weaker than those for the optically selected galaxies, consistent with the visual impression of looser clusters in the IRAS sample. We find that the N-point correlation functions can be expressed as the symmetric sum of products of N - 1 two-point functions, although the correlations above the four-point function are consistent with zero. The coefficients are consistent with the hierarchical clustering scenario as modeled by Hamilton and by Schaeffer, although for v~ -1, and with the predictions of secondary infall. Publication: The Astrophysical Journal Pub Date: July 1992 DOI: 10.1086/171561 Bibcode: 1992ApJ...394...87M Keywords: Computational Astrophysics; Galactic Clusters; Infrared Astronomy Satellite; Astronomical Spectroscopy; Autocorrelation; Correlation Coefficients; Galactic Structure; Astrophysics; GALAXIES: CLUSTERING; INFRARED: GALAXIES; METHODS: NUMERICAL full text sources ADS |

The efficiency of galaxy formation in clusters

Whether the observed trend in the efficiency of galaxy formation between poor and rich clusters is consistent with CDM-biased galaxy formation scenarios or with other hierarchical clustering scenarios described by a power-law fluctuation spectrum from galaxy to rich cluster scales in a universe with density parameter Omega = 1. A greater statistical correlation is found between galaxy and poor cluster masses than between galaxy and rich cluster masses due to the cluster proximity in filter length. This is the fundamental reason why galaxy formation is more efficient in poor clusters than in rich ones. It is found that the observed trend in the efficiency of galaxy formation and the observed values can be produced within the context of CDM-based galaxy formation scenarios or other scenarios involving flat spectra if X-ray luminous clusters are typically 2-3 sigma objects.

Abundance of rich clusters of galaxies - A test for cosmological parameters

The Press-Schechter formalism for deriving distribution functions for mass and X-ray temperature (or velocity dispersion) of rich clusters of galaxies in hierarchical clustering scenarios is extended to generaI cosmologies with arbitrary values of cosmological density parameter and cosmological constant. These distribution functions are shown to provide important diagnostics for both the fluctuation spectrum and for the fundamental cosmological parameters. The distribution functions for low-density CDM models fall less steeply of at the high end compared with the «standard» CDM model. This brings them closer to the observational results

The mass distribution of groups and clusters of galaxies

We reformulate the collapse theories for the mass distribution of cosmic structures-of which the theory of Press and Schechter constitutes the archetype in terms of a rate equation with finite collapse and survival times. We then derive the mass distribution and the optical luminosity function expected for groups and clusters of galaxies in these hierarchical clustering scenarios, considering both the collapse of high peaks in the initial overdensity field and the subsequent mass infall. We finally discuss the transition from direct collapse to merging and aggregation theories.

The origin of the mass, disk-to-halo mass ratio, and L-V relation of spiral galaxies

A model is presented in which spiral galaxies only form when t(c) is roughly equal to t(f) in a hot component of the protogalactic gas. This assumption, along with a disk stability criterion, predicts a range of spiral galaxy masses roughly consistent with observation. The nature of the cooling function for a primordial plasma implies that in less massive galaxies, more gas must fragment in the halo to preserve t(c) roughly equal to t(f). Consequently, less gas survives to form the disk, so that the disk-to-halo mass ratio increases with disk mass and hence galaxy luminosity. The canonical L proportional to V exp 4 relation can be reproduced by the model, and the apparent change in the slope of this relation also arises naturally. In the hierarchical clustering scenario, the model requires that all spirals formed at about the same epoch. These results support earlier claims that much of the dark matter observed in the universe is baryonic and probably formed during protogalactic collapse. 38 refs.

The radius-mass relation for clusters of galaxies - Cosmological scenarios versus observations

The relationship between the characteristic radii and masses (luminosities) of rich clusters of galaxies is studied using N-body simulations in comparison with observations. Various scenarios for the formation of the large-scale structure of the universe within the gravitational instability picture are considered, including hierarchical clustering, pancake, and hybrid scenarios. A well-defined radius-mass relation is found for the simulated clusters in each model, the slope of which depends on the form of the initial density fluctuation spectrum. The radius-luminosity relation is also studied for a sample of 29 Abell clusters, and a well-defined correlation is found to exist, R is proportional to L^0.51 +/= 0.07^. If M/L is roughly the same among clusters, and if the distributions of light and mass are similar, then best agreement with the observations is found for an initial spectrum with an effective slope -1 <= n <= 0 on the scale of protoclusters, as is expected in such scenarios as cold dark matter, isocurvature baryonic models, and certain hybrid scenarios. The pancake scenario, on the other hand, may have difficulty reproducing the observed radius-luminosity relation.

Radiation pressure in intergalactic clouds

The possible effects of radiation pressure by trapped photons in optically thick pregalactic or intergalactic clouds are examined. While the radiation pressure is negligible in protogalaxies which emerge from hierarchical clustering scenarios as in the cold dark matter scenario, it could suppress galaxy formation near QSOs in high-density pancakes. This could provide the local antibiasing mechanism necessary in the neutrino scenario. The possible role of radiation pressure in the intervening absorption systems which are observed along lines of sight to QSOs is discussed. Such pressure may be the crucial mechanism which distinguishes between the low neutral column density, metal-poor clouds and the high column density, metal-rich clouds. 27 refs.

Orientation of first-ranked galaxies in rich clusters

We present new evidence that first ranked galaxies are aligned with their parent cluster and with the direction of the nearest neighbour cluster (scale 15h −1 Mpc). The effect is stronger for cD and gE galaxies than for first-ranked galaxies of later type. The relevance of this result for different galaxy formation scenarios is discussed. In hierarchical clustering scenarios like the cold dark matter theory, galactic halos and clusters of galaxies are expected to have moderate asphericity. We present some numerical results of an on-going study of the dissipationless collapse of moderately aspherical systems. Our results indicate that the central part of the collapsed and virialized system does show the large scale elongation imposed by the initial conditions. It is pointed out that this may have important implications also for the properties of disk galaxies in dark halos.

Cosmological alignment of clusters of galaxies with their surroundings - A problem for cold dark matter?

view Abstract Citations (38) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cosmological Alignment of Clusters of Galaxies with Their Surroundings: A Problem for Cold Dark Matter? West, Michael J. ; Dekel, Avishai ; Oemler, Augustus, Jr. Abstract The alignment of clusters of galaxies with the surrounding galaxy distribution is found, using N-body simulations, to be a useful discriminant between certain cosmogonies within the gravitational instability picture. The observed large-scale alignments of the Shane-Wirtanen galaxy counts with Abell clusters places strong constraints on the form of the initial density fluctuation spectrum, requiring either a coherence length in the spectrum on a scale of a few tens of megaparsecs, as in pancake scenarios, or a flat (n <= -2) power spectrum on such large scales. The observed alignments are in conflict with hierarchical clustering scenarios such as the cold dark matter (CDM) model. This reflects a general deficiency of large-scale "filamentary" structure in CDM. Cluster ellipticities, contrary to alignments, seem to be rather insensitive to the cosmological initial conditions. Publication: The Astrophysical Journal Pub Date: January 1989 DOI: 10.1086/166991 Bibcode: 1989ApJ...336...46W Keywords: Cosmology; Dark Matter; Galactic Clusters; Galactic Evolution; Elliptical Galaxies; Power Spectra; Spatial Distribution; Astrophysics; COSMOLOGY; GALAXIES: CLUSTERING; GALAXIES: FORMATION full text sources ADS |

XVI. THE MASS MULTIPLICITY FUNCTION FROM NON-GAUSSIAN PRIMORDIAL PERTURBATIONS

ABSTRACT The mass multiplicity function of cosmic structures is considered for a general statistic of density perturbations in the framework of the hierarchical clustering scenario. In particular hierarchical and scale-invariant distributions are analyzed. The effect of the bias constraint on the mass function is also discussed.

Cold, warm, or hot dark matter - Biased galaxy formation and pancakes

view Abstract Citations (55) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cold, Warm, or Hot Dark Matter: Biased Galaxy Formation and Pancakes Schaeffer, Richard ; Silk, Joseph Abstract The distribution of galaxies, clusters, and pancakes that arises from the growth of primordial fluctuations is calculated. All current candidates for the gravitational mass of the universe are reviewed and classified according to the generic type of fluctuation spectrum. Particle candidates depend on the nature of the dark, weakly interacting matter which can be either "hot" (adiabatic fluctuations of massive neutrinos or baryons which dominate a low-{OMEGA} universe), "warm" (adiabatic fluctuations in a baryon-dominated universe with {OMEGA} = 1 or in a universe dominated by one of the new particles predicted by supersymmetric theories with a mass ~1 keV), or "cold" (axions or stable supersymmetric particles with mass >>1 keV, or isocurvature fluctuations in a universe dominated by baryons). We compare, for these various possibilities, a number of predictions with observations: the galaxy luminosity function, the cluster multiplicity function, and the large-scale velocity field as well as the galaxy correlations. We consider only galaxies that are primordial objects and form within a hierarchical clustering scenario, even if the initial spectrum is adiabatic and has the usual damping length. The formation of pancakes is associated with scales which are just about to turn nonlinear at the present epoch. In particular, we do not require galaxy formation to be associated with pancake collapse and fragmentation. The popular candidates for the dominant constituent of the universe can be expected to meet the requirements imposed by the various observations under conditions ranging from {OMEGA}h^0.55^> 0.3 and an average epoch of galaxy formation <z> ~ 5 if the fluctuation spectrum has power on all scales ("cold" dark matter) up to {OMEGA}h^0.55^> 0.7 and <z> ~ 1 if damping on galaxy scales was severe. This excludes only the adiabatic baryon scenario in a low-{OMEGA} universe, but allows massive neutrinos to produce a number of primordial galaxies that, although far too small to account for normal galaxies, could (marginally) account for the presence of quasars and rare, very luminous, galaxies at high redshift. Cold dark matter fluctuation spectra with no damping length at all may overproduce low-mass objects, whereas warm dark matter can provide a galaxy distribution that is very close to the observed galaxy counts. Publication: The Astrophysical Journal Pub Date: September 1988 DOI: 10.1086/166624 Bibcode: 1988ApJ...332....1S Keywords: Dark Matter; Density Distribution; Galactic Clusters; Galactic Evolution; Mass Distribution; Baryons; Fourier Transformation; Normal Density Functions; Red Shift; Astrophysics; EARLY UNIVERSE; GALAXIES: FORMATION; NEUTRINOS full text sources ADS |

The spatial correlation function of galaxies confronted with theoretical scenarios

view Abstract Citations (69) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The spatial correlation function of galaxies confronted with theoretical scenarios Dekel, A. ; Aarseth, S. J. Abstract The authors study the two-point spatial correlation function of galaxies, ξ(r), in a series of numerical simulations of cosmological clustering scenarios in comparison with ξ(r) obtained from redshift samples. The evolution of ξ(r) is evaluated in new large-scale 10000-body simulations of the pancake scenarios. The observed excess of correlation in the range 2 - 10 Mpc h-1, which does not agree with a pure hierarchical clustering scenario, is reproduced in a scenario where λ = 30±5 Mpc h-1 superclusters collapse first to flat (or elongated) pancakes. The shape of ξ(r) is reproduced provided that the first pancaking occurred at z ≤ 0.5 if Ω = 1 and z < 2 if Ω0 = 0.1. This result indicates that galaxy formation preceded supercluster formation. Constraints on the spectrum of density perturbations after decoupling are obtained and the origin of such a spectrum with respect to the particles that dominate the mass of the universe is considered. Publication: The Astrophysical Journal Pub Date: August 1984 DOI: 10.1086/162269 Bibcode: 1984ApJ...283....1D Keywords: Big Bang Cosmology; Correlation; Galactic Clusters; Spatial Distribution; Astronomical Models; Galactic Evolution; Orthogonality; Perturbation Theory; Shapes; Astrophysics full text sources ADS |