Galaxy Fluctuations Research Articles

Steps toward the Power Spectrum of Matter. II. The Biasing Correction with σ8Normalization

We suggest a new method to determine the bias parameter of galaxies relative to matter. The method is based on the assumption that gravity is the dominating force which determines the formation of the structure in the universe. Because of gravitational instability, matter flows out of underdense regions toward overdense regions. To form a galaxy, the density of matter within a certain radius must exceed a critical value (Press-Schechter limit); thus galaxy formation is a threshold process. In low-density environments (voids) galaxies do not form and matter remains in primordial form. We estimate the value of the threshold density which divides the matter into two populations, a low-density population in voids and a clustered population in high-density regions. We investigate the influence of the presence of these two populations on the power spectrum of matter and galaxies. We find that the power spectrum of clustered particles (galaxies) is similar to the power spectrum of matter. We show that the fraction of total matter in the clustered population determines the difference between amplitudes of fluctuations of matter and galaxies, i.e., the bias factor. To determine the fraction of matter in voids and clustered population we perform numerical simulations. The fraction of matter in galaxies at the present epoch is found using a calibration through the σ8 parameter. We find σ8 = 0.89 ± 0.09 for galaxies, σ8 = 0.68 ± 0.09 for matter, and bgal = 1.3 ± 0.13, the biasing factor of the clustered matter (galaxies) relative to all matter.

TheLX‐TRelation and Temperature Function for Nearby Clusters Revisited

The X-ray luminosity-temperature relation for nearby T 3.5-10 keV clusters is rederived using new ASCA temperatures and ROSAT luminosities. Both quantities are derived by directly excluding the cooling flow regions. This correction results in a greatly reduced scatter in the LX-T relation; cooling flow clusters are similar to others outside the small cooling flow regions. For a fit of the form Lbol ∝ Tα, we obtain α = 2.64 ± 0.27 (90%) and a residual rms scatter in log Lbol of 0.10. The derived relation can be directly compared to theoretical predictions that do not include radiative cooling. It also provides an accurate reference point for future evolution searches and comparison to cooler clusters. The new temperatures and LX-T relation together with a newly selected cluster sample are used to update the temperature function at z ~ 0.05. The resulting function is generally higher and flatter than, although within the errors of, the previous estimates by Edge and coworkers and Henry and Arnaud (as rederived by Eke and coworkers). For a qualitative estimate of constraints that the new data place on the density fluctuation spectrum, we apply the Press-Schechter formalism for Ω0 = 1 and 0.3. For Ω0 = 1, assuming cluster isothermality, the temperature function implies σ8 = 0.55 ± 0.03, while taking into account the observed cluster temperature profiles, σ8 = 0.51 ± 0.03, consistent with the previously derived range. The dependence of σ8 on Ω0 is different from the earlier results because of our treatment of the slope of the fluctuation spectrum, n, as a free parameter. For the considered values of Ω0, n = -(2.0-2.3) ± 0.3, somewhat steeper than that derived from the earlier temperature function data, in agreement with the local slope of the galaxy fluctuation spectrum from the Automatic Plate Measuring Facility (APM) survey, and significantly steeper than the standard cold dark matter prediction.

Power spectrum analysis of the Stromlo—APM redshift survey

We test estimators of the galaxy power spectrum $P(k)$ against simulated galaxy catalogues constructed from N-body simulations and we derive formulae to correct for biases. These estimators are then applied to compute the power spectrum of galaxies in the Stromlo-APM redshift survey. We test whether the amplitude of $P(k)$ depends on galaxy luminosity, but find no significant luminosity dependence except at absolute magnitudes brighter than $M_{\bj} = -20.3$, ($H_{0} = 100 \kms$) where there is some evidence for a rise in the amplitude of $P(k)$. By comparing the redshift space power spectrum of the Stromlo-APM survey with the real space power spectrum determined from the parent APM Galaxy Survey, we attempt to measure the distortion in the shape of $P(k)$ caused by galaxy peculiar motions. We find some evidence for an effect, but the errors are large and do not exclude a value of $\beta = \Omega^{0.6}/b = 1$, where $\Omega$ is the cosmological density parameter and $b$ is the linear biasing parameter relating galaxy fluctuations to those in the mass, $\left(\delta \rho/\rho\right)_{gal} = b \left(\delta \rho/\rho\right)_{m}$. The shape of the Stromlo-APM power spectrum is consistent with that determined from the CfA-2 survey, but has a slightly higher amplitude by a factor of about 1.4 than the power spectrum of IRAS galaxies.

Large-Scale Clustering from Non-Gaussian Texture Models

A basic ingredient for understanding large-scale fluctuations in our present-day universe is the initial conditions. Using N-body simulations, we compare the clustering that arises from Gaussian and non-Gaussian initial conditions. The latter is motivated by a global texture model which has initial J-order correlations J close to the strongly non-Gaussian regime J2J/2. The final amplitudes SJ ≡ J/2J − 1 in the non-Gaussian (texture) model evolve slowly toward the (Gaussian) gravitational predictions but, even at σ8 = 1, are still significantly larger, showing a characteristic minimum with a sharp increase in SJ with increasing scales. This minimum, which is between 10 and 15 h-1 Mpc, depending on the normalization, separates the regime where gravity starts dominating the evolution from the one in which the initial conditions are the dominant effect. In comparing this result with galaxy clustering observations, one has to take into account biasing, i.e., how galaxy fluctuations trace matter fluctuations. Although biasing could change the amplitudes, we show that the possible distortions to the shape of SJ are typically small. In contrast to the non-Gaussian (texture) predictions, we find no significant minimum or rise in SJ obtained from the APM Galaxy Survey.

Large-Scale Clustering from Non-Gaussian Texture Models

A basic ingredient to understand large scale fluctuations in our present day Universe is the initial conditions. Using N-body simulations, we compare the clustering that arises from Gaussian and non-Gaussian initial conditions. The latter is motivated by a global texture model which has initial $J$-order correlations $\xibar_J$ close to the strongly non-Gaussian regime $\xibar_J \simeq \xibar_2^{J/2}$. The final amplitudes $S_J \equiv \xibar_J/\xibar_2^{J-1}$ in the non-Gaussian (texture) model evolves slowly towards the (Gaussian) gravitational predictions but, even at $\sigma_8=1$, are still significantly larger, showing a characteristic minimum with a sharp increase in $S_J$ with increasing scales. This minimum, which is between $10$ and $15 \Mpc$, depending on the normalization, separates the regime where gravity starts dominating the evolution from the one in which the initial conditions are the dominant effect. In comparing this results with galaxy clustering observations, one has to take into account {\it biasing}, i.e. how galaxy fluctuations trace matter fluctuations. Although biasing could change the amplitudes, we show that the possible distortions to the shape of $S_J$ are typically small. In contrast to the non-Gaussian (texture) predictions, we find no significant minimum or rise in $S_J$ obtained from the APM Galaxy Survey.

High-order galaxy correlation functions in the APM Galaxy Survey

We estimate J-point galaxy averaged correlation functions $\wbar_J(\theta)$ for $J=2,...,9$, in a sample of the APM Galaxy Survey with more than $1.3 \times 10^6$ galaxies and a depth $ \calD \sim 400 \Mpc$. The hierarchical amplitudes $s_J=\wbar_J/\wbar_2^{J-1}$ are roughly constant, up to $J=9$, between $0.5 \Mpc$ and $2 \Mpc$ and decrease slowly for larger scales. At scales larger than $7 \Mpc$ we find strong similarities between the statistical properties of the galaxy fluctuations and the theoretical properties of matter fluctuations evolving under the influence of gravity in an expanding universe on assumption that the initial fluctuations are small and Gaussian. This is most easily explained if at large scales there is no significant biasing between matter and galaxy fluctuations. The comparison of the skewness in the CfA and SSRS catalogues with comparable sub-samples of the APM indicates that the volume of a ``fair sample'' has to be much larger that the one in the combined CfA/SSRS catalogues.

Statistical distribution of number-density fluctuations of galaxies

Statistical distribution of number-density fluctuations of galaxies

Galaxies and gas in a cold dark matter universe

We use a combined gravity/hydrodynamics code to simulate the formation of structure in a random 22 Mpc cube of a cold dark matter universe. Adiabatic compression and shocks heat much of the gas to temperatures of 10^6^-10^7^ K, but a fraction of the gas cools radiatively to about 10^4^ K and condenses into discrete, highly overdense lumps. We identify these lumps with galaxies. The high-mass end of their baryonic mass function fits the form of the observed galaxy luminosity function. They retain independent identities after their dark halos merge, so gravitational clustering produces groups of galaxies embedded in relatively smooth envelopes of hot gas and dark matter. The galaxy correlation function is approximately an r^-2.1^ power law from separations of 35 kpc to 7 Mpc. Galaxy fluctuations are biased relative to dark matter fluctuations by a factor b~ 1.5. We find no significant "velocity bias" between galaxies and dark matter particles. However, virial analysis of the simulation's richest group leads to an estimated {OMEGA}~0.3, even though the simulation adopts {OMEGA} = 1.

N-point correlation functions in the CfA and SSRS redshift distribution of galaxies

Using counts in cells, we estimate the volume-average N-point galaxy correlation functions for N = 2, 3, and 4, in redshift samples of the CfA and SSRS catalogs. Volume-limited samples of different sizes are used to study the uncertainties at different scales, the shot noise, and the problem with the boundaries. The hierarchical constants S3 and S4 agree well in all samples in CfA and SSRS, with average S3 = 194 +/- 0.07 and S4 = 4.56 +/- 0.53. We compare these results with estimates obtained from angular catalogs and recent analysis over IRAS samples. The amplitudes SJ seem larger in real space than in redshift space, although the values from the angular analysis correspond to smaller scales, where we might expect larger nonperturbative effects. It is also found that S3 and S4 are smaller for IRAS than for optical galaxies. This, together with the fact that IRAS galaxies have smaller amplitude for the above correlation functions, indicates that the density fluctuations of IRAS galaxies cannot be simply proportional to the density fluctuations of optical galaxies, i.e., biasing has to be nonlinear between them.

Statistics of the cosmic Mach number from numerical simulations of a cold dark matter universe

view Abstract Citations (126) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Statistics of the Cosmic Mach Number from Numerical Simulations of a Cold Dark Matter Universe Suto, Yasushi ; Cen, Renyue ; Ostriker, Jeremiah P. Abstract Results are presented of an analysis of the cosmic Mach number, M, the ratio of the streaming velocity, v, to the random velocity dispersion, sigma, of galaxies in a given patch of the universe, which was performed on the basis of hydrodynamical simulations of the cold dark matter scenario. Galaxy formation is modeled by application of detailed physical processes rather than by the ad hoc assumption of 'bias' between dark matter and galaxy fluctuations. The correlation between M and sigma is found to be very weak for both components. No evidence is found for a physical 'velocity bias' in the quantities which appear in the definition of M. Standard cold-dark-matter-dominated universes are in conflict, at a statistically significant level, with the available observation, in that they predict a Mach number considerably lower than is observed. Publication: The Astrophysical Journal Pub Date: August 1992 DOI: 10.1086/171626 Bibcode: 1992ApJ...395....1S Keywords: Dark Matter; Digital Simulation; Galactic Evolution; Mach Number; Statistics; Computational Astrophysics; Velocity Distribution; Astrophysics; COSMOLOGY: THEORY; COSMOLOGY: DARK MATTER; HYDRODYNAMICS; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE full text sources ADS |

Dipole Anisotropy in the Lick galaxy catalogue

The Shane & Wirtanen (SW) galaxy catalog, as reduced by Seldner et al (SSGP), is used to calculate the dipole vector of the galaxy distribution. The catalogue covers 86% of the North and 53% of the South Galactic cap (totally 8.8 steradians) and contains about 810,000 galaxies binned in 10′ × 10′ cells with magnitude limit mB ∼ 18.8. Dipoles have been found in the IRAS and in an optical catalogue based on the ESO, UGC and MCG catalogues, with average depths of ∼ 100 h−1 and ∼ 50 h−1 Mpc respectively. The direction of these dipoles is consistent with that of the microwave background dipole which means that the structures responsible for the dipole are present within the limits of the shallower catalogue and dominate the large-scale morpology of the galaxy distribution in both catalogues. It can therefore be expected that these structures will be ‘washed out’ by more distant structures dominating the deeper SW catalogue. The characteristic depth of the SW catalogue is 360 h−1 and the median depth of a cluster sample, identified from the SW catalogue by an objective proccedure, is ∼ 180 h−1 Mpc. Even if a dipole is found there is no apparent reason for it to point towards the MWB dipole direction since other galaxy fluctuations, comparable in size with those responsible for the MWB dipole, should be present in the SW catalogue if the Cosmological Principle is relevant on scales traced by the catalogue.