The Scaling of the Redshift Power Spectrum: Observations from the Las Campanas Redshift Survey

In a recent paper we have studied the redshift power spectrum $P^S(k,\mu)$ in three CDM models with the help of high resolution simulations. Here we apply the method to the largest available redshift survey, the Las Campanas Redshift Survey (LCRS). The basic model is to express $P^S(k,\mu)$ as a product of three factors P^S(k,\mu)=P^R(k)(1+\beta\mu^2)^2 D(k,\mu). Here $\mu$ is the cosine of the angle between the wave vector and the line of sight. The damping function $D$ for the range of scales accessible to an accurate analysis of the LCRS is well approximated by the Lorentz factor D=[1+{1\over 2}(k\mu\sigma_{12})^2]^{-1}. We have investigated different values for $\beta$ ($\beta=0.4$, 0.5, 0.6), and measured $P^R(k)$ and $\sigma_{12}(k)$ from $P^S(k,\mu)$ for different values of $\mu$. The velocity dispersion $\sigma_{12}(k)$ is nearly a constant from $k=0.5$ to 3 \mpci. The average value for this range is 510\pm 70 \kms. The power spectrum $P^R(k)$ decreases with $k$ approximately with $k^{-1.7}$ for $k$ between 0.1 and 4 \mpci. The statistical significance of the results, and the error bars, are found with the help of mock samples constructed from a large set of high resolution simulations. A flat, low-density ($\Omega_0=0.2$) CDM model can give a good fit to the data, if a scale-dependent special bias scheme is used which we have called the cluster-under-weighted bias (Jing et al.).Comment: accepted for publication in MNRAS, 20 pages with 7 figure

Low Redshift QSO Lyman alpha Absorption Line Systems Associated with Galaxies

In this paper we present Monte-Carlo simulations of Lyman alpha absorption systems which originate in galactic haloes, galaxy discs and dark matter (DM) satellites around big central haloes. It is found that for strong Lyman alpha absorption lines galactic haloes and satellites can explain ~20% and 40% of the line number density of QSO absorption line key project respectively. If big galaxies indeed possess such large numbers of DM satellites and they possess gas, these satellites may play an important role for strong Lyman alpha lines. However the predicted number density of Lyman-limit systems by satellites is \~0.1 (per unit redshift), which is four times smaller than that by halo clouds. Including galactic haloes, satellites and HI discs of spirals, the predicted number density of strong lines can be as much as 60% of the HST result. The models can also predict all of the observed Lyman-limit systems. The average covering factor within 250 kpc/h is estimated to be ~0.36. And the effective absorption radius of a galaxy is estimated to be ~150 kpc/h. The models predict W_r propto rho^{-0.5} L_B^{0.15} (1+z)^{-0.5}. We study the selection effects of selection criteria similar to the imaging and spectroscopic surveys. We simulate mock observations through known QSO lines-of-sight and find that selection effects can statistically tighten the dependence of line width on projected distance. (abridged)Comment: 23 pages, 9 postscript figures; references updated, minor change in section

Thermal Diagnostics with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory: A Validated Method for Differential Emission Measure Inversions

We present a new method for performing differential emission measure (DEM) inversions on narrow-band EUV images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). The method yields positive definite DEM solutions by solving a linear program. This method has been validated against a diverse set of thermal models of varying complexity and realism. These include (1) idealized gaussian DEM distributions, (2) 3D models of NOAA Active Region 11158 comprising quasi-steady loop atmospheres in a non-linear force-free field, and (3) thermodynamic models from a fully-compressible, 3D MHD simulation of AR corona formation following magnetic flux emergence. We then present results from the application of the method to AIA observations of Active Region 11158, comparing the region's thermal structure on two successive solar rotations. Additionally, we show how the DEM inversion method can be adapted to simultaneously invert AIA and XRT data, and how supplementing AIA data with the latter improves the inversion result. The speed of the method allows for routine production of DEM maps, thus facilitating science studies that require tracking of the thermal structure of the solar corona in time and space.Comment: 21 pages, 18 figures, accepted for publication in Ap

The dependence of the pairwise velocity dispersion on galaxy properties

(abridged) We present measurements of the pairwise velocity dispersion (PVD) for different classes of galaxies in the Sloan Digital Sky Survey. For a sample of about 200,000 galaxies, we study the dependence of the PVD on galaxy properties such as luminosity, stellar mass (M_*), colour (g-r), 4000A break strength (D4000), concentration index (C), and stellar surface mass density (\mu_*). The luminosity dependence of the PVD is in good agreement with the results of Jing & B\"orner (2004) for the 2dFGRS catalog. The value of \sigma_{12} measured at k=1 h/Mpc decreases as a function of increasing galaxy luminosity for galaxies fainter than L*, before increasing again for the most luminous galaxies in our sample. Each of the galaxy subsamples selected according to luminosity or stellar mass is divided into two further subsamples according to colour, D4000, C and \mu_*. We find that galaxies with redder colours and higher D4000, C, and \mu_* values have larger PVDs on all scales and at all luminosities/stellar masses. The dependence of the PVD on parameters related to recent star formation(colour, D4000) is stronger than on parameters related to galaxy structure (C, \mu_*), especially on small scales and for faint galaxies. The reddest galaxies and galaxies with high surface mass densities and intermediate concentrations have the highest pairwise peculiar velocities, i.e. these move in the strongest gravitational fields. We conclude that the faint red population located in rich clusters is responsible for the high PVD values that are measured for low-luminosity galaxies on small scales.Comment: 14 pages, 13 figures; reference updated and text slightly changed to match the published version; data of measurements of power spectrum and PVD available at http://www.mpa-garching.mpg.de/~leech/papers/clustering

Slow relative motion of IRAS galaxies at small separations: implications for galaxy formation models

We report on the measurement of the two-point correlation function and the pairwise peculiar velocity of galaxies in the IRAS PSCz survey. The real space two-point correlation function can be fitted to a power law $\xi(r) = (r_0/r)^{\gamma}$ with $\gamma=1.69$ and r_0=3.70 \mpc. The pairwise peculiar velocity dispersion $\sigma_{12}(r_p)$ is close to 400 \kms at r_p=3\mpc and decreases to about 150 \kms at r_p \approx 0.2 \mpc. These values are significantly lower than those obtained from the Las Campanas Redshift Survey, but agree very well with the results of blue galaxies reported by the SDSS team later on. We have constructed mock samples from N-body simulations with a cluster-weighted bias and from the theoretically constructed GIF catalog. We find that the two-point correlation function of the mock galaxies can be brought into agreemnt with the observed result, but the model does not reduce the velocity dispersions of galaxies to the level measured in the PSCz data. Thus we conclude that the peculiar velocity dispersions of the PSCz galaxies require a biasing model which substantially reduces the peculiar velocity dispersion on small scales relative to their spatial clustering. The results imply that either the cosmogony model needs to be revised or the velocity bias is important for the velocity dispersion of the IRAS galaxies.Comment: 4 pages, contribution to the proceedings "A New Era in Cosmology", ASP conference series, eds. T. Shanks and N. Metcalf