Power law correlations in galaxy distribution and finite volume effects from the Sloan Digital Sky Survey Data Release Four

We discuss the estimation of galaxy correlation properties in several volume limited samples, in different sky regions, obtained from the Fourth Data Release of the Sloan Digital Sky Survey. The small scale properties are characterized through the determination of the nearest neighbor probability distribution. By using a very conservative statistical analysis, in the range of scales [0.5,~30] Mpc/h we detect power-law correlations in the conditional density in redshift space, with an exponent \gamma=1.0 \pm 0.1. This behavior is stable in all different samples we considered thus it does not depend on galaxy luminosity. In the range of scales [~30,~100] Mpc/h we find evidences for systematic unaveraged fluctuations and we discuss in detail the problems induced by finite volume effects on the determination of the conditional density. We conclude that in such range of scales there is an evidence for a smaller power-law index of the conditional density. However we cannot distinguish between two possibilities: (i) that a crossover to homogeneity (corresponding to \gamma=0 in the conditional density) occurs before 100 Mpc/h, (ii) that correlations extend to scales of order 100 Mpc/h (with a smaller exponent 0 < \gamma <1). We emphasize that galaxy distributions in these samples present large fluctuations at the largest scales probed, corresponding to the presence of large scale structures extending up to the boundaries of the present survey. Finally we discuss several differences between the behavior of the conditional density in mock galaxy catalogs built from cosmological N-body simulations and real data. We discuss some theoretical implications of such a fact considering also the super-homogeneous features of primordial density fields.Comment: 13 pages, 19 figures, to be publsihed in Astronomy and Astrophysic

Observation of a Transition from BCS to HTSC-like Superconductivity in Ba_{1-x}K_xBiO_3 Single Crystals

A study of temperature dependences of the upper critical field B_{c2}(T) and surface impedance Z(T)=R(T)+iX(T) in Ba_{1-x}K_xBiO_3 single crystals that have transition temperatures in the range 6 x>0.4) reveals a transition from BCS to unusual type of superconductivity. B_{c2}(T) curves corresponding to the crystals that have T_c>20 K have positive curvature (like in some HTSC), and those of the crystals with T_c<15 K fall on the usual Werthamer-Helfand-Hohenberg curve. R(T) and X(T) dependences of the crystals with T_c~30 K and T_c~11 K are respectively linear (like in HTSC) and exponential (BCS) in the temperature range T << T_c. The experimental results are discussed in connection with the extended saddle point model by Abrikosov.Comment: 5 pages, 5 figure

UGC 7388: a galaxy with two tidal loops

We present the results of spectroscopic and morphological studies of the galaxy UGC7388 with the 8.1-m Gemini North telescope. Judging by its observed characteristics, UGC7388 is a giant late-type spiral galaxy seen almost edge-on. The main body of the galaxy is surrounded by two faint (\mu(B) ~ 24 and \mu(B) ~ 25.5) extended (~20-30 kpc) loop-like structures. A large-scale rotation of the brighter loop about the main galaxy has been detected. We discuss the assumption that the tidal disruption of a relatively massive companion is observed in the case of UGC7388. A detailed study and modeling of the observed structure of this unique galaxy can give important information about the influence of the absorption of massive companions on the galactic disks and about the structure of the dark halo around UGC7388.Comment: 8 pages, 5 figure

Breaking the self-averaging properties of spatial galaxy fluctuations in the Sloan Digital Sky Survey - Data Release Six

Statistical analyses of finite sample distributions usually assume that fluctuations are self-averaging, i.e. that they are statistically similar in different regions of the given sample volume. By using the scale-length method, we test whether this assumption is satisfied in several samples of the Sloan Digital Sky Survey Data Release Six. We find that the probability density function (PDF) of conditional fluctuations, filtered on large enough spatial scales (i.e., r>30 Mpc/h), shows relevant systematic variations in different sub-volumes of the survey. Instead for scales r<30 Mpc/h the PDF is statistically stable, and its first moment presents scaling behavior with a negative exponent around one. Thus while up to 30 Mpc/h galaxy structures have well-defined power-law correlations, on larger scales it is not possible to consider whole sample average quantities as meaningful and useful statistical descriptors. This situation is due to the fact that galaxy structures correspond to density fluctuations which are too large in amplitude and too extended in space to be self-averaging on such large scales inside the sample volumes: galaxy distribution is inhomogeneous up to the largest scales, i.e. r ~ 100 Mpc/h, probed by the SDSS samples. We show that cosmological corrections, as K-corrections and standard evolutionary corrections, do not qualitatively change the relevant behaviors. Finally we show that the large amplitude galaxy fluctuations observed in the SDSS samples are at odds with the predictions of the standard LCDM model of structure formation.(Abridged version).Comment: 32 pages, 28 figures, accepted for publication in Astronomy and Astrophysics. A higher resolution version is available at http://pil.phys.uniroma1.it/~sylos/fsl_highlights.html . Version v2 has been corrected to match the published on

A List of Galaxies for Gravitational Wave Searches

We present a list of galaxies within 100 Mpc, which we call the Gravitational Wave Galaxy Catalogue (GWGC), that is currently being used in follow-up searches of electromagnetic counterparts from gravitational wave searches. Due to the time constraints of rapid follow-up, a locally available catalogue of reduced, homogenized data is required. To achieve this we used four existing catalogues: an updated version of the Tully Nearby Galaxy Catalog, the Catalog of Neighboring Galaxies, the V8k catalogue and HyperLEDA. The GWGC contains information on sky position, distance, blue magnitude, major and minor diameters, position angle, and galaxy type for 53,255 galaxies. Errors on these quantities are either taken directly from the literature or estimated based on our understanding of the uncertainties associated with the measurement method. By using the PGC numbering system developed for HyperLEDA, the catalogue has a reduced level of degeneracies compared to catalogues with a similar purpose and is easily updated. We also include 150 Milky Way globular clusters. Finally, we compare the GWGC to previously used catalogues, and find the GWGC to be more complete within 100 Mpc due to our use of more up-to-date input catalogues and the fact that we have not made a blue luminosity cut.Comment: Accepted for publication in Classical and Quantum Gravity, 13 pages, 7 figure

Galaxy distribution and extreme value statistics

We consider the conditional galaxy density around each galaxy, and study its fluctuations in the newest samples of the Sloan Digital Sky Survey Data Release 7. Over a large range of scales, both the average conditional density and its variance show a nontrivial scaling behavior, which resembles to criticality. The density depends, for 10 < r < 80 Mpc/h, only weakly (logarithmically) on the system size. Correspondingly, we find that the density fluctuations follow the Gumbel distribution of extreme value statistics. This distribution is clearly distinguishable from a Gaussian distribution, which would arise for a homogeneous spatial galaxy configuration. We also point out similarities between the galaxy distribution and critical systems of statistical physics

The Sloan Great Wall. Morphology and galaxy content

We present the results of the study of the morphology and galaxy content of the Sloan Great Wall (SGW). We use the luminosity density field to determine superclusters in the SGW, and the fourth Minkowski functional V_3 and the morphological signature (the K_1-K_2 shapefinders curve) to show the different morphologies of the SGW, from a single filament to a multibranching, clumpy planar system. The richest supercluster in the SGW, SCl~126 and especially its core resemble a very rich filament, while another rich supercluster in the SGW, SCl~111, resembles a "multispider" - an assembly of high density regions connected by chains of galaxies. Using Minkowski functionals we study the substructure of individual galaxy populations determined by their color in these superclusters. We assess the statistical significance of the results with the halo model and smoothed bootstrap. We study the galaxy content and the properties of groups of galaxies in two richest superclusters of the SGW, paying special attention to bright red galaxies (BRGs) and to the first ranked galaxies in SGW groups. About 1/3 of BRGs are spirals. The scatter of colors of elliptical BRGs is smaller than that of spiral BRGs. About half of BRGs and of first ranked galaxies in groups have large peculiar velocities. Groups with elliptical BRGs as their first ranked galaxies populate superclusters more uniformly than the groups, which have a spiral BRG as its first ranked galaxy. The galaxy and group content of the core of the supercluster SCl~126 shows several differences in comparison with the outskirts of this supercluster and with the supercluster SCl~111. Our results suggest that the formation history and evolution of individual neighbour superclusters in the SGW has been different.Comment: Comments: 26 pages, 20 figures, accepted for publication in Ap

New Approaches To Photometric Redshift Prediction Via Gaussian Process Regression In The Sloan Digital Sky Survey

Expanding upon the work of Way and Srivastava 2006 we demonstrate how the use of training sets of comparable size continue to make Gaussian process regression (GPR) a competitive approach to that of neural networks and other least-squares fitting methods. This is possible via new large size matrix inversion techniques developed for Gaussian processes (GPs) that do not require that the kernel matrix be sparse. This development, combined with a neural-network kernel function appears to give superior results for this problem. Our best fit results for the Sloan Digital Sky Survey (SDSS) Main Galaxy Sample using u,g,r,i,z filters gives an rms error of 0.0201 while our results for the same filters in the luminous red galaxy sample yield 0.0220. We also demonstrate that there appears to be a minimum number of training-set galaxies needed to obtain the optimal fit when using our GPR rank-reduction methods. We find that morphological information included with many photometric surveys appears, for the most part, to make the photometric redshift evaluation slightly worse rather than better. This would indicate that most morphological information simply adds noise from the GP point of view in the data used herein. In addition, we show that cross-match catalog results involving combinations of the Two Micron All Sky Survey, SDSS, and Galaxy Evolution Explorer have to be evaluated in the context of the resulting cross-match magnitude and redshift distribution. Otherwise one may be misled into overly optimistic conclusions.Comment: 32 pages, ApJ in Press, 2 new figures, 1 new table of comparison methods, updated discussion, references and typos to reflect version in Pres

Identifying dynamically young galaxy groups via wide-angle tail galaxies: A case study in the COSMOS field at z=0.53

We present an analysis of a wide-angle tail (WAT) radio galaxy located in a galaxy group in the COSMOS field at a redshift of z=0.53 (hereafter CWAT-02). We find that the host galaxy of CWAT-02 is the brightest galaxy in the group, although it does not coincide with the center of mass of the system. Estimating a) the velocity of CWAT-02, relative to the intra-cluster medium (ICM), and b) the line-of-sight peculiar velocity of CWAT-02's host galaxy, relative to the average velocity of the group, we find that both values are higher than those expected for a dominant galaxy in a relaxed system. This suggests that CWAT-02's host group is dynamically young and likely in the process of an ongoing group merger. Our results are consistent with previous findings showing that the presence of a wide-angle tail galaxy in a galaxy group or cluster can be used as an indicator of dynamically young non-relaxed systems. Taking the unrelaxed state of CWAT-02's host group into account, we discuss the impact of radio-AGN heating from CWAT-02 onto its environment, in the context of the missing baryon problem in galaxy groups. Our analysis strengthens recent results suggesting that radio-AGN heating may be powerful enough to expel baryons from galaxy groups.Comment: 8 pages, 6 figures, 1 table. Accepted for publication in Ap