Groups of galaxies in the SDSS Data Release 7

We apply a friends-of-friends algorithm to an enhanced SDSS DR12 spectroscopic catalog, including redshift from the literature to construct a catalog of 1588 compact groups of galaxies containing 5178 member galaxies and covering the redshift range 0.01 < z < 0.19. This catalog contains 18 times as many systems and reaches 3 times the depth of the similar catalog of Barton et al. We construct catalogs from both magnitude-limited and volume-limited galaxy samples. Like Barton et al. we omit the frequently applied isolation criterion in the compact group selection algorithm. Thus the groups selected by fixed projected spatial and rest-frame line-of-sight velocity separation produce a catalog of groups with a redshift-independent median size. In contrast to previous catalogs, the enhanced SDSS DR12 catalog (including galaxies with r < 14.5) includes many systems with z ≲ 0.05. The volume-limited samples are unique to this study. The compact group candidates in these samples have a median stellar mass independent of redshift. Groups with velocity dispersion ≲100 show abundant evidence for ongoing dynamical interactions among the members. The number density of the volume-limited catalogs agrees with previous catalogs at the lowest redshifts but decreases as the redshift increases. The SDSS fiber placement constraints limit the catalog’s completeness. In spite of this issue, the volume-limited catalogs provide a promising basis for detailed spatially resolved probes of the impact of galaxy–galaxy interactions within similar dense systems over a broad redshift range.

Flux-limited and volume-limited galaxy samples are constructed from the Sloan Digital Sky Survey (SDSS) data releases DR4, DR6 and DR7 for statistical analysis. The two-point correlation functions ξ(s), monopole of three-point correlation functions ζ0, projected two-point correlation function wp and pairwise velocity dispersion σ12 are measured to test if galaxy samples are fair for these statistics. We find that with the increment of sky coverage of subsequent data releases in SDSS, ξ(s) of the flux-limited sample is extremely robust and insensitive to local structures at low redshift. However, for volume-limited samples fainter than L* at large scales s ≳ 10 h−1 Mpc, the deviation of ξ(s) from different SDSS data releases (DR7, DR6 and DR4) increases with the increment of absolute magnitude. The case of ζ0(s) is similar to that of ξ(s). In the weakly nonlinear regime, there is no agreement between ζ0 of different data releases in all luminosity bins. Furthermore, wp of volume-limited samples of DR7 in luminosity bins fainter than −Mr,0.1 = [18.5, 19.5] are significantly larger and σ12 of the two faintest volume-limited samples of DR7 display a very different scale dependence than results from DR4 and DR6. Our findings call for caution in understanding clustering analysis results of SDSS faint galaxy samples and higher order statistics of SDSS volume-limited samples in the weakly nonlinear regime. The first zero-crossing points of ξ(s) from volume-limited samples are also investigated and discussed.

Using the Berlind et al. algorithm and the Davis et al. algorithm, we find that the mean velocity dispersion, virial radius, and virial mass of the group catalogs identified in the volume-limited sample are much smaller than those identified in the flux-limited sample. Our study shows that these properties of groups are heavily influenced by the relative values of the linking parameters and that there may be no values for the linking lengths that will work perfectly for every sample. In addition, we note that the luminosity distribution of member galaxies of groups identified by different algorithms is nearly the same, but member galaxies of groups identified using the linking lengths b⊥ = 0.14, b∥ = 0.75 have a higher proportion of blue galaxies and a lower proportion of red galaxies than member galaxies of groups identified using the linking length b = 0.2, and the early-type fraction of member galaxies of groups identified using the linking length b = 0.2 is higher than that of member galaxies of groups identified using the linking lengths b⊥ = 0.14, b∥ = 0.75.

Aims. We intend to compile a new galaxy group and cluster sample of the latest available SDSS data, adding several parameter for the purpose of studying the supercluster network, galaxy and group evolution, and their connection to the surrounding environment. Methods. We used a modified friends-of-friends (FoF) method with a variable linking length in the transverse and radial directions to eliminate selection effects and to find reliably as many groups as possible. Using the galaxies as a basis, we calculated the luminosity density field. Results. We create a new catalogue of groups and clusters for the SDSS data release 8 sample. We find and add environmental parameters to our catalogue, together with other galaxy parameters (e.g., morphology), missing from our previous catalogues. We take into account various selection effects caused by a magnitude limited galaxy sample. Our final sample contains 576493 galaxies and 77858 groups. The group catalogue is available at http://www.aai.ee/~elmo/dr8groups/ and from the Strasbourg Astronomical Data Center (CDS).

We present the results of our study of the peculiar motions for 41 galaxy clusters and groups in the regions of the Hercules and Leo superclusters (0.027 < z < 0.045). For this purpose, based on SDSS (Data Release 8) data, we compiled a sample of early-type galaxies in the investigated clusters and groups, constructed their fundamental planes (FPs), and determined independent distances and peculiar velocities. The Hubble law between the radial velocities galaxy clusters and the distances derived from the FPs holds for the rich Hercules supercluster as a whole. At the same time, however, significant peculiar motions along the line of sight with rms deviations of 736 ± 50 and 584 ± 50 km s−1 are observed within this supercluster and in its immediate neighborhood. In the poor Leo supercluster, the rms deviation is also large, 625 ± 70 km s−1. Its Hubble diagram exhibits the approach of galaxy clusters and groups along the line of sight relative to the most massive cluster A1185. In the immediate neighborhood of the Leo supercluster (virtually in its field), the rms deviation of the peculiar velocities is minimal, 287 ± 60 km s−1.

We use cosmic microwave background (CMB) temperature maps from the 500 deg2 SPTpol survey to measure the stacked lensing convergence of galaxy clusters from the Dark Energy Survey (DES) Year-3 redMaPPer (RM) cluster catalog. The lensing signal is extracted through a modified quadratic estimator designed to be unbiased by the thermal Sunyaev–Zel’dovich (tSZ) effect. The modified estimator uses a tSZ-free map, constructed from the SPTpol 95 and 150 GHz data sets, to estimate the background CMB gradient. For lensing reconstruction, we employ two versions of the RM catalog: a flux-limited sample containing 4003 clusters and a volume-limited sample with 1741 clusters. We detect lensing at a significance of 8.7σ(6.7σ) with the flux (volume)–limited sample. By modeling the reconstructed convergence using the Navarro–Frenk–White profile, we find the average lensing masses to be and for the volume- and flux-limited samples, respectively. The systematic error budget is much smaller than the statistical uncertainty and is dominated by the uncertainties in the RM cluster centroids. We use the volume-limited sample to calibrate the normalization of the mass-richness scaling relation, and find a result consistent with the galaxy weak-lensing measurements from DES.

We report the results of the spatial two-point correlation functioncc(r) for the new X- ray galaxy cluster survey REFLEX, which comprises of 452 X-ray selected clusters (449 with redshifts) detected by the ROSAT satellite during the ROSAT All-Sky-Survey (RASS). The REFLEX cluster sample is flux limited to 3 × 10 12 erg s 1 cm 2 in the ROSAT energy band (0.1 − 2.4 keV) and spans 3 decades in X-ray luminosity (10 42 −10 45 h 2 erg s 1 ), containing galaxy groups and rich clusters out to a redshift z ≤ 0.3. Covering a contiguous area of 4.24 sr REFLEX is the largest X-ray cluster sample to date for which spatial clustering has been analysed. Correlation studies using clusters selected on the basis of their X-ray emission are particularly interesting as they are largely free from the projection biases inherent to optical studies. For the entire flux-limited sample we find that the correlation length (the scale at which the correlation amplitude passes through unity) r0 ≃ 20h 1 Mpc. For example, if a power-law fit is made to �(r) over the range 4 − 40h 1 Mpc then r0 = 18.8 ± 0.9. An indication of the robustness of this result comes from the high degree of isotropy seen in the clustering pattern on scales close to the correlation length. On larger scalescc(r) deviates from a power-law, crossing zero at ≃ 45h 1 Mpc. From an examination of 5 volume-limited cluster sub-samples we find no significant trend of r0 with limiting X-ray luminosity. A comparison with recent model predictions for the clustering properties of X-ray flux-limited samples, indicates that Cold Dark Matter models with the matter density m = 1 fail to produce sufficient clustering to account for the data, while m ≃ 0.3 models provide an excellent fit.

Despite decades of study, it remains unclear whether there are distinct radio-loud and radio-quiet populations of quasi-stellar objects (QSOs). Early studies were limited by inhomogeneous QSO samples, inadequate sensitivity to probe the radio-quiet population, and degeneracy between redshift and luminosity for flux-density-limited samples. Our new 6 GHz EVLA observations allow us for the first time to obtain nearly complete (97%) radio detections in a volume-limited color-selected sample of 179 QSOs more luminous than M_i = -23 from the Sloan Digital Sky Survey (SDSS) Data Release Seven in the narrow redshift range 0.2 < z < 0.3. The dramatic improvement in radio continuum sensitivity made possible with the new EVLA allows us, in 35 minutes of integration, to detect sources as faint as 20 microJy, or log[L_6 (W/Hz)] ~ 21.5 at z = 0.25, well below the radio luminosity, log[L_6 (W/Hz)] ~ 22.5, that separates star-forming galaxies from radio-loud active galactic nuclei (AGNs) driven by accretion onto a super-massive black hole. We calculate the radio luminosity function (RLF) for these QSOs using three constraints: (a) EVLA 6 GHz observations for log[L_6 (W/Hz)] < 23.5, (b) NRAO-VLA Sky Survey (NVSS) observations for log[L_6 (W/Hz)] > 23.5, and (c) the total number of SDSS QSOs in our volume-limited sample. We show that the RLF can be explained as a superposition of two populations, dominated by AGNs at the bright end and star formation in the QSO host galaxies at the faint end.

Using two volume-limited Main galaxy samples of the Sloan Digital Sky Survey Data Release 8 above and below the value of M* r , we have explored some properties of AGNs. In the luminous volume-limited sample, 9.5% is composite galaxies (C) and 5.0% AGNs, while in the faint volume-limited sample, these fractions are 7.2% and 2.9%, respectively. The C and AGN fractions increase with luminosity. We also note that the fraction of star-forming galaxies decreases with luminosity, but the fraction of low signal-to-noise star-forming galaxies slightly increases with luminosity. In two volume-limited samples, the fraction of star-forming galaxies decreases with density, while the AGN fraction is nearly insensitive to the local environment. If composite galaxies (C) are classified as AGNs, then the fraction of AGNs in the luminous volume-limited sample declines with increasing local density. When focusing on some properties of AGN host galaxies, we find that galaxies hosting AGNs are preferentially more massive and have high concentration indices. In the faint volume-limited sample, AGN host galaxies preferentially have low star formation rates (SFRs) and specific SFRs (SSFRs). However, in the luminous volume-limited sample, AGN host galaxies preferentially have moderate SFRs and SSFRs.

We present results of the study of peculiar motions of 57 clusters and groups of galaxies in the regions of the Corona Borealis (CrB), Bootes (Boo), Z 5029/A 1424, A 1190, A 1750/A 1809 superclusters of galaxies and 20 galaxy clusters located beyond massive structures (0.05 z 0.10). Using the SDSS Data Release 8 data, a sample of early-type galaxies was compiled in the systems under study, their Fundamental Planes (FP) were built and relative distances and peculiar velocities were determined.

We present the first systematic 1.4 GHz Very Large Array radio continuum survey of fossil galaxy group candidates. These are virialized systems believed to have assembled over a gigayear in the past through the merging of galaxy group members into a single, isolated, massive elliptical galaxy and featuring an extended hot X-ray halo. We use new photometric and spectroscopic data from SDSS Data Release 7 to determine that three of the candidates are clearly not fossil groups. Of the remaining 30 candidates, 67% contain a radio-loud (L_1.4GHz > 10^23 W Hz^-1) active galactic nucleus (AGN) at the center of their dominant elliptical galaxy. We find a weak correlation between the radio luminosity of the AGN and the X-ray luminosity of the halo suggesting that the AGN contributes to energy deposition into the intragroup medium. We only find a correlation between the radio and optical luminosity of the central elliptical galaxy when we include X-ray selected, elliptically dominated non-fossil groups, indicating a weak relationship between AGN strength and the mass assembly history of the groups. The dominant elliptical galaxy of fossil groups is on average roughly an order of magnitude more luminous than normal group elliptical galaxies in optical, X-ray, and radio luminosities and our findings are consistent with previous results that the radio-loud fraction in elliptical galaxies is linked to the stellar mass of a population. The current level of activity in fossil groups suggests that AGN fueling continues long after the last major merger. We discuss several possibilities for fueling the AGN at the present epoch.

We characterize the spatial properties of the Sagittarius dwarf galaxy tidal debris, both primary and secondary (bifurcated) tidal tails, in the south Galactic cap. The Sagittarius dwarf galaxy is currently being ripped apart by tidal forces from the Milky Way galaxy. The spatial density of turnoff stars from the Sloan Digital Sky Survey Data Release 8 are fit using statistical photometric parallax with half a petaFLOPS of computing power from the MilkWay@home volunteer computing platform. The secondary tail appears to be significantly wider than the originally detected primary tail. These results are compared with the leading tidal tail stream density measured in the north Galactic cap. This research was funded by NSF grant AST 10-09670. ! SDSS DR 8 MSTO Stars in the south Galactic Cap The technique of Statistical Photometric Parallax (Newberg 2013) was developed to use the large data sets available from the Sloan Digital Sky Survey. In Newby et al 2013, the spatial density of Main Sequence Turn Off (MSTO) stars in the Sagittarius dwarf tidal stream was measured from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), using statistical photometric parallax. The publication of the SDSS Data Release 8 (SDSS DR8), extended the the coverage of the survey by 3000 square degrees, particularly in the south Galactic Cap (Aihara et al. 2011). ! To map the spatial density of halo stars in the south, we chose stars from the SDSS DR 8 with the selection criteria described in Cole 2008 and summarized in Newby et al 2013 as; galactic latitude b 0.4 . These criteria select stars low enough in latitude to be out of the galactic plane, This data set contained 1,690,858 MSTO stars, which were separated into files corresponding to the SDSS stripes. The stripes 79 through 86 were selected for analysis containing approximately 500,00 MSTO stars. ! ! ! ! ! Figure 1 A density plot of SDSS DR 8 MSTO stars in the south Galactic Cap plotted in Galactic l b coordinates. The Sagittarius Tidal debris can be seen in orange and red between l of 120° and 180°. ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Figure 2 A density plot of SDSS DR8 MSTO stars in stripe 82 with distance from the Sun in kpc plotted on the radial axis and SDSS Great Circle coordinate μ plotted as the polar angle. The SDDS coordinate ν which represents the 2.5° scanning width of the survey is flattened in this graph. ! Milkyway@Home Separation: MSTO stars were chosen to map the spatial structures of the halo, as they have an average absolute magnitude of 4.20 (Newberg and Yanny 2006). The data set for each stripe form a wedge (r, μ, ν). The smooth component of the halo is modeled using the Hernquist density profile with a flattening parameter q and scale length r0. The tidal streams are modeled as cylinders with a Gaussian density about the center. Each cylinder is fit to 6 parameters:the distance to the cylinder center from the Sun R, the position of the cylinder center in SDSS Great Circle coordinate μ, the width of the stream σ, the orientation of the cylinder relative to the galactic x and z coordinates (ɸ, θ) and the weight e that determines the number of stars. This is illustrated in the figure above from Newby 2013. ! Cole et al. in 2008 developed a maximum likelihood algorithm to determine the best fit parameters for each SDSS stripe. Each stripe needs to be fit to 8 to 20 parameters depending on the number of streams that are being fit, typically 1 to 3 streams. The large parameter space and the large number of stars require massive computing resources. MilkyWay@home uses the Berkeley Open Infrastructure for Network Computing (BOINC) to distribute the work amongst volunteers across the globe. This volunteer network is currently running at ~0.5 PetaFLOPS. ! We used MilkyWay@home to characterize the spatial properties for three streams across the dataset. Two streams were used for the primary and secondary tidal tails, while the third stream was used for “garbage collection.”

The primary objective of this work is to create an all sky extinction map of the Milky Way galaxy. We have cross-matched the Sloan Digital Sky Survey (SDSS data release 8) photometric observations with that of Galaxy Evolution Explorer (GALEX data release 6). This provides a wide range of wavelength coverage from Far Ultra-Violet through the optical spectrum and gives one unique SDSS source for every GALEX source. We discuss a sample of ∼32000 objects in the north galactic pole (≥75° latitude) from this combined database. The Castelli and Kurucz Atlas was fit to the photometric observations of each star, best fit being determined using a chi-square test. Best fit parameters provide the spectral type and extinction towards each of the objects. The shift in magnitude obtained during the best-fit can be used to determine the distance to each of the stars. With this data, a comprehensive extinction map can be made for the high-latitude objects and later extended to all-sky.

We constrain deviations from general relativity (GR) including both redshift and scale dependencies in the modified gravity (MG) parameters. In particular, we employ the under-used binning approach and compare the results to functional forms. We use available datasets such as Cosmic Microwave Background (CMB) from Planck 2018, Baryonic Acoustic Oscillations (BAO) and Redshift Space Distortions (BAO/RSD) from the BOSS Data Release 12, the 6DF Galaxy Survey, the SDSS Data Release 7 Main Galaxy Sample, the correlation of Lyman-α forest absorption and quasars from SDSS-DR14, Supernova Type Ia (SNe) from the Pantheon compilation, and DES Y1 data. Moreover, in order to maximize the constraining power from available datasets, we analyze MG models where we alternatively set some of the MG parameters to their GR values and vary the others. Using functional forms, we find an up to 3.5-σ tension with GR in Σ (while μ is fixed) when using Planck+SNe+BAO+BAO/RSD; this goes away when lensing data is included, i.e. CMB lensing and DES Y1 (CMBL+DES) . Using different binning methods, we find that a tension with respect to GR above 2-σ in the (high-z, high-k) bin is persistent even when including CMBL+DES to Planck+SNe+BAO+BAO/RSD . Also, we find another tension above 2-σ in the (low-z, high-k) bin, but that can be reduced with the addition of lensing data. Furthermore, we perform a model comparison using the Deviance Information Criterion statistical tool and find that the MG model (μ=1, Σ) is weakly favored by the data compared to ΛCDM, except when DES data is included. Another noteworthy result is that we find that the binning methods do not agree with the widely-used functional parameterization where the MG parameters are proportional to ΩDE(a), and this is clearly apparent in the high-z and high-k regime where this parameterization underestimates the deviations from GR.

We selected a sample of luminous infrared galaxies by cross-identification of the Faint Source Catalogue (FSC) and Point Source Catalogue (PSC) of the IRAS Survey with the Second Data Release of the SDSS. The size of our sample is 1267 for FSC and 427 for PSC by using the 2σ significance level cross-section. The ``likelihood ratio method is used to estimate the individual's reliability and for defining two more reliable subsamples (908 for FSC and 356 for PSC). A catalog of infrared, optical and radio data is compiled and will be used in further work. Some statistical results show that luminous infrared galaxies are quite different from ultra-luminous infrared galaxies. The AGN fractions of galaxies at different infrared luminosities and the radio–infrared correlations are consistent with the previous studies.