Sloan Digital Sky Survey DR7 Research Articles

ANDROMEDA XXIX: A NEW DWARF SPHEROIDAL GALAXY 200 kpc FROM ANDROMEDA

We report the discovery of a new dwarf galaxy, Andromeda XXIX, using data from the recently-released Sloan Digital Sky Survey DR8, and confirmed by Gemini North telescope Multi-Object Spectrograph imaging data. And XXIX appears to be a dwarf spheroidal galaxy, separated on the sky by a little more than 15 degrees from M31, with a distance inferred from the tip of the red giant branch of 730kpc+/-75kpc, corresponding to a three dimensional separation from M31 of between 205kpc and 227kpc (close to M31's virial radius). Its absolute magnitude, as determined by comparison to the red giant branch luminosity function of the Draco dwarf spheroidal, is M_V = -8.3+/-0.4. And XXIX's stellar populations appear very similar to Draco's; consequently, we estimate a metallicity for And XXIX of [Fe/H]\sim-1.8. The half-light radius of And XXIX is 360pc+/-60pc and its ellipticity is 0.35+/-0.06, typical of dwarf satellites of the Milky Way and M31 at this absolute magnitude range.

SPIRAL ARM MORPHOLOGY IN CLUSTER ENVIRONMENT

We examine the dependence of the morphology of spiral galaxies on the environment using the KIAS Value Added Galaxy Catalog (VAGC) which is derived from the Sloan Digital Sky Survey (SDSS) DR7. Our goal is to understand whether the local environment or global conditions dominate in determining the morphology of spiral galaxies. For the analysis, we conduct a morphological classiflcation of galaxies in 20 X-ray selected Abell clusters up to z»0.06, using SDSS color images and the X-ray data from the Northern ROSAT All-Sky (NORAS) catalog. We analyze the distribution of arm classes along the clustercentric radius as well as that of Hubble types. To segregate the efiect of local environment from the global environment, we compare the morphological distribution of galaxies in two X-lay luminosity groups, the low-Lx clusters (Lx 1:8 £ 10 44 erg/s). We flnd that the morphology-clustercentric relation prevails in the cluster environment although there is a brake near the cluster virial radius. The grand design arms comprise about 40% of the cluster spiral galaxies with a weak morphology-clustercentric radius relation for the arm classes, in the sense that ∞occulent galaxies tend to increase outward, regardless of the X-ray luminosity. From the cumulative radial distribution of cluster galaxies, we found that the low-Lx clusters are fully virialized while the high-Lx clusters are not.

Publicly available database for spectral line measurements of SDSS DR7 galaxies

AbstractWe present a new database of absorption and emission-line measurements based on the Sloan Digital Sky Survey (SDSS) 7th data release of galaxies within a redshift of 0.2. Using the publicly available penalized pixel-fitting (pPXF) and gas and absorption line fitting (gandalf) codes, our work improve the existing measurements for stellar kinematics, the strength of various absorption line features, and the flux and width of the emissions from different species of ionised gas. Most notable of our work is that, we provide quality of the fit to assess reliability of the measurements. The quality assessment can be highly effective for finding new classes of objects. For example, based on the quality assessment around the Ha and [NII] nebular lines, we found approximately 1% of the SDSS spectra which classified as galaxies by the SDSS pipeline are in fact type I Seyfert AGN. This paper presents a summary of the recent paper, Oh et al.(2011). The database is publicly available at http://gem.yonsei.ac.kr/ossy/.

HIGH-RESOLUTION VERY LARGE ARRAY IMAGING OF SLOAN DIGITAL SKY SURVEY STRIPE 82 AT 1.4 GHz

We present a high-resolution radio survey of the Sloan Digital Sky Survey (SDSS) Southern Equatorial Stripe, a.k.a. Stripe 82. This 1.4 GHz survey was conducted with the Very Large Array (VLA) primarily in the A-configuration, with supplemental B-configuration data to increase sensitivity to extended structure. The survey has an angular resolution of 1.8" and achieves a median rms noise of 52 microJy/bm over 92 deg^2. This is the deepest 1.4 GHz survey to achieve this large of an area, filling a gap in the phase space between small, deep and large, shallow surveys. It also serves as a pilot project for a larger high-resolution survey with the Expanded Very Large Array (EVLA). We discuss the technical design of the survey and details of the observations, and we outline our method for data reduction. We present a catalog of 17,969 isolated radio components, for an overall source density of ~195 sources/deg^2. The astrometric accuracy of the data is excellent, with an internal check utilizing multiply-observed sources yielding an rms scatter of 0.19" in both right ascension and declination. A comparison to the SDSS DR7 Quasar Catalog further confirms that the astrometry is well tied to the optical reference frame, with mean offsets of 0.02" +/- 0.01" in right ascension, and 0.01" +/- 0.02" in declination. A check of our photometry reveals a small, negative CLEAN-like bias on the level of 35 microJy. We report on the catalog completeness, finding that 97% of FIRST-detected quasars are recovered in the new Stripe 82 radio catalog, while faint, extended sources are more likely to be resolved out by the resolution bias. We conclude with a discussion of the optical counterparts to the catalog sources, including 76 newly-detected radio quasars. The full catalog as well as a search page and cutout server are available online at http://third.ucllnl.org/cgi-bin/stripe82cutout.

Galaxy morphology, luminosity, and environment in the SDSS DR7

We study the influence of the environment on the evolution of galaxies by investigating the luminosity function (LF) of galaxies of different morphological types and colours at different environmental density levels. We construct the LFs separately for galaxies of different morphology (spiral and elliptical) and of different colours (red and blue) using data from the Sloan Digital Sky Survey (SDSS), correcting the luminosities for the intrinsic absorption. We use the global luminosity density field to define different environments, and analyse the environmental dependence of galaxy morphology and colour. The smoothed bootstrap method is used to calculate confidence regions of the derived luminosity functions. We find a strong environmental dependency for the LF of elliptical galaxies. The LF of spiral galaxies is almost environment independent, suggesting that spiral galaxy formation mechanisms are similar in different environments. Absorption by the intrinsic dust influences the bright-end of the LF of spiral galaxies. After attenuation correction, the brightest spiral galaxies are still about 0.5 mag less luminous than the brightest elliptical galaxies, except in the least dense environment, where spiral galaxies dominate the LF at every luminosity. Despite the extent of the SDSS survey, the influence of single rich superclusters is present in the galactic LF of the densest environment.

NEAR-INFRARED PHOTOMETRIC PROPERTIES OF 130,000 QUASARS: AN SDSS-UKIDSS-MATCHED CATALOG

We present a catalog of over 130,000 quasar candidates with near-infrared (NIR) photometric properties, with an areal coverage of approximately 1200 deg2. This is achieved by matching the Sloan Digital Sky Survey (SDSS) in the optical ugriz bands to the UKIRT Infrared Digital Sky Survey (UKIDSS) Large Area Survey (LAS) in the NIR YJHK bands. We match the ≈1 million SDSS DR6 Photometric Quasar catalog to Data Release 3 of the UKIDSS LAS (ULAS) and produce a catalog with 130,827 objects with detections in one or more NIR bands, of which 74,351 objects have optical and K-band detections and 42,133 objects have the full nine-band photometry. The majority (∼85%) of the SDSS objects were not matched simply because these were not covered by the ULAS. The positional standard deviation of the SDSS Quasar to ULAS matches is δR.A. = 01370 and δdecl. = 01314. We find an absolute systematic astrometric offset between the SDSS Quasar catalog and the UKIDSS LAS, of |R.A.offset| = 0025 and |decl.offset| = 0040; we suggest the nature of this offset to be due to the matching of catalog, rather than image, level data. Our matched catalog has a surface density of ≈53 deg−2 for K ⩽ 18.27 objects; tests using our matched catalog, along with data from the UKIDSS Deep Extragalactic Survey, imply that our limiting magnitude is i ≈ 20.6. Color–redshift diagrams, for the optical and NIR, show a close agreement between our matched catalog and recent quasar color models at redshift z ≲ 2.0, while at higher redshifts, the models generally appear to be bluer than the mean observed quasar colors. The gJK and giK color spaces are used to examine methods of differentiating between stars and (mid-redshift) quasars, the key to currently ongoing quasar surveys. Finally, we report on the NIR photometric properties of high, z > 4.6, and very high, z > 5.7, redshift previously discovered quasars.

GALEX observations of quasar variability in the ultraviolet

Using archival observations recorded over a 5+ year timeframe with the NASA Galaxy Evolution Explorer (GALEX) satellite, we present a study of the ultraviolet (UV) variability of 4360 quasars of redshifts up to z=2.5 that have optical counterparts in the Sloan Digital Sky Survey DR5 spectroscopic catalog. The observed changes in both the far UV (FUV: 1350-1785A) and near UV (NUV: 1770-2830A) AB magnitudes as a function of time may help differentiate between models of the emission mechanisms thought to operate in these active galaxies. A list of NUV and FUV variable quasars was derived from the UV light-curves of sources with 5 or more observational visits by GALEX that spanned a time-frame greater than 3 months. By measuring the error in the derived mean UV magnitude from the series of GALEX observations for each source, quasars whose UV variability was greater than the 3-sigma variance from the mean observed value were deemed to be (intrinsically) UV variable. This conservative selection criterion (which was applied to both FUV and NUV observations) resulted in identifying 550 NUV and 371 FUV quasars as being statistically significant UV variable objects.

Study of Blue Compact Dwarf Galaxies at Redshift 0.2 ~ 0.5 Using SDSS DR7

We present a mass-metallicity (M–Z) relation of 412 blue compact dwarf galaxies at z = 0.2 ~ 0.5 using the Sloan Digital Sky Survey (SDSS) DR7 spectroscopic data. We found a variation of mass-metallicity relations depending on the redshift in which metallicity evolution of low mass galaxies appear to be stronger than that of high mass galaxies. The specific star formation rate (SSFR) shows tight correlation with galaxy mass in the sense that low mass galaxies have substantially high SSFR. We also discuss environmental effect in the SSFR vs galaxy mass relation. At a given mass, merging system shows systematically higher SSFR than isolated one.

The quasar mass-luminosity plane - II. High mass turn-off evolution and a synchronization puzzle

We use 62,185 quasars from the Sloan Digital Sky Survey DR5 sample and standard virial mass scaling laws based on the widths of H beta, Mg II, and C IV lines and adjacent continuum luminosities to explore the maximum mass of quasars as a function of redshift, which we find to be sharp and evolving. This evolution is in the sense that high-mass black holes cease their luminous accretion at higher redshift than lower-mass black holes. Further, turnoff for quasars at any given mass is more highly synchronized than would be expected given the dynamics of their host galaxies. We investigate potential signatures of the quasar turnoff mechanism, including a dearth of high-mass quasars at low Eddington ratio. These new results allow a closer examination of several common assumptions used in modeling quasar accretion and turnoff.

Fossil evidence for spin alignment of Sloan Digital Sky Survey galaxies in filaments

We search for and find fossil evidence that the distribution of the spin axes of galaxies in cosmic web filaments relative to their host filaments are not randomly distributed. This would indicate that the action of large scale tidal torques effected the alignments of galaxies located in cosmic filaments. To this end, we constructed a catalogue of clean filaments containing edge-on galaxies. We started by applying the Multiscale Morphology Filter (MMF) technique to the galaxies in a redshift-distortion corrected version of the Sloan Digital Sky Survey DR5. From that sample we extracted those 426 filaments that contained edge-on galaxies (b/a < 0.2). These filaments were then visually classified relative to a variety of quality criteria. Statistical analysis using "feature measures" indicates that the distribution of orientations of these edge-on galaxies relative to their parent filament deviate significantly from what would be expected on the basis of a random distribution of orientations. The interpretation of this result may not be immediately apparent, but it is easy to identify a population of 14 objects whose spin axes are aligned perpendicular to the spine of the parent filament (\cos \theta < 0.2). The candidate objects are found in relatively less dense filaments. This might be expected since galaxies in such locations suffer less interaction with surrounding galaxies, and consequently better preserve their tidally induced orientations relative to the parent filament. The technique of searching for fossil evidence of alignment yields relatively few candidate objects, but it does not suffer from the dilution effects inherent in correlation analysis of large samples.

Kinematic and chemical properties of five open clusters based on SDSS DR7

We present metallicities and radial velocities for five old open clusters (NGC 6791, NGC 2420, NGC 2682, NGC 2158, and NGC 7789) using data from the seventh public data release of the Sloan Digital Sky Survey (SDSS), which includes the directed stellar program SEGUE: Sloan Extension For Galactic Understanding and Exploration. The radial velocities are used to calculate cluster membership probabilities for stars in each cluster region. NGC 6791, NGC 2420, NGC 2682, NGC 2158 and NGC 7789 are found to have mean metallicities [Fe/H] = +0.08 ± 0.09, −0.38 ± 0.11, −0.08 ± 0.05, −0.41 ± 0.13 and −0.19 ± 0.13 dex (s.d.), respectively. The mean radial velocities for NGC 6791, NGC 2420, NGC 2682, NGC 2158 and NGC 7789 are Vr = −45.9 ± 0.2, +76.1 ± 0.2, +35.0 ± 0.2, +26.9 ± 0.2 and −48.2 ± 0.2 km s−1(s.e.m.), respectively. We have compared our results with the values from literatures, and found that our metallicity of NGC 6791 is significantly underestimated (by about 0.3 dex) and our radial velocities of the open clusters agree well with the values derived using high-resolution spectroscopy.

The scale of homogeneity of the galaxy distribution in SDSS DR6

Abstract The assumption that the Universe, on sufficiently large scales, is homogeneous and isotropic is crucial to our current understanding of cosmology. In this Letter, we test if the observed galaxy distribution is actually homogeneous on large scales. We have carried out a multifractal analysis of the galaxy distribution in a volume-limited subsample from the Sloan Digital Sky Survey (SDSS) Data Release 6. This considers the scaling properties of different moments of galaxy number counts in spheres of varying radius, r, centred on galaxies. This analysis gives the spectrum of generalized dimension Dq(r), where q &amp;gt; 0 quantifies the scaling properties in overdense regions and q &amp;lt; 0 in underdense regions. We expect Dq(r) = 3 for a homogeneous, random point distribution. In our analysis, we have determined Dq(r) in the range −4 ≤q≤ 4 and 7 ≤r≤ 98 h−1 Mpc. In addition to the SDSS data, we have analysed several random samples which are homogeneous by construction. Simulated galaxy samples generated from dark matter N-body simulations and the Millennium Run were also analysed. The SDSS data is considered to be homogeneous if the measured Dq is consistent with that of the random samples. We find that the galaxy distribution becomes homogeneous at a length-scale between 60 and 70 h−1 Mpc. The galaxy distribution, we find, is homogeneous at length-scales greater than 70 h−1 Mpc. This is consistent with earlier works which find the transition to homogeneity at around 70 h−1 Mpc.

Applying the Halo Model to Large Scale Structure Measurements of the Luminous Red Galaxies: SDSS DR7 Preliminary Results

The non-trivial relationship between observations of galaxy positions in redshift space and the underlying matter field complicates our ability to determine the linear theory power spectrum and extract cosmological information from galaxy surveys. The Sloan Digital Sky Survey (SDSS) Luminous Red Galaxy (LRG) catalog has the potential to place powerful constraints on cosmological parameters. LRGs are bright, highly biased tracers of large-scale structure. However, because they are highly biased, the non-linear contribution of satellite galaxies to the galaxy power spectrum is large and Fingers-of-God are significant. We propose an new approach to recovering the matter field from galaxy observations. Our approach is to use halos rather than galaxies to trace the underlying mass distribution. We identify Fingers-of-God (FOGs) and replace each FOG with a single halo object. This removes the nonlinear contribution of satellite galaxies, the one-halo term. We test our method on a large set of high-fidelity mock SDSS LRG catalogs and present consistency checks between the mock and LRG DR7 reconstructed halo density fields. We present preliminary cosmological constraints from the LRG DR7 reconstructed halo density field power spectrum. Finally, we summarize the potential gains in cosmological parameter constraints using our approach and the largest remaining sources of systematic errors.

Unknown selection effect simulates redshift periodicity in quasar number counts from Sloan Digital Sky Survey

Discrete Fourier analysis on the quasar number count, as a function of redshift, z, calculated from the Sloan Digital Sky Survey DR6 release appears to indicate that quasars have preferred periodic redshifts with redshift intervals of 0.258, 0.312, 0.44, 0.63, and 1.1. However the same periods are found in the mean of the zConf parameter used to flag the reliability of the spectroscopic measurements. It follows that these redshift periods must result from some selection effect, as yet undetermined. It does not signal any intrinsic (quantized) redshifts in the quasars in Sloan survey data. However this result does not rule out the possibility as found in earlier studies of other data.

Spectral-basedk-corrections and implications for the colour-magnitude relation of E/S0s and its evolution

We select a sample of 70 378 E/S0 (early-type) galaxies at 0 < z < 0.36 from the Sloan Digital Sky Survey (SDSS), excluding disc and star-forming galaxies. We estimate g and r magnitudes in the observer- and rest frames directly from the SDSS DR6 spectra; this provides an object-by-object estimate of the k-correction. Observer-frame colours can also be estimated from the imaging photometry. However, in this case, rest-frame colours, and hence k-corrections, must be inferred from fitting to data in other band passes as well as to stellar population synthesis models. There are small (a few 0.01 mag) discrepancies between the spectra and imaging photometry, particularly for galaxies with low signal-to-noise ratios in the spectra and more negative eclass spectral classifications. We correct for these, and then use the k-corrections from the spectra to study the evolution of the rest-frame colour–magnitude relation (CMR) and colour–σ relation (CσR). Both the CMR and CσR evolve bluewards with increasing redshift, approximately in agreement with passive evolution models with age ∼12 Gyr. The rate of evolution is sensitive to the k-corrections. Using k-corrections from the Coleman, Wu & Weedman (CWW) template spectrum, rather than the observed SDSS spectra, gives a CMR with much more evolution; k-corrections from Blanton & Roweis, which are based on fitting Bruzual & Charlot models with solar abundance ratios to the ugriz colours, give less/no evolution. However, the slope and zero-point of CMR depends on whether colours were defined in fixed physical or angular apertures, a consequence of the fact that the centres of these objects tend to be redder: the relation is steeper for fixed angular apertures. These colour gradients must be accounted for when estimating CMR evolution with fixed angular apertures. On the other hand, although the zero-point of the CσR depends on the aperture in which the colour was defined, the slope does not, suggesting that colour gradients are correlated with residuals from the σ–Mr relation. Since these residuals are age indicators, our findings suggest that colour gradients depend on the age of the stellar population.

Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from SDSS DR7

AbstractThe structure and kinematics of the recognized stellar components of the Milky Way are explored, based on well-determined atmospheric parameters and kinematic quantities for 32360 “calibration stars” from the Sloan Digital Sky Survey (SDSS) and its first extension, (SDSS-II), which included the sub-survey SEGUE: Sloan Extension for Galactic Understanding and Exploration. Full space motions for a sub-sample of 16920 stars, exploring a local volume within 4 kpc of the Sun, are used to derive velocity ellipsoids for the inner- and outer-halo components of the Galaxy, as well as for the canonical thick-disk and proposed metal-weak thick-disk populations. This new sample of calibration stars represents an increase of 60% relative to the numbers used in a previous analysis. A Maximum Likelihood analysis of a local sub-sample of 16920 calibration stars has been developed in order to extract kinematic information for the major Galactic components (thick disk, inner halo, and outer halo), as well as for the elusive metal-weak thick disk (MWTD). We measure velocity ellipsoids for the thick disk, the MWTD, the inner halo, and the outer halo, demonstrate that the MWTD may be a component that is kinematically and chemically independent of the canonical thick disk (and put limits on the metallicity range of the MWTD), and derive the inferred spatial density profiles of the inner/outer halo components. We also present evidence for tilts in the velocity ellipsoids for stars in our sample as a function of height above the plane, for several ranges in metallicity, and confirm the shift of the observed metallicity distribution function (MDF) from the inner-halo to the outer-halo dominated sample.

Cosmological constraints from the clustering of the Sloan Digital Sky Survey DR7 luminous red galaxies

We present the power spectrum of the reconstructed halo density field derived from a sample of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey Seventh Data Release (DR7). The halo power spectrum has a direct connection to the underlying dark matter power for k <= 0.2 h/Mpc, well into the quasi-linear regime. This enables us to use a factor of ~8 more modes in the cosmological analysis than an analysis with kmax = 0.1 h/Mpc, as was adopted in the SDSS team analysis of the DR4 LRG sample (Tegmark et al. 2006). The observed halo power spectrum for 0.02 < k < 0.2 h/Mpc is well-fit by our model: chi^2 = 39.6 for 40 degrees of freedom for the best fit LCDM model. We find \Omega_m h^2 * (n_s/0.96)^0.13 = 0.141^{+0.009}_{-0.012} for a power law primordial power spectrum with spectral index n_s and \Omega_b h^2 = 0.02265 fixed, consistent with CMB measurements. The halo power spectrum also constrains the ratio of the comoving sound horizon at the baryon-drag epoch to an effective distance to z=0.35: r_s/D_V(0.35) = 0.1097^{+0.0039}_{-0.0042}. Combining the halo power spectrum measurement with the WMAP 5 year results, for the flat LCDM model we find \Omega_m = 0.289 +/- 0.019 and H_0 = 69.4 +/- 1.6 km/s/Mpc. Allowing for massive neutrinos in LCDM, we find \sum m_{\nu} < 0.62 eV at the 95% confidence level. If we instead consider the effective number of relativistic species Neff as a free parameter, we find Neff = 4.8^{+1.8}_{-1.7}. Combining also with the Kowalski et al. (2008) supernova sample, we find \Omega_{tot} = 1.011 +/- 0.009 and w = -0.99 +/- 0.11 for an open cosmology with constant dark energy equation of state w.

Estimating the redshift distribution of photometric galaxy samples - II. Applications and tests of a new method

In Lima et al. we presented a new method for estimating the redshift distribution, N(z), of a photometric galaxy sample, using photometric observables and weighted sampling from a spectroscopic subsample of the data. In this paper, we extend this method and explore various applications of it, using both simulations and real data from the Sloan Digital Sky Survey (SDSS). In addition to estimating the redshift distribution for an entire sample, the weighting method enables accurate estimates of the redshift probability distribution, p(z), for each galaxy in a photometric sample. Use of p(z) in cosmological analyses can substantially reduce biases associated with traditional photometric redshifts, in which a single redshift estimate is associated with each galaxy. The weighting procedure also naturally indicates which galaxies in the photometric sample are expected to have accurate redshift estimates, namely those that lie in regions of photometric-observable space that are well sampled by the spectroscopic subsample. In addition to providing a method that has some advantages over standard photo-z estimates, the weights method can also be used in conjunction with photo-z estimates e.g. by providing improved estimation of N(z) via deconvolution of N(zphot) and improved estimates of photo-z scatter and bias. We present a publicly available p(z) catalogue for ∼78 million SDSS DR7 galaxies.

Alignment between galaxies and large-scale structure

Based on the Sloan Digital Sky Survey DR6 (SDSS) and the Millennium Simulation (MS), we investigate the alignment between galaxies and large-scale structure. For this purpose, we develop two new statistical tools, namely the alignment correlation function and the cos(2θ)-statistic. The former is a two-dimensional extension of the traditional two-point correlation function and the latter is related to the ellipticity correlation function used for cosmic shear measurements. Both are based on the cross correlation between a sample of galaxies with orientations and a reference sample which represents the large-scale structure. We apply the new statistics to the SDSS galaxy catalog. The alignment correlation function reveals an overabundance of reference galaxies along the major axes of red, luminous (L ≲ L*) galaxies out to projected separations of 60 h-1 Mpc. The signal increases with central galaxy luminosity. No alignment signal is detected for blue galaxies. The cos(2θ)-statistic yields very similar results. Starting from a MS semi-analytic galaxy catalog, we assign an orientation to each red, luminous and central galaxy, based on that of the central region of the host halo (with size similar to that of the stellar galaxy). As an alternative, we use the orientation of the host halo itself. We find a mean projected misalignment between a halo and its central region of ∼ 25°. The misalignment decreases slightly with increasing luminosity of the central galaxy. Using the orientations and luminosities of the semi-analytic galaxies, we repeat our alignment analysis on mock surveys of the MS. Agreement with the SDSS results is good if the central orientations are used. Predictions using the halo orientations as proxies for central galaxy orientations overestimate the observed alignment by more than a factor of 2. Finally, the large volume of the MS allows us to generate a two-dimensional map of the alignment correlation function, which shows the reference galaxy distribution to be flattened parallel to the orientations of red luminous galaxies with axis ratios of ∼ 0.5 and ∼ 0.75 for halo and central orientations, respectively. These ratios are almost independent of scale out to 60 h-1 Mpc.

Compact groups in theory and practice - III. Compact groups of galaxies in the Sixth Data Release of the Sloan Digital Sky Survey

We present the largest publicly available catalogue of compact groups of galaxies identified using the original selection criteria of Hickson, selected from the Sixth Data Release (DR6) of the Sloan Digital Sky Survey (SDSS). We identify 2297 compact groups down to a limiting magnitude of r = 18 (~0.24groups degree^{-2}), and 74791 compact groups down to a limiting magnitude of r = 21 (~6.7groups degree^{-2}). This represents 0.9% of all galaxies in the SDSS DR6 at these magnitude levels. Contamination due to gross photometric errors has been removed from the bright sample of groups, and we estimate it is present in the large sample at the 14% level. Spectroscopic information is available for 4131 galaxies in the bright catalogue (43% completeness), and we find that the median redshift of these groups is z_{med} = 0.09. The median line-of-sight velocity dispersion within the compact groups from the bright catalogue is sigma_{LOS} ~ 230km/s and their typical inter-galactic separations are of order 50 - 100kpc. We show that the fraction of groups with interloping galaxies identified as members is in good agreement with the predictions from our previous study of a mock galaxy catalogue, and we demonstrate how to select compact groups such that the interloper fraction is well defined and minimized. This observational dataset is ideal for large statistical studies of compact groups, the role of environment on galaxy evolution, and the effect of galaxy interactions in determining galaxy morphology.