Search

We investigate the strength of ultraviolet Fe II emission in fainter quasars compared with brighter quasars for 1.0 ~ 25 Ang. there is a universal (i.e. for quasars in general) strengthening of W2400 with decreasing intrinsic luminosity, L3000. (2) In conjunction with previous work by Clowes et al., we find that there is a further, differential, strengthening of W2400 with decreasing L3000 for those quasars that are members of Large Quasar Groups (LQGs). (3) We find that increasingly strong W2400 tends to be associated with decreasing FWHM of the neighbouring Mg II {\lambda}2798 broad emission line. (4) We suggest that the dependence of W2400 on L3000 arises from Ly{\alpha} fluorescence. (5) We find that stronger W2400 tends to be associated with smaller virial estimates from Shen et al. of the mass of the central black hole, by a factor ~ 2 between the ultrastrong emitters and the weak. Stronger W2400 emission would correspond to smaller black holes that are still growing. The differential effect for LQG members might then arise from preferentially younger quasars in the LQG environments., Comment: Accepted for publication in MNRAS. 21 pages, 11 figures

In this Letter, we investigate the very large-scale environments of cluster-scale triplets of type 1 Seyfert galaxies, lower luminosity counterparts of quasars, detected at z < 0.2 in the fourth Data Release of the Sloan Digital Sky Survey. The search for cluster scale triplets (object-object separations

This paper describes an algorithm based on the 2D approach of Allard and Fraley that uses Voronoi tessellation and a non-parametric maximum likelihood estimator. We have designed a 3D version of this algorithm which detects multiple clusters of points immersed in background noise; its application to the detection of galaxy clusters in redshift space, using the astronomical database of the 2-degree Field Galaxy Redshift Survey, is presented and discussed. Adopting as a benchmark a particular set of catalogued clusters of galaxies, we find that the proposed algorithm recognizes the location of ∼ 67% of the clusters. Three variants of the algorithm were assessed to deal with the elongation of the clusters in the radial direction of observation introduced by the astronomical distance indicator; their merits and limitations are discussed. We address separately the detection of the galaxy cluster location and the detection of galaxy cluster members, both of them having an anisotropic space as their search domain. In the case of detection of galaxy cluster members, a second stage of detection was incorporated in order to improve the results. © 2006 IEEE.

A Large Quasar Group (LQG) of particularly large size and high membership has been identified in the DR7QSO catalogue of the Sloan Digital Sky Survey. It has characteristic size (volume^1/3) ~ 500 Mpc (proper size, present epoch), longest dimension ~ 1240 Mpc, membership of 73 quasars, and mean redshift = 1.27. In terms of both size and membership it is the most extreme LQG found in the DR7QSO catalogue for the redshift range 1.0, Accepted for publication in Monthly Notices of the Royal Astronomical Society. 9 pages, 3 figures

We investigate whether radio-quiet quasars (RQQs) with z < 0.3 (and predominantly of low luminosity) are located preferentially in specific regions with respect to the centres and boundaries of neighbouring galaxy clusters. This way of characterizing the environment of RQQs differs from previous studies, which relied on the galaxy excess statistics within small radii around the quasars. For the detection of galaxy clusters we use a robust, semiparametric method based on a maximum likelihood estimate applied to Voronoi tessellation and enhanced by a colour-cut approach, allowing boundary determination and redshift estimates. We find that most of the RQQs reside within 3 h -1 Mpc of the centre of a galaxy cluster with comparable redshift and that none of them lies in the core itself. About 20 per cent of the investigated quasars reside between two galaxy clusters, which are possibly at an early stage of merger. Consequently, we suggest that quasars found in rich environments are associated with cluster mergers whereas those found in poorer environments are associated with infall towards a cluster. The information on larger scales provided by our analysis thus allows a clearer interpretation of the diverse environments that have for many years been reported in the literature for smaller scales. We discuss our findings in the context of existing quasar formation models and suggest that at least two formation mechanisms coexist. Additionally, we confirm, using multiple data sets, that low-redshift quasars follow a narrow channel of width ∼10 h -1 Mpc around the large-scale structure (LSS) traced by galaxy clusters, in agreement with the first report of this effect by Söchting, Clowes and Campusano. Such a result, if it applies to quasars at higher redshifts, has the potential to explain the clustering of quasars on scales

This paper presents a large sample of intermediate- to high-redshift galaxy groups and clusters detected using a fully automated search in the Cosmic Evolution Survey field. The detection algorithm is based on density peak extraction from a density distribution sampled using Voronoi tessellation within overlapping slices in the photometric redshift space. The cluster catalogue contains 1780 structures covering the redshift range 0.2 0.4 and many even below this threshold show very prominent substructure indicating that z~ 0.4 marks the slow emergence of virialized clusters in this field in agreement with published findings for other regions of the sky. The redshift distribution of detected structures shows strong variations with prominent peaks suggesting the presence of large-scale structures across the whole range covered by this catalogue. Supercluster candidates have been identified at redshifts z= 0.35, 0.72, 0.94, 1.12, 1.27, 1.45, 2.0 and 2.52. At z= 2.9 we identified a compact agglomeration of galaxy groups and clusters suggesting the presence of another supercluster-like structure which has been the highest redshift candidate so far. Out of the nine supercluster candidates found in this study, six are new detections. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

We identify 1901 galaxy clusters (N g ≥ 2) with the VoML+G algorithm (Paper I) on the Two-Degree Field Galaxy Redshift Survey. We present the 341 clusters with at least 10 galaxies that are within 0.009 < z < 0.14 (the Catalog), of which 254 (~75%) have counterparts in the literature (NED), with the remainder (87) plausibly "new" because of incompleteness of previous searches or unusual galaxy contents. The 207 clusters within z = 0.04–0.09 are used to study the properties of the galaxy systems in the nearby universe, including their galaxy contents parameterized by the late-type galaxy fractions (f L ). For this nearly complete cluster subsample, we find the following: (i) 63% are dominated by early-type galaxies (i.e., the late-type-poor clusters, f L < 0.5) with corresponding mean multiplicity and logarithmic virial mass (in units of M ⊙) of 22 ± 1 and 12.91 ± 0.04, respectively; and (ii) 37% are dominated by late-type galaxies (i.e., the late-type-rich clusters, f L ≥ 0.5) with corresponding mean multiplicity and logarithmic virial mass (in units of M ⊙) of 15.7 ± 0.9 and 12.66 ± 0.07, respectively. The statistical analysis of the late-type fraction distribution supports, with a 3σ confidence level, the presence of two population components. It is suggested that the late-type-poor galaxy systems reflect and extend the class of Abell-APM-EDCC clusters and that the late-type-rich systems (~one-third of the total) belong to a new, previously unappreciated class. The late-type-rich clusters, on average high mass-to-light ratio systems, appear to be more clustered on large scales than the late-type-poor clusters. A class of late-type-rich clusters is not predicted by current theory.

We have conducted an optical and infrared imaging in the neighbourhoods of 4 triplets of quasars. R, z', J and Ks images were obtained with MOSAIC II and ISPI at Cerro Tololo Interamerican Observatory. Accurate relative photometry and astrometry were obtained from these images for subsequent use in deriving photometric redshifts. We analyzed the homogeneity and depth of the photometric catalog by comparing with results coming from the literature. The good agreement shows that our magnitudes are reliable to study large scale structure reaching limiting magnitudes of R = 24.5, z' = 22.5, J = 20.5 and Ks = 19.0. With this catalog we can study the neighbourhoods of the triplets of quasars searching for galaxy overdensities such as groups and galaxy clusters., The paper contains 12 figures and 3 tables

We present the catalogue of the Mg II absorption systems detected at a high significance level using an automated search algorithm in the spectra of quasars from the twelfth data release of the Sloan Digital Sky Survey. A total of 266,433 background quasars were searched for the presence of absorption systems in their spectra. The continuum modelling for the quasar spectra was performed using a mean filter. A pseudo-continuum derived using a median filter was used to trace the emission lines. The absorption system catalogue contains 39,694 Mg II systems detected at a 6.0, 3.0$\sigma$ level respectively for the two lines of the doublet. The catalogue was constrained to an absorption line redshift of 0.35 $\le$ z$_{2796}$ $\le$ 2.3. The rest-frame equivalent width of the $\lambda$2796 line ranges between 0.2 $\le$ W$_r$ $\le$ 6.2 $\AA$. Using Gaussian-noise only simulations we estimate a false positive rate of 7.7 per cent in the catalogue. We measured the number density $\partial N^{2796}/\partial z$ of Mg II absorbers and find evidence for steeper evolution of the systems with W$_r \ge$ 1.2 $\AA$ at low redshifts (z$_{2796}$ $\le$ 1.0), consistent with other earlier studies. A suite of null tests over the redshift range 0.5 $\le$ z$_{2796}$ $\le$ 1.5 was used to study the presence of systematics and selection effects like the dependence of the number density evolution of the absorption systems on the properties of the background quasar spectra. The null tests do not indicate the presence of any selection effects in the absorption catalogue if the quasars with spectral signal-to-noise level less than 5.0 are removed. The resultant catalogue contains 36,981 absorption systems. The Mg II absorption catalogue is publicly available and can be downloaded from the link http://srini.ph.unimelb.edu.au/mgii.php, Comment: Updated references, typos; Published in MNRAS; 15 pages, 14 figures, 2 tables

We are looking at quasar environment in the context of large-scale structure (LSS) —a new approach, giving a more informed interpretation of quasar-galaxy correlations.This paper presents our first results for a sample of z ∼ 0.3 quasars. We are usingVoronoitessellation applied in colour (B J −R) slices for the detection of galaxyclustersand the minimal spanning tree (MST) to delineate the large-scale structure. This newcluster-detection method allows us to find reliably galaxy clusters at z < 0.3 fromSuperCOSMOS measurements of UK Schmidt plates. By reconstructing the large-scale structure in a relatively narrow redshift band (0.2 < z < 0.3), we show thatquasars follow the large-scale structure traced by galaxy clusters. None of the quasarsin ourradio-quiet sample is located in the central areaofa galaxycluster. Two quasars,found in a very rich environment, are actually located between two very close galaxyclusters, consistent with results on z ∼ 1 quasars suggesting that cluster mergers maybe involved in one of the quasar formation mechanisms.Key words: galaxies: clusters: general – quasars: general – cosmology: large-scalestructure of Universe.

This paper is the first in a series, presenting a new galaxy cluster finder based on a three-dimensional Voronoi Tesselation plus a maximum likelihood estimator, followed by gapping-filtering in radial velocity(VoML+G). The scientific aim of the series is a reassessment of the diversity of optical clusters in the local universe. A mock galaxy database mimicking the southern strip of the magnitude(blue)-limited 2dF Galaxy Redshift Survey (2dFGRS), for the redshift range 0.009 < z < 0.22, is built on the basis of the Millennium Simulation of the LCDM cosmology and a reference catalog of "Millennium clusters," spannning across the 1.0 × 1012–1.0 × 1015 M ⊙ h −1 dark matter (DM) halo mass range, is recorded. The validation of VoML+G is performed through its application to the mock data and the ensuing determination of the completeness and purity of the cluster detections by comparison with the reference catalog. The execution of VoML+G over the 2dFGRS mock data identified 1614 clusters, 22% with N g ≥ 10, 64 percent with 10 > N g ≥ 5, and 14% with N g < 5. The ensemble of VoML+G clusters has a ~59% completeness and a ~66% purity, whereas the subsample with N g ≥ 10, to z ~ 0.14, has greatly improved mean rates of ~75% and ~90%, respectively. The VoML+G cluster velocity dispersions are found to be compatible with those corresponding to "Millennium clusters" over the 300–1000 km s−1 interval, i.e., for cluster halo masses in excess of ~3.0 × 1013 M ⊙ h −1.

We have investigated the strength of ultraviolet Fe II emission from quasars within the environments of Large Quasar Groups (LQGs) in comparison with quasars elsewhere, for 1.1 = 45 Ang. (more precisely, ultrastrong-plus with W2400 >= 44 Ang.) have preferred nearest-neighbour separations of ~ 30-50 Mpc to the adjacent quasar of any W2400 strength. No such effect is seen for the ultrastrong emitters that are not in LQGs. The possibilities for increasing the strength of the Fe II emission appear to be iron abundance, Ly-alpha fluorescence, and microturbulence, and probably all of these operate. The dense environment of the LQGs may have led to an increased rate of star formation and an enhanced abundance of iron in the nuclei of galaxies. Similarly the dense environment may have led to more active blackholes and increased Ly-alpha fluorescence. The preferred nearest-neighbour separation for the stronger emitters would appear to suggest a dynamical component, such as microturbulence. In one particular LQG, the Huge-LQG (the largest structure known in the early universe), six of the seven strongest emitters very obviously form three pairings within the total of 73 members., Accepted for publication in Monthly Notices of the Royal Astronomical Society. 11 pages, 7 figures

We present evidence for a skewed distribution of UV FeII emission in quasars within candidate overdense regions spanning spatial scales of ~ 50 Mpc at 1.11 < z < 1.67, compared to quasars in field environments at comparable redshifts. The overdense regions have an excess of high equivalent width sources (W2400 > 42 ��), and a dearth of low equivalent width sources. There are various possible explanations for this effect, including dust, Ly��fluorescence, microturbulence, and iron abundance. We find that the most plausible of these is enhanced iron abundance in the overdense regions, consistent with an enhanced star formation rate in the overdense regions compared to the field., 9 pages, 5 figures, accepted for publication in MNRAS