Mean Redshift Research Articles

The Edinburgh-Durham Southern Galaxy Catalogue - VII. The Edinburgh-Milano cluster redshift survey

In this paper, we present the redshifts of the galaxies and galaxy clusters which form the Edinburgh-Milano (EM) cluster redshift survey. A total of 777 galaxy redshifts have been measured in 94 clusters extracted from the digitized Edinburgh-Durham Cluster Catalogue. We also present the radial velocity dispersions for 37 clusters. Observational and data reduction techniques are discussed in detail, together with the strategy adopted to determine the mean redshift of a cluster and to identify and discard plausible phantom clusters. Some 10 per cent of our clusters show heavy contamination, indicating that projection is a serious problem for optically selected, rich clusters. The median velocity dispersion estimated for a subsample of richness |$R\ge1$| clusters is |$742\pm63\enspace\text{km s}^{-1}$|⁠. From a simple comparison with Ω=1 cold dark matter models of structure formation, these results favour a biasing parameter b=1.6–2.0 and are inconsistent with a bias outside of the range b=1.3-2.5.

The redshift-space neighborhoods of 36 loose groups of galaxies. 1: The data

We have selected 36 loose groups of galaxies (RGH89) with at least five members, and with mean redshift average value of CZ is greater than 3200 km/s. These groups all lie within the first two slices of the CfA redshift survey 8(sup h) less than or equal to alpha less than or equal to 17(sup h) and 26.5 deg less than or equal to delta less than or equal to 38.5 deg). For each of these groups, we define the redshift-space neighborhood as a region centered on the group coordinates and delimited by a circle of projected radius R(sub cir) = 1.5/h Mpc on the sky, and by a velocity interval delta (sub cz) = 3000 km/s. Here we give the redshifts of 334 galaxies in these redshift-space neighborhoods. For completeness, we also give the redshifts of the 232 original members. These data include 199 new redshifts. We demonstrate that these samples of fainter galaxies significantly increase the number of members.

The morphology of faint galaxies in Medium Deep Survey images using WFPC2

First results from Hubble Space Telescope (HST) Medium Deep Survey images taken with Wide Field/Planetary Camera-2 (WFPC2) demonstrate that galaxy classifications can be reliably performed to magnitudes I814 approximately less than 22.0 in the F815W band. Published spectroscopic surveys to this depth indicate a mean redshift of bar-z approximately 0.5. We have classified over 200 galaxies in nine WFPC2 fields according to a basic morphological scheme. The majority of these faint galaxies appear to be similar to regular Hubble-sequence examples observed at low redshift. To the precision of our classification scheme, the relative proportion of spheroidal and disk systems of normal appearance is as expected from nearby samples, indicating that the bulk of the local galaxy population was in place at half the Hubble time. However, the most intriguing result is the relatively high proportion (approximately 40%) of objects which are in some way anomalous, and which may be of relevance in understanding the origin of the familiar excess population of faint galaxies established by others. These diverse objects include apparently interacting pairs whose multiple structure is only revealed with HST's angular resolution, galaxies with superluminous star-forming regions, diffuse low surface brightness galaxies of various forms, and compact galaxies. These anomalous galaxies contribute a substantial fraction of the excess counts at our limiting magnitude, and may provide insights into the 'faint blue galaxy' problem.

Obscuration, orientation, and the infrared properties of radio-loud active galaxies

We report on a study of the mid- and far-infrared (MFIR) properties of several different classes of radio-loud active galactic nuclei (AGNs) using the IRAS database. Our goal is to try to improve the understanding of the possible relationships between the diverse classes of AGNs. The MFIR and radio properties of radio-loud AGNs are especially useful in this regard, since (excluding the blazar class, which we do not study here) the radio emission is thought to be emitted isotropically, and the radio and MFIR radiation should be much less affected by dust obscuration than radiation at shorter wavelengths. We have first compared samples of 3CR broad-line radio galaxies (BLRGs) and narrow-line radio galaxies (NLRGs) matched in radio flux and mean redshift. We find that the BLRGs are stronger than the NLRGs by a factor of 4-5 in their mid-IR emission but are similar to the NLRGs in the far-IR. This is qualitatively consistent with recent 'unification' models for NLRGs and BLRGs which invoke thermal MFIR emission from dusty 'obscuring tori,' but there may be an additional source of far-IR emission present in the more luminous broad-line objects (the radio-loud quasars) studied previously by Heckman, Chambers & Postman (1992). We have also compared samples of Fanaroff-Riley class I (FRI) and Fanaroff-Riley class II (FRII) radio galaxies matched in radio flux and redshift. The FRII galaxies are stronger MFIR emitters than the FRI galaxies by a factor of about 4. This is consistent with suggestions that the central engine in FRI galaxies produces relatively little radiant energy per unit jet power (expecially since we find that the weak MFIR emission from the FRI galaxies may not be powered by the AGN). Comparing samples of gigahertz-peaked spectrum (GPS) and compact steep spectrum (CSS) sources versus non-GPS-CSS sources, we find that the GPS-CSS and non-GPS-CSS sources have similar MFIR strengths. This suggests that the efficiency of the conversion of jet kinetic energy into radio emission is not much higher in the GPS-CSS sources, contrary to some theoretical predictions. Overall, we find that the MFIR and radio powers of all the classes of radio-loud AGNs we have studied correlate well with one another over a range of about 10(exp 3) in power. This is most naturally understood if the MFIR is primarily powered by the AGN in most highly luminous radio-loud AGNs. However, other processes (starbursts or the intracluster medium) may contribute significantly in the less radio-luminous radio galaxies.

Gravitational lensing of distant field galaxies by rich clusters - I. Faint galaxy redshift distributions

{}From deep optical images of three clusters selected by virtue of their X-ray luminosity and/or optical richness (1455+22; $z=0.26$, 0016+16; $z=0.55$ and 1603+43; $z=0.89$), we construct statistically-complete samples of faint field galaxies ($I \leq 25$) suitable for probing the effects of gravitational lensing. By selecting clusters across a wide redshift range we separate the effects of the mean redshift distribution of the faint field population well beyond spectroscopic limits and the distribution of dark matter in the lensing clusters. A significant lensing signature is seen in the two lower redshift clusters whose X-ray properties are well-constrained. Based on these and dynamical data, it is straightforward to rule out field redshift distributions for $I \leq 25$ which have a significant low redshift excess compared to the no evolution prediction, such as would be expected if the number counts at faint limits were dominated by low-$z$ dwarf systems. The degree to which we can constrain any high redshift tail to the no evolution redshift distribution depends on the distribution of dark matter in the most distant lensing cluster. In the second paper in this series, we use the lensing signal to reconstruct the full two-dimensional mass distribution in the clusters and, together with high resolution X-ray images, demonstrate that their structural properties are well-understood. The principal result is therefore the absence of a dominant low-$z$ dwarf population to $I \leq25$.

Absolute redshifts in the C iv 1548 � line in the transition region of the quiet Sun

Observations with the UVSP instrument on the SMM spacecraft were made at the polar limb and disk center for the accurate determination of Doppler shifts of the Civ 1548 A emission line formed at 105 K in the transition region of the quiet Sun. Individual data points representing 3 arc sec square pixels yield both redshifts and blueshifts, but the mean values from four different days of observations are toward the red. The mean redshifts are in the range 4–8 km s-1 and are produced by nearly vertically directed flows; the uncertainty associated with the mean values correspond to ±0.5 km s-1. The redshift increases with brightness of the Civ line.

Physical pairs of quasars with large separation on the sky

Quasars and quasar-like objects from the Palomar bright quasar survey show an apparent aggregation and pairing. Nearest neighbor statistical tests here show physical pairing at confidence levels up to 99.8%. At their redshift distance, however, the separation of the pairs is in the range (30–150) h −1 Mpc. Such separations are an order of magnitude larger than the dimensions of superclusters. Using the mean redshifts of the pairs as a measure of their distance yields a three-dimensional average distribution in space. For those quasars with separations ≲50 h −1 Mpc this distribution is not spherical, but elongated toward the observer. If this elongation is caused by peculiar velocities it would have to arise from the expansion of the quasars away from each other. Such deviations from the Hubble flow would have to be ≳2000 km s −1. In an independent analysis performed on two other samples of quasars, Fang (private communication) derived peculiar velocities of 600 to 1800 km s −1 for weakly clustered pairs at separations of ≲ 100 h −1 Mpc.

A statistically complete survey for arc-like features in images of distant rich clusters of galaxies

Using data from an earlier search for supernovae in distant clusters, we have constructed a homogeneous set of V images for 19 rich clusters with mean redshift z¯¯¯=0.32z¯=0.32. By considering the images above a fixed surface brightness limit, the data for extended arc-like features which might arise from gravitational lensing of background sources. A list of 20 candidate arcs is presented. We examine the usefulness of such a catalogue for deriving the background source redshift distribution, NzNz. Whilst the number and shape distribution of arc candidates is consistent with the lensing hypothesis, cluster velocity dispersions of very high precision would be needed to provide useful constraints on the fraction of high-redshift galaxies to faint limits. We show how, in principle, a likelihood ratio test based on the radial distribution of arcs in a single well-studied cluster could determine whether a significant fraction of the faint galaxy population is at high redshift. This test also provides a means of determining accurate cluster velocity dispersions at any redshift.

The emission-line properties of steep radio spectrum quasars

Data on the emission-line properties of a sample of 45 intermediate redshift quasars with steep radio spectra are presented and compared with those of a sample of flat radio spectrum sources of comparable mean luminosity and redshift. The steep-spectrum sources have, on average, smaller widths of the C IV 1549 A and semiforbidden C III 1909 A line profiles, larger Mg II 2800 A equivalent widths, and smaller (He II and semiforbidden O III)/C IV intensity ratios than the flat-spectrum sources. The steep-spectrum sources have C IV profiles that tend to be asymmetric to the red, while those in the flat-spectrum sources are asymmetric to the blue. The Baldwin effect and the correlation between the Mg II-C IV and semiforbidden C III-C IV redshift differences reported elsewhere are also present both within the combined sample and among the steep-spectrum objects alone. These results support proposed unified models of steep and flat radio spectrum quasars. 56 refs.

Spectroscopy and multicolor photometry of the medium redshift southern cluster of galaxies A3639

Spectroscopic observations at 360-700 nm and photometric observations in the Gunn g, r, and i bands are reported for Abell 3639. Data obtained using the EFOSC with CCD detectors at the Cassegrain focus of the 3.6-m telescope at ESO during 1987 and 1988 are presented in extensive tables, graphs, and sample spectra and characterized in detail. Ten of 14 cluster-core galaxies studied are identified as cluster members, giving a mean redshift of 0.1496 + or - 0.0027 and a velocity dispersion of sigma = 538 + 212 or - 128 km/sec. No evidence of an extended envelope is found for the dominant galaxy. Investigation of the luminosity function of cluster galaxies down to M(r) = -18 in an area of radius about 500 kpc and elimination of background galaxies on the basis of their position in the color-color diagram gives a total of 105 galaxies in the sample. 28 refs.

Evolving starburst galaxies, faint number counts, and the 2kev background

We model blue and 60 μm extragalactic number counts and redshift distributions with evolutionary models in which galaxies periodically undergo transient starburst events. We find support for the hypothesis that the same starburst phenomenon is responsible for the excess source counts seen at both wavelengths, though observed at lower mean redshift in the far infrared than in the blue. The soft X-ray emission from these evolving starburst events is not likely to exceed the 2 kev background unless the evolutionary rate is greater than (1+ z) 3 (luminosity or density evolution) and the redshift of galaxy formation is greater than about 5.

Weakly barred early-type ringed galaxies. II - The double-ringed S0(+) galaxy NGC 7187

This paper presents BVR CCD surface photometry of NGC 7187, a southern S0^+^ galaxy possessing a bright circular inner ring and a faint elongated outer ring. The rings both have a contrast comparable to those observed in typical (R)SB(r)-type galaxies, yet there is no obvious bar crossing the inner ring as in such galaxies. Instead, there are two nearly orthogonal, small oval zones in the bulge region, one of which may be a nuclear bar. The inner ring is a narrow zone of slightly enhanced blue colors and is therefore a place where star formation has been favored in the disk. An asymmetry in the outer ring region suggests that the galaxy has suffered some interaction, possibly associated with membership in a group whose mean redshift is 2500 km s^-1^. The origin of the two rings is uncertain. If they are resonance rings, then NGC 7187 would be a possible example of a galaxy which once had a stronger, more important bar than it has now. This bar could have evolved to a more axisymmetric state through an internal interaction, leaving the rings and an inner lens as signatures of its presence. However, the data do not exclude completely the possibility that both rings are a result of a collision or merger/accretion of one or more smaller galaxies with an S0, in which case NGC 7187 would more properly belong into the pure ring galaxy or Hoag-type ring galaxy classes. The main argument against this possibility is that NGC 7187 does not resemble the more prototypical members of those classes. Other considerations are discussed in some detail.

Internal redshift difference and central mass in QSOs

view Abstract Citations (32) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Internal Redshift Difference and Central Mass in QSOs Zheng, Wei ; Sulentic, Jack W. Abstract The redshifts of broad Hβ and narrow [O III] emission lines are measured and compared in 31 low-redshift QSOs with weak or absent optical Fe II features. The redshifts are often not the same for the broad and narrow lines. The distribution of peak redshift differences for our sample is centered near zero. At larger widths the distribution is skewed, suggesting that Hβ more often shows a higher redshift. A mean redshift difference at FWHM of ~ 300 km s^-1^ is found between broad Hβ and the narrow [O III] lines. This value appears to set an upper limit to the central mass, on the order of 10^9^ M_sun_, and further to imply the dominance of radiative acceleration in the broad line region of these QSOs. Publication: The Astrophysical Journal Pub Date: February 1990 DOI: 10.1086/168407 Bibcode: 1990ApJ...350..512Z Keywords: Active Galactic Nuclei; Forbidden Transitions; H Beta Line; Mass Distribution; Quasars; Red Shift; Iron; Line Spectra; Oxygen Spectra; Astrophysics; GALAXIES: REDSHIFTS; QUASARS full text sources ADS | data products NED (31) SIMBAD (30)

Detection of systematic gravitational lens galaxy image alignments - Mapping dark matter in galaxy clusters

view Abstract Citations (420) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of Systematic Gravitational Lens Galaxy Image Alignments: Mapping Dark Matter in Galaxy Clusters Tyson, J. A. ; Valdes, F. ; Wenk, R. A. Abstract A gravitational lens distorts most background galaxies by stretching along a circle centered on the lens. This systematic alignment of 20-60 faint background galaxy images has been detected, centered on foreground galaxy clusters of high velocity dispersion. The background galaxy population is selected by its extreme blue B-R color. At a limiting surface brightness of 29 B mag arcsec^-2^, there are over 30 background galaxies arcmin^-2^ mag^-1^ anywhere in the sky, which is sufficient to map statistically the dark matter distribution in a foreground cluster. Pattern recognition software generates a two-dimensional lens distortion map. Initial results for the high-velocity dispersion clusters A1689 and CL 1409+52 are presented here. The dark matter is apparently correlated (center and radial extent) with the cluster red light, suggestive of a baryonic origin or dissipative coupling. The existence of a high percentage of lens-distorted faint blue galaxies sets a lower limit of approximately 0.9 to this background galaxy population mean redshift. Publication: The Astrophysical Journal Pub Date: January 1990 DOI: 10.1086/185636 Bibcode: 1990ApJ...349L...1T Keywords: Dark Matter; Galactic Clusters; Gravitational Lenses; Brightness Distribution; Charge Coupled Devices; Faint Objects; Red Shift; Spatial Distribution; Astrophysics; COSMOLOGY; DARK MATTER; GALAXIES: CLUSTERING; GALAXIES: REDSHIFTS; GRAVITATION full text sources ADS | data products SIMBAD (2) NED (1)

The distribution of galaxies in the direction of the 'Great Attractor'

THE discovery of galaxy streaming motions of up to 600 km s−1 towards Hydra–Centaurus has led to the suggestion1 that this motion is induced by a concentration of galaxies (the 'Great Attractor') centred on galactic coordinates l = 307°, b = 9° at a distance corresponding to 4,350 ± 350 km s−1 in the Hubble Flow. Here we present a map of ~17,000 galaxies down to BJ = 17m in an extended region of the sky in this direction. The clusters of galaxies identified from the map are shown to constitute two distinct concentrations in redshift (see, for example, ref. 2). We explore the possibility that the further of the two concentrations, centred near l = 312°, b = 31°, at a mean redshift of 14,000 km s−1, is a major contributor to the peculiar motion of the Local Group. This is unlikely because the contribution to the dipole anisotropy of extragalactic light due to galaxies in this region, beyond a diameter limit of 1.3 arcmin, is about 10% of that due to nearer galaxies, and the concentration would need to have an extremely high mass-to-light ratio. Superclusters as massive as this would cause appreciable anisotropies in the cosmic microwave background radiation.

Damped Lyman-alpha absorption by disk galaxies with large redshifts. III - Intermediate-resolution spectroscopy

New intermediate-resolution spectroscopy for certain members of a sample of 68 moderate- to high-redshift QSOs are presented. These new data have been obtained to supplement previously published lower resolution survey data, which we refer to as the Lick survey, in order to identify H I disk galaxies with large redshifts that give rise to absorption lines in the spectra of background QSOs. Data have been obtained with the MMT and the CTIO 4 m telescope for six QSOs at 1.0-2.4 A resolution. Absorption-line lists are presented, and evidence supporting the identification of seven damped Lyα absorption systems in the six QSOs is discussed. Six of the seven damped Lyα systems have neutral hydrogen column densities N(H I) >= 2 x 10^20^ cm^-2^. The damped Lyα systems have absorption properties which include those expected when high-redshift galaxy disks cosmologically intervene along our sight line to QSOs. Therefore, these systems are sometimes termed "Lyα disk" systems. The properties of the damped Lyα systems are discussed and contrasted with other QSO absorption-line systems. About half of the damped Lyα systems would have been missed if they had been searched for on the basis of a spectroscopic survey for C IV absorption alone. Damped Lyα systems are distinguished because they have low ions (i.e., low ionization species) which are generally stronger than the high ions (i.e., high ionization species), they sometimes have other metal line systems clustering around them, and they are the only systems which exhibit 21 cm absorption. In order to investigate these systems further, a standard curve-of-growth analysis was performed when possible. We suggest that damped Lyα systems arise when a sight line intercepts a high-redshift protogalaxy disk containing a quiescent cloud component characterized by high column density and low effective velocity dispersion. At the same time, the sight line usually intercepts a broader turbulent component, which we identify as the halo, characterized by much lower column density and higher effective velocity dispersion. The turbulent component may contain both low and high ions but about half the time only the low ions are strong enough to be detected. In two of the damped Lyα systems only a quiescent component, with effective velocity dispersion σ < 10 km s^-1^, appears to be present. Based on the restrictive assumption that these are single quiescent components, abundances between 4 x 10^-3^ and 1 times solar values are derived. These determinations should be considered strictly lower limits, provided that our metal line identifications and measurements (sometimes in the presence of Lyα forest absorption) are substantially correct. A summary of the incidence of damped Lyα absorption in the Lick survey sample of 68 QSOs is also presented. At least 16 damped Lyα systems with column densities N(H I) >= 2 x 10^20^ are found over a cosmological path length {DELTA}z = 56 (dN/dz >= 0.29 +/- 0.07) at a mean redshift z = 2.24.

A survey of Lyman-limit absorption in the spectra of 59 high-redshift QSOs

view Abstract Citations (341) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Survey of Lyman-Limit Absorption in the Spectra of 59 High-Redshift QSOs Sargent, Wallace L. W. ; Steidel, Charles C. ; Boksenberg, A. Abstract We have obtained spectra of 59 QSOs with redshifts in the range 2.75 <= z_em_ <= 4.11 at a resolution of 4 A in the wavelength range λλ 3150-4700 and 6 A in the range λλ4600-7000. The spectra are suitable for crude spectrophotometry. The signal-to-nolse ratio is typically 30-50 in the blue and 50-70 in the red. The objects constitute an unbiased sample as far as Lyman-limit detection is concerned. We list emission-line equivalent widths and fluxes and use the new data to derive new emission redshifts. Power-law fits to the overall continuum of the form f_v_ is proportional to v^-α^ for the spectrum longward of Lyman-α emission result in a range of slopes 0.18 <= α <= 1.51, with a mean value <α> = 0.78 +/- 0.04 (error in the mean). Known radio sources have a significantly steeper mean slope than the remainder. There is a significant correlation between the rest equivalent width of the C IV emission line and absolute luminosity (the "Baldwin effect"). Absorption lines were measured longward of Lyman-α emission, and redshifts were determined. It was possible to derive an unbiased sample of redshifts based on the Mg II λλ2796, 2803 doublet. We found 62 Mg II absorption redshifts in 58 QSOs. These were used to improve the statistics of Mg II absorption in the range 0.7 <= z_abs_ <= 1.45. A total of 37 Lyman-limit systems (LLSs) were identified in the 59 QSOs; three QSOs each had two LLSs. All the LLSs have τ >= 1.5. It was possible to measure the size of the Lyman discontinuity in 12 cases; these had 1.5 <= τ <= 3.5. In the case of all the other LLSs, τ was indeterminate but greater than 3.0. By taking all single LLSs and the highest redshift one in the three QSOs with more than one we derived an unbiased sample of 33 LLSs with τ >= 1.5 in the rpectra of 54 QSOs. Survival statistics were used to derive a mean density per unit redshift for LLSs of N = 1.91 +/- 0.33 at <z_LLS_> = 2.950. A hazard plot and the Kaplan-Meier estimator were used to show that the distribution of LLSs is Poissonian. Our data were combined with published samples to produce an extended sample of 54 LLSs in 90 QSOs covering the range 0.67 <= z_LLS_ <= 3.58. The resulting number density as a function of redshift has the form N(z) is proportional to (1 + z)^0.68+/-0.54^ which is consistent with either q_0_ = 0 or q_0_ = 1/2 and no evolution in the properties of the absorbing objects. We estimate that the Lyman-α forest clouds only contribute N(z) ~ 0.3 to the total at z ~ 3.5, and less at lower redshifts. Thus, we believe that most of the LLSs are produced by galaxies. About two-thirds of the LLSs have associated heavy-element absorption systems, usually identifled by the C IV doublet, in these low-resolution spectra. A plot of the distribution of the apparent ejection velocity β is flat over the whole observed range; thus, there is no significant tendency for LLSs to congregate around z_em_. The selection of QSOs was not biased against BAL objects. Three unambiguous and five possible BAL QSOs were found; they constitute 5.1%-13.6% of the total sample. Six "damped Lyman-α" lines with H I column densities in excess of 1.0 x 10^21^ cm^-2^ were found; the correrponding density per unit redshift is N_DLA_ = 0.09 +/- 0.04 at a mean redshift (< z_abs_> = 2.62, larger than the local density of disks with the same H I column density. Our data from the LLSs and from the damped Lyman-α sample were used to determine the distribution of H I column density at the high end. As found by Tytler, the H I column density distribution over the range 13.25 <= log N(H I) <= 22.0 can be represented as a single power law, although formally the fit is not very good. Publication: The Astrophysical Journal Supplement Series Pub Date: April 1989 DOI: 10.1086/191326 Bibcode: 1989ApJS...69..703S Keywords: Intergalactic Media; Lyman Spectra; Quasars; Red Shift; Signal To Noise Ratios; Emission Spectra; Line Spectra; Lyman Alpha Radiation; Astrophysics; ATOMIC PROCESSES; QUASARS; COSMOLOGY; GALAXIES: INTERGALACTIC MEDIUM; LINE PROFILES full text sources ADS | data products NED (295) SIMBAD (61)

Optical identifications of radio sources from the Molonglo deep surveys - I. The declination Formula portion of the first deep survey

Optical identifications have been sought for 304 radio sources from the declination |$-{20}^{^{\circ} }$| region of the first Molonglo deep survey. Optical objects have been measured using the Palomar sky survey to an accuracy of 1 arcsec and magnitude estimates are to 0.4 mag. Finding charts are given for all fields with optical objects within |$\pm 2\sigma$| of the radio position. Sixteen QSOs have been confirmed (details have been reported elsewhere) amongst the blue stellar-like objects and spectroscopy is complete to mo = 19.5. The mean magnitude of the QSOs is 19.2 and the mean redshift is 1.34. A study of the background densities of objects indicates that about half of the remaining blue stellar-like objects are QSOs and about 80 per cent of the galaxies are associated with the radio emission. The QSOs are amongst the fainter and the bluer of the blue stellar-like objects. There is no evidence for any average offset between the radio and optical positions.

Clustering in the quasar distribution

We analyse the two-dimensional distribution of quasars for clustering. In an area of 40 square degrees centered at α = 1 h 44 m , δ = −40°00', we found 1092 quasar candidates in a systematic search with slitless spectra. A two-dimensional power spectrum analysis gave the result that the quasars are weakly clustered, with a clustering scale of about 0.5°. This corresponds to 35 Mpc at the mean redshift of 2.

Correlation of QSO absorption lines in universes dominated by cold dark matter

Theoretical predictions for the redshift correlations between QSO absorption-line systems are investigated in the context of 'cold dark matter' cosmological models. Particles in 'particle-mesh' N-body simulations are interpreted as absorbing clouds at epochs corresponding to mean redshifts, z, of 0.0, 1.25, and 3.0. The velocity correlation function for absorbing clouds is found by passing lines-of-sight through the systems and computing velocity differences for those particles which lie close to the lines. It depends strongly on z and Omega but only weakly, if at all, on the number density, diameter or mass of the clouds. Two interpretations are possible: (1) the heavy element absorption systems are associated with galaxies which are an unbiased sample of the mass distribution in an Omega(0) = 0.2 universe or (2) the Lyman-alpha absorbers are an unbiased sample of the mass in an Omega(0) = 1 universe and the heavy-element absorption systems, like galaxies, are more strongly clustered than the mass.