The 2dF Galaxy Redshift Survey: a targeted study of catalogued clusters of galaxies     

Roberto De Propris  Warrick J. Couch  Matthew Colless  Gavin B. Dalton  Chris Collins  Carlton M. Baugh  Joss Bland-Hawthorn  Terry Bridges  Russell Cannon  Shaun Cole  Nicholas Cross  Kathryn Deeley  Simon P. Driver  George Efstathiou  Richard S. Ellis  Carlos S. Frenk  Karl Glazebrook  Carole Jackson  Ofer Lahav  Ian Lewis  Stuart Lumsden  Steve Maddox  Darren Madgwick  Stephen Moody  Peder Norberg  John A. Peacock  Will Percival  Bruce A. Peterson  Will Sutherland  Keith Taylor 《Monthly notices of the Royal Astronomical Society》2002,329(1):87-101

We have carried out a study of known clusters within the 2dF Galaxy Redshift Survey (2dFGRS) observed areas and have identified 431 Abell, 173 APM and 343 EDCC clusters. Precise redshifts, velocity dispersions and new centroids have been measured for the majority of these objects, and this information is used to study the completeness of these catalogues, the level of contamination from foreground and background structures along the cluster's line of sight, the space density of the clusters as a function of redshift, and their velocity dispersion distributions. We find that the Abell and EDCC catalogues are contaminated at the level of about 10 per cent, whereas the APM catalogue suffers only 5 per cent contamination. If we use the original catalogue centroids, the level of contamination rises to approximately 15 per cent for the Abell and EDCC catalogues, showing that the presence of foreground and background groups may alter the richness of clusters in these catalogues. There is a deficiency of clusters at     that may correspond to a large underdensity in the Southern hemisphere. From the cumulative distribution of velocity dispersions for these clusters, we derive a space density of     clusters of     This result is used to constrain models for structure formation; our data favour low-density cosmologies, subject to the usual assumptions concerning the shape and normalization of the power spectrum.  相似文献   

Compositional analyses of lunar pyroclastic deposits   总被引：1，自引：0，他引：1  

Lisa R Gaddis  Matthew I Staid  B.Ray Hawke 《Icarus》2003,161(2):262-280

The 5-band Clementine UVVIS data at ∼100 m/pixel were used to examine the compositions of 75 large and small lunar pyroclastic deposits (LPDs), and these were compared to representative lunar maria and highlands deposits. Results show that the albedo, spectral color, and inferred composition of most LPDs are similar to those of low-titanium, mature lunar maria. These LPDs may have consisted largely of fragmented basalt, with substantial components of iron-bearing mafic minerals (pyroxenes, olivine) and smaller amounts (if any) of volcanic glass. Several smaller LPDs also show substantial highland components. Three classes of very large deposits can be distinguished from most LPDs and from each other on the basis of crystallinity and possible titanium content of their pyroclastic components. One class has spectral properties that are dominated by high-titanium, crystallized “black beads” (e.g., Taurus-Littrow), a second consists of a mixture of high-titanium glasses and beads with a higher glass/bead ratio (Sulpicius Gallus) than that of Taurus-Littrow, and a third has a significant component of quenched iron-bearing volcanic glasses (Aristarchus) with possible moderate titanium contents. Although areally extensive, these three classes of very large pyroclastic deposits compose only 20 of the 75 deposits studied (∼27%), and eruption of such materials was thus likely to have been less frequent on the Moon.  相似文献   

Can redistribution of material by sputtering explain the hemispheric dichotomy of europa?     

Matthew S. Tiscareno  Paul E. Geissler 《Icarus》2003,161(1):90-101

Sputtering and redeposition make up a significant geologic process on Europa and may contribute to the observed color differences between the leading and trailing hemispheres. Sputtering is particularly efficient on Europa as an erosive process, due to the high rate of ion bombardment from the Io plasma torus, together with Europa’s easily sputtered icy surface. We estimate the global average sputtering erosion rate on Europa at 14.7 mm Myr⁻¹. However, 42% to 86% of sputtered water molecules survive to redeposit onto the surface again. Due to gravitational escape and removal by electron-impact ionization, the number of redepositing particles cannot overcome sputtering erosion, and the global average result of sputtering is net erosion. However, neither sputtering nor redeposition is globally uniform, and differences in the global distributions of the two processes can result locally in net deposition.We propose that Europa’s hemispheric color dichotomy might be explained by net deposition on the leading hemisphere, which may obscure the non-ice signature by covering it with a thin water frost. To test this hypothesis, we have created a simulated model of the sputtering erosion/redeposition process on Europa. Our objectives are to determine the conditions under which net deposition occurs on the leading hemisphere and to evaluate the effects on this process of Jupiter’s gravity, of Europa’s rotation, and of the loss of water molecules to the jovian magnetosphere. We have followed the trajectories of hundreds of thousands of simulated sputtered water molecules in a Monte Carlo process, evolving their orbits under the gravity of both Europa and Jupiter. We have performed this model multiple times, in order to explore the effects of different assumptions of the global distribution of impacting ions, as well as of the sputtered particle ejection velocity. Based upon our results, we conclude that net deposition occurs under a wide range of conditions, making sputtering erosion and redeposition a plausible explanation for the hemispheric color dichotomy of Europa.  相似文献   

The formation of a star cluster: predicting the properties of stars and brown dwarfs   总被引：2，自引：0，他引：2  

Matthew R. Bate  Ian A. Bonnell  Volker Bromm 《Monthly notices of the Royal Astronomical Society》2003,339(3):577-599

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The 2dF Galaxy Redshift Survey: luminosity functions by density environment and galaxy type     

Darren J. Croton  Glennys R. Farrar  Peder Norberg  Matthew Colless  John A. Peacock  I. K. Baldry  C. M. Baugh  J. Bland-Hawthorn  T. Bridges  R. Cannon  S. Cole  C. Collins  W. Couch  G. Dalton  R. De Propris  S. P. Driver  G. Efstathiou  R. S. Ellis  C. S. Frenk  K. Glazebrook  C. Jackson  O. Lahav  I. Lewis  S. Lumsden  S. Maddox  D. Madgwick  B. A. Peterson  W. Sutherland  K. Taylor 《Monthly notices of the Royal Astronomical Society》2005,356(3):1155-1167

We use the 2dF Galaxy Redshift Survey to measure the dependence of the b _J-band galaxy luminosity function on large-scale environment, defined by density contrast in spheres of radius  8  h ⁻¹ Mpc  , and on spectral type, determined from principal component analysis. We find that the galaxy populations at both extremes of density differ significantly from that at the mean density. The population in voids is dominated by late types and shows, relative to the mean, a deficit of galaxies that becomes increasingly pronounced at magnitudes brighter than   M _{b J}−5log₁₀ h ≲−18.5  . In contrast, cluster regions have a relative excess of very bright early-type galaxies with   M _{b J}−5log₁₀ h ≲−21  . Differences in the mid- to faint-end population between environments are significant: at   M _{b J}−5log₁₀ h =−18  early- and late-type cluster galaxies show comparable abundances, whereas in voids the late types dominate by almost an order of magnitude. We find that the luminosity functions measured in all density environments, from voids to clusters, can be approximated by Schechter functions with parameters that vary smoothly with local density, but in a fashion that differs strikingly for early- and late-type galaxies. These observed variations, combined with our finding that the faint-end slope of the overall luminosity function depends at most weakly on density environment, may prove to be a significant challenge for models of galaxy formation.  相似文献   

Smoothed particle hydrodynamics with radiative transfer in the flux-limited diffusion approximation     

Stuart C. Whitehouse  Matthew R. Bate 《Monthly notices of the Royal Astronomical Society》2004,353(4):1078-1094

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Thinking Geographically     

Matthew Henry 《New Zealand geographer》2003,59(2):64-64

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The 2dF Galaxy Redshift Survey: correlation functions, peculiar velocities and the matter density of the Universe   总被引：1，自引：0，他引：1  

Ed Hawkins  Steve Maddox  Shaun Cole  Ofer Lahav  Darren S. Madgwick  Peder Norberg  John A. Peacock  Ivan K. Baldry  Carlton M. Baugh  Joss Bland-Hawthorn  Terry Bridges  Russell Cannon  Matthew Colless  Chris Collins  Warrick Couch  Gavin Dalton  Roberto De Propris  Simon P. Driver  George Efstathiou  Richard S. Ellis  Carlos S. Frenk  Karl Glazebrook  Carole Jackson  Bryn Jones  Ian Lewis  Stuart Lumsden  Will Percival  Bruce A. Peterson  Will Sutherland  Keith Taylor 《Monthly notices of the Royal Astronomical Society》2003,346(1):78-96

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Stellar encounters involving massive stars in young clusters     

Melvyn B. Davies  Matthew R. Bate  Ian A. Bonnell  Vernon C. Bailey  Christopher A. Tout 《Monthly notices of the Royal Astronomical Society》2006,370(4):2038-2046

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Saturn's dynamic D ring     

Matthew M. Hedman  Joseph A. Burns  Mark R. Showalter  Philip D. Nicholson  Matthew S. Tiscareno  Bonnie J. Buratti  Roger Clark 《Icarus》2007,188(1):89-107

The Cassini spacecraft has provided the first clear images of the D ring since the Voyager missions. These observations show that the structure of the D ring has undergone significant changes over the last 25 years. The brightest of the three ringlets seen in the Voyager images (named D72), has transformed from a narrow, <40-km wide ringlet to a much broader and more diffuse 250-km wide feature. In addition, its center of light has shifted inwards by over 200 km relative to other features in the D ring. Cassini also finds that the locations of other narrow features in the D ring and the structure of the diffuse material in the D ring differ from those measured by Voyager. Furthermore, Cassini has detected additional ringlets and structures in the D ring that were not observed by Voyager. These include a sheet of material just interior to the inner edge of the C ring that is only observable at phase angles below about 60°. New photometric and spectroscopic data from the ISS (Imaging Science Subsystem) and VIMS (Visual and Infrared Mapping Spectrometer) instruments onboard Cassini show the D ring contains a variety of different particle populations with typical particle sizes ranging from 1 to 100 microns. High-resolution images reveal fine-scale structures in the D ring that appear to be variable in time and/or longitude. Particularly interesting is a remarkably regular, periodic structure with a wavelength of ∼30 km extending between orbital radii of 73,200 and 74,000 km. A similar structure was previously observed in 1995 during the occultation of the star GSC5249-01240, at which time it had a wavelength of ∼60 km. We interpret this structure as a periodic vertical corrugation in the D ring produced by differential nodal regression of an initially inclined ring. We speculate that this structure may have formed in response to an impact with a comet or meteoroid in early 1984.  相似文献