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21.
Santiago Torres Enrique García-Berro reas Burkert Jordi Isern 《Monthly notices of the Royal Astronomical Society》2001,328(2):492-500
In this paper we analyse the consequences in the white dwarf population of a hypothetical merger episode in our Galactic disc. We have studied several different merging scenarios with our Monte Carlo simulator. For each one of these scenarios we have derived the main characteristics of the resulting white dwarf population and we have compared them with the available observational data, namely the white dwarf luminosity function and the kinematic properties of the white dwarf population. Our results indicate that very recent (less than ∼6 Gyr ago) and massive (∼16 per cent of the mass of our Galaxy) merger episodes are quite unlikely in view of the available kinematical properties of the disc white dwarf population. Smaller merger episodes (of the order of ∼4 per cent of the mass of our Galaxy) are, however, compatible with our current knowledge of those kinematical properties. Finally, we prove that the white dwarf luminosity function is quite insensitive to such a merger episode. 相似文献
22.
Rolando Dünner reas Reisenegger rés Meza † Pablo A. Araya Hernán Quintana 《Monthly notices of the Royal Astronomical Society》2007,376(4):1577-1587
An exponentially expanding Universe, possibly governed by a cosmological constant, forces gravitationally bound structures to become more and more isolated, eventually becoming causally disconnected from each other and forming so-called 'island universes'. This new scenario reformulates the question about which will be the largest structures that will remain gravitationally bound, together with requiring a systematic tool that can be used to recognize the limits and mass of these structures from observational data, namely redshift surveys of galaxies. Here we present a method, based on the spherical collapse model and N -body simulations, by which we can estimate the limits of bound structures as observed in redshift space. The method is based on a theoretical criterion presented in a previous paper that determines the mean density contrast that a spherical shell must have in order to be marginally bound to the massive structure within it. Understanding the kinematics of the system, we translated the real-space limiting conditions of this 'critical' shell to redshift space, producing a projected velocity envelope that only depends on the density profile of the structure. From it we created a redshift-space version of the density contrast that we called 'density estimator', which can be calibrated from N -body simulations for a reasonable projected velocity envelope template, and used to estimate the limits and mass of a structure only from its redshift-space coordinates. 相似文献
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24.
Seb Oliver Robert G. Mann Ruth Carballo Alberto Franceschini Michael Rowan-Robinson Maria Kontizas Anastasios Dapergolas Evanghelos Kontizas Aprajita Verma David Elbaz Gian Luigi Granato Laura Silva Dimitra Rigopoulou J. Ignacio Gonzalez-Serrano Steve Serjeant reas Efstathiou Paul P. van der Werf 《Monthly notices of the Royal Astronomical Society》2002,332(3):536-548
We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15 μm with the ISOCAM instrument on board the Infrared Space Observatory ( ISO ). The final map in each band was constructed by the co-addition of four independent rasters, registered using bright sources securely detected in all rasters, with the absolute astrometry being defined by a radio source detected at both 6.7 and 15 μm. We sought detections of bright sources in a circular region of radius 2.5 arcmin at the centre of each map, in a manner that simulations indicated would produce highly reliable and complete source catalogues using simple selection criteria. Merging source lists in the two bands yielded a catalogue of 35 distinct sources, which we calibrated photometrically using photospheric models of late-type stars detected in our data. We present extragalactic source count results in both bands, and discuss the constraints that they impose on models of galaxy evolution, given the volume of space sampled by this galaxy population. 相似文献
25.
Elena D'Onghia reas Burkert Giuseppe Murante Sadegh Khochfar 《Monthly notices of the Royal Astronomical Society》2006,372(4):1525-1530
The processes are investigated by which gas loses its angular momentum during the protogalactic collapse phase, leading to disc galaxies that are too compact with respect to the observations. High-resolution N -body/SPH simulations in a cosmological context are presented including cold gas and dark matter (DM). A halo with quiet merging activity since redshift z ∼ 3.8 and with a high-spin parameter is analysed that should be an ideal candidate for the formation of an extended galactic disc. We show that the gas and the DM have similar specific angular momenta until a merger event occurs at z ∼ 2 with a mass ratio of 5:1. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and dynamical friction processes falls quickly into the centre. In contrast, gas infall through small subclumps or accretion does not lead to catastrophic angular momentum loss. In fact, a new extended disc begins to form from gas that was not involved in the 5:1 merger event and that falls in subsequently. We argue that the angular momentum problem of disc galaxy formation is a merger problem: in cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1 are expected to occur in almost all galaxies. We suggest that energetic feedback processes could in principle solve this problem, however only if the heating occurs at the time or shortly before the last substantial merger event. Good candidates for such a coordinated feedback would be a merger-triggered starburst or central black hole heating. If a large fraction of the low angular momentum gas would be ejected, late-type galaxies could form with a dominant extended disc component, resulting from late infall, a small bulge-to-disc ratio and a low baryon fraction, in agreement with observations. 相似文献
26.
Jens Wittenbrink Bernd Lehmann Michael Wiedenbeck Alex Wallianos reas Dietrich Carlos Palacios 《地学学报》2009,21(2):111-118
Quartz-hosted melt inclusions from latite dykes of the Eocene El Salvador copper porphyry system in northern Chile display wide ranges in both boron concentration (15–155 p.p.m. B) and isotope composition (δ11 B −7 to +12‰; n = 10), likely reflecting slab-derived fluid input from seawater-altered oceanic crust. In contrast, the major Miocene tin-silver and tin porphyry systems in the Bolivian back-arc region (Cerro Rico de Potosi, Chorolque, Llallagua) have distinctly different melt inclusion compositions with δ11 B of −11.4 ± 2.7‰ ( n = 10), and magmatic boron enrichment up to several hundred p.p.m. B. The `seawater' signature in the El Salvador melt inclusions explains the oxidized mineral assemblage of the copper porphyry system, as opposed to the more reduced nature of the Bolivian tin porphyry systems, which reflect intracrustal melting of pelitic rocks. 相似文献
27.
Chiara Mastropietro reas Burkert Ben Moore 《Monthly notices of the Royal Astronomical Society》2009,399(4):2004-2020
We use high-resolution N -body/smoothed particle hydrodynamics (SPH) simulations to study the hydrodynamical interaction between the Large Magellanic Cloud (LMC) and the hot halo of the Milky Way. We investigate whether ram pressure acting on the satellite's interstellar medium can explain the peculiarities observed in the H i distribution and the location of the recent star formation activity.
Due to the present nearly edge-on orientation of the disc with respect to the orbital motion, compression at the leading edge can explain the high density region observed in H i at the south-east border. In the case of a face-on disc (according to Mastropietro the LMC was moving almost face-on before the last perigalactic passage), ram pressure directed perpendicular to the disc produces a clumpy structure characterized by voids and high density filaments that resemble those observed by the Parkes H i survey. As a consequence of the very recent edge-on motion, the Hα emission is mainly concentrated on the eastern side where 30 Doradus and most of the supergiant shells are located, although some Hα complexes form a patchy distribution on the entire disc. In this scenario, only the youngest stellar complexes show a progression in age along the leading border of the disc. 相似文献
Due to the present nearly edge-on orientation of the disc with respect to the orbital motion, compression at the leading edge can explain the high density region observed in H i at the south-east border. In the case of a face-on disc (according to Mastropietro the LMC was moving almost face-on before the last perigalactic passage), ram pressure directed perpendicular to the disc produces a clumpy structure characterized by voids and high density filaments that resemble those observed by the Parkes H i survey. As a consequence of the very recent edge-on motion, the Hα emission is mainly concentrated on the eastern side where 30 Doradus and most of the supergiant shells are located, although some Hα complexes form a patchy distribution on the entire disc. In this scenario, only the youngest stellar complexes show a progression in age along the leading border of the disc. 相似文献
28.
Enhancing Geo-Service Chaining through Deep Service Descriptions 总被引:2,自引:0,他引:2
29.
Further Characterisation of the 91500 Zircon Crystal 总被引:28,自引:2,他引:28
Michael Wiedenbeck John M. Hanchar William H. Peck Paul Sylvester John Valley Martin Whitehouse reas Kronz Yuichi Morishita Lutz Nasdala J. Fiebig I. Franchi J.-P. Girard R.C. Greenwood R. Hinton N. Kita P.R.D. Mason M. Norman M. Ogasawara P.M. Piccoli D. Rhede H. Satoh B. Schulz-Dobrick O. Skår MJ. Spicuzza K. Terada A. Tindle S. Togashi T. Vennemann Q. Xie Y.-F. Zheng 《Geostandards and Geoanalytical Research》2004,28(1):9-39
This paper reports the results from a second characterisation of the 91500 zircon, including data from electron probe microanalysis, laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS) and laser fluorination analyses. The focus of this initiative was to establish the suitability of this large single zircon crystal for calibrating in situ analyses of the rare earth elements and oxygen isotopes, as well as to provide working values for key geochemical systems. In addition to extensive testing of the chemical and structural homogeneity of this sample, the occurrence of banding in 91500 in both backscattered electron and cathodoluminescence images is described in detail. Blind intercomparison data reported by both LA-ICP-MS and SIMS laboratories indicate that only small systematic differences exist between the data sets provided by these two techniques. Furthermore, the use of NIST SRM 610 glass as the calibrant for SIMS analyses was found to introduce little or no systematic error into the results for zircon. Based on both laser fluorination and SIMS data, zircon 91500 seems to be very well suited for calibrating in situ oxygen isotopic analyses. 相似文献
30.
Tadashi Kito Christine Thomas reas Rietbrock Edward J. Garnero Stuart E. J. Nippress Andy E. Heath 《Geophysical Journal International》2008,174(3):1019-1028
Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method—the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core–mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10–15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection. 相似文献