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31.
Richard de Grijs Michiel Kregel Karen H. Wesson † 《Monthly notices of the Royal Astronomical Society》2001,324(4):1074-1086
We present the first results of a systematic analysis of radially truncated exponential discs for four galaxies of a complete sample of disc-dominated edge-on spiral galaxies.
The discs of our sample galaxies are truncated at similar radii on either side of their centres. With the possible exception of the disc of ESO 416-G25, it appears that the truncations in our sample galaxies are closely symmetric, in terms of both their sharpness and the truncation length. However, the truncations occur over a larger region and not as abruptly as found in previous studies.
We show that the truncated luminosity distributions of our sample galaxies, if also present in the mass distributions, comfortably meet the requirements for longevity. The formation and maintenance of disc truncations are probably closely related to stability requirements for galactic discs. 相似文献
The discs of our sample galaxies are truncated at similar radii on either side of their centres. With the possible exception of the disc of ESO 416-G25, it appears that the truncations in our sample galaxies are closely symmetric, in terms of both their sharpness and the truncation length. However, the truncations occur over a larger region and not as abruptly as found in previous studies.
We show that the truncated luminosity distributions of our sample galaxies, if also present in the mass distributions, comfortably meet the requirements for longevity. The formation and maintenance of disc truncations are probably closely related to stability requirements for galactic discs. 相似文献
32.
33.
Paul S. Wesson 《Astrophysics and Space Science》1977,46(2):321-326
The possibility is discussed of using the latitude-dependent cutoff in the intensity and flux of cosmic ray particles reaching the surface of a planet to investigate ancient magnetic fields in the Moon, Mars and the Earth. In the last case, the method could provide a validity test for conventional palaeomagnetism. 相似文献
34.
Paul S. Wesson 《Astrophysics and Space Science》1975,37(1):235-256
Attention is given to four reasons for believing that the upper limit on the rotation of the Universe ω set by isotropy of the 3K background may not be appropriate to the local system because of its hierarchical structure. In particular, recent work of Rubinet al. (1973) on the anisotropy of Hubble's parameter (H) as determined by certain galaxies is examined. The anisotropy inH is a 1st order harmonic effect, inconsistent with an origin in an acceleration of the expansion of the Universe (U α;4≠0), but explicable as being due to a large peculiar velocity of the Local Group. This compromises limits set on ω by isotropy of the 3K field, as does the realization that only weak limits can be set if the last-scattering surface (z *) is notz *→∞ but is at smallz * (as expected in a hierarchy). In a rotating Universe, the 3-spaces of constant density cannot be orthogonal on the world lines of matter: a number test of Gödell based on this is generalized and applied (after consideration of Galactic obscuration) to the local Universe, by taking data on clusters of galaxies from the Abell and Zwicky catalogues. Data from the former give only a marginally significant result for the component ω1 of ω in one direction, but a bootstrap argument is applied which takes significance over from Abell's data (considered as a class of galaxies) to Zwicky's data (taken as a class of clusters), giving a statistically significant result on the hypothesis that clusters are the fundamental units of the Universe: it seems likely that ω1r?(const)r-n with 0?n?1 over the interval 500–1000 Mpc (H=60 km s?1 Mpc?1) with a total rotation of ω<2ω1, and ω1 = 1.2 (+0.25) x 10-18 s-1 evaluated on data out to 103 Mpc. Strictly, the quoted value of the rotation only applies to a region of space that in some sense has an isotropic limit: if the actual hierarchy has a large density-dependence away from a local origin (i.e., large thinning factor), then the numerical value of the rotation is smaller than the quoted value but still finite and significant. 相似文献
35.
Paul S. Wesson 《Astrophysics and Space Science》1975,32(2):315-330
While Euclidean models with uniform matter density have a number of radio sources of flux density greater thanF at frequencyv that varies asN(>F, v)1
F
–3/2, hierarchical models with = 0
r
–2 haveN(>F,)F
–1/2 (Section 1). Since the observed dependency isN(>F,)F
–1.8, severe density and/or luminosity evolution must be present in a workable hierarchical cosmology (Section 2). The same argument applies (Section 3) to the number of sources of apparent luminosity greater thanl,N(>l) and (Section 4) to the number of sources within redshift distancez from the local origin. To give agreement with empirical data demandsq
o=+1 and large first and second derivatives with respect to time of the number source density (Section 5). The adoption ofq
o=+1 allows one to show (Section 6) that a Lemaitre-type hierarchical Universe with a long coasting or waiting time can give agreement with observations of the numbers of QSO's etc. if the age of the Universe is more than 1013 yr. The dependence of the effective Hubble parameter onk(t),
(t) andR (Section 7) leads one to suggest that ak=0,
=0 hierarchy with 0 might be the simplest acceptable form of model Universe. Section 8 (Conclusion) points out that further data on source count anisotropies should allow the component levels of the hierarchy to be delineated. 相似文献
36.
Intermediate-term observations preceding earthquakes of magnitude 5.7 or greater in California from 1975 through 1986 suggest that: (1) The sudden appearance of earthquakes in a previously inactive area indicates an increased likelihood of a significant earthquake in that area for a period from days to years; (2) these larger earthquakes tend to occur towards the ends of creeping fault segments; (3) one large earthquake in a region increases the likelihood of a subsequent significant event in the adjacent area; and (4) marginal evidence for the occurrence of a regional deformation event suggests that such events increase the probability of earthquake occurrence throughout the entire area. A common element in many of these observed patterns appears to be the transmission and amplification of tectonic stress changes by the mechanism of fault creep, and suggests that surface fault creep is a sensitive indicator of changes in stress. The preceding critieria are used to construct a preliminary forecast of the likely locations of significant earthquakes over the next decade. 相似文献
37.
The interaction of dust grains with each other in a finite-temperature solar nebula are examined, taking into account the important fact that such grains would carry net steady-state charges like those of grains in interstellar clouds. This charge is given by the well-known Spitzer relation. It provides a screening mechanism that operates during accretion and results in bodies of differing compositions depending on the local temperature in the nebula. In a typical nebula, it is found that planetesimals of 0.1–102-cm size form in a time of order 106–107 years. These planetesimals are of iron and stone and mixed composition in the inner solar system, but of mixed composition only in the outer solar system. The predictions of this type of charged-dust accretion can be compared to known data on meteorites and the composition of the planets. 相似文献
38.
An exact solution of Einstein's field equations is given representing an inhomogeneous sphere of matter in static isothermal equilibrium with a density profile of the form R
–2. A perturbation analysis shows that initial small density inhomogeneities grow self-similarly in a non-extreme configuration for which general relativistic effects are important. The perturbed configuration develops asymptotically like the expanding solution to Einstein's equations considered previously by Henriksen and Wesson, and the present solution is therefore identified as the initial stage of that evolving model. As a cosmology, the solutions in tandem provide an expanding cosmology that did not begin as a Big Bang, but evolved from an inhomogeneous primeval atom, tending at late epochs to a model with homogeneous density. The new static solution has applications also in other branches of astrophysics. 相似文献
39.
Einstein's equations for a rotating pressure-free space-time are reduced to a system of four first-order non-linear ordinary differential equations in one self-similar dimensionless variable. Numerical results are given for the vacuum solution. A compatible thin disk can be specified by a surface density and an angular velocity . Self-similarity as a statement of the absence of scales implies that and can be written as=c
2/4Gr, =c/r, and demands that and be pure numbers. 相似文献
40.
The influence of subclustering in rich clusters of galaxies is examined using results from numericaln-body experiments. It is found that, under some conditions, the standard virial theorem is satisfied. No physical missing mass is needed because its role is replaced by the gravitational energy of the subclustering. We find that, in the Coma cluster, this effect masquerades as a missing mass about 7 times that of the physical mass, so that the apparent extant virial discrepancy (M
VT/M8) in this cluster is explained. 相似文献