共查询到20条相似文献，搜索用时 187 毫秒
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Phillip Helbig 《Monthly notices of the Royal Astronomical Society》1998,298(2):395-398
Recently, Park &38; Gott claimed that there is a statistically significant, strong, negative correlation between the image separation Δθ and source redshift z s for gravitational lenses. This is somewhat puzzling if one believes in a flat ( k = 0) universe, since in this case the typical image separation is expected to be independent of the source redshift, while one expects a negative correlation in a k = −1 universe and a positive one in a k = +1 universe. Park &38; Gott explored several effects that could cause the observed correlation, but no combination of these can explain the observations with a realistic scenario. Here, I explore this test further in three ways. First, I show that in an inhomogeneous universe a negative correlation is expected regardless of the value of k . Secondly, I test whether the Δθ– z s relation can be used as a test to determine λ0 and Ω0 , rather than just the sign of k . Thirdly, I compare the results of the test from the Park &38; Gott sample with those using other samples of gravitational lenses, which can illuminate (unknown) selection effects and probe the usefulness of the Δθ– z s relation as a cosmological test. 相似文献
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We present a two-dimensional version of the classical one-dimensional Kolmogorov–Smirnov (KS) test, extending an earlier idea due to Peacock and an implementation proposed by Fasano and Franceschini. The two-dimensional KS test is used to optimize the goodness of fit in an iterative source-detection scheme for astronomical images. The method is applied to a ROSAT /HRI X-ray image of the post-core-collapse globular cluster NGC 6397 to determine the most probable source distribution in the cluster core. Comparisons to other widely used source-detection methods, and to a Chandra image of the same field, show that our iteration scheme is superior in measuring statistics-limited sources in severely crowded fields. 相似文献
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Stéphane Rauzy Martin A. Hendry† Kenton D'Mellow 《Monthly notices of the Royal Astronomical Society》2001,328(4):1016-1026
We propose a test which allows us to check whether a luminosity function model can account for the intrinsic luminosity distribution of a magnitude–redshift sample complete in apparent magnitude. No a priori assumptions are required concerning the redshift-space distribution of the sources, so neither the clustering nor an eventual evolution of the mean number density of the galaxies affects the conclusions of the goodness-of-fit test. The statistical efficiency of the test, if used as a fitting technique for estimating the best-fitting solution of a parametric luminosity function model, is comparable to the efficiency of standard maximum likelihood fitting techniques. The goodness-of-fit test presents however a major improvement compared with fitting techniques in general: the capacity to assess the adequacy of the proposed parametric model to the data. The computational implementation of this new test is straightforward. Its potential is illustrated on the Southern Sky Redshift Survey of da Costa et al. 相似文献
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Coles Pearson Borgani Plionis & Moscardini 《Monthly notices of the Royal Astronomical Society》1998,294(2):245-258
We study the geometry and topology of the large-scale structure traced by galaxy clusters in numerical simulations of a box of side 320 h −1 Mpc, and compare them with available data on real clusters. The simulations we use are generated by the Zel'dovich approximation, using the same methods as we have used in the first three papers in this series. We consider the following models to see if there are measurable differences in the topology and geometry of the superclustering they produce: (i) the standard cold dark matter model (SCDM); (ii) a CDM model with Ω0 = 0.2 (OCDM); (iii) a CDM model with a 'tilted' power spectrum having n = 0.7 (TCDM); (iv) a CDM model with a very low Hubble constant, h = 0.3 (LOWH); (v) a model with mixed CDM and HDM (CHDM); (vi) a flat low-density CDM model with Ω0 = 0.2 and a non-zero cosmological Λ term (ΛCDM). We analyse these models using a variety of statistical tests based on the analysis of: (i) the Euler–Poincaré characteristic; (ii) percolation properties; (iii) the minimal spanning tree construction. Taking all these tests together we find that the best-fitting model is ΛCDM and, indeed, the others do not appear to be consistent with the data. Our results demonstrate that despite their biased and extremely sparse sampling of the cosmological density field, it is possible to use clusters to probe subtle statistical diagnostics of models, which go far beyond the low-order correlation functions usually applied to study superclustering. 相似文献
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L. S. Lyubimkov S. I. Rostopchin T. M. Rachkovskaya D. B. Poklad D. L. Lambert 《Monthly notices of the Royal Astronomical Society》2005,358(1):193-201
From high-resolution spectra a non-local thermodynamic equilibrium analysis of the Mg ii 4481.2-Å feature is implemented for 52 early and medium local B stars on the main sequence (MS). The influence of the neighbouring line Al iii 4479.9-Å is considered. The magnesium abundance is determined; it is found that log ɛ(Mg) = 7.67 ± 0.21 on average. It is shown that uncertainties in the microturbulent parameter Vt are the main source of errors in log ɛ(Mg) . When using 36 stars with the most reliable Vt values derived from O ii and N ii lines, we obtain the mean abundance log ɛ(Mg) = 7.59 ± 0.15 . The latter value is precisely confirmed for several hot B stars from an analysis of the Mg ii 7877-Å weak line. The derived abundance log ɛ(Mg) = 7.59 ± 0.15 is in excellent agreement with the solar magnesium abundance log ɛ⊙ (Mg) = 7.55 ± 0.02 , as well as with the proto-Sun abundance log ɛ ps (Mg) = 7.62 ± 0.02 . Thus, it is confirmed that the Sun and the B-type MS stars in our neighbourhood have the same metallicity. 相似文献
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V. Castellani S. Degl'Innocenti P. G. Prada Moroni 《Monthly notices of the Royal Astronomical Society》2001,320(1):66-72
We compare theoretical stellar models for main sequence (MS) stars with the Hipparcos data base for the Hyades cluster to give a warning against the uncritical use of available theoretical scenarios and to show how formal MS fittings can be fortuitous if not fictitious. Moreover, we find that none of the current theoretical scenarios appears able to account for an observed mismatch between theoretical predictions and observations of the coolest Hyades MS stars. Finally, we show that current theoretical models probably give too faint He burning luminosities unlike the case of less massive He burning models, with degenerate progenitors, which have been suggested to suffer the opposite discrepancy. 相似文献
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We investigate the role that dry mergers play in the build-up of massive galaxies within the cold dark matter paradigm. Implementing an empirical shut-off mass scale for star formation, we find a nearly constant dry merger rate of ∼6 × 10−5 Mpc−3 Gyr−1 at z ≤ 1 and a steep decline at larger z . Less than half of these mergers are between two galaxies that are morphologically classified as early-types, and the other half is mostly between an early- and late-type galaxy. Latter are prime candidates for the origin of tidal features around red elliptical galaxies. The introduction of a transition mass scale for star formation has a strong impact on the evolution of galaxies, allowing them to grow above a characteristic mass scale of M *, c ∼ 6.3 × 1010 M⊙ by mergers only. As a consequence of this transition, we find that around M *, c , the fraction of 1:1 mergers is enhanced with respect to unequal mass major mergers. This suggests that it is possible to detect the existence of a transition mass scale by measuring the relative contribution of equal mass mergers to unequal mass mergers as a function of galaxy mass. The evolution of the high-mass end of the luminosity function is mainly driven by dry mergers at low z . We however find that only 10–20 per cent of galaxies more massive than M *, c experience dry major mergers within their last Gyr at any given redshift z ≤ 1 . 相似文献
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D. Harari S. Mollerach E. Roulet 《Monthly notices of the Royal Astronomical Society》2009,394(2):916-922
We analyse in detail the two-dimensional Kolmogorov–Smirnov test as a tool to learn about the distribution of the sources of the ultra-high energy cosmic rays. We confront, in particular, models based on active galactic nuclei observed in X-rays, galaxies observed in H i and isotropic distributions, discussing how this method can be used not only to reject isotropy but also to support or reject specific source models, extending results obtained recently in the literature. 相似文献
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A statistical model is formulated that enables one to analyse jointly the times between maxima and minima in the light curves of monoperiodic pulsating stars. It is shown that the combination of both sets of data into one leads to analyses that are more sensitive. Illustrative applications to the American Association of Variable Star Observers data for a number of long-period variables demonstrate the usefulness of the approach. 相似文献
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S. Cassisi V. Castellani P. Ciarcelluti G. Piotto M. Zoccali 《Monthly notices of the Royal Astronomical Society》2000,315(4):679-688
We make use of the 'Next Generation' model atmospheres of Allard et al. and Hauschildt, Allard & Baron to compute theoretical models for low- and very-low-mass stars for selected metallicities in the range Z =0.0002 to 0.002. On this basis, we present theoretical predictions covering the sequence of H-burning stars as observed in Galactic globulars from the faint end of the main sequence up to, and beyond, the cluster turn-off. The role played by the new model atmospheres is discussed, showing that present models appear in excellent agreement with models by Baraffe et al. as computed on a quite similar physical basis. One finds that the theoretical mass–luminosity relations based on this updated set of models are in good agreement with the empirical data provided by Henry & McCarthy. Comparison with HST observation discloses that the location on the colour–magnitude diagram of the lower main sequence in Galactic globular clusters appears again in good agreement with the predicted sensitive dependence of these sequences on the cluster metallicity. 相似文献
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Margrethe Wold Mark Lacy Per B. Lilje Stephen Serjeant 《Monthly notices of the Royal Astronomical Society》2001,323(1):231-247
We quantify the galaxy environments around a sample of 0.5≤ z ≤0.8 radio-quiet quasars using the amplitude of the spatial galaxy–quasar correlation function, B gq . The quasars exist in a wide variety of environments; some sources are located in clusters as rich as Abell class 1–2 clusters, whereas others exist in environments comparable to the field. We find that, on average, the quasars prefer poorer clusters of ≈Abell class 0, which suggests that quasars are biased tracers of mass compared with galaxies. The mean B gq for the sample is found to be indistinguishable from the mean amplitude for a sample of radio-loud quasars matched in redshift and optical luminosity. These observations are consistent with recent studies of the hosts of radio-quiet quasars at low to intermediate redshifts, and suggest that the mechanism for the production of powerful radio jets in radio-loud quasars is controlled by processes deep within the active galactic nucleus itself, and is unrelated to the nature of the hosts or their environments. 相似文献